US2800443A - Method of chromium plating - Google Patents

Description

higher strength may bealso-plated.

United States Patent METHOD OF .CI-IROMIUM PLATING Jesse E. Stareck, Royal Oak, Edgar J." Seyb,'*Jr.; Oak Park, and Frank Passal, Detroit, Mich.,- assignorsyby 'mesne assignments, to Metal & Thermit Corporation, New York, N. Y., a corporation of New Jersey 'No Drawing. Application" July 26, 1955, :Serial No;524',578

11 Claims. "(CL 204-51) *This "invention relates to an improved method for chromium plating 'hard', high strength steels. Conventional chromium plating of such-steels is accompanied by such ahigh loss of fatigue strength that failure ofthe steel usually occurs under repeated loading; moveover, the loss in'fatigue strengthincreases with increasing strength of the steel. *This unfavorable effect of conventional chromiumplating has naturally limited the usefulness of these plated steels.

A-principal object or the inventionis to provide a method for chromium"platingh'ard, high strength steel without substantial loss'of fatigue strength. "The method is particularlydirected-to the'platingof steel articles for use at ordinary temperatures under conditions of repeated stress and strain that approach the fatigue limit of the steel. It has been found that ahigh fatigue strength may be obtained in the plated article if the chromium deposited on-it contains a large number of fine cracks such that it exhibits a low residual stress, The resultant chromium plated steel enjoys the protection against corrosion and -wear, and the hardnessand low resistance to friction,

- a. s. i., is passed-to the article as cathode inanaqueous bath having a chromic acid, CrOz, content of 100 to 400 .g./l., preferably l50 to-300 g./l.; and containing dissolved sulfate, SO4=,,as catalyst. -The ratio of 0193 to .dissolved S04 is 40:1 to 100: 1, and the bath temperature is 90 to 110 F. A wider temperature range, namely, 90-to 130 F., may be employed for baths having a CrO3/SO4 ratio of 40:1 to85 :1. Chromium is. plated on the article to forma coating at least 1 mil thick.

The chromic acidmay be supplied to the bath as such and the sulfate inthe form of anysuitable compound, such as sulfuric acid.

At CrOs concentrations f300 to 400 g./l. the bath may be made to be self-regulating with respect to the sulfate concentration. Such a bath comprises CIO3 and, as the source of the sulfate, calcium sulfate in an amount sufficient to saturate the bath and to provide an undissolved excess of calcium sulfate. As this saturation concentration of dissolved sulfate is higher than required; a soluble calcium compound is added in an amountsufficient to suppress the concentration of dissolved sulfate from its unsuppressed saturation concentration to a "lower value corresponding to the required CrO3/SO4 ratio. Thus, as dissolved sulfate is removed from the'bath, as by drag out losses, sufiicient excess calcium sulfate goes into solution to replace the lost sulfate. Lower tempera tures and ratios are preferred for this type of bath; thus, in the case-of a CrO3 concentration of 300 g./l., a ratio of 40:1 to 60:1 is quite satisfactory. Suitable soluble ,g./l. for a ratio'of to 1. g./l. of CrOa a 40 to 1 ratio-and a 60 to 1 ratio canbe '2 calcium suppressor compounds for providing dissolved calcium are calcium oxide, calcium .hydroxide,'calcium carbonate, calcium dichromate, and calcium ,chromate. For example, for baths ontaining 400 g./l. of CrOs there is added sufi'icient calcium sulfate to the solution to saturate it and then 7'g./l.'of'dissolved suppressor calcium are added to give a ratio of CrOa-to S04 of 40 to 1; 14 g./l. of dissolved suppressor calcