US2835616A - Procedure for the manufacture of oxalate coatings on metals - Google Patents

Description

United State Fatent O PROCEDURE FOR THE MANUFACTURE OF OXALATE COATINGS UN METALS Werner Rausch and Franz Gonnert, Frankfurt am Main, Germany, assignors to Farker Rust Proof Company, Detroit, Mich, a corporation of Michigan No Drawing. Application March 17, 1954 Serial No. 416,962

7 Claims. or. 148-614) The present invention relates generally to the chemical coating of metals such as iron, carbon steel, and stainless steel. More particularly, the invention relates to a complete method including pretreatment of the metal and the application of such coatings from solutions of oxalic acid. The method to be described herein is particularly applicable to the coating of stainless or alloy steel-s to facilitate drawing, spinning, and other metal forming operations.

In the manufacture of oxalate coated iron, steel and stainless or alloy steel articles, it is essential that a perfectly clean surface be exposed to the bath. When the metal is rusted and scaled, it has been treated heretofore, prior to immersion in the oxalic acid solution, with a de-rusting or de-scaling acid pickling treatment, wherein the rust or scale layers are removed. It has also been reported that with the stainless or alloy steels, a further pretreatment in a bath of molten alkali, following the acid pickling treatment, results in a further improved oxalate coating thereon. The molten alkali pretreatment, however, has the disadvantage that it requires special equipment, and in addition, is dangerous to workers in the plant.

The chemical coatings formed on metals protect the metal surface satisfactorily during most forming operations. Great difficulty is frequently encountered, however, duringwire-drawing operations either because apparently tightly adherent coatings flake off or are scraped off on the leading edge of the die or the coating does not seem to have the ability to properly lubricate the metal While in the die. Thus, the coating must not only be of a correct weight and be tightly adherent to the metal surface, but also it must have the property to cling to the etal in spite of deformation of the metal and the intense rubbing action and form a thin lubricating film between metal and die. With the stainless steels, coatings of this desirable type are difficult to obtain. coating both drawing speeds and the reduction-per-draw must be reduced, and a rough surface is usually obtained.

it has now been found that the Weight, quality and adherence of the oxalate coatings can be significantly improved if the metal, which has been subjected to a prior acid pickling treatment or other equivalent cleaning pretreatment, is given an additional pretreatment in an aqueous alkaline solution, before applying the oxalate coating. Oxalate coatings applied to the so-treated metal are especially improved on the stainless or alloy steels, and also with ordinary iron and carbon steels. The improvement in all cases includes the development of a thicker coating, a more tightly adherent coating and the elimination of flaked and loose coatings. In addition, the oxalate coatings produced by this method have a signiiicantly improved lubricating quality which is especially valuable in Wire-drawing or tube-forming operations.

While in most cases the alkaline pretreatment step of the present invention will be applied to metal surfaces which have been subjected to a prior acid-pickling treat- Without such a- Patented May 20, lo e? ment, it is also applicable to metal surfaces which have been flame cleaned or the like where such a treatment makes pickling unnecessary. When the flame cleaned metal surface is treated in the alkaline solution, an improved, heavier and more adherent oxalate coating is obtained. Thus, the alkaline pretreatment step is not necessarily restricted to metals which have been subjected to an initial acid-pickling step, although its principal application appears to be on acid-pickled metal surfaces.

The alkaline pretreatment is conducted according to the invention by contacting a rustand scale-free metal surface with an aqueous solution of alkali, for example, sodium hydroxide, potassium hydroxide, sodium carbonate, or of any other strongly alkaline-reacting salts of the alkali-metal hydroxides such as tertiary-sodium phosphate. in the usual case, however, a sodium hydroxide solution is much preferred. The alkaline solution is dilute, any concentration between 5 and 50% producing an improvement in the oxalate coating step. More preferred concentrations when using the hydroxides are between 5 and with best results being obtained at 10 to 15%. An improvement in coating weight of at least 30 to 40% and frequently of as much as or more is obtained with an acid-pickled, alkaline-rinsed metal as compared to that obtained from metal that has had only the acid-pickling treatment. A particularly preferred alkaline bath of the invention contains additional small amounts of one or more activating anions selected from the class consisting of cyanide and thiocyanate. Very small proportions of either or both of the latter, for example, from as little as 02% to as much as 1%, effect a further increase in the Weight of coating deposited in a given time from a given oxalate solution, of from 50 to 100%. More preferred concentrations for the activating anion are from 0.06 to 0.5%. In all cases the adherence and lubricating quality of the coating are markedly increased irrespective of the Weight thereof.

