WO1995004607A1

WO1995004607A1 - Lead-free galvanizing technique

Info

Publication number: WO1995004607A1
Application number: PCT/US1994/008826
Authority: WO
Inventors: Yum Gerenrot; David Leychkis; Thomas L. Ranck; James L. Griffin; Gary Stefanick
Original assignee: Ferro Technologies, Inc.
Priority date: 1993-08-05
Filing date: 1994-08-04
Publication date: 1995-02-16
Also published as: AU7554394A

Abstract

A process for galvanizing steel articles comprises applying an aqueous preflux to the surfaces of the articles to be galvanized, preheating each of the articles in a non-reducing atmosphere to dry the preflux and impart significant additional energy in the form of heat content thereto over and above the amount of energy imparted to the articles as a result of drying the preflux, and applying molten zinc to the surfaces to be galvanized to form a galvanized coating thereon.

Description

LEAD-FREE GALVANIZING TECHNIQUE CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of application Serial No.

08/102,570, filed August 5, 1993 and also application Serial No. , filed June 21, 1994 (attorney docket 19328/00112). The disclosures of these applications are incorporated herein by reference.

BACKGROUND OF THE INVENTION

The present invention relates to galvanizing steel.

Galvanizing steel articles is a well-known technique for corrosion protection utilized by industry worldwide. In this process, a solidified layer of zinc is formed on the article surfaces by dipping the article into a molten zinc bath. The zinc coating layer formed as a result is strongly adhered to the surfaces of the article by an iron/zinc intermetallic alloy which forms during the galvanizing process.

It is well known that oxides and other foreign materials ("soil") on the surfaces of the steel article interfere with the chemistry of the galvanizing process and prevent formation of a uniform, continuous, void-free coating. Accordingly, various techniques and combinations of techniques have been adopted in industry to reduce, eliminate, or at least accommodate, oxides and soil as much as possible.

For many decades lead has been used as a very important galvanizing bath component in both batch and continuous galvanizing processes. Lead significantly reduces surface tension of molten zinc and increases its fluidity. This results in better wetting of the steel surface to be coated and higher drainage after withdrawal of the parts from the kettle.

Lead also creates a specific crystalline pattern in the galvanized surface, so called spangles, which do not influence corrosion resistance and have only an aesthetic advantage.

By far the greatest benefit of lead, however, is that it makes galvanizing less demanding and less sensitive to many unfavorable circumstances such as insufficient pickling and cleaning, the presence of a high degree of rust and moisture and so on. Accordingly, while lead has traditionally been added for the purpose of reducing surface tension and increasing fluidity, an important effect of lead has nonetheless been that it allows the galvanizing process to "accommodate" more oxides and soil than if lead were absent.

Unfortunately, lead addition is no longer suitable in connection with producing galvanized pipe and other articles intended to handle drinking water. When zinc corrodes, small but significant amounts of lead leach into water passing through galvanized pipe. For this reason, use of lead galvanized pipe for drinking water has been outlawed in a number of states, beginning in 1995.

Many attempts have been made in the past to galvanize steel articles without lead. Unfortunately, all such attempts, other than those based on aluminum as discussed below, have failed. As a result, the best that can be obtained today in continuous industrial processes, in terms of reducing lead content, is experienced in the production of steel strip in which the lead content of the galvanized bath has been reduced to 0.2% . In batch industrial operations, the amount of lead in the zinc kettle remains at its traditional value of 0.5 to 1.0%.

Improvement in the properties of galvanized products can be achieved by alloying zinc with aluminum. Addition of 5% aluminum produces an alloy with the lowest melting temperature. This alloy exhibits improved drainage properties relative to pure zinc. Moreover, galvanized coatings produced from this zinc-aluminum alloy have greater corrosion resistance, improved formability and better paintability than those formed from pure zinc. Furthermore, galvanized coatings free of lead can be made with this technology. However, zinc-aluminum galvanizing is known to be particularly sensitive to surface cleanliness so that various difficulties, such as insufficient steel surface wetting and the like, are often encountered when zinc-aluminum alloys are used in galvanizing. This, in turn, leads to the production of coatings with bare spots and voids. These difficulties may be attributable to the absence of lead on one hand, and aluminum oxide emerging on the steel surfaces on the other. The actual mechanism of this phenomenon is still unknown.

Thus, low lead, lead-free and zinc-aluminum galvanizing have the common feature that they are very sensitive to steel surface cleanliness, that is the presence of oxides, metal fines, oil and so on.

As mentioned above, many techniques and combinations of techniques have been adopted in industry to reduce, eliminate, or at least accommodate, oxides and soil as much as possible. In essentially all these processes, organic soil, that is, oil, grease, rust preventive compounds, is first removed by contacting the surface to be coated with alkaline aqueous wash (alkaline cleaning). This may be accompanied by brush scrubbing and/or electrocleaning, if desired. Then follows rinsing with water, contacting the surface with an acidic aqueous wash for removing iron fines and oxides (picking), and finally rising with water again.

All these cleaning-pickling-rinsing procedures are common for most galvanizing techniques and are carried out in industrial practice more or less accurately.

One of the most widely used galvanizing techniques for dealing with oxides and soil, used in combination with the above cleaning, pickling, rinsing procedure, is the application of a preflux to the article surfaces prior to the galvanizing operation. Such prefluxes normally comprise aqueous zinc chloride and also typically contain ammonium chloride as well. The presence of zinc chloride and preferably ammonium chloride has been found to improve wetability of the article surfaces by molten zinc and thereby promote formation of a uniform, continuous, void-free coating.

Another galvanizing technique, which may be used with or without prefluxing, is to cover the galvanizing bath with a top flux. Top fluxes also typically are composed of zinc chloride, and usually ammonium chloride as well, but in this case these salts are molten in form and present floating on the top of the galvanizing bath. The purpose of a top flux, like a preflux, is to supply zinc chloride and preferably ammonium chloride to the system to aid wetability during galvanizing. In this case, all surface oxides and soil which are left after cleaning- pickling-rinsing are removed when the steel part passes through the top flux layer and is dipped into the galvanizing kettle. Top fluxes have the further advantage that they reduce or eliminate spattering when the steel article is dipped into the galvanizing bath, which oftentimes occurs if the article is still wet with rinse water or preflux.

Another known galvanizing technique also used in combination with the above cleaning, pickling, rinsing pretreatment is preheating. Typically, preheating is done only to dry the article surfaces prior to dipping into the galvanizing bath. In certain processes, however, namely continuous processes using zinc-aluminum alloys as the galvanizing medium, preheating is done under more vigorous conditions in reducing atmospheres. This not only eliminates reoxidation of previously cleaned, pickled and rinsed surfaces but, also, it is believed, actually removes any residual surface oxides and soil that might still be present. Vigorous preheating, i.e., under conditions more severe than necessary merely to dry the article, also represents a convenient way to add heat to the galvanizing bath and thereby reduce the heat load on the system used to keep the galvanizing bath at the appropriate temperatures. As mentioned above, galvanizing with aluminum-zinc alloys has become increasingly popular in modern times. For example, U.S. 4,448,748, the disclosure of which is incorporated herein by reference, describes a technique for using aluminum as a processing aid in galvanizing in which 5 % aluminum and a small amount of rare earth metals in the form of mischmetals are incorporated into the galvanizing bath. The alloy so formed, which is known as "Galfan," is so effective that the vast majority of galvanized steel strip made today worldwide is produced using this alloy and technology.

However, this technique is effective only if practiced in a continuous galvanizing line in which the article to be galvanized is heated in a furnace having a reducing atmosphere between the final rinsing and galvanizing steps.

Another technique used for producing zinc-aluminum galvanized coatings comprises electro-coating the steel articles with a thin (0.5-0.7 μm) layer of zinc (hereafter "prelayer"), drying in a furnace with an air atmosphere and then dipping the precoated article into the galvanizing kettle. This technique, together with Galfan, is now widely used in industry for hot-dip coating of steel tubing in continuous lines and, to a lesser extent for the production of steel strip. Although this technique does not require continuous processing with reducing atmospheres, it is inherently disadvantageous because of the additional metal-coating step required.

Industrially, galvanizing is practiced either in batch operation or continuously. Continuous operation is typically practiced on articles amenable to this type of operation such as wire, sheet, strip, tubing, and the like. In continuous operation, transfer of the articles between successive treatments steps is done continuously and automatically, with operating personnel being present to monitor operations and fix problems if they occur. Usually, production volumes in continuous operations are high, and transfer between successive treatments steps is very rapid. For example, in a continuous galvanizing line involving application of an aqueous preflux followed by drying in a furnace, the period of time elapsing between removal of the article from the preflux tank and dipping in the galvanizing bath is usually on the order of 10 to 30 seconds, usually not more . than one minute.

