US3206324A - Method and pre-flux for coating ferrous metals with nickel prior to galvanizing - Google Patents

Description

Sept. 14, 1965 J. R. DAESEN 3,206,324

METHOD AND FIRE-FLUX FOR COATING FERROUS METALS WITH NICKEL PRIOR TO GALVANIZING Filed June 22,' 1961 FERROUS METAL TO BE GALVANIZED SURFACE CLEANING TREATMENT TREATMENT WITH AQUEOUS SOLUTION CONTAINING ALKALI HALIDE SALT, NICKEL CHLORIDE AND A SUBSTANCE CONTAINING A FREE ALDEHYDE GROUP INTRODUCTION OF TREATED METAL INTO GALVANIZING BATH WITHDRAWAL OF GALVANIZED METAL FROM GALVANIZING BATH IN VENTOR.

JOHN R. DAESEN ATTORNEYS United States Patent 3 206,324 METHOD AND PRE-hLUX FOR COATING FER- ROUS METALS WITH NICKEL PRIOR TO GAL- VANIZING John R. Daesen, 131 E. Cuttriss Place, Park Ridge, Ill. Filed June 22, 1961, Ser. No. 118,784 14 Claims. (Cl. 117-51) This invention relates to the preparation of ferrous metals for the operation of coating with zinc or zinc alloys by immersion in molten baths of the coating metal, commonly called hot dip galvanizing, and to a novel flux which enables a thin layer of reduced nickel to be deposited on ferrous metals.

Hot dip galvanized coatings consist essentially of a layer or layers of zinc-iron alloy next to a ferrous base and an outer layer which is substantially the composition of the molten bath in which the steel base has been immersed.

The relative amounts and the distribution of the various layers of differing composition have a profound effect on the physical properties of the coating, including ductility and smoothness of coating, and a definite, though less universally recognized, effect on the chemical characteristics of the coating, specifically with reference to resistance to corrosion. Accordingly, many means have been developed to control the relative amounts and distribution of these different components of the coating.

Most of the hot dip galvanized steel sheet and strip now produced is coated in a molten bath of zinc to which suflicient aluminum has been added (.07 to .15 percent) to restrain the formation of zinc-iron layers to the extent that they form only a few percent of the total coating.

In the galvanizing of metal with baths containing insufficient aluminum to greatly restrict alloying (.02 percent aluminum or less), commonly called conventional hot-dip galvanizing, it has been recognized that a more moderate control of the development of the zinc-iron layers during the coating operation may be secured by a number of means including low temperature of coating bath, use of steel low in carbon, silicon and phosphorus, annealing the steel base before galvanizing, avoidance of severe pickling which leaves a smut of metallic ferrous particles on the surface, thorough rinsing of all ferrous salts from the base before entrance to the molten bath, and cooling the work promptly as it is withdrawn from the molten coating bath.

Many liquid solutions, called pre-fiuxes, some of them proprietary, have been used to coat the ferrous base material (after cleaning or pickling and before dipping in the molten zinc) with a layer of salts, usually zinc and ammonium chlorides. These solutions have a dual function which to some extent is self contradictory. During the time of exposure of the pre-fiuxed steel to the air, these solutions are intended toprevent dissolution to or re-oxidation of the cleaned steel surface, while during the immersion of the steel in the molten bath, the preflux material reacts with any zinc or iron oxide formed on the bath or on the work to permit clean steel to make contact with clean zinc. Ineviably during atmospheric exposure and entrance into the molten bath, the pre-flux solutions also attack to some extent the clean steel of the base thereby contributing in varying degree to an increased amount of zinc-iron alloy in the coating and often to a proliferation of it irregularly towards the surface of the coating.

Dissolution of the steel base occurs in the floating cover of frothy flux, which is maintained (except in so-called dry galvanizing) on the entrance surface of the zinc bath.

The art of practical hot dip galvanizing consists largely 3,205,324 Patented Sept. 14, 1965 "ice in the selection of materials for and the manipulation of the Work through these preparatory pre-fiux baths and molten flux covers to achieve, for the particular product in each case, the needed cleaning action without undue dissolution of the steel base; for this results in uneconomically high percentages of zinc-iron dross in the bath, or unfavorable amounts and distribution of zinciron alloys in the coating.

If the steel base is coated with nickel before being immersed in molten zinc, it not only will be protected from oxidation during the interval between pickling and dipping in molten zinc, but its reaction rate with the zinc bath, and the growth of the various alloy layers by diffusion will be so reduced that a substantial and useful improvement in surface appearance, physical properties and corrosion resistance of the coating will result. In addition, the production of zinc-iron dross in the bath will be materially reduced.

This advantageous effect is secured even though the amount of nickel deposited be so small that none of it remains in unreacted condition on the steel base after the immersion in molten zinc. This improved coating is therefore not of the type described by Schoonmaker et al. in US. Patent 2,315,740, etc., where the outer zinc coating acted as a sealer of pits or imperfections in an underlying nickel coat.

Nickel may be applied to a steel base by electro-deposition, by chemical deposition without the use of electric current as in hypophosphite-containing baths or the cyanide-containing so-called black nickel solution with or without the use of an externally connected electrode.

Unalloyed nickel deposits are superior for the purpose to those containing phosphorus or sulphur, especially when the nickel deposit is thin and of course only thin nickel coatings, of the order of of a mil thick are economically feasible when commercial operations are considered.

Regardless of the extent of prior recognition, acceptance or disclosure of any of the above considerations, and the limitation these may be held to impose on the novelty of my discovery, I have found by considerable experimentation a new means of application of metallic nickel to a steel base, never before disclosed or known to my knowledge, that has such advantages in economy of operation and investment in material and equipment that it makes commercially attractive for the first time, the control of the reaction between zinc and iron in the hot dip galvanizing operation through the use of nickel coating so as to produce coatings that are superior in appear anec, physical properties and resistance to corrosion in atmospheric exposure to coatings not so treated. These and other objects are achieved according to the present invention by providing a pre-flux solution comprising nickelous chloride, an alkali metal halide, and a substance containing a free aldehydic group for use in a galvanizing process.

The use of the easily oxidized aldehydes for the reduction of metals from aqueous solutions of their salts, depositing them on metallic or non-metallic surfaces has long been known. Examples of such known process include the production of silver mirrors by the action of invert sugar on silver nitrate solutions, or the formation of copper layers from the action of formaldehyde on cuprotartrate solution in the presence of silver. (Misciattelli, US. Patent 2,183,202.) Typical among the substances which contain a free aldehydic group and which may be used in accordance with the present invention are aldehydes, aldoses, and carbohydrates.

Precipitation of nickel from nickel chloride solutions by sugar or glucose is an effective means of depositing sufficient nickel for the accomplishment of this improvement on a steel base, without the use of electric current or an externally connected electrode to provide an electrolytic potential. Prior to my invention, however, it has not been possible to secure preparatory coatings of nickel that were satisfactory for the control of the reaction between the steel base and the molten zinc in hot dip galvanizing except in highly concentrated nickel chloride solutions of 35 percent or more by Weight The excessive cost of large tanks (commercial installations may hold 3,000 to 6,000 gallons for the immersion of large articles) of such an expensive solution is commercially prohibitive, considering risk of loss by leakage, polution and carry-over as well as interest on investment.

A great many attempts to discover a solution effective for the deposition of the required amount and quality of nickel, while avoiding the excessive cost of solutions high in nickel chloride content have been made, but many of the manipulations that suggest themselves were found ineffective when tried.

Incorporation of nickel chloride in solutions of the usual zinc-ammonium chloride used as a pre-dip flux failed to yield the desired results. Additions of aluminum chloride to nickel chloride solutions of decreasing concentration inhibited the deposition of nickel. The use of boric acid, citric acid or tartaric acid was found ineffectual for securing the desired result.

Manipulation of the hydrogen ion concentration of lower concentration nickel chloride solution form a pH of 3 to a pH of 8 did not result in satisfactory deposits.

Reasoning that since other manipulations did not result in a deposit of the desired coating, the near saturation, with respect to nickel chloride, of the high concentration solutions might be the cause of ready deposition of the nickel, trials were made of solutions in which one half or three quarters of the nickel chloride hexahydrate were replaced by like amounts of highly soluble salts of alkali halides. The results were successful. A

It was later found that successful coatings were deposited in solutions containing glucose as a reducing agent with nickel chloride concentrations as low as 5 percent by weight of nickelous chloride hexahydrates, even when the alkali halide content was reduced to as little as 5 percent by Weight. Lower percentages of nickelous chloride hexahydrate or alkali halides were not investigated since the exact point between and percent at which the effect ceases would not be of great practical importance, but the recitation of 5 percent is of course meant to include the smal range below 5 percent in which the effect is operative.

The alkali halide, sodium and potassium chloride in normal solution have electrical resistivities of 11.6 and 8.94 ohms cm. respectively. These are greater than the resistivity of mineral acids and alkali hydroxides, neither of which are useful for depositing nickel for the end desired. It is believed that these acids and bases would be effective if other processes such as acid attack of the ferrous metal did not occur. With these exceptions however, the alkali halides are lower in electrical resistivity in normal solution than all other commercially useful salts.

Whether the success of the use of alkali halides in solutions containing low concentrations of nickel chlorides and a reducing agent is due to the improvement in electrical conductivity they confer to the solution (which may be important in small local region of deposition even though no electrical current is imposed as in electroplating), or due to making the solution more nearly saturated for the nickel ions by the addition of compounds containing more negative metals combined with the common chloride ion or some other cause is not known. Several eifects may be operating.

Be sure that as it may, I have discovered that the failure of lowconcentration solutions of nickel chloride to be reduced by aldehydes or substances containing an aldehydic group ;;(one oxygen and, one hydrogen atom at- ,7 5 percent water tached to a carbon atom), can be remedied at low cost by the use of an inexpensive, highly soluble alkali halide salt; sodium chloride is eminently suitable for this purpose. The solutions are effective from normal room temperature to 200 degrees F. In the lower part of the range, deposition is slower. At temperatures over degrees F., consideration must be given to evaporation of the solution.

An example of my invention is as follows. A piece of hot rolled, low carbon steel sheet, .100 thick was pickled in an 8 percent solution of sulfuric acid for 15 minutes at F., rinsed in running water, and immersed for 30 minutes (at 150 F.) in a solution containing, by weight:

5 percent nickel chloride hexahydrate 10 percent sodium chloride 10 per cent glucose 75 percent water On removal it was allowed to drain for 5 minutes and was then immersed promptly through a layer of zincammonium chloride flux cover into a bath of molten zinc at 855 F. for a period of two minutes. It was then withdrawn through a surface uncovered by flux and cooled in the air.

Visual examination showed the outer surface of the galvanized sheet to be bright and smooth, and examination of the structure of the coating under the microscope showed it to consist of dense compact alloy layers for about one-third of the thickness adjoining the steel base, while the outer two-thirds of the thickness of the coating was zinc of the composition of the galvanizing bath, with no proliferation or branching out of the outer alloy layers irregularly into the outer zinc layer, as is commonly found in conventional hot dip galvanized coatings (aluminum less than .03 percent) produced without the restraining effect of a nickel pre-coat.

The resistivity of the pro-treating solution of this example measured at room temperature was found to be about 8 ohms cm. A solution containing 5 percent by weight of nickel chloride hexahydrate in water was found to have an electrical resistivity, on the other hand, of about 26 ohms cm.; and a 10 percent solution had a resistivity of 17 ohms cm.

In a second example, similar procedure and hot dip galvanizing were used with a pro-treatment bath, following pickling and rinsing, of the following composition by weight:

5 percent nickel chloride hexahydrate 5 percent sodium chloride 30 percent glucose 60 percent water Similar results were obtained as in the first example. The use of the higher content of glucose appears to have an advantage in providing an oxidation resisting film, when there is a delay in immersion of the pre-treated article in the molten zinc bath, but glucose contents as low as 10 percent give satisfactory results in the above solution.