The oxalic acid solution applied to metal surfaces having the pretreatment of the present invention is not critical. If the solution produces an adherent coating on the metal, the weight, quality and adherenceof coating obtained on an alkaline-rinsed metal surface will be improved. The coating will be more uniform, finer grained, more tightly adherent and more useful in metal forming operations, particularly in wire drawing or the mandrel drawing of tubes. Thus, the usual iron-containing oxalic acid solutions such as the oxalic acid-iron oxalate solutions activated by ferric ion may be utilized. Iron-free oxalic acid solutions activated by halide ions, particularly silicofluoride ions may also be used. The iron-containing solutions may contain chloride, bromide, thiocyanate or ferricyanide ions and also oxygen-containing sulfur compounds such as thiosulfates, sulfites and the like. Both the ironfree and iron-containing solutions may contain oxidizing agents such as sodium chlorate or bromate, sodium nitrate or nitrite, hydrogen peroxide, organic nitro compounds such as meta-nitrobenzene sodium sulfonate and the like. Much preferred are the ferric ion activated oxalic acid solutions containing 0.4 to 15% total iron, at least 0.002% ferric ion and from about 2 to 16% of chloride 1011.

The method of this invention is applicable to the treatment of the surface of any metal selected from the class consisting ofiron, chromium, nickel, and alloys of at least any two of these metals. Thus, the method is applicable to ordinary soft iron, to carbon steels, to chrome-, nickelor chrome-nickel steels, to essentially pure chromium, essentially pure nickel, and to nickel-chrome or chrome-nickel alloys low in iron, or entirely free thereof. An example of the high nickel-low chromium-low iron alloys is Inconel X.

Several of the more important general types of alloysincluded in this broad definition are the austenitic, martensitic, and ferritic stainless steels. The 'austenitic stainless steels contain a major proportion of iron, from 16 to 26% chromium, and from 6 to 22% nickel, with the total of the alloying elements being at least 23 The martenistic stainless steels contain from 4 to 18% chromium and nickel, if present, not more than 2 or 3%. The ferritic stainless steels differ slightly from the martensitic since they usually contain slightly greater amounts'of chromium, no nickel and a relatively greater variety and higher proportions of other alloying elements.

Other alloys which may be coated by the method of this invention contain major proportions of nickel, minor proportions of chromium and little or no iron. The latter are unusually resistant to corrosion and are used for making clad steel sheets. A fuller description of the composition and properties of these and other corrosionresistant metals included in the above class may be found in the ASTM Metals Handbook 1948 or later edition. All of these and others are satisfactorily coated by the method of this invention. The stainless steels, since they are relatively difficult to coat, are the most important class of alloys to which the invention may be applied. With the especially diflicult-to-coat austenitic stainless steels, the greatest improvement in coating quality and weight is obtained.

In the method of this invention, a metal surface should have at least two pretreatment steps before the oxalate coating is applied. The first step is a pre-clean step which may consist of any treatment adapted to remove soil, grease and rust or scale. This first pretreatment step may consist in a flame cleaning, solvent washing, molten salt treatment and/ or an acid-pickling treatment. If the metal is not rusted or sealed, a simple flame treatment or solvent wiping or washing to remove all traces of grease will be sufiicient. If rusted or sealed, the metal surface should be pickled in acid. For the latter purpose, exceptionally thick and adherent scale layers can be loosened by a molten salt treatment, for example in Hookers salt and the so-treated surface then pickled in acid. The acidpickling solution will consist of dilute solutions of strong mineral acids such as nitric acid, hydrofluoric acid, sulphuric acid, hydrochloric acid and others, and mixtures of these and other acids, or solutions of these acids with soluble salts such as sodium chloride, ferric sulphate, and others. Preferred rust and scale removing pickling baths are as follows, in order of their preference:

(2) HNO HF 3 HNO HCI (4) H SO NaCl (with or without HNO The above pickling solutions are applied to the metal at total concentrations between 5 and 20%, more preferably about and at temperatures between 68 and 176 F., preferably at ordinary temperatures of from 68 to 90 F. The ferric sulfate of bath (1) is used in concentrations of from 4 to 7% together with 1% to 2% or more hydrofluoric acid. The time of immersion will vary considerably according to the bath strength and temperature with efficient cleaning being usually obtained in from 5 to 60 minutes, more preferably to 50 minutes.

The thus-cleaned metal preferably is first given a thorough Water rinsing before being subjected to the alkaline pro-rinse treatment of this invention. Improved coatings are obtained when the metal is left in contact with the alkaline rinsing solution for as little as one minute. No significant improvement is obtained beyond about 15 minutes contact. The alkaline rinsing bath may be at any convenient room temperature and may be warmed slightly, if desired. Temperaturesof from 84 to 194 F. are satisfactory,'with 140 to 194 being preferred;

Following the alkaline prerinse treatment, the metal is again thoroughly rinsed in cold water and then immersed in a conventional oxalate coating bath. The latter bath is applied to the metal at any convenient temperature from room temperature F.) to about 190 F. Practical processing times of from 1 to 15 minutes are obtained at temperatures between and F. After rinsing thoroughly with water, it is preferred to immediately dry the oxalate coated surface in order to reduce the attack of the corrosive atmosphere usually prevailing in the pickling room. The resulting coating is a suitable paint base and it protects the metal against corrosion and during metal working operations.

The thus-coated metal usually should be lubricated before being subjected to metal working operations. Ordinary sodium soaps as well as the alkaline earth and aluminum stearates may be applied dry or from solutions or emulsions and the surface again dried. The metal also may be limed when such treatment is permissible. The metal is then ready for the drawing, spinning or other metal forming operation.

The invention will now be more fully described with reference to certain specific examples. The latter are intended as being merely illustrative and not as restricting the invention. Unless otherwise specified, concentrations in the following examples and elsewhere herein are expressed as percent w./vol.

Example I A series of tube sections of ordinary 18/8 chromenickel stainless steel were subjected first to an acidpickling treatment until smooth in an aqueous bath containing 11% nitric acid and 1.5% hydrofluoric acid, the temperature of the pickling bath being 113 F. and the time of immersion being from 30 to 50 minutes. The pickled tube sections were then thoroughly rinsed in running Water. A number of these acid-pickled sections were immediately dried and immersed for 10 minutes at 122 F. in an oxalate coating bath having the following composition:

Grams/liter Oxalic acid 196 Iron (total) 70 NaCl In all cases, the coatings on these tube sections were irregular and there were clear uncoated areas or spots on the surface. The coated tubes (Tests A) were set aside for determination of the total weight of coating.

Others of the acid-pickled and water-rinsed stainless steel tube sections were immersed for 10 minutes in a bath containing 15% of sodium hydroxide and which was maintained at 176 F. The tubes were again rinsed with cold water and oxalate coated in the same manner (Tests B). The coatings in this case, however, were complete, uniform, tightly adherent and considerably heavier than those of (A) above.

A third group of the acid-pickled, water-rinsed stainless steel tube segments or sections were immersed in a bath containing 15% sodium hydroxide and 1% potassium cyanide. As before, the bath temperature was 176 F. and the time of immersion was 10 minutes. After rinsing and drying these tubes were then oxalate coated as before (Tests C). The resulting coatings again were heavy, complete, uniform and tightly adherent.

A' last group of the acid-pickled, water-rinsed tube sections were immersed for 10 minutes in a bath at 68 F., the bath containing 15% sodium hydroxide and 1% potassium thiocyanate. After rinsing in water and drying these tubes (Tests D) were oxalate coated as before. The coatings obtained, like those of Tests B and C, were heavy, complete, uniform and tightly adherent.

From various quantitative tests, the weight of the adherent coating layer on all these tube sections was determined. Summarized, the results are as follows:

Weight of coating mg./dm. (ave.)

From the above, it is readily seen that coatings in Tests It through D were complete coatings while those of Tests A were irregular and failed to completely coat the metal. In addition, all of the B-D coatings were from 53 to 84% heavier than that of Tests A. When the coated stainless steel tubes of Tests A through D were drawn, however, it was found that the coatings'of Tests A were highly unsatisfactory in that they were not adequately adherent to the metal and they flaked or were scraped off the metal when the latter were drawn through the die. The coatings of Tests B through D, by contrast, were sufficiently adherent to the metal to be a great aid in the tube drawing operation. With the latter, higher drawing speeds were utilized and greater total reduction in cross section was permitted.

Example ll Ordinary 18/8 stainless steel is subjected to two different preliminary acid-pickling baths. The pickling baths and conditions'were as follows:

( 1 Ferric sulfate percent 6 HF do 1.5 Temperature F 158 Time minutes 60 (2) Nitric acid percent 11 HF do.. 1.5 Temperature F 113 Time minutes 30 to 50 After immersing in the pickling baths, the metal in each case was thoroughly rinsed with cold water and then immersed for 8 minutes in alkaline rinse baths containing 10% sodium hydroxide and in most cases varying amounts of either sodium cyanide or potassium thiocyanate. After the alkaline rinse, the metal was again thoroughly rinsed with water and then oxalate coated in a coating bath similar to that of Example I. The weight of coatings on each panel was determined to be as follows:

From the above it is seen that in every case the pre-rinse in plain sodium hydroxide solution produced an improvement in coating Weight and that all pre-rinsing solutions tested which contained 0.06 to 1% of NaCN or KCNS produced still further improvements in coating weight. As the concentration of NaCN or KCNS in the pre-rinse was increased, the weight of coating increased rapidly to a maximum between 0.06 and 0.5% and then slowly decreased, although in all cases the weight of coating obtained with the alkaline pro-rinsing baths was greater than those obtained without such a treatment.

Example III In a similar fashion pre-rinsing treatments varying from 15 to 50% by volume of sodium hydroxide and activated with 1% potassium cyanide or potassium thiocyanate 'were applied to a number of common German corrosion- Several bundles of 6 mm. coiled wire of RNO 18 German chrome steel (18% chromium) and Anoxin 4 (18% chromium, 10% nickel and 2% molybdenum) were de-scalecl by pickling for six hours in dilute HCl-HF followed by a complete pickling pretreatment in 14% hitric acid and finally given a thorough rinsing in cold Water. Several of these bundles were given an oxalate coating at 113 to 122 F. in a bath similar to that of Example I; The coatings were gray-green and thin.

The remaining bundles of pickled wire were immersed for 10 minutes at 176 F. in a bath containing 15% sodium hydroxide plus 1% potassium or sodium cyanide. After rinsing in water, the bundles were then given an oxalate coating as before. The coatings obtained were dusty, yellowish to gray-green in color, and they were uniform and heavy.

When these two groups of round wire were subjected to drawing to an octagonal profile greatly differing results were obtained. With the wire not subjected to the alkaline pro-rinse, the wire turned sharp on the first extrustion. By contrast, the wire that had been alkaline pre-rinsed before being oxalate coated could be drawn from 5.5 mm. to 3.3 mm. in octagonal profile and from 5.5 to 2.25 mm. in round profile with a total reduction in area of 83%.

In a similar fashion 18/8 stainless steel wire was pickled in HNO -HF and then in HNO given a pre-rinse bath treatment in 15% NaOH containing 1.0% KCN, and coated with oxalate as before. The coatings were dusty and almost a gray-yellow in color. When drawn on multiple dry extrusion machines (12 extrusions) with a dry soap lubricant, reduction of from 0.8 to 0.28 mm. (total reduction 83%) were obtained. Without the alkaline pre-rinse the oxalate coated wire could not be drawn in this manner.

While there have been disclosed certain preferred ways of carrying out the invention, it is desired not to be limited solely thereto, and it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. A method of producing oxalate coatings on stainless steels, which coatings greatly improve metal-drawing operations, which method comprises subjecting the surface of the metal to a series of treatments in the order: 1) an acid-pickling treatment to remove corrosion and scale, (2) an alkaline rinse in a solution containing from 5 to 25% of an alkali-metal hydroxide and from 0.02 to 1% of an activating ion selected from the 7 class consisting of cyanide and thiocyanate, and (3) contacting the resulting acid-pickled, alkaline-rinsed metal surface with an oxalate coating solution until a uniform and tightly-adherent coating is form-ed thereon.

2. A method as defined in claim 1 and further characterized in that the acid-pickling treatment is conducted in a bath of nitric and hydrofluoric acids.

3. A method as claimed in claim 1 and further characterized in that the alkaline rinsing solution contains from to 25% sodium hydroxide and 0.06 to 0.5% of an alkali-metal cyanide.

4. A method as claimed in claim 1 and further characterized in that the alkaline rinsing solution contains from 5 to 25% sodium hydroxide and 0.06 to 0.5% alkali-metal thiocyanate.

5. A method of producing oxalate coatings on stainless steels, which coatings greatly improve metal-drawing operations, which method comprises subjecting the surface of the metal to a series of treatments in the order: (1) an acid-pickling treatment in an aqueous solution containing ferric sulfate and hydrofluoric acid, (2) an alkaline rinse in a solution containing from 5 to 25 of an alkali-metal hydroxide and 0.02 to 1% of an activating ion selected from the class consisting of cyanide and thiocyanate, and (3) contacting the resulting acid-pickled, alkaline-rinsed metal surface with an oxalic acid solution containing ferric ion activator until a uniform, adherent coating is formed thereon.

6. A method as claimed in claim 5 and further characterized in that the alkaline-rinsing solution contains from 5 to 25% of sodium hydroxide.

7. A method of producing oxalate coatings on metals selected from the class consisting of iron, chromium, nickel and alloys of at least any two of these metals, which method comprises subjecting the surface of the metal to a series of treatments in the order: (1) an acidpickling treatment to remove corrosion and scale, (2) an alkaline rinse in a solution containing from 5 to of an alkaline compound selected from the class consisting of the alkali-metal hydroxides and their strongly alkaline-acting salts and from 0.02% to 1% of an activating ion selected from the class consisting of cyanide and thiocyanate, and (3) contacting the resulting acidpickling, alkaline-rinsed metal surface with an oxalate coating solution until a uniform and tightly-adherent coating is formed thereon.

References Cited in the file of this patent UNITED STATES PATENTS 2,116,954 Singer May 10,1938

2,338,045 Leonard Dec. 28, 1943 2,577,887 Gibson Dec. 11, 1951 FOREIGN PATENTS 661,385 Great Britain Nov. 21, 1951 OTHER REFERENCES Starr: Finishing Magnesium, Metal Finishing, October 1952, pages 62-64.

Claims (1)

1. A METHOD OF PRODUCING OXALATE COATINGS ON STAINLESS STEELS, WHICH COATINGS GREATLY IMPROVE METAL-DRAWING OPERATIONS, WHICH METHOD COMPRISES SUBJECTING THE SURFACE OF THE METAL TO A SERIES OF TREATMENTS IN THE ORDER: (1) AN ACID-PICKLING TREATMENT TO REMOVE CORROSION AND SCALE, (2) AN ALKALINE RINSE IN A SOLUTION CONTAINING FROM 5 TO 25% OF AN ALKALI-METAL HYDROXIDE AND FROM 0.02 TO 1% OF AN ACTIVATING ION SELECTED FROM THE CLASS CONSISTING OF CYANIDE AND THIOCYANATE, AND (3) CONTACTING THE RESULTING ACID-PICKLED, ALKALINE-RINSED METAL SURFACE WITH AN OXALATE COATING SOLUTION UNTIL A UNIFORM AND TIGHTLY-ADHERENT COATING IS FORMED THEREON.