Batch operations are considerably different. Batch operations are favored where production volumes are lower and the parts to be galvanized are more complex in shape. For example, various fabricated steel items, structural steel shapes and pipe are advantageously galvanized in batch operations. In batch operations, the parts to be processed are manually transferred to each successive treatment step in batches, with little or no automation being involved. This means that the time each piece resides in a particular treatment step is much longer than in continuous operation, and even more significantly, the time between successive treatment steps is much wider in variance than in continuous operation.

For example, in a typical batch process for galvanizing steel pipe, a batch of as many as 100 pipe after being dipped together in a preflux bath is transferred by means of a manually operated crane to a table for feeding, one at a time, into the galvanizing bath. In this procedure, individual pipe can sit on the table anywhere from five to thirty minutes before being dipped into the galvanizing batch, depending on how many pipe are already on the table from a previous batch and also on whether a particular pipe is at the beginning or end of the batch in terms of the order in which the pipe are dipped into the galvanizing bath.

Because of the procedural and scale differences between batch and continuous operations, techniques particularly useful in one type of operation are not necessarily useful in the other. For example, the above-noted technique of using a reducing furnace is restricted to continuous operation only, at least on a commercial or industrial scale. In continuous processes, the delay time between final rinse and galvanizing is very short, and accordingly it is in this environment where there is very little time for the parts to reoxidize that heating in reducing atmospheres is believed to be sufficiently effective in preventing oxidation. Also, the high production rates involved in continuous processes make preheating a valuable aid in supplying make-up heat to the galvanizing bath. In batch processes, delay times are much longer and moreover production rates, and hence the rate of heat energy depletion of the galvanizing bath, are much lower.

In any event, at the present time, commercial processes for producing high quality, continuous void-free coatings either require the galvanized coating to contain significant amounts of lead or, in the alternative, adopt one or more processing aids, such as continuous operation with reducing furnaces or electrocoating of a zinc prelayer, to be successful. Accordingly, there is no technique available at the present time for producing low lead (0.3% or less, preferably 0.2% or less, more preferably 0.1 % or less) or lead-free (0.003% or less, lead) galvanized coatings, without the use of a zinc or other metal prelayer, which can be carried out industrially in batch operation or in continuous operation without a reducing furnace.

Accordingly, it is an object of the present invention to provide a new galvanizing process for producing uniform, continuous, void-free coatings on steel articles using low lead or lead-free galvanizing baths which can be carried out in batch operation or continuously, either with or without a reducing furnace, and which do not require a zinc or other prelayer prior to application of the galvanized coating.

In addition, it is a further object of the present invention to provide a new galvanizing process as described above which can be practiced with galvanizing baths in which aluminum is an optional component, i.e. with low lead or lead-free galvanizing baths which contain aluminum or which are free of aluminum, as desired. In addition, it is a further object of the present invention to provide novel aqueous prefluxes as well as novel top fluxes for use in such processes.

And, still further, it is also an object of the present invention to provide a novel procedural technique for carrying out the inventive process, particularly in the batch mode of operation.

SUMMARY OF THE INVENTION

These and other objects are made possible by the present invention which is based on the discovery that the incipiently-formed solidified zinc layer, that is, the solidified layer of zinc which inherently forms on the surfaces of a steel article by freezing immediately upon dipping of the article into the galvanizing bath, plays an important role in the galvanizing process and that reducing the time it takes this incipiently-formed solidified zinc layer to remelt significantly improves the quality of hot-dip coatings produced by this process.

In particular, it has been observed that wetting of the steel surfaces will not occur while the incipiently-formed frozen zinc layer remains in solid form and consequently there will be no interactions between iron and zinc and hence no formation of the intermetallic layer, during this time. Moreover, it has also been recognized that if this frozen zinc layer remains in existence too long, unwanted competing chemical reactions will occur at the article's surfaces, particularly if aluminum is present in the galvanizing bath, which will deleteriously affect the quality of the galvanized coating ultimately produced.

Accordingly, the present invention adopts one or more techniques to promote rapid remelting of the incipiently-formed frozen zinc layer to thereby allow wetting of the article surfaces and formation of the desired iron-zinc metallic bond before significant adverse chemical reactions can occur at the article's surfaces.

The particular technique adopted by the present invention for this purpose usually involves preheating the steel article after application of the preflux. By increasing the heat content of the steel article through heating prior to dipping in the galvanizing bath, less heat from the bath itself is necessary for remelting of the frozen zinc layer and accordingly less time is required for this remelting operation.

To facilitate this preheating procedure, the present invention also provides novel, thermally-resistant prefluxes. These prefluxes are particularly resistant to degradation at high temperature, and thereby allow heating of the steel article to higher temperatures for longer periods of time than conventional prefluxes. This, in turn, enables more heat to be imparted to the steel article prior to galvanizing and thereby enables the incipiently-formed frozen zinc layer to melt even faster.

In addition, the present invention provides additional novel prefluxes especially useful in zinc-aluminum galvanizing, which when combined with preheating in accordance with the present invention, makes possible the production of aluminum containing galvanized coatings without using furnaces with reducing atmospheres or zinc prelayers.

Furthermore, the present invention also provides novel top-fluxes which make low-lead or lead-free galvanizing less demanding as to steel surface cleanliness and thereby allow the production of continuous, void-free zinc coatings with preheating using the thermally resistant prefluxes of the invention or conventional prefluxes.

Accordingly, by adopting the appropriate combination of preheating procedure and preflux composition, it is possible in accordance with the present invention to produce uniform, continuous, void-free low-lead or lead-free zinc coatings, with or without aluminum, by batch operation or continuous operation not involving a furnace with reducing atmosphere and also not involving the application of a zinc or other metal prelayer. DETAILED DESCRIPTION

Surface Cleaning and Pickling

As well appreciated by those skilled in the art, it is important in any galvanizing process for the surface of the article to be galvanized to be cleaned before prefluxing. But the quantitative level of cleanliness has not been discussed in the art before. For different coating processes, different levels of cleanliness are demanded. What is clean for painting, is not clean for electroplating, and what is clean for conventional galvanizing, is not clean enough for zinc-aluminum hot-dip coating.

As previously mentioned, low lead, lead-free and zinc-aluminum galvanizing are especially sensitive to contamination of the surface to be coated. Accordingly, in developing the present invention research was conducted to determine quantitatively the levels of cleanliness which are acceptable for low- level, lead-free and zinc-aluminum galvanizing. For this purpose a technique known as evaporative rate analysis was used. In accordance with this technique, a predetermined amount of a volatile radio-active compound is deposited on the surface to be tested, and the rate of evaporation of this compound is measured by a detector. Any organic soil on the article surfaces retards evaporation of the radioactive compound, while on the contrary, evaporation from clean surfaces proceeds very quickly. The accuracy of this technique is very high, and as little as 0.01 μg/cm² of organic soil can be detected.

In accordance with the present invention, it is desirable that, regardless of the particular embodiment or feature of the invention adopted, the surface to be galvanized have a residual (that is after cleaning) soil content of no more than 0.8 μg/cm². This corresponds to a cleanliness such that there is no waterbreak. Preferably, the articles to be galvanized should have a residual soil content of no more than about 0.4 μg/cm².

Techniques for achieving this degree of surface cleanliness are known. Conventionally, such techniques involve alkaline cleaning, followed by aqueous rinsing, pickling in acid and finally aqueous rinse. Although all of these procedures are well known, the following description is presented for the purpose of completeness.

Alkaline cleaning can conveniently be carried out with an aqueous alkaline composition also containing phosphates and silicates as builders as well as various surfactants. The free alkalinity of such aqueous cleaners can very broadly, with free alkalinity ranges on the order of 1.2 to 2.4% being typical.

Next, the article is pickled by immersing the article in aqueous hydrochloric or sulfuric acid, usually at a temperature from ambient to about 60°C. Acid concentrations on the order of 5 to 10% are normally employed, although more concentrated acids can be used. The time of pickling typically ranges from 5 to 30 seconds, more typically 10 to 15 second.

In order to prevent over-pickling, it is also conventional to include in the pickling liquid at least one corrosion inhibitor, typically a cationic or amphoteric surface active agent. Typically, such inhibitors are present in the amount of about 0.02 to 0.2%, preferably 0.05 to 0.1 %.

Pickling can be accomplished simply by dipping the article in the pickling tank. Additional processing steps can also be employed. For example, the article can be agitated either mechanically or ultrasonically, and/or an electric current can be passed through the article for electropickling. As appreciated by those skilled in the art, these additional processing aids usually shorten pickling time significantly.

After acid pickling, the articles are again rinsed with water to remove residual acid.

In any event, by following the foregoing procedures or combinations thereof, it will be possible to easily clean the surfaces of the article to be galvanized to have the desired soil contamination of no greater than 0.8 μg/cm², preferably no greater than 0.4 μg/cm². Preheating As previously mentioned, the presence of lead in a zinc galvanizing kettle somehow compensates for bad galvanizing practices. More than that, lead allows good coating to be obtained on insufficiently pickled and cleaned surfaces. Sometimes, steel surfaces with heavy rust can be galvanized when lead is present, although adhesion, of course, is bad. As a whole, lead makes galvanizing a very undemanding and forgiving process.

This positive influence of lead is usually ascribed to a reduction in the surface tension of molten zinc from 760 to 520 dyn/cm at 1 wt. % lead. But it is believed that the high wetting ability of zinc containing lead cannot be explained solely by reason of surface tension drop. That is why heat transfer and metallurgical aspects of galvanizing were investigated in an attempt to identify lead's influence.

It is well known when a steel part at ambient temperatures is immersed in molten zinc, a layer of solidified metal is incipiently formed on the steel surface. It is also known that the bigger part mass and the lower temperature of the kettle and the part, the thicker is the frozen layer.

The effect of this phenomenon is believed to be as follows: When the steel part is dipped into the galvanizing bath, and during the time when the incipiently-formed frozen zinc layer remains in solid form, there is no wetting of the article surface by molten zinc and hence no adhesion of frozen zinc to the steel surface. This can be easily proved, for example, by withdrawing a panel from the kettle and cooling. When this is done shortly after the article is dipped into the galvanizing bath, a thick (3 to 5 mm) frozen zinc layer is separated from the panel. A dark steel surface with flux remnants thereon is revealed.

As appreciated by those skilled in the art a steel part when placed in a galvanized kettle, is heated by the heat content of the molten zinc and the frozen zinc layer on the part surface is gradually melted. When this layer is totally melted, the metal surface becomes wetted by molten zinc and growth of the zinc- iron intermetallic alloy starts. In developing the present invention, the foregoing phenomenon was studied in detail. In this study, a series of experiments was conducted. In each experiment, steel panels measuring 4 x 150 x 150 mm, after cleaning, rinsing, pickling, rinsing and dipping in a conventional aqueous preflux, were dried in an oven with an atmosphere of air at 120-150°C. The dried panels were then cooled to ambient temperature, and then immersed in a molten galvanizing bath. The panels were then individually withdrawn from the bath at intervals of two seconds, with the total time in the bath for all panels ranging from 4 to 20 seconds. The panels were then visually inspected to determine if the incipiently formed frozen zinc layer thereon had remelted. A number of different experiments was conducted at various different conditions, in particular at kettle temperatures of 440°C, 454°C and 468°C and at lead levels of 0.0%, 0.2%, 0.5% and 1.0%.

It was discovered that, at each temperature, the rate of remelting of the frozen zinc layer was directly proportional to the lead content. The higher the lead level, the faster the frozen zinc layer remelted.

For example, in a kettle without lead, at 454°C the frozen zinc layer totally remelted in 12 to 15 seconds. However, when 1 % lead was included in the molten zinc, melting time was reduced to 5 to 7 seconds. This shows that a very important effect of lead, in galvanizing, is to reduce the remelting time of the incipiently-formed frozen zinc layer.

For comparison, a steel panel after dipping into a conventional preflux was preheated to 200°C rather than 100° - 120°C and then immersed in a galvanizing bath maintained at 454°C and containing no lead. In this case, it was found that the frozen zinc layer totally remelted after 5 to 6 seconds. In effect, this shows that preheating to 200 °C accomplishes the same result, in terms of the time taken for remelting the incipiently-formed frozen zinc layer, as including 1 % lead in the galvanizing bath. This, in turn, suggests that preheating will add significant heat to steel articles galvanized on an industrial scale and, as a result, will enable significant reduction in lead content in galvanizing baths without deleterious effect.

The phenomenon described above may be explained by lead liquation at the grain boundaries during crystallization (lead solubility in solid zinc does not exceed 0.003%), and its remelting by heating after the panel is immersed in the kettle but before the panel reaches kettle temperature. This means easier zinc grain separation and faster frozen layer remelting. There may exist some other explanation why lead accelerates zinc remelting, but it should be clearly understood that lead dramatically reduces the time period between immersing the panel in the kettle and its wetting by molten zinc which occurs when the frozen zinc layer is totally remelted.

It is well known, that liquid prefluxes do not protect steel article surfaces from oxidation. If a steel article is not dried immediately after withdrawal from the preflux tank, it becomes green (ferrous hydroxide is created) and then red (ferric hydroxide is formed).

In accordance with the present invention, it has been further found that even if the article after pifefluxing is immediately dried, steel surface oxidation nonetheless takes place, though to a lesser degree. Thus, by using an optically stimulated electron emission technique (OSEE), it has been discovered that steel surfaces carrying dried preflux continue to be oxidized and further that the oxidation process proceeds faster when the steel part carrying the preflux is preheated in a furnace. In particular, the higher the preheating temperature, then the heavier the oxide film will be on the steel surface under the preflux layer. These iron oxides (but not rust) are reduced, when the flux melts and acts like hydrochloric acid.

Now, if a cold or insufficiently warm part is immersed in molten zinc and a layer of frozen zinc is created, some time will pass until this layer is melted. During this time, the flux may be destroyed or at least removed from the article surface before it can fulfill its function. In conventional galvanizing, this does not matter. However, in low lead or no lead systems, the steel surfaces cannot be sufficiently wetted by the molten zinc and consequently continuous, void-free zinc coatings cannot be formed.

In accordance with the invention, however, if the steel article after prefluxing is dried and preheated to a temperature of 150-350°C (depending on article mass) the incipiently-formed frozen zinc layer will either not form at all, or if formed, will remelt in several seconds. As a result, the molten flux components are capable of reducing all oxides, as intended, which in turn promotes good wetting of the steel surfaces by the molten zinc and hence production of high quality coatings.

In zinc-aluminum alloy galvanizing, the advantages of preheating in accordance with the present invention are even more pronounced.

When aluminum is present in the galvanizing bath, it is believed that aluminum chloride, A1C1₃ is formed by the following reactions

(1) Zn + 2NH₄C1 - 2NH₃ t + ZnCl₂ + H₂t

(2) Zn + 2NH₄C1 - ZnCl₂(NH₃)₂ + ZnCl₂ + H₂, and

(3) 2A1 + 3ZnCl₂ - 2A1C1₃ + 3Zn and further that the aluminum chloride so formed readily reacts with ferrous oxide by the following reaction

(4) 3FeO + 2A1C1₃ - Al₂O₃ + 3FeCl₂ + 370 Kcal/mol. Because aluminum chloride is a gas, it is believed that the aluminum chloride formed as above remains trapped on the article's surfaces by the incipiently-formed frozen zinc layer when the article is dipped into the galvanizing bath. Reaction (4) is therefore driven to its thermodynamic conclusion if the incipiently-formed frozen zinc layer is not remelted, and the article surfaces thereby become coated with aluminum oxide and cannot be wetted by molten zinc.

In accordance with the present invention, certain novel prefluxes are also provided which retard the formation of aluminum chloride. Accordingly, when articles bearing such prefluxes are used in aluminum-zinc alloy galvanizing, preheating in accordance with the present invention allows the incipiently-formed zinc layer to be removed quick enough so that little or no formation of aluminum chloride takes place on the article's surfaces. This, in turn, prevents substantial formation of aluminum oxide on the article surfaces and thereby promotes the formation of good quality coatings.

Thus, in accordance with the present invention, preheating is used as an important procedural step in galvanizing, particularly with low-lead, lead-free and zinc-aluminum galvanizing baths. As used herein, "preheating" means more than merely applying heat to the surfaces of the article to remove moisture therefrom. Rather, "preheating", at least as it relates to applying heat to an article whose surfaces are wet with a liquid such as rinse water or aqueous preflux, refers to the application of sufficient heat to first dry the article surfaces and thereafter to impart further heat energy to the article so as to increase its heat content.

In other words, when an article whose surfaces contain a liquid such as a rinse water or an aqueous preflux is heated, one result of the heating operation is to evaporate moisture from the liquid and thereby dry the article surfaces. However, some of the energy used for this purpose will inherently transfer to the article itself, as heat content, meaning that as a result of the drying operation, the heat content of the article will inherently increase at least to some extent. In accordance with the present invention, however, preheating of articles carrying liquids on their surfaces is carried out beyond the point necessary to just dry the article surfaces. As a result, significant additional heat is imparted to the article over and above the amount of heat transferred to the article incident drying of its surfaces.

Exactly how much heat this might be depends on many factors and is measurable, in absolute terms, only with great difficulty or not at all. In accordance with the invention, therefore, the surface temperature of the article to be galvanized, which can be determined fairly easily with a thermocouple, for example, is used as one indicator of what this increase in heat content should be. Following this procedure, preheating should be done in accordance with the present invention until the article surfaces exhibit a temperature of at least 120°C, preferably 150-350°C.

Another indicator of the amount of preheating to which the articles should be subjected in accordance with the present invention is the amount of time it takes for the incipiently-formed frozen zinc layer to melt. In accordance with the invention, the amount of preheating used is preferably enough so that the incipiently-formed frozen zinc layer melts within one minute, more preferably within about 30 seconds of immersion of the steel article in the galvanizing bath. When the galvanizing bath contains aluminum, melting times on the order of 10 seconds or less, for example, 2 to 5 seconds, are preferred. This melting time can be easily determined by visual inspection.

Accordingly, the present invention, in one embodiment, provides a novel technique for reducing the amount of lead necessary for producing high quality galvanized coatings in batch operation or continuous operation, the technique comprising applying an aqueous preflux to the article to be galvanized and thereafter preheating the article to dry the preflux and, in addition, impart significant additional energy in the form of heat content to the article over and above the amount of energy imparted to the article as a result of simple drying.

Thermally-Resistant Prefluxes

As well appreciated by those skilled in the art, conventional prefluxes containing, for example, only zinc chloride and ammonium chloride are unable to withstand high temperature preheating (350°C) for short periods of time (1 minute or less) or lower temperature heating (e.g. 250°C) for longer periods of time (3 to 15 minutes, for example). Accordingly, the amount of heat that can be imparted to the steel articles in accordance with the inventive technique of vigorous preheating will be limited, as a practical matter, if conventional prefluxes are used. Accordingly, in another aspect of the present invention, a series of novel prefluxes exhibiting higher heat resistance than conventional prefluxes is provided.

These prefluxes, which are described in our prior application serial number , filed June 21, 1994 (Attorney Docket No. 19328/00112), comprise zinc chloride and ammonium chloride mixture, preferably in aqueous form, to which has been added about 0.1 to 1.0%, preferably 0.5 to 0.8%, boric acid for the purpose of enhancing thermal resistance. This concentration range of boric acid is critical and lies beyond the scope of U.S. Patent No. 3,740,275 (5 to 15%). This concentration range (0.1 to 1.0%) produces an unexpected result - it increases preflux thermal stability, which is not disclosed in the cited U.S. patent. Increasing boric acid concentration significantly over 1.0% causes precipitation of some zinc-boric acid reaction products and decreases the wetability of molten zinc even if containing lead.

In addition to boric acid, other analogous compounds can also be used, such as the salts of boric acid, especially the alkali and alkaline earth metal salts, particularly the sodium and potassium salts. Salts of other cations which do not adversely affect the other ingredients of the system can also be employed.

It has also been determined that ammonium chloride should preferably be present in the prefluxes of this embodiment of the invention. Otherwise, the surfaces of the steel article to be galvanized will not be sufficiently wetted by the molten zinc. Thus, while any amounts of zinc chloride and ammonium chloride can be included in these prefluxes, it is preferred that they contain 8 to 30%, more preferably 15 to 20% zinc chloride and 2 to 20% ammonium chloride.

Moreover, it has also been found that the fluidity of these prefluxes can be improved, and hence wetting of the steel surface by the molten zinc bath, by including in the preflux a metal halide, particularly a chloride of an alkali or alkaline earth metal such as potassium, sodium, calcium, magnesium, and others. Preferably, the amount of such metal halide included in the preflux is 0.1 to 5 % , more preferably about 0.1 to 2%.

It is well known that steel parts corrode in aqueous prefluxes and that iron accumulates in the preflux tank. It is also known that such iron accumulation causes reduced wetting of the steel surfaces by molten zinc in the galvanizing bath and that this problem is particularly acute when lead is absent from the system. Maintaining the preflux at elevated temperature, for example 80 to 90°C, only exacerbates the problem. Accordingly, to deal with this problem, it is preferable in accordance with this embodiment of the invention to include in the preflux an amino derivative corrosion inhibitor.

Amino derivative corrosion inhibitors are commonly added to pickling tanks for their corrosion protection properties. They are not used, however, in prefluxes. Rather, various types of surfactants commonly employed in prefluxes, primarily for improving wetability, are also relied on to promote corrosion inhibition.

In accordance with the present invention, it has also been found that inclusion of such amino derivative corrosion inhibitors in the prefluxes of this embodiment of the invention can reduce the rate of iron accumulation in the preflux tank by more than 2 to 3 times as compared with surfactants conventionally used in aqueous prefluxes.

By "amino derivative corrosion inhibitors" is meant a compound which inhibits the oxidation of steel particularly in acid environments and which also contains an amino group. Aliphatic alkyl amines and quaternary ammonium salts (preferably C_λ to C alkyls) are examples of the type of amino compounds which are useful. Specific examples of useful compounds are hexamethylenediamine tetra, hexapotassium hexamethylenediamine and alkyl dimethyl quaternary ammonium nitrate. The amount of this inhibitor may vary in the range of about 0.02 to 2.0%, preferably 0.1 to 2.0%, more preferably 0.5 to 1.0%. Inhibitor use in accordance with this embodiment of the present invention is more important in low lead and lead-free galvanizing, which processes are very sensitive to high level of iron in the preflux.

The efficiency of the inhibitor used in this embodiment of the invention can be increased by also including in the preflux a nonionic surfactant which, when combined with the other ingredients therein, produces a preflux having a surface tension of 27 to 30 dyn/cm. Normally this translates to a non-nonionic surfactant concentration of about 0.02 to 2.0%, preferably 0.5 to 1.0%. Essentially any type of nonionic surfactant can be used for this purpose. Examples of suitable surfactants for this purpose are ethoxylated alcohols such as nonyl phenol ethoxylate, other alkyl phenols such as Triton X-102 and Triton N- 101, both available from Union Carbide, and block copolymers of ethylene oxide and propylene oxide, such as L-44 available from BASF.

The thermally resistant prefluxes of this embodiment exhibit excellent thermal resistance properties. As mentioned above, conventional prefluxes are unable to withstand high temperature preheating (350 °C) for short periods of time (less than 1 minute), which would be appropriate for continuous processes, or relatively longer preheating periods of time (3 to 15 minutes) at lower temperatures (250°C), which would be appropriate for batch technology. The prefluxes of this embodiment of the invention, however, can withstand such rigorous preheating and will not decompose or bum when heated to 350 °C for 1 minute or 200 to 250°C for 3 to 15 minutes. Thus, the prefluxes of this embodiment are particularly useful when vigorous preheating conditions are employed, for example, at 180° to 350 °C for one minute or more depending on the specific temperature employed.

It should also be understood that the thermally resistant prefluxes of this embodiment of the invention can also function successfully at lower preheating temperatures (120 to 180°C or lower), since they are universal in coverage in terms of their temperature preheating range. They can also be used with conventional galvanizing zinc baths, e.g., containing up to 1 % or more lead, or containing 0.2 to 1 % lead, although they are particularly useful in low lead and lead-free galvanizing. They can also be used with galvanizing baths containing aluminum, although it is preferable to keep the aluminum content below about 0.2%.

This embodiment of the present invention, based on the use of the inventive high temperature prefluxes, is illustrated by the following examples:

Example 1

Steel panels measuring 4 x 50 x 150 mm in size and after cleaning, pickling and rinsing, are immersed in a preflux aqueous solution at a temperature of 80°C and containing 17% zinc chloride, 5.0% ammonium chloride, 0.5% boric acid, 0.05% nonionic surfactant Merpol HCS (Dupont) and 0.04% inhibitor Ethomeen (Akzo US). The panels are then heated for 5 minutes in air to 250°C and dipped in molten Special High Grade zinc (0.003% lead) at 455 °C. Steel surface wetting and zinc coating adhesion are perfect.

Example 2

On a continuous, industrial galvanizing line, a steel strip 1200 mm wide and 0.5 mm thick is cleaned in an alkaline cleaner, pickled in 10% HC1 aqueous solution, rinsed and immersed in an aqueous preflux solution at a temperature of 80°C, the preflux containing 15% zinc chloride, 2.0% ammonium chloride, 0.8% boric acid, 0.2% sodium chloride, 0.2% potassium chloride, 0.08% nonionic surfactant Merpol HCS, and 0.05% inhibitor Ethomeen. The strip is heated in a tower furnace in an air atmosphere to 320 to 340 °C for 40 seconds and then dipped into molten zinc with 0.1 % lead at a temperature of 460°C. The quality of the zinc coating, as well as its adhesion, is very good.

Thus, this embodiment of the present invention provides among other things, a novel preflux comprising about 8 to 30%, preferably 15 to 20%, zinc chloride, about 2 to 20% ammonium chloride, about 0.1 to 1.0%, preferably 0.5 to 0.8%, boric acid or salt thereof, optionally and preferably about 0.1 to 5% of a fluidity modifying agent comprising an alkali or alkaline earth metal halide, preferably chloride, optionally and preferably, about 0.1 to 2.0%, more preferably 0.5 to 1.0% , of an amino derivative corrosion inhibitor, optionally and preferably about 0.1 to 2.0%, more preferably 0.5 to 1.0%, of a nonionic surfactant, with the balance being a suitable carrier such as water.

In addition, this embodiment also provides an improved galvanizing process in which the article to be galvanized, after first being coated with the foregoing preflux, is preheated to a temperature of 180 to 350°C or more, preferably a temperature of at least 350°C or more for at least 30 seconds, or a temperature of at least 200 to 250°C for 3 to 15 minutes, and then contacted with low lead or lead-free molten zinc for galvanizing, the molten zinc preferably containing no more than 0.2% aluminum or being free of aluminum, as desired.

For convenience, the preflux compositions of this embodiment, like all other embodiments of this invention, can be formulated as concentrates to be diluted by the ultimate user, if desired. An example of a concentrate illustrating this embodiment of the invention comprises about 35 % zinc chloride, about 3.5 % ammonium chloride, about 1.75% boric acid, about 0.6% KC1, about 0.6% NaCl, about 0.05% inhibitor and about 0.02% surfactant.

Fluxes for Zinc-Aluminum Galvanizing

As appreciated by those skilled in the art, it is far more difficult to provide uniform, continuous, void-free galvanized coatings when aluminum is included in the galvanizing bath. The effect of aluminum can be noticed at concentrations starting as low as 0.2% but the most adverse influence is realized at concentrations levels of 3-4% and higher.

Although not wishing to be bound any theory, it is believed that the adverse effect of aluminum is due to the fact that aluminum oxide forms on the article surfaces before the surfaces can be wet with molten zinc. As mentioned above, the most thermodynamically favored way of aluminum oxide formation is through the reaction (4), i.e., the reaction of aluminum chloride with ferrous oxide. Aluminum chloride, in turn, is formed by reaction (3) because of the stronger chemical affinity of aluminum to chlorine than of zinc to chlorine. In any event, if evolution of A1C1₃ could be somehow eliminated or at least substantially retarded until the incipiently-formed frozen zinc layer on the steel article melted, then the surface to be galvanized would be free from aluminum oxide and hence good wetting by the molten zinc metal would be possible.

It is already known that halides of alkali and alkaline earth metals such as sodium, potassium, calcium, magnesium and so on do not react with aluminum and no aluminum chloride is created. This happens because the chemical affinity of these metals to chlorine is much higher than that of aluminum.

In accordance with the invention, it has been discovered that when a halide of an alkali or alkaline earth metal is added to zinc chloride and the resultant mixture is melted, the average chemical affinity to chlorine of the mixture so-created becomes higher than affinity of zinc by itself to chlorine. In addition, it has also been found that this increased affinity is enough to retard formation of aluminum chloride, at least for a short time. Moreover, it has been still further found that a synergistic effect occurs when halides of different metals are added simultaneously to zinc chloride, in that a stronger retarding effect is realized with mixtures of metal halides as compared to only one metal halide being used in the same amount.

A series of very simple experiments illustrates this effect. If KC1, NaCl, CaCl₂, MgCl₂ or their mixture in powder form is put on the surface of a molten zinc bath containing 5 % aluminum, practically no smoke is generated. However, when zinc chloride powder is laid on the molten zinc-aluminum alloy, it melts and a lot of white smoke, which in effect is gaseous A1C1₃, is generated. Finally, the amount of smoke is dramatically reduced, if one or several of said salts are added to zinc chloride and melted in the same way. Accordingly, in accordance with the present invention, a family of prefluxes has been developed for zinc-aluminum galvanizing, in which the aluminum content may reach 10% or more, both for continuous and batch processes. Some of these fluxes are described in our prior application serial number 08/102,570, filed August 5, 1993.

The main component of these prefluxes is zinc chloride which is present in the range of 6-30%, preferably 8-25%. The amount of ammonium chloride present may vary from 0-15%, depending on the nature of articles to be coated, available equipment and so on.

In accordance with this embodiment of the invention, the deleterious action of aluminum is counteracted by adding to zinc chloride or zinc chloride/ammonium chloride mixtures one or more halides of the alkali or alkaline earth metals, or any other metal, which increases the average chemical affinity of the molten mixture to chlorine.

Preferably, the alkali and alkaline earth metal halides used in this embodiment are chlorides, preferably KC1, NaCl, MgCl₂, or CaCl₂. Moreover, the amount of these salts may vary from 0.2% to 10% , more preferably 0.5-6% .

A mixture of several said halides is more beneficial to increase the aluminum-ameliorating effect of these prefluxes. Particularly preferred is a mixture of KC1, NaCl, and MgCl₂/CaCl₂ being present in a KCl/NaCl/MgCl₂ or KCl/NaCl/CaCl₂ ratio of 0.1-0.5/0.5-3/0.5-3, with the total amount of such package being present in a ratio to the zinc chloride in the preflux of 1:20 to 1:4.

The novel prefluxes of this embodiment of the invention also preferably contain stannous chloride, SnCl₂. It has been found that stannous chloride improves wetability of steel surfaces by molten zinc-aluminum alloy and acts similarly to lead in zinc. SnCl₂ may be added to the preflux of this embodiment in any amount, although amounts greater than 3% would demand very low pH to keep this salt in the solution. Preferably, the amount of stannous chloride is 0.1-5% more preferably 0.2-2% with the SnCl₂/ZnCl₂ in the preflux preferably being 1:100 to 1:10.

It is also preferable in this embodiment of the invention that the pH of the preflux be maintained in the fairly acidic, but not overly acidic, range. PH's of 2.5 to 5.5 are preferred, with pH's of 3.5 to 4.5 being particularly preferred. Adding hydrochloric acid to the other ingredients of the preflux is an easy way to adjust pH, with 0.2 to 2.0% of hydrochloric acid addition usually being sufficient. Other acids which do not adversely affect the properties of the preflux can also be used.

It is also preferable for the aqueous prefluxes of this embodiment of the invention to contain an inhibitor as well as a nonionic surfactant. The same amino derivative corrosion inhibitors and nonionic surfactants as described above can also be used herein, with the amounts of these components preferably being about >0.0 to 2.0%, more preferably 0.05 to 0.2% in the case of the inhibitor and about 0.02 to 1.0%, more preferably 0.04 to 0.1%, in the case of the nonionic surfactant. As in the earlier mentioned prefluxes, the inhibitor and nonionic surfactant in these prefluxes also serve to increase the wetting ability of the prefluxes and prevent iron accumulation in the preflux tank due to corrosion of the articles to be galvanized.

The prefluxes of this embodiment can be used in any galvanizing process since, like the previously described prefluxes, they are also universal in terms of applicability. However, they are particularly suitable for use in combination with the preheating feature of the present invention, as described above. In particular, the combination of the preheating feature of this invention and the use of the special prefluxes of this invention allows aluminum-containing galvanized coatings containing little lead (low lead) or no lead (lead-free) to be produced by processes not requiring application of a metal prelayer or continuous operation with a reducing furnace, as required in the prior art. Thus, in accordance with this aspect of the invention, aluminum-containing galvanized coatings can be produced without application of a metal prelayer first, both in batch operation as well as in continuous operation without using a reducing atmosphere in the preheating step. The following working examples are presented to illustrate this embodiment of the present invention.

Example 3

Panels of cold rolled, rust and scale-free steel strip measuring 150 x 50 x 0.2 mm and having 45 μg/cm² of soil were electrocleaned in a phosphated cleaner with a surfactant package having 1.2% free alkalinity at 85°C. To simulate the conditions of a commercial continuous galvanizing line, a direct current of 7.5 A/dm² and polarity reverses were used. Four polarity reverses were used spaced 0.3 seconds apart and total cleaning time was 2.4 seconds Then the panels were rinsed by hot water brushing and then pickled in 10% HCl containing 0.05% cationic type inhibitor at 60° C for 10 seconds. After rinsing in cold water, the panels were immersed in a preflux solution having a pH of 4.3 and containing 17% zinc chloride, 2% ammonium chloride, 0.4% potassium chloride, 1.0% sodium chloride, 1.0% magnesium chloride 1.0%, 0.05% inhibitor Alkaminox T-12 (available from Rhone Poulenc), 0.04% nonionic surfactant Merpol HCS and the balance water.

The panels were then heated in an electric oven in an air atmosphere to 150°C and dipped in molten zinc containing 5 % aluminum at 435 °C. The quality of the coating was very good.

Example 4

Steel tubing having an outer diameter of 9 mm and a wall thickness of 0.4 mm was electrocleaned on a continuous line at a speed of 50 m/min in alkaline cleaner at a temperature of 80°C and a current density of 50 A/dm² for 0.6 seconds. The cleaned tubing was then rinsed, pickled in 18% HCl, rinsed and immersed in a preflux tank at 80°C. The preflux had a pH of 0.8 and contained 19% zinc chloride, 0.8% potassium chloride, 0.8% sodium chloride, 0.8% magnesium chloride, 0.6% stannous chloride, 1.4% hydrochloric acid, 0.04% inhibitor Alkazinc O (available from Phone Poulenc), 0.1 % nonionic surfactant Merpol HCS, and the balance water.

The tubing was then heated by an induction heater in an atmosphere of air to 250°C and dipped into a kettle containing Galfan (95% zinc and 5% aluminum) at 430°C. Coating quality was very good, there being no uncoated spots and the coating exhibiting good adhesion.

Example 5

A variety of small parts including eye-nuts, bolts, washers and the like, with the largest cross-sectional size being 15 mm, were cleaned by soaking in alkaline cleaner having a free alkalinity of 6% at 80°C for 7-8 minutes, rinsed, pickled in 10% HCl at 60°C for 1 minute, rinsed and immersed in an aqueous preflux maintained at 80°C, having a pH of 4 and containing 15.0% zinc chloride, 10% ammonium chloride, 0.5% potassium chloride, 1.0% sodium chloride, 1.0% magnesium chloride, 3.0% stannous chloride, 4.0% hydrochloric acid, 0.05% inhibitor Alkazinc O, 0.2% nonionic surfactant Merpol HCS, and the balance water.

The parts were kept in the preflux tank for 3 minutes. Then they were dried and heated in an electric oven in air to 220 °C and dipped into a molten zinc galvanizing bath containing zinc and 4.5% aluminum at 440°C. After the parts were withdrawn, they were put in a centrifuge to remove excess of molten metal. The quality of the coatings was high even on thread surfaces.

Top Fluxes

In addition to novel prefluxes, the present invention also provides a number of novel top fluxes for use in the galvanizing operation. As appreciated by those skilled in the art, top fluxes are widely used in galvanizing, particularly in pipe and conduit galvanizing, to prevent molten metal spattering and oxidation. If the surfaces of articles to be galvanized are not cleaned or pickled properly, or if the preflux performs poorly, a top flux will nonetheless allow sufficient wetting by the molten zinc and hence production of a good galvanized coating.

As discussed previously, lead-free zinc is very sensitive to article surface contamination and to obtain continuous void-free coatings the amount of soil should not exceed 0.8 μg/cm², preferably 0.4-0.6 μg/cm².

In accordance with this aspect of the present invention, it has been found that such demands as to surface cleanliness may be reduced somewhat by adopting an integrated procedure which involves applying a thermally-resistant preflux such as those described herein, then preheating to a temperature of 150- 250°C, and finally using the top flux of the present invention. Following this procedure, steel articles having on their surfaces as much as 2.5 μg/cm² of soil can be coated by lead free or low lead zinc without any voids being and bare spots.

The novel top flux of this invention, however, can also be used either with conventional prefluxes or with other thermally resistant prefluxes. In the latter case, as in the case of using the thermally resistant preflux of this invention, steel articles can be preheated to a much higher temperature, which in turn increases equipment productivity, saves energy for running the galvanizing bath and also increases bath useful life.

In this regard, it is known in the art that conventional top fluxes rapidly become saturated with oxygen from chemically bound oxygen on the steel surfaces, from oxygen in the air and from moisture. Aluminum and zinc oxides are then accumulated in the top flux due to the presence of oxygen. As a result, the top flux becomes very viscous, sticks to the article surfaces and creates black spots in the galvanized coating. This deleterious effect of thick top fluxes is very pronounced with low lead or lead-free galvanizing baths.

A further advantage of the top fluxes of the present invention, then, is that they preserve a good working consistency in the galvanizing bath for a long time (4-8 hours). In particular, galvanizing baths carrying a top flux of the present invention do not become viscous, do not stick to the article surfaces to be coated, but do provide good wetting action by the molten zinc for a long period of time.

A further subject of the present invention, therefore, is top flux formulations. In accordance with one aspect of this feature of the present invention, such top fluxes comprise about 30 to 90%, preferably about 40 to 70%, more preferably 60 to 65%, zinc chloride, about 10-55%, preferably 25- 45%, more preferably 20 to 40%, ammonium chloride and 0.1 to 2.5%, preferably about 0.2-1.5%, more preferably 0.1 to 1.0%, of a compound referred to hereinafter as a "deoxidizer".

By deoxidizer is meant any compound, be it of inorganic or organic origin, which, when present in the molten top flux, exhibits a higher chemical affinity for oxygen than zinc or iron, and thereby binds oxygen or zinc in preference to the zinc or iron. Of course, the specific compound or material selected for this purpose should not introduce wanted elements in the system such as those which would interfere chemically with the galvanizing process. Also, it is preferable to avoid compounds and materials which would increase flux viscosity as this would adversely affect one of the main flux functions. Selecting an appropriate compound or material for exhibiting a deoxidizing function is every day practice in metallurgy and can be easily accomplished for the present invention by those skilled in the art of galvanizing based on the above criteria using routine experimentation. Examples of suitable inorganic compounds for this purpose are silicon carbide, bismuth, carbon in the form of graphite and so on. Examples of suitable compounds and materials of organic origin are sawdust, charcoal, and many carbon containing chemical compounds such as hydrocarbons, carbohydrates, etc.

In a preferred embodiment of this aspect of the present invention, the top flux also contains chromium chloride, i.e. CrCl₃ in an amount of about 0.1 to 3.5%, preferably 0.5 to 3.0%, more preferably about 0.8 to 1.5%. In accordance with this aspect of the invention, it has been found that adhesion of the zinc coating to the steel surface is improved with CrCl₃ being present. Although not wishing to be bound to any theory, it is believed this beneficial effect occurs as a result of the steel surfaces absorbing some amount of chromium chloride as they pass through the top flux, which in turn is reduced to metallic chromium in the molten zinc bath.

It is also desirable to include in the top flux of this aspect of the invention some organic foaming agent in the range of about 0.1 to 2.5%, preferably 0.2-1.8%, more preferably 0.4 to 1.0%, even more preferably 0.6 to 0.9%. Essentially any type of organic compound which will exhibit foaming properties when charged into molten galvanizing top fluxes can be used for this purpose, and many types of such compounds are well known in the art. Examples of appropriate foaming agents are described in U.S. 2,473,579, for example, the disclosure of which is incorporated herein by reference. Preferred foaming agents are 1 ,4-diamino anthraquinone, purpurin, 2-chloro-l ,4-dihydroxy anthraquinone, β-sulfonic acid anthraquinone and phthalic anhydride.

The foregoing foaming agents, being organic in nature, partially serve as deoxidizers. For the purpose of this invention, however, they are not considered as "deoxidizers" as described above since they serve the additional function of promoting foaming.

The novel top fluxes of this invention also contain alkali and alkaline earth metal halides, more preferably chlorides, such as NaCl, KC1, MgCl₂, and CaCl₂ in the range of 0.5 to 10% , more preferably about 1 to 8.0% . These salts, either alone or in combination, impart high fluidity (low viscosity) to the invention top fluxes and further prevent the top fluxes from sticking to the steel surface to be coated. Especially preferred are fluidity modifying salts comprising the combination of about 0.5 to 2.5% NaCl and 0.5 to 2.5% KC1.

It is also preferred in this embodiment of the invention that the molten zinc used for galvanizing contain at least some aluminum, e.g., at least about 0.01 % , preferably at least about 0.1 % , or more, to promote improved adhesion.

Top-fluxes as described above are illustrated by the following example:

Example 6

Steel rings having an inside diameter of 50mm, a wall thickness of 5mm and a length of 40mm and carrying heavy scale were soaked in a high silicate alkaline cleaner having 1.4% free alkalinity at 80°C for five minutes. Then the rings were pickled for 45 minutes in 15% HCl solution to remove scale, rinsed and immersed in a thermally resistant aqueous preflux of the present invention maintained at 60° C, having a pH 4 and containing 18.0% zinc chloride, 3.0% ammonium chloride, 1.5% borax (sodium salt of boric acid), 0.15% sodium chloride, 0.15% potassium chloride, 0.05% Merpol HCS and 0.02% Ethomeen.

After withdrawal from the preflux the rings were dried, preheated to 280°C in an air atmosphere and immersed in molten Special High Grade zinc (0.003% lead) at 440°C through a top flux, comprising 58% zinc chloride, 37.7% ammonium chloride, 1.0% chromium chloride, 0.2% graphite powder, 1.25% sodium chloride, 1.25% potassium chloride, and 0.6% purpurin.

The zinc coating obtained had no bare spots and very good adhesion.

Example 7

Steel pipe 6-8 m long and 19-100 mm in diameter were cleaned in alkaline cleaner, pickled to remove rust, rinsed and immersed in a preflux solution at 80°C containing 20% zinc chloride, 22% ammonium chloride, 0.1 % Merpol SCH, and 0.05% Ethomeen. After withdrawal from the preflux tank, the pipe were dried and preheated for 6 to 7 minutes in air to 180°C, after which they were immersed in molten Special High Grade zinc (0.016% lead) through a top-flux comprising: 60.9% zinc chloride, 35.0% ammonium chloride, 1.5% chromium chloride, 0.1 % silicon carbide, 2.0% potassium chloride and 0.5% 1,4-diamino antraquinone. All pipes exhibited good zinc coatings without black spots on either the outer or inner surfaces, as well as very good adhesion.

Thus, the present invention further provides novel top-fluxes comprising about 30 to 90%, preferably about 40 to 70%, zinc chloride, about 10 to 55%, preferably 25 to 45%, ammonium chloride, about 0.1 to 3.5%, preferably 0.5 to 3.0%, chromium chloride, about O.l to 2.5%, preferably 0.2 to 1.5% deoxidizer, about 0.5 to 10% more preferably about 1 to 8% of one or more alkali or alkaline earth metal halides, preferably one or more of KC1, NaCl, CaCl₂ and MgCl₂, and about 0.1 to 2.5%, preferably 0.4 to 1.8% organic foaming agent.

Combination of Process Steps

The foregoing has described various features of the present invention individually. It will be appreciated, however, that in practice these features will be used in various different combinations depending on the type of process desired, i.e. batch or continuous, the type of articles to be galvanized, the equipment available, the composition of the galvanizing bath (i.e. , with or without aluminum, normal lead content, low lead or lead-free) and so forth.

For example, preheating in a furnace having a non-reducing atmosphere can be employed in connection with an otherwise conventional galvanizing process, batch or continuous, to reduce the melting time of the incipiently-formed frozen zinc layer in the galvanizing bath and thereby allow reduction of the amount of lead in the bath. Or, instead of using a conventional preflux in such procedure, the novel prefluxes of the present invention can be employed as they will exhibit improved thermal stability. This, of course, will allow even greater heat content to be imparted to the steel article, which in turn will allow even greater reduction in the lead content of the galvanizing bath, as explained above.

In addition, in such processes, any of the novel top fluxes of the present invention can also be used, or no top flux at all.

In the same way, the top fluxes of the present invention and also the prefluxes of the present invention can be used separately, with other top fluxes, prefluxes and procedures, or combined with the other top fluxes, prefluxes and procedures of the present invention, as desired.

The present invention finds particular applicability in batch operations where control of the prefluxing delay time (time between withdrawal of the article from the preflux tank and dipping of the article into the galvanizing tank) is much less precise and further wherein variance in delay time from piece to piece is much wider.

An example of a batch pipe galvanizing process embodying features of the present invention would involve (1) application of a conventional low temperature preflux, comprising 20% zinc chloride and 15% ammonium chloride to a batch of 100 pipe, for example, (2) transferring the batch of prefluxed pipes onto a holding table, (3) passing the pipes through a furnace having a non- reducing atmosphere, for example, an atmosphere of air, for heating the pipe to a temperature of about 180°C over a period of 6-8 minutes, and (4) charging the pipes one by one into a molten zinc galvanizing bath containing 0.1 % or less lead, with not more than 0.2% aluminum, the galvanizing bath having thereon the top flux described, for example, in the above Example 6.

Another example of a batch operation for galvanizing pipe in accordance with the present invention, for example, would involve (1) applying the preflux of Example 2, (2) transferring the pipes through a furnace, having an air atmosphere, where they are heated to 220 to 250°C for 5 to 10 minutes, and (3) depositing the pipes so preheated individually or serially, into a galvanizing bath comprising about 0.003 to 0.1 % lead and 0.01 to 0.1 % aluminum, the galvanizing bath having no top flux thereon.

The present invention is broadly applicable to continuous galvanizing operations as well as batch processes. Vigorous preheating to increase the heat content of the articles to be galvanized in accordance with the present invention enables reduction of the lead content of the galvanizing bath used therein regardless of whether the atmosphere of the furnace is reducing or not. Furthermore, use of the prefluxes of the present invention allow zinc aluminum alloys (like Galfan) to be used in galvanizing without also requiring preheating in a reducing atmosphere or zinc electroplating. Moreover, use of the thermally resistant prefluxes of the present invention also allow existing preheating/drying furnaces in continuous lines to be operated at higher temperatures and more rigorous conditions than previously possible, which in turn allows a reduction in kettle temperature and increase in equipment productivity leading to lower cost and lesser kettle wear.

Unless otherwise specified, all temperatures referred to herein are the temperature of the steel article on which the preflux is deposited, not the oven temperature into which the steel article is placed. The temperature of the steel article itself can be easily determined by means of a thermocouple attached to the surface of the article. Also, all concentrations given herein are in weight percents and based on the total weight of the composition being referred to, unless otherwise specified. Furthermore, all ratios are weight ratios unless otherwise indicated.

Also, as used herein, "industrial" or "industrial scale" or in "industry" means that the process referred to is practiced on a scale such that the products of the process, i.e., the galvanized products, can be sold at a commercial profit over their fully-allocated cost of manufacture. Such processes are typically practiced over an indefinite time frame, in factories using equipment dedicated over its useful life for that purpose and in equipment large enough so that economies of scale allow a commercial profit to be made. Such processes are distinguished from laboratory or research processes or experiments, which are typically conducted for the primary purpose of developing or discovering information, which are practiced only once or a few times and then discontinued, and which are conducted in expensive laboratory equipment used for a variety of different purposes over its useful life. Typically, the products produced by these processes cannot be sold at a profit, since the cost of conducting the laboratory experiments in which they are made vastly exceeds their commercial value.

Although only a few embodiments of the present invention have been described above, it should be appreciated that many modifications can be made without departing from the spirit and scope of the invention. For example, although the foregoing description indicates that certain of the prefluxes described above are particularly useful with particular types of galvanizing baths, it should be appreciated that any of these prefluxes can be used with any of these galvanizing baths although some will perform better than others depending on the particular galvanizing bath composition employed. All such modifications are intended to be included within the scope of the present invention, which is to be limited only by the following claims.

Claims

WE CLAIM:

1. A process for galvanizing steel articles comprising

(a) applying an aqueous preflux to the surfaces of the articles to be galvanized,

(b) preheating each of said articles in a non-reducing atmosphere to dry said preflux and impart significant additional energy in the form of heat content thereto over and above the amount of energy imparted to said articles as a result of drying said preflux, and

(c) applying molten zinc to said surfaces to form a galvanized coating thereon.

2. The process of claim 1 , wherein preheating is accomplished under conditions rigorous enough so that the frozen zinc layer formed on said surfaces immediately upon contact thereof with molten zinc is remelted within 1 minute of said contact.

3. The process of claim 2 , wherein preheating is accomplished under conditions rigorous enough so that the frozen zinc layer formed on said surfaces immediately upon contact thereof with molten zinc is remelted within 30 seconds of said contact.

4. The process of claim 3, wherein said process is a batch process in which said articles are arranged in a batch and contacted together in said batch with said preflux.

5. The process of claim 4, wherein the articles in said batch are contacted serially with said molten zinc.

6. The process of claim 3, wherein said process is a continuous process in which said articles are subjected to steps (a), (b) and (c) serially.

7. The process of claim 3, wherein said process is practiced on an industrial scale.

8. A process for reducing the lead content of a zinc galvanizing bath for use in galvanizing steel articles in which a batch of said articles is contacted with an aqueous preflux for depositing a layer of said preflux on the surfaces to be galvanized and thereafter said articles are fed into a galvanizing bath for application of a galvanized coating thereon, said process comprising preheating said articles to a temperature above 120°C to increase the heat content of said articles to thereby reduce the melting time of the frozen zinc layer inherently formed on such articles when they are dipped into said galvanizing bath.

9. The process of claim 8 wherein said articles are preheated to a temperature of 150 to 350 °C for increasing the heat content thereof, wherein preheating is done under conditions vigorous enough so that the frozen zinc layer on each article melts within one minute of immersion of that article in said galvanizing bath and further wherein the preflux deposited on said surfaces comprises about 8 to 30% zinc chloride, about 2 to 20% ammonium chloride, sufficient boric acid or salt thereof to increase the thermal stability of said preflux without causing significant precipitation of zinc-boric acid reaction products, and water.