The electrical resistivity of the pre-treat-ing solution of the second example, measured at room temperature, was found to be about 12 ohms cm. This is substantially the resistivity of a solution containing 40 percent nickel chloride hexahydrate, 20 percent glucose and 40 percent water by weight, which was also effective in producing a satisfactory nickel coating for securing the described benefits on subsequent galvanizing.

In a third example, after customary pickling and rinsing, a sheet of hot rolled, low carbon sheet steel .100 thick was immersed for 20 minutes (at 180 F.) in a solution containing, by weight:

10 percent nickelous chloride hexahydrate 5 percent potassium chloride 10 percent glucose and rinsed in water. An adherent deposit of nickel was obtained. After passing the sheet through a molten flux of zinc-ammonium chloride into a molten zinc bath at 850 F. for two minutes and cooling in air, the surface of the coating was found to be bright and smooth, and microscopic examination of the coating in section showed it to consist of dense, compact alloy layers comprising less than half of the thickness of coating with an outer layer of zinc of the composition of the galvanizing bath. There was no proliferation or branching out of the alloy layers into the outer zinc layer.

The resistivity of the pre-treating solution of this example, measured at room temperature, was found to be about 9 ohms cm.

In a fourth example, after customary pickling and rinsing, a piece of hot rolled, low carbon sheet steel .100" thick was immersed for 20 minutes in a solution at 180 F. containing, by weigh:

5 percent nickelous chloride hexahydrate 10 percent sodium chloride 5 :percent iso-butyraldehyde 80 percent water and rinsed in water. An adherent deposit of nickel was obtained. After rinsing, the sheet was passed through a molten flux cover of zinc-ammonium chloride into a molten zinc bath at 850 F. for two minutes, removed through a surface uncovered by flux, and air cooled. The resulting coating was similar to that secured in other examples described.

The resistivity of the pro-treating solution of this fourth example, measured at room temperature, was found to be about 7 ohms cm.

Further experiments indicated that calcium chloride, barium chloride and magnesium chloride satisfactorily facilitate the coating of nickel on a ferrous metal from solutions having less than 35 percent by weight nickelous chloride.

Other halide salts were found effective in securing the desired results in solutions containing 10 percent by weight of nickel chloride hexahydrate With an aldehyde in water. Some of the resistivities obtained are indicated below:

Addit-ion Resistivity, ohms cm. 10 percent barium chloride 10 percent calcium chloride 10 percent magnesium chloride 10 percent ammonium chloride 10 percent potassium bromide 10 percent potassium iodide 5 percent sodium bi-fiuoride 11.

In general, the chlorides appear to be the most effective in deposition of a satisfactory pre-coat. The bromide salt was less effective but appeared superior to the iodide and the bifiuoride. This supports the belief that deposition of nickel in the form desired is facilitated by introduction, by means of the addition salt, of the common ion, chlorine, to the nickel chloride solution. going alkali metal (including ammonium in this sense) and alkaline earth metal halide salts, therefore, when used in an amount such as to yield aqueous solution having electrical resistivity of less than 12 ohm cm. measured at room temperature, may be used.

Since in no case Were satisfactory nickel deposits secured with nickel chloride solutions containing less than 35 percent nickel chloride hexahydrate except where the resistivity was lowered by additions of a soluble salt yielding low resistivity solutions, it appears that the use of such means for improving electrical conductivity is a significant control in the lowering of the nickel chloride The forecontent required for nickel coating to values that are reasonable and commercially feasible.

Having now particularly described and disclosed the nature of my invention and the manner in which the same is to be performed, I claim:

1. A method of coating a ferrous metal with a thin coating of nickel prior to hot dip galvanizing, comprising, pickling the ferrous metal, and passing the metal through an aqueous pre-flux solution corn-prising from 5 percent to 35 percent by weight of nickelous chloride hexahydrate, at least 5 percent by weight of at least one halide salt selected from the group consisting of ammonium halides, alkali metal halides and alkaline earth meatl halides, and at least 10 percent by weight of an aldose.

2. A method of coating a ferrous metal as claimed in claim 1, in which the halide salt is a chloride.

3. A method of coating a ferrous metal as claimed in claim 2, in which the halide salt is sodium chloride.

4. A method of coating ferrous metals prior to hot dip galvanizing which comprises the step of passing a pickled ferrous metal through an aqueous prerflux solution containing less than 35% by weight nickelous chloride, at least 5% by weight of a substance containing a free alaldehydic group and at least 5% by weight of at least one soluble halide salt selected from the group consisting of ammonium halides, alkali metal halides and alkaline earth metal halides.

5. A method according to claim 4 wherein said halide salt is an alkali metal chloride.

6. A method according to claim 4 wherein said halide salt is an alkaline earth metal chloride.

7. A method according to claim 4 wherein said substance containing a free aldehydic group is at least one selected from the group consisting of aldehydes, carbohydrates and aldoses.

8. A method of galvanizing a ferrous metal comprising pickling said ferrous metal, passing the pickled metal through an aqueous pre-flux solution comprising an aqueous solution of less than 35% by weight of nickelous chloride hexahydrate, at least 5% by weight of a substance having a .free aldehydic group, and at least 5% by weight of at least one soluble halide salt selected from the group consisting of ammonium halides, alkali metal halides and alkaline earth metal halides, and subsequently passing the pre-fluxed metal through a galvanizing bath.

9. A method according to claim 8, wherein the aqueous pre-flux solution contains from 5 to 35% by Weight of nickelous chloride hexahydrate, from 5 to 10% by weight of sodium chloride, and from 5 to 30% by Weight of a substance having a free aldehydic group.

10. A pro-flux composition for coating ferrous metals with a thin coating of nickel prior to galvanizing comprising an aqueous solution of less than 35% by Weight nickelous chloride, at lea-st 5% by weight of a substance having a free aldehydic group, and at least 5% by weight of at least one soluble halide salt selected from the group consisting of ammonium halides, alkali metal halides and alkaline earth metal halides.

11. A pre-flux composition according to claim 10 wherein the substance having a free aldehydic group is at least one selected from the group of aldehydes, aldoses and carbohydrates and wherein the halide salt is a chloride.

12. A pre-fiux composition according to claim 11 wherein the chloride salt is sodium chloride.

13. An aqueous solution of from 5 to 35 by weight of nickelous chloride hexahydrate, from 5 to 10% by weight sodium chloride and from 5 to 30% by weight (References on following page) 8 References Clted by the Exammer OTHER REFERENCES I UNITED STATES PATENTS I Wein: Gold Films, part 2, The Glass Industry, June 12,191,813 2/40 Brown 117-130 X 959, 2,315,740 4/43 Schoonmaker et a1. 20440 X 5 2,762,723 9/56 Talmey et aL WILLIAM D. MARTIN, Pnmary Examzner. 2,940,870 6/ 6O Baldwin 11752 MURRAY KATZ, Examiner.

2,999,770 9/61 Gutzeit 1O6-1 X 3,015,858 1/62 Hendricks 11735 X

Claims (1)

1. 8. A METHOD OF GALVANIZING A FERROUS METAL COMPRISING PICKLING SAID FERROUS METAL, PASSING THE PICKLED METAL THROUGH AN AQUEOUS PRE-FLUX SOLUTION COMPRISING AN AQUEOUS SOLUTION OF LESS THAN 35% BY WEIGHT OF NICKELOUS CHLORIDE HEXAHYDRATE, AT LEAST 5% BY WEIGHT OF A SUBSTANCE HAVING A FREE ALDEHYDIC GROP, AND AT LEAST 5% BY WEIGHT OF AT LEAST ONE SOLUBLE HALIEDE SALT SELECTED FROM THE GROUP CONSISTING OF AMMONIUM HALIDES, ALKALI METAL HALIDE AND ALKALINE EARTH METAL HALIDES, AND SUBSEQUENTLY PASSING THE PRE-FLUXED METAL THROGH A GALVANIZING BATH.

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