GB2507310A - Flux composition for hot dip galvanization of ferrous materials - Google Patents

Abstract

A flux composition for treating a metal surface comprises (a) more than 40 and less than 70 weight % zinc chloride, (b) 10 to 30 weight % ammonium chloride, (c) more than 6 and less than 30 weight % of a set of at least two alkali metal chlorides including sodium chloride and potassium chloride, (d) from 0 to 2 weight % lead chloride, and (e) from 0 to 15 weight % tin chloride. The KCl:NaCI weight ratio of the set of at least two alkali metal chlorides ranges from 2.0 to 8.0. A fluxing bath comprising this flux composition dissolved in water for use in galvanizing processes is also described. The galvanisation process may be a batch or continuous process. It is particularly well suited for galvanising ferrous materials including iron and steel, particularly metal articles such as iron or steel long products and flat products including wires, plates, coils, rods, reinforcing bars, tubes, strips and sheets. The galvanising bath may comprise from 4 to 24 wt % aluminium, from 0.5 to 6 wt % magnesium, and balance zinc.

Description

FLUX COMPOSITIONS FOR HOT DIP GALVANIZATION

FIELD OF THE INVENTION

The present invention relates to the field of galvanization, more specifically hot dip galvanization or hot-dip zinc coating. In particular the present invention relates to the galvanization of ferrous materials such as, but not limited to, iron, cast iron, steel and cast steel. More particularly the present invention relates to a range of novel flux compositions for treating the surface of a ferrous material such as iron and steel before it is dipped into a zinc-based molten bath. The present invention also relates to galvanization processes, in particular hot dip galvanization, making use of the novel flux compositions in at least one process step. The present invention also relates to galvanized products, including galvanized ferrous products, made by a galvanization process wherein the product surface has been treated with the novel flux compositions.

BACKGROUND OF THE INVENTION

The importance of providing protection against corrosion for ferrous (e.g. iron or steel) articles used outdoors such as, but not limited to, fences, wires, bolts, cast iron elbows and automobile parts is well known, and coating a ferrous material with zinc is a very effective and economical means for accomplishing this goal. Zinc coatings are commonly applied by dipping or passing the article to be coated through a molten bath of the metal. This operation is termed "galvanizing", "hot galvanizing" or "hot-dip galvanizing" to distinguish it from zinc electroplating processes. In this process, a solidified layer of zinc is formed on the article surface and the zinc coating layer formed as a result is strongly adhered to the surface 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 surface 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.

Improvement in the properties of galvanized steel products can be achieved by alloying zinc with aluminum and/or magnesium. Addition of 5% by weight aluminum produces an alloy with a lower melting temperature which 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.

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.

As mentioned above, many techniques and combinations of techniques have been adopted in the steel industry in order 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 an alkaline aqueous wash (alkaline cleaning). This may advantageously be accompanied by additional techniques such as, but not limited to, brush scrubbing, ultrasound treatment and/or electro-cleaning, if desired.

Then follows rinsing with water, contacting the surface with an acidic aqueous wash for removing iron fines and oxides (pickling), and finally rinsing with water again. All these cleaning-pickling-rinsing procedures are common for most steel galvanizing techniques and are carried out in industrial practice more or less accurately.

Another pre-treatment method used for high strength steels, steels with high carbon contents, cast iron and cast steels is a mechanical cleaning method called blasting. In this method, rust and dirt are removed from the steel or iron surface by the projection of small shots and grits onto this surface. Depending on the shape, the size and the thickness of the steel parts to be treated, different blasting machines are used such as, but not limited to, a tumble blasting machine for bolts, a tunnel blasting machine for automotive parts, and the like.

There are two main galvanizing techniques used on cleaned metal (e.g. iron or steel) parts: (1) the fluxing method, and (2) the annealing furnace method.

The first galvanizing technique, i.e. the fluxing method, may itself be divided into two categories, the dry fluxing method and the wet fluxing method.

The dry fluxing method, which may be used in combination with one or more of the above cleaning, pickling, rinsing or blasting procedures, creates a salt layer on the ferrous metal surface by dipping the metal part into an aqueous bath containing chloride salts, called a "pre-fiux". Afterwards, this layer is dried prior to the galvanizing operation, thus protecting the steel surface from re-oxidation until its entrance in a molten zinc bath. Such pre-fluxes normally comprise aqueous zinc chloride and may also typically contain ammonium chloride as well. The presence of zinc chloride and preferably ammonium chloride has been found to improve wettability of the steel article surface by molten zinc and thereby promote formation of a uniform, continuous, void-free coating.

The concept of the wet fluxing method 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 and are floating on the top of the galvanizing bath. The purpose of a top flux, like a pre-flux, is to supply zinc chloride and preferably ammonium chloride to the system to aid wettability 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 iron or steel article is dipped into the galvanizing bath, which oftentimes occurs if the article is still wet with rinse water or pre-fiux.

The wet fluxing method has several disadvantages such as, but not limited to, consuming much more zinc than the dry fluxing method, producing much more fumes, and the like. Therefore, the majority of galvanizing plants today have switched their process to the dry fluxing method.

Below is provided a summary of the annealing furnace method. In continuous processes using zinc or zinc-aluminum or zinc-aluminum-magnesium alloys as the galvanizing medium, annealing is done under a reducing atmosphere such as, but not limited to a mixture of nitrogen and hydrogen gas. This not only eliminates re-oxidation of previously cleaned, pickled and rinsed steel surfaces but, also, it is believed, actually removes any residual surface oxides and soil that might still be present. The majority of steel coils are today galvanized according this technology. A very important requirement is that the coil is leaving the annealing furnace by continuously going directly into the molten zinc without any contact with air. However this requirement makes it extremely difficult if not impossible to use this technology for shaped parts. It is also very difficult to use it for steel wire since wires are breaking too often and the annealing furnace method does not allow discontinuity.

Another technique used for producing zinc-aluminum galvanized coatings comprises electro-coating the steel articles with a thin (i.e. 0.5 -0.7 pm) layer of zinc (hereafter "pre-layer"), drying in a furnace with an air atmosphere and then dipping the pre-coated article into the galvanizing kettle.

This technique, is 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 steel galvanizing production line involving application of an aqueous pre-flux followed by drying in a furnace, the period of time elapsing between removal of the article from the pre-flux tank and dipping in the galvanizing bath is usually about 10 to 60 seconds, instaed of to 60 minutes for a batch process.

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 pipes after being dipped together in a pre-flux bath is transferred by means of a manually operated crane to a table for feeding, one at a time, 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 use of a reducing furnace is restricted to continuous operation only, at least on a commercial or industrial scale.. 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.

There is also a need to combine good formability with the enhanced corrosion protection of the ferrous metal article. However, before such a zinc-based alloy coating with high amounts of aluminum (and optionally magnesium) can be introduced into the general galvanizing industry, the following difficulties have to be overcome: -high-aluminum content zinc alloys can hardly be produced using the standard zinc-ammonium chloride flux. Fluxes based on metallic Cu or Bi deposit have been proposed earlier, but the possibility of copper or bismuth leaching into the zinc bath is not an attractive one. Thus, better fluxes are needed.

-high-aluminum content alloys tend to form outbursts of zinc-iron rn intermetallic alloy which are detrimental at a later stage in the galvanizing process. This phenomenon leads to very thick, uncontrolled and rough coatings. Control of the outbursting effect is absolutely essential.

-wettability issues were previously reported in Zn-Al alloys with high-aluminum content, possibly due to a higher surface tension than pure zinc. Hence bare spots due to poor wetting of the steel are easily formed. There is therefore a need to lower the surface tension of the melt.

-a poor control of coating thickness was reported. in Zn-Al alloys with high-aluminum content, possibly depending upon parameters such as, but not limited to, the temperature, the flux composition, the dipping time, the steel quality and others.

From the above statements it is clear that a lot of technical problems remain to be solved in the galvanizing industry.

European Patent EP 1.352.100-B describes a flux for hot dip galvanization comprising -60 to 80 wt. % (percent by weight) of zinc chloride (ZnCI2); -7 to 20 wt. % of ammonium chloride (NH4CI); -2 to 20 wt. % of at least one alkali or alkaline earth metal salt; -0.1 to 5 wt. % of a least one of the following compounds: NiCI2, CoCI2, MnC12; and -0.1 to 1.5 wt. % of at least one of the following compounds: PbCI2, SnCI2, SbCI3,BiCI3.

Preferably this flux comprises 6% by weight of NaCI and 2% by weight of KCI.

International Patent Application No. WO 2007/146161 describes a method of galvanizing with a molten zinc-alloy comprising the steps of: -immersing a ferrous material to be coated in a flux bath in an independent vessel thereby creating a flux coated ferrous material, and -thereafter immersing the flux coated ferrous material into a molten zinc-aluminum alloy bath in a separate vessel to be coated with a zinc-aluminum alloy layer thereby creating a coating on the ferrous material, wherein the molten zinc-aluminum alloy is a zinc alloy of a high aluminum content comprising 10% -40% by weight of aluminum, at least 0.2% by weight of silicon, and the balance being zinc and optionally comprising one or more additional elements selected from the group consisting of magnesium and a rare earth element.

In the first step of this method, the flux bath may comprise from 10 to weight % zinc chloride, 1 to 15 weight % ammonium chloride, 1 to 15 weight % of an alkali metal chloride, a surfactant and an acidic component such that the flux has a final pH of 1.5 or less. In another embodiment of the first step of this method, the flux bath may be as previously defined in EP 1.352.100-B. In one embodiment of the first step of this method, the flux comprises 6% by weight of NaCI and 2% by weight of KCI.

Japanese patent publication No. 2001/049414 describes a method for simply producing a hot-dip Zn-Mg-Al base alloy coated steel sheet excellent in corrosion resistance with one hot-dipping step in the atmosphere by making use of a flux containing 61 -80 wt% (percent by weight) of zinc chloride (ZnCI2), 5 to 20 wt. % of ammonium chloride (NH4CI), 5 to 15 wt. % of one or more kinds among chloride, fluoride or silicafluoride of alkali or alkaline earth

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metal elements, and 0.01 to 5 wt. % of one or more kinds among chlorides of Sn, Pb, In, TI, Sb or Bi. More specifically, table 1 of JP 2001/049414 discloses various flux compositions within the above definition and with a KCI/NaCI weight ratio ranging from 0.38 to 0.60 which, when applied to a steel sheet in a molten alloy bath comprising 0.05 -7 wt.% Mg, 0.01 -20 wt.% Al and the balance being zinc, provide a good plating ability, no pin hole, no dross, and flat. By contrast, table 1 of JP 2001/049414 discloses a flux composition within the above definition and with a KCI/NaCI weight ratio of 1.0 which, when applied to a steel sheet in a molten alloy bath comprising 1 wt.% Mg, 5 wt.% rn Al and the balance being zinc, provides a poor plating ability, pin hole defect, some dross, and poorly flat.

Thus, the common teaching of all three above documents is a preference for a KCI/NaCI weight ratio well below 1.0 in the fluxing composition. Although the methods described in the three above documents have brought some improvements to the previous state of the art, they have still not resolved most of the technical problems outlined hereinbefore.

Consequently there is still a need in the art for improved fluxing compositions and galvanizing methods making use thereof.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a flux composition that makes it possible to produce continuous, more uniform, smoother and void-free coatings on metal articles, in particular iron or steel articles, of any shape and size by hot dip galvanization with pure zinc or zinc alloys, in particular zinc-aluminum alloys and zinc-aluminum-magnesium alloys of any composition. It has surprisingly been found that this can be achieved by providing potassium and sodium chlorides in a KCI/NaCI weight ratio well above 1.0 in the flux composition. Most of the hereinabove stated problems are thus solved by a flux composition as defined in claim 1.

DETAILED DESCRIPTION OF THE INVENTION

The main feature of the present invention is the recognition that huge improvements in galvanization of metals, in particular iron and steel, can be achieved when starting from a flux composition having a set of at least two alkali metal chlorides including sodium chloride and potassium chloride, provided that the KCI/NaCI weight ratio of said set of at least two alkali metal chlorides ranges from 2.0 to 8.0 This main feature is associated with specific amounts of the other components of the flux composition, as is manifested in claim 1. Each of these parameters of the present invention will now be presented in details.

In the following, certain terms and expressions should be understood according to the definitions below: The terms "hot dip galvanization", unless specified otherwise, is meant to designate the corrosion treatment of a metal article such as, but not limited to, an iron or steel article by dipping into a molten bath of pure zinc or a zinc-alloy, in continuous or batch operation, for a sufficient period of time to create a protective layer at the surface of said article.

The term "pure zinc" as used herein and unless specified otherwise, refers zinc galvanizing baths that may contain trace amounts of some additives such as for instance antimony, bismuth, nickel or cobalt. This is in contrast with zinc alloys that contain significant amounts of one or more other metals such as aluminum or magnesium.

In the following the different percentages relate to the proportion by weight of each component with respect to the total weight (100%) of the flux composition. This implies that not all maximum or not all minimum percentages can be present at the same time, in order for their sum to match to 100% by weight.

As defined in claim 1 the flux composition of this invention comprises, as an essential feature, a set of at least two alkali metal chlorides including sodium chloride and potassium chloride, provided that the KCI/NaCI weight ratio of said set of at least two alkali metal chlorides ranges from 2.0 to 8.0 Various specific embodiments of the flux composition of this invention are defined in claims 2 to 11 and are further presented in details.

In one embodiment of this invention, the specified KCI/NaCI weight ratio is associated with the presence of lead chloride in the flux composition. The proportion of lead chloride may be at least 0.1 weight %, or at least 0.4 weight % or at least 0.7 weight % of the flux composition. In another embodiment of this invention, the proportion of lead chloride in the flux composition may be at most 2 weight%, or at most 1.5 weight % or at most 1.2 weight %. In a specific embodiment of this invention, the proportion of lead chloride in the flux composition is from 0.8 to 1.1 weight %.

In one embodiment of this invention, the specified KCI/NaCI weight ratio is associated with the presence of tin chloride in the flux composition. The proportion of tin chloride in the flux composition may be at least 2 weight % or at least 3.5 weight % or at least 7 weight %. In another embodiment of this invention, the proportion of tin chloride in the flux composition is at most 14 weight %.

In one embodiment of this invention, the combined amounts of lead chloride and tin chloride represent at least 2.5 weight %, or at most 14 weight % of the flux composition. In another embodiment of this invention, the flux composition may further comprise other salts of lead and/or tin, such as the fluoride, or other chemicals that are inevitable impurities present in commercial sources of lead chloride and/or tin chloride.

In one aspect of this invention, the specified KCI/NaCI weight ratio is combined with specified proportions of all other chlorides that make it possible to produce continuous, more uniform, smoother and void-free coatings on metal, in particular iron or steel, articles by galvanization, in particular hot dip galvanization, processes with molten zinc or zinc-based alloys, especially in batch operation or continuously.

In one aspect of this invention, the specified KCI/NaCI weight ratio in the flux composition is combined with more than 40 and less than 70 weight % zinc chloride. In one embodiment of this invention, the proportion of zinc chloride in the flux composition is at least 45 weight % or at least 50 weight %.

In another embodiment of this invention, the proportion of zinc chloride in the flux composition is at most 65 weight % or at most 62 weight %. These selected proportions of ZnCI2 are capable, in combination with the specified KCI/NaCI weight ratio in the flux composition, to ensure a good coating of the metal article to be galvanized and to effectively prevent oxidation of the metal article during subsequent process steps such as drying, i.e. prior to galvanization itself.

In one aspect of this invention, the specified KCI/NaCI weight ratio in the flux composition is combined with 10 to 30 weight % ammonium chloride.

In one embodiment of this invention, the proportion of ammonium chloride in the flux composition is at least 13 weight % or at least 17 weight %. In another embodiment of this invention, the proportion of ammonium chloride in the flux composition is at most 26 weight % or at most 22 weight %. The optimum is proportion of NH4CI may be determined by the skilled person so as to achieve a sufficient etching effect during hot dipping to remove residual rust or poorly pickled spots, while however avoiding the formation of black spots, i.e. uncoated areas of the metal article. The optimum proportion of NH4CI may be determined by the skilled person without extensive experimentation, depending upon parameters such as the type of metal to be galvanized and the weight proportions of the other metal chlorides in the flux composition, by simply using the experimental evidence shown in the following examples. In some circumstances it may be useful to substitute a minor part of ammonium chloride with one or more alkyl quaternary ammonium salt wherein at least one alkyl group has from 8 to 18 carbon atoms such as described in [P 0488.423, for instance an alkyl-trimethylammonium chloride (e.g. trimethyllaurylammonium chloride) or dialyldimethylammonium chloride.

In one aspect of this invention, the specified KCI/NaCI weight ratio in the flux composition is further combined with the presence of suitable amounts of alkali or alkaline earth metal halides. These halides are preferably or predominantly chlorides (still fluorides may be useful as well), and the alkali or alkaline earth metals are advantageously selected (sorted in decreasing order of preference in each metal class) from the group consisting of Na, K, Li, Cs, Mg, Ca, Sr and Ba. The flux composition shall advantageously comprise a mixture of these alkali or alkaline earth metal halides, since such mixtures tend to increase the average chemical affinity of the molten mixture towards chlorine and to provide a synergistic effect allows to better and more accurately control the melting point and the viscosity of the molten salts and hence the wettability. In one embodiment of this invention, the mixture of alkali or alkaline earth metal halides is a set of at least two alkali metal chlorides and represents from 6 to 30 weight % of the flux composition. In another embodiment of this invention, the set of at least two alkali metal chlorides includes sodium chloride and potassium chloride as major components. In another embodiment of this invention, the set of at least two alkali metal chlorides (e.g. including sodium chloride and potassium chloride as major components) represents at least 12 weight % or at least 15 weight of the flux composition. In another embodiment of this invention, the set of at least two alkali metal chlorides (e.g. including sodium chloride and potassium chloride as major components) represents at most 25 weight %, or at most 21 weight %, of the flux composition. In a specific embodiment of this invention, the proportion of the at least two alkali metal chlorides (e.g. including sodium chloride and potassium chloride as major components) in the flux composition is from 20 weight % to 25 weight %. Magnesium chloride and/or calcium chloride may be present as well as minor components in each of the above stated embodiments.

In one aspect of this invention, the specified KCI/NaCI weight ratio in the flux composition is further combined with suitable amounts of one or more other metal chlorides such as, but not limited to, nickel chloride. For instance, examples below demonstrate that the presence of up to 1 weight % nickel chloride is not detrimental to the behavior of the flux composition of the present invention in terms of quality of the coating obtained after hot dip galvanization. Other metal chlorides that may be present include bismuth chloride, antimony chloride and the like.

In order to achieve the above stated advantages in the best possible way, the ratio between these alkali or alkaline earth metal halides in their mixtures is not without importance. It has also been surprisingly found that flux compositions wherein the mixture of alkali or alkaline earth metal halides is a set of at least two alkali metal chlorides including sodium chloride and potassium chloride in a KCI/NaCI weight ratio from 2.0 to 8.0 exhibit useful or even outstanding properties. In one embodiment of the present invention, the KCI/NaCI weight ratio may for instance be from 3.5 to 5.0 In other aspects of this invention, the specified respective KCI/NaCI weight ratio in the flux composition is further combined with the presence of other additives. Preferably such additives are functional additives, i.e. they participate in tuning or improving some desirable properties of the flux composition. Several classes of additives meet such definition and will be presented in details below.

For instance the flux composition of this invention may further comprise at least one nonionic surfactant or wetting agent which, when combined with the other ingredients therein, is capable of achieving a predetermined desirable surface tension. Essentially any type of nonionic surfactant, preferably liquid water-soluble nonionic surfactant, can be used for this purpose. Examples of suitable nonionic surfactants include, but are not limited to, ethoxylated alcohols such as nonyl phenol ethoxylate, other alkyl phenols such as Triton X-102 and Triton NiOl (both commercially available from Union Carbide), block copolymers of ethylene oxide and propylene oxide such as [-44 (commercially available from BASF), and tertiary amine ethoxylates such as based on coco amines (commercially available as Ethomeen from AKZO NOBEL). Other suitable non-ionic surfactants include polyethoxylated and polypropoxylated derivatives of alkylphenols, fatty alcohols, fatty acids, aliphatic amines or amides containing at least 12 carbon atoms in the molecule, alkylarenesulphonates and dialkylsulphosuccinates, such as polyglycol ether derivatives of aliphatic and cycloaliphatic alcohols, saturated and unsaturated fatty acids and alkylphenols, said derivatives preferably containing 3 to 10 glycol ether groups and 8 to 20 carbon atoms in the (aliphatic) hydrocarbon moiety and 6 to 18 carbon atoms in the alkyl moiety of the alkyiphenol. Further suitable non-ionic surfactants are water-soluble adducts of polyethylene oxide with poylypropylene glycol, ethylene-diaminopolypropylene glycol containing 1 to 10 carbon atoms in the alkyl chain, which adducts contain 20 to 250 ethyleneglycol ether groups and/or 10 to 100 propyleneglycol ether groups, and mixtures thereof. Such compounds usually contain from 1 to 5 ethyleneglycol (EO) units per propyleneglycol unit.

Representative examples of non-ionic surfactants are nonylphenol-polyethoxyethanol, castor oil polyglycolic ethers, polypropylene-polyethylene io oxide adducts, tributyl-phenoxypolyethoxyethanol, polyethylene-glycol and octylphenoxypoly-ethoxyethanol. Fatty acid esters of polyethylene sorbitan (such as polyoxyethylene sorbitan trioleate), glycerol, sorbitan, sucrose and pentaerythritol, and mixtures thereof, are also suitable non-ionic surfactants.

Low foaming wetting agents such as, but not limited to, the ternary mixtures described in U.S. Patent No. 7,560,494 are also suitable. Commercially available non-ionic surfactants of the above-mentioned types include those marketed by Zschimmer & Schwarz GmbH & Co KG (Lahnstein, Germany) under the trade names OXETAL, ZUSOLAT and PROPETAL, and those marketed by Alfa Kimya (Istanbul, Turkey) under the trade name NEIZER SB II. Various grades of other suitable non-ionic surfactants are commercially available under the trade name MERPOL, as detailed below.

The hydrophilic-lipophilic balance (HLB) of said at least one nonionic surfactant is not a critical parameter of this invention and may be appropriately selected by the skilled person within a wide range from 3 to 18, for instance from 6 to 16. For instance the HLB of MERPOL-A is about 6 to 7, the HLB of MERPOL-SE is 11, and the HLB of MERPOL-HCS is 15. Mixtures of such surfactants may be used as well.

Another relevant feature of the nonionic surtactant is its cloud point.

Preferably the cloud point of the nonionic surfactant (i.e. the temperature where the mixture starts to phase separate, thus becoming cloudy, as may me determined for instance according to the ASTM D2024-09 standard test method; this behavior is characteristic of non-ionic surfactants containing polyoxyethylene chains, which exhibit reverse solubility versus temperature behavior in water and therefore "cloud out" at some point as the temperature is raised; glycols demonstrating this behavior are known as "cloud-point glycols") should be higher than the flux working temperature as defined below with respect to the use ot a fluxing bath in a hot dip galvanization process.

Preferably the cloud point of the nonionic surfactant should be higher than 90°C.

Suitable amounts of such nonionic surfactants are well known from the skilled person and usually range from 0.02 to 2.0 weight %, preferably from 0 0.5 to 1.0 weight %, of the composition, depending upon the selected type of compound.

The flux composition of the present invention may further comprise at least one corrosion inhibitor. By "corrosion inhibitor" is meant herein a compound which inhibits the oxidation of steel particularly in oxidative or acidic conditions. In one embodiment of this invention, the corrosion inhibitor includes at least an amino group. Inclusion of such amino derivative corrosion inhibitors in the flux compositions of this embodiment of the invention can significantly reduce the rate of iron accumulation in the flux tank. By "amino derivative corrosion inhibitor" is meant herein a compound which inhibits the oxidation of steel and which also contains an amino group. Aliphatic alkyl amines and quaternary ammonium salts (preferably containing four independently selected alkyl groups with 1 to 12 carbon atoms) such as, but not limited to, alkyl dimethyl quaternary ammonium nitrate are suitable examples of this type of amino compounds. Other suitable examples include hexamethylenediamines. In another embodiment of this invention, the corrosion inhibitor includes at least one hydroxyl group, or both a hydroxyl group and an amino group. Suitable inhibitors of the latter type are also well known to those skilled in the art. Suitable amounts of the corrosion inhibitor are well known from the skilled person and usually range from 0.02 to 2.0 weight %, preferably from 0.1 to 1.5 weight %, or from 0.2 to 1.0 weight %, depending upon the selected type of compound.

The flux compositions of the present invention may comprise both at least one corrosion inhibitor as defined hereinabove and at least one nonionic surfactant or wetting agent as defined hereinabove.

The flux compositions of the present invention may be produced by a wide range of methods. They can simply be produced by mixing, preferably thoroughly mixing, the essential components (i.e. zinc chloride, ammonium chloride, alkali metal chlorides, lead chloride and tin chloride) and, if need be, the optional ingredients (i.e. corrosion inhibitor(s) and/or nonionic surfactant(s)) in any possible order in one or more mixing steps. When lead chloride is present in the flux composition, then the flux compositions of the present invention may also be produced by a sequence of at least two steps, wherein one step comprises the dissolution of lead chloride in ammonium chloride or sodium chloride or a mixture thereof, and wherein in a further step the solution of lead chloride in ammonium chloride or sodium chloride or a mixture thereof is then mixed with the other essential components and, if need be, the optional ingredients of the composition. In one embodiment of the latter production method, dissolution of lead chloride is effected in the presence of water. In another embodiment of the latter production method, it has been found useful to dissolve an amount ranging from 8 to 35 gIl lead chloride in an aqueous mixture comprising from 150 to 450 gIl ammonium chloride and/or or sodium chloride and the balance being water. In particular the latter dissolution step may be performed at a temperature ranging from 55°C to 75°C for a period of time ranging from 4 to 30 minutes and preferably with stirring.

A significant advantage of the flux composition of the present invention is its broad field of applicability. As mentioned hereinbefore, the present flux composition is particularly suitable for batch hot dip galvanizing processes using a wide range of zinc alloys but also pure zinc. Moreover, the present flux can also be used in continuous galvanizing processes using either zinc-aluminum or zinc-aluminum-magnesium or pure zinc baths, for galvanizing a wide range of metal pieces, e.g. wires, pipes, tubes or coils (sheets), especially made from ferrous materials like iron and steel.

According to another aspect, the present invention thus relates to a fluxing bath for galvanization, in particular hot dip galvanization, wherein a suitable amount of a flux composition according to any one of the above defined embodiments is dissolved in water or an aqueous medium. Methods of dissolving in water a flux composition based on zinc chloride, ammonium chloride, alkali metal chlorides and one or more chlorides of a transition metal are generally well known in the art. The total concentration of components of the flux composition in the fluxing bath may range within very wide limits such as, but not limited to, between 200 and 750 gIl, preferably between 350 and rn 750 gIl, most preferably between 500 and 750 gIl. This fluxing bath is particularly adapted for hot dip galvanizing processes using zinc-aluminum baths, but can also be used with pure zinc galvanizing baths, either in batch or continuous operation.

For use in hot dip galvanization processes, the fluxing bath of the present invention should advantageously be maintained at a temperature between 50°C and 90°C, preferably between 60°C and 90°C, most preferably between 65°C and 85°C.

According to a further aspect of the present invention, a process for the hot dip galvanization of a metal article, preferably an iron or steel article, is proposed. The process comprises a step of treating, e.g. immersing, said article in a fluxing bath according to any one of the above defined embodiments. Preferably, in discontinuous (batch) operation, said treatment step is performed for a period of time ranging from 0.01 to 30 minutes, depending upon operating parameters such as, but not limited to, the composition of the fluxing bath, the composition of the metal (e.g. steel) to be galvanized, the shape and/or size of the article, and the temperature of the fluxing bath. In another embodiment of batch operation of the present invention, the treatment (e.g. immersion) time may range from 0.03 to 20 minutes, or from 0.5 to 15 minutes, or from 1 to 10 minutes. As is well known to the skilled person, the treatment time may widely vary from one article to the other: the shorter times (close to or even below 0.1 minute) are suitable for wires, whereas the longer times (closer to 15 minutes or more) are more suitable for instance for rods.

In continuous operation, the metal treatment step (immersion into the fluxing bath) may be performed at a dipping speed from about 0.5 to 10 mfminute, preferably from 1 to 5 m/minute. Whether in batch or continuous operation, preferably said metal treatment step (immersion into the fluxing bath) may be performed at a temperature ranging from about 50°C to 90°C, preferably between 60°C and 90°C, most preferably between 70°C and 85°C.

Practically, any metal surface susceptible to corrosion, e.g. any type of iron or steel article may be treated in this way. The shape (flat or not), geometry or size of the metal article are not critical parameters of the present invention. The article to be galvanized may be a so-called long product. As used herein the terms "long product", as opposed to flat products, refers to products with one dimension (length) being at least 10 times higher than the two other dimensions such as, but not limited to, wires (coiled or not, for making e.g. bolts and fences), rods, bobbins, reinforcing bars, tubes (welded or seamless), rails, structural shapes (e.g. I-beams, H-beams, [-beams, T-beams and the like), or pipes of any dimensions. The metal article to be galvanized may also be, without limitation, in the form of a flat product such as, but not limited to, plates, sheets, panels and strips.

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 fluxing. Techniques for achieving a desirable degree of surface cleanliness are well known in the art. 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 vary broadly. Thus at an initial process step, the metal article is submitted to cleaning (degreasing) in a degreasing bath. The latter may advantageously be an ultrasonic, alkali degreasing bath. Then, in a second step, the metal article is rinsed. At further steps the metal article is submitted to a pickling treatment and then rinsed. Next, the article is pickled by immersing the article into an aqueous strongly acidic medium, e.g. a water-soluble inorganic acid such as, but not limited to, hydrochloric acid, sulfuric acid, hydrofluoric acid, phosphoric acid, nitric acid and mixtures thereof in any suitable proportions. In a manner which is well known to the skilled person, the choice of the primary acid used for pickling depends upon parameters such as the speed at which pickling is desired and the type of steel, in particular the alloy content in carbon steels.

Pickling is usually performed at a temperature ranging from about 15°C to 60"C. Acid concentrations on the order of 5 to 12% by weight are normally employed, although more concentrated acids can also be used. The time of pickling typically ranges from about 5 to 30 seconds, more typically 10 to 15 seconds.

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 one or more corrosion inhibitors are present in an amount ranging from about 0.02 to 1.0 weight %, for instance from 0.05 to 0.5 weight %, depending upon the type of corrosion inhibitor. The pickling bath may further include significant amounts, e.g. up to about 15% by weight, of one or more iron chlorides Pickling can be accomplished simply by dipping the article into a pickling tank containing the pickling bath. 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 electro-pickling. As known from those skilled in the art, these additional processing means usually shorten pickling time significantly. It is clear that these pre-treatment steps may be repeated individually or by cycle if needed until the desirable degree of cleanliness is achieved.

Then shortly, preferably immediately, after the cleaning steps, the metal (e.g. steel) article is treated with, e.g. immersed into, a fluxing bath in accordance with the present invention in order to form a protective film on its surface as described hereinbefore.

The fluxed metal (e.g. iron or steel) article, i.e. after immersion into the fluxing bath during the appropriate period of time, is preferably subsequently dried. Drying may be effected, according to prior art conditions, by transferring the fluxed metal article through a furnace having an air atmosphere, for instance a forced air stream, where it is heated at a temperature from 220°C to 250°C until its surface exhibited a temperature between 170°C and 200°C, e.g. for 5 to 10 minutes. However it has also been surprisingly found that milder heating conditions may be more appropriate when the fluxing composition is defined according to the first aspect of the present invention, or any particular embodiment thereof.

Thus it has been found that it may be sufficient for the surface of the metal (e.g. steel) article to exhibit a temperature from 100° to 200°C during the drying step. This can be achieved for instance by setting a heating temperature ranging from 100°C to 200°C, e.g. ranging from 125 °C to 190 °C, or from 140 °C to 170 °C. This can also be achieved by using a poorly oxidative atmosphere during the drying step. In one embodiment of the present invention, the temperature at the surface of the steel article to be dried may range from 125°C to 150°C. In another embodiment of this invention, depending upon the selected drying temperature, drying may be effected for a period of time ranging from about 0.5 to 10 minutes, or from 1 to less than 5 minutes. In another embodiment of this invention, drying may be effected in specific gas atmospheres such as, but not limited to a water-depleted air atmosphere, a water-depleted nitrogen atmosphere, or a water-depleted nitrogen-enriched air atmosphere (e.g. wherein the nitrogen content is above 20%).

At a next step of the galvanization process, the fluxed and dried metal (e.g. steel) article may be dipped into a molten zinc-based galvanizing bath to form a metal coating thereon. As is well known to the person skilled in the art, the dipping time may be suitably defined depending upon a set of parameters including, but not restricted to, the size and shape of the article, the desired coating thickness, and the exact composition of the zinc bath, in particular its aluminum content (when a Zn-Al alloy is used as the galvanizing bath) or magnesium content (when a Zn-Al-Mg alloy is used as the galvanizing bath).

In one embodiment of the present invention, the molten zinc-based galvanizing bath may comprise (a) from 4 to 24 weight % aluminum, (b) from 0.5 to 6 weight% magnesium, and (c) the rest being essentially zinc. In another embodiment ot the present invention, the molten zinc-based galvanizing bath may comprise tiny amounts (i.e. below 1.0 weight %) or trace amounts (i.e. unavoidable impurities) of other elements such as, but not limited to, silicium, tin, lead, titanium or vanadium. In another embodiment of the present invention, the molten zinc-based galvanizing bath may be agitated during a part of this treatment step. During this process step, the zinc-based galvanizing bath is preferably maintained at a temperature ranging from 360°C to 600°C. It has been surprisingly found that with the flux composition of the present invention it is possible to lower the temperature of the dipping step whilst obtaining thin protective coating layers of a good quality, i.e. which are capable of maintaining their protective effect for an extended period of time such as five years or more, or even 10 years or more, depending upon the type of environmental conditions (air humidity, temperature, and so on). Thus in one embodiment of the present invention, the molten zinc-based galvanizing bath is kept at a temperature ranging from 350°C to 550°C, or preferably from 380 to 520°C, the optimum temperature depending upon the content of aluminum and/or magnesium optionally present in the zinc-based bath. In one embodiment of the present invention, the thickness of the protective coating layer obtained by carrying out the dipping step on a metal article, e.g. an iron or steel article, that has been pre-treated with the flux composition of this invention may range from 5 to 50 pm, for instance from 8 to 30 pm. This can be appropriately selected by the skilled person, depending upon a set of parameters including the thickness and/or shape of the metal article, the environmental conditions to which the metal article is supposed to withstand, the expected durability in time of the protective coating layer formed, and so on. In a particular embodiment, the hot dip galvanization process is such that dipping is performed at a temperature between 380 and 440°C and said molten zinc-based galvanizing bath comprising (a) from 4 to 7 weight % aluminum, (b) from 0.5 to 3 weight % magnesium, and (c) the rest being essentially zinc.

Finally, the metal article, e.g. the iron or steel article, is removed from the galvanizing bath and cooled. This cooling step may conveniently be carried out either by dipping the galvanized metal article in water or simply by allowing it to cool down in air.

The present hot dip galvanization process has been found to allow the continuous or batch deposition of thinner, more uniform, smoother and void- free, protective coating layers on iron or steel articles, especially when a zinc-aluminum or zinc-aluminum-magnesium galvanizing bath was employed.

Moreover, pure zinc galvanizing baths may also be used in the process of the present invention.

Moreover the process of the present invention is well adapted to galvanize steel articles made from a large variety of steel grades, in particular, but not limited to, steel articles having a carbon content up to 0.25 weight %, a phosphorous content between 0.005 and 0.1 weight % and a silicon content between 0.0005 and 0.5 weight %, as well as stainless steel. The classification of steel grades is well known to the skilled person, in particular through the Society of Automotive Engineers (SAE). In one embodiment of the present invention, the metal may be a chromium/nickel or chromium/nickel/molybdenum steel susceptible to corrosion. Suitable examples thereof are the steel grades known as AlSI 304 (*1.4301), AISI 304L (1.4307, 1.4306), AISI 316 (1.4401), AISI 316L (1.4404, 1.4435), AISI31 6Ti (1.4571), or AISI 904L (1.4539) [*1.xxxx = according to DIN 10027- 2]. In another embodiment of the present invention, the metal may be a steel grade referenced as 5235JR (according EN 10025) or S46OMC (according EN 10149-2) The following examples are given for understanding and illustrating the invention and should not be construed as limiting the scope of the invention, which is defined only by the appended claims.

EXAMPLE 1 -general procedure for galvanization at 440°C A plate (2 mm thick, 100 mm wide and 150 mm long) made from the steel grade S235JR (weight contents: 0.114 % carbon, 0.025 % silicium, 0.394 % manganese, 0.012 % phosphorus, 0.016 % sulfur, 0.037 % chromium, 0.045 % nickel, 0.004 % molybdenum, 0.041 % aluminum and 0.040 % copper) was pre-treated according the following pre-treatment sequential procedure: -first alkaline degreasing by means of SOLVOPOL SOP (50 gIl) and a tenside mixture EMULGATOR SEP (10 gIl), both commercially available from Lutter Galvanotechnik GmbH, at 65°C for 20 minutes; -rinsing with water; -pickling in a hydrochloric acid based bath (composition: 10 wt% HCI, 12 wt% FeCI2)at 25°C for 1 hour; -rinsing with water; -second degreasing for 10 minutes in a degreasing bath with the same chemical composition as first step above -rinsing with water; -second pickling for 10 minutes in a pickling bath with the same chemical composition as above; -rinsing with water, fluxing in a flux composition as described in one of the following tables for 180 seconds at a concentration of 650 gIl and 0,3% by weight Netzer 4 (a non-ionic wetting agent commercially available from Lutter Galvanotechnick GmbH); -drying at 100 -150°C for 200 seconds; -galvanizing for 3 minutes at 440°C at a dipping speed of 1.4 mlminute in a zinc-based bath comprising 5,0% by weight aluminum, 1,0% by weight magnesium, trace amounts of silicium and lead, the balance being zinc; and -cooling in air.

EXAMPLES 2 to 17-steel treatment with illustrative flux compositions of this invention before galvanizing at 440°C The experimental procedure of example 1 has been repeated with various flux compositions wherein the proportions of the various chloride components are as listed in table 1. The coating quality has been assessed by a team of three persons evaluating the percentage (expressed on a scale from io 0 to 100) of the steel surface that is perfectly coated with the alloy, the value indicated in the last column of table 1 below being the average of these three individual notations. The coating quality has been assessed while keeping the fluxing bath at 72°C (examples 1 to 10, no asterisk) or at 80°C (examples 11 to 17, marked with an asterisk).

Table I

Ex. ZnCI2 NH4CI NaCI % KCI % SnCI2 PbCI2 Coating % quality 1* 59 20 3 12 4 1 75 2 60 20 3 12 4 1 90 3* 52.5 17.5 3 12 13 1 75 4 53 18 3 12 13 1 80 5* 52 21 4 17 4 1 70 6 52.5 21.5 4 17 4 1 60 7 60.5 12 4.5 18 4 1 60 8 57 19 3 12 8 1 85 9 59 20 4.5 11.5 4 1 70 59 20 2.5 13.5 4 1 70 11 61.3 20.4 3.1 12.3 2 1 95* 12 55 25 3 12 4 1 95* 13 56.1 25.5 3.1 12.2 2 1 90* 14 50 30 3 12 4 1 60* 54.1 18 2.7 20.7 3.6 0.9 70 * 16 62.5 20.8 3.2 12.5 0 1 80 * 17 57.3 26 3.2 12.5 0 1 85*

Table 1 (end)

* The flux compositions of examples 1, 3 and 5 additionally contain 1 weight % NiCI2 to match up to 100% by weight.

COMPARATIVE EXAMPLE 18 The experimental procedure of example 1 has been repeated with a flux composition comprising 60 weight% zinc chloride, 20 weight% ammonium chloride, 10 weight% sodium chloride, 5 weight% potassium chloride and 5 weight% tin chloride,. The coating quality has been assessed by the same methodology as in the previous examples and has found been found 20%.

io This comparative example demonstrates that when a KCI/NaCI weight ratio of 1/3 is used as in the prior art, then the coating quality is significantly lower than for examples Ito 17.

EXAMPLE 19 -general procedure for galvanization at 520°C The sequential procedure of example 1 is repeated, the treatment step with a fluxing composition being performed at 80°C, except that in the penultimate step galvanizing was effected at 520°C at a dipping speed of 4 mfminute in a zinc-based bath comprising 20.0% by weight aluminum, and 1.0% or 2.0% or 4.0 % by weight magnesium, trace amounts of silicium and lead, the balance being zinc.

EXAMPLES 20 to 25 -steel treatment with illustrative flux comrositions of this invention before galvanizing at 520°C The experimental procedure of example 19 has been repeated with various flux compositions wherein the proportions of the various chloride components are as listed in table 2 below. The coating quality has been assessed by the same methodology as in the previous examples.

Table 2

Ex. ZnCI2 NH4CI NaCI % KCI % SnCl2 PbCI2 Coating % % % quality 60 20 3 12 4 1 95 21 57 19 3 12 8 1 80 22 61.3 20.4 3.1 12.3 2 1 85 23 55 25 3 12 4 1 80 24 56.1 25.5 3.1 12.2 2 1 85 54.1 18 2.7 20.7 3.6 0.9 75

Table 2 (end)

EXAMPLE 26 -general procedure for galvanization at 460°C The sequential procedure of example 1 was repeated, the treatment step with a fluxing composition being performed at 80°C, except that in the penultimate step galvanizing was effected at 460°C at a dipping speed of 4 is m/minute in a zinc-based bath comprising 11,0% by weight aluminum, 3,0% by weight magnesium, trace amounts of silicium and lead, the balance being zinc.

EXAMPLES 27 to 29 -steel treatment with illustrative flux comrositions of this invention before galvanizing at 460°C The experimental procedure of example 26 has been repeated with various flux compositions wherein the proportions of the various chloride components are as listed in table 3 below. The coating quality has been assessed by the same methodology as in the previous examples.

Table 3

Ex. ZnCI2 NH4CI NaCI % KCI % SnCl2 PbCI2 Coating % % % quality 27 61.3 20.4 3.1 12.3 2 1 95 28 55 25 3 12 4 1 95 29 56.1 25.5 3.1 12.2 2 1 95 As a summary, examples 20-25 and 27-29 demonstrate that the present invention achieves outstanding coating quality whatever the composition of the zinc-based galvanization bath may be.