10. The process of claim 9 wherein preheating is done in a furnace having a non-reducing atmosphere.

11. The process of claim 8 wherein preheating is accomplished at conditions vigorous enough so that said melting time is reduced to no more than 10 seconds when said galvanizing bath has a lead content of 0.3% or less.

12. A preflux for galvanizing steel comprising about 8 to 30% zinc chloride, about 2 to 20% ammonium chloride, sufficient boric acid or salt thereof to increase preflux thermal stability without causing significant precipitation of zinc-boric acid reaction products, and water.

13. The preflux of claim 12, wherein said preflux contains about 0.1 to 1.0% boric acid or salt thereof.

14. The preflux of claim 13, wherein said preflux further comprises about 0.1 to 5.0% alkali or alkaline earth metal halide.

15. The preflux of claim 14 wherein said alkali or alkaline earth metal halite comprises at least one of NaCl, KC1, MgCl₂ and CaCl₂.

16. The preflux of claim 12, wherein said prefluxes further comprises about 0.1 to 2.0% amino derivative corrosion inhibitor and about 0.02 to 2.0% nonionic surfactant.

17. A process for galvanizing the surface of a steel article, said process comprising

(1) applying the preflux of claim 1 to said surface to form a preflux layer thereon; (2) preheating said article to a temperature above about 180°C for drying said preflux and imparting heat to said article; and thereafter

(3) coating said surface with molten zinc containing no more than 0.3% lead.

18. The process of claim 17, wherein said article is heated to a temperature of about 200 to 350°C in step (2).

19. The process of claim 17 wherein said molten zinc contains up to 0.2% of aluminum.

20. The process of claim 17 wherein preheating is accomplished at conditions vigorous enough so that the incipiently-formed frozen zinc layer formed upon contact of molten zinc with said surface melts within about 10 seconds of such contact.

21. A batch process for galvanizing pipe comprising

(a) cleaning the surfaces of the pipe to be galvanized to a surface cleanliness of 2.5 μg/cm² soil or less,

(b) applying to said surfaces the preflux of claim 12,

(c) passing said pipe through a furnace for heating said pipe to a temperature of at least 180°C, and

(d) charging said pipe into a galvanizing bath having a lead content of 0.1 % or less.

22. The process of claim 21 , wherein said galvanizing bath has no top flux thereon.

23. The process of claim 21 , wherein said galvanizing bath is covered with a top flux comprising about 30 to 90% zinc chloride, about 10 to 55% ammonium chloride, about 0.1 to 3.5% chromium chloride, about 0.1 to 2.5% deoxidizer, about 0.1 to 2.5% foaming agent, and about 0.5 to 10% of a fluidity modifying agent comprising at least one alkali and alkaline earth metal.

24. The process of claim 21, wherein said surface cleanliness is 0.8 μg/cm² soil or less.

25. A preflux for use in galvanizing steel comprising 6 to 30% zinc chloride, 0 to 15% ammonium chloride, 0.2 to 10% of a fluidity modifying agent comprising an alkali or alkaline earth metal halide or mixtures thereof, 0.01 to 5% stannous chloride and water.

26. The preflux of claim 25 wherein said preflux comprises about 10 to 25% zinc chloride, about 0 to 10% ammonium chloride, about 0.02 to 2% of an amino derivative corrosion inhibitor, about 0.02 to 2 % of a nonionic surfactant and about 0.2 to 2% stannous chloride, wherein the ratio of said alkali or alkaline earth metal halides to zinc chloride in said preflux is 1:20 to 1:4, wherein the ratio of stannous chloride to zinc chloride in said preflux is 1: 100 to 1:10, and wherein said preflux is aqueous and has a pH of about 2.5 to 5.5.

27. The preflux of claim 26 wherein said preflux contains about 2 to 5% of the combination of potassium chloride, sodium chloride and at least one of magnesium chloride and calcium chloride.

28. The preflux of claim 27 wherein the ratio of potassium chloride to sodium chloride to magnesium chloride and/or calcium chloride in said preflux is 0.1-0.5/0.5-3/0.5-3, and further wherein the total amount of said potassium chloride, sodium chloride, magnesium chloride and calcium chloride in said preflux is about 2 to 5% of said preflux.

29. A preflux for use in galvanizing steel comprising 6 to 30% zinc chloride, 0 to 15% ammonium chloride, 0.2 to 10% of a fluidity modifying agent comprising potassium chloride, sodium chloride and at least one of magnesium chloride and calcium chloride and water, and wherein the ratio of potassium chloride to sodium chloride to magnesium chloride and/or calcium chloride is 0.1- 0.5/0.5-3/0.5-3.

30. The prefulx of claim 29, wherein the ratio of said fluidity modifying agent to zinc chloride in said preflux is 1:20 to 1:4.

31. The preflux of claim 29, wherein said composition contains 1 to 5% of a fluidity modifying agent comprising KCl, NaCl and at least one of MgCl₂ and CaCl₂.

32. The preflux of claim 31, wherein said fluidity modifying agent comprises KCl, NaCl and MgCl₂ in a ratio of about 1:10: 11.

33. The composition of claim 31, wherein said fluidity modifying agent comprises KCl, NaCl and CaCl₂ in a ratio of about 1:4:5.

34. A process for forming a zinc/aluminum coating on a surface of a steel article, said processing comprising

(a) contacting said surface with the preflux of claim 25 to form a layer of said preflux thereon,

(b) preheating said steel article to a temperature of 150 to 250° for drying said preflux and imparting heat into said article, and (c) coating said surface with molten zinc containing at least 0.2% weight aluminum.

35. The process of claim 34 wherein said molten zinc has a lead content of no greater than 0.2%.

36. The process of claim 34 wherein said molten zinc bath contains 3 to 15% aluminum, and further wherein preheating in step (b) is vigorous enough so that the solidified zinc layer inherently formed when said article is contacted with molten zinc melts within 10 after initiation of such contact.

37. The process of claim 36 wherein the aluminum content of said molten zinc is about 5%.

38. A top-flux for use in galvanizing steel, said top-flux comprising about 30 to 90% zinc chloride, about 10 to 55% ammonium chloride, about 0.1 to 3.5% chromium chloride, about 0.1 to 2.5% deoxidizer, about 0.1 to 2.5% foaming agent, and about 0.5 to 10% of a fluidity modifying agent comprising at least one alkali or alkaline earth metal.

39. The top-flux of claim 38, wherein said deoxidizer is carbon, a sulfur-free organic compound, a cellulosic material, silicon carbide or bismuth.

40. The top-flux of claim 38, wherein said foaming agent is 1,4- diamino anthraquinone, purpurin, 2-chloro-l,4-dihydroxy anthraquinone, β- sulfonic acid anthraquinone or phthalic anhydride.

41. The top-flux of claim 38, wherein said fluidity modifying agent is at least one of KCl, NaCl, MgCl₂ and CaCl₂.

42. The top-flux of claim 41, wherein said top-flux comprises 50 to 70% zinc chloride, about 25 to 45% ammonium chloride, about 0.5 to 3.0% chromium chloride, about 0.1 to 1.0% deoxidizer, and about 0.6 to 0.9% organic foaming agent.

43. The top-flux of claim 38, wherein said top-flux comprises about 30 to 90% zinc chloride, about 10 to 55% ammonium chloride, about 0.1 to 3.5% chromium chloride, about 0.1 to 1.0% deoxidizer, and about 0.2 to 1.0% foaming agent.

44. The top-flux of claim 43, wherein said deoxidizer is graphite, charcoal, silicone carbide or bismuth.

45. A galvanizing bath for galvanizing steel comprising

(a) a molten zinc bath containing no more than 0.3% lead, and

(b) a molten layer of top-flux on said molten zinc bath, said top-flux comprising the top-flux of claim 38.

46. A process for galvanizing the surface of a steel article, said process comprising

(a) applying to said surface a preflux containing zinc chloride and ammonium chloride,

(b) drying and preheating said steel article sufficiently so that when said article is contacted with molten zinc the incipiently-formed frozen zinc layer made thereby melts within 10 seconds of said contact, and

(c) immersing said article in the galvanizing bath of claim 45.

47. The process of claim 46, wherein said preflux is the preflux of claim 12.

48. A batch process for galvanizing pipe comprising

(a) cleaning the surfaces of said pipe to be galvanized to a surface cleanliness of 2.5 μg/cm² soil or less,

(b) applying to said surfaces a preflux containing zinc chloride and ammonium chloride,

(c) passing said pipe through a furnace for heating said pipe to a temperature of at least about 120 °C, and

(d) charging said pipes into a galvanizing bath having a lead content of 0.1 % or less, said galvanizing bath being covered with the top flux of claim 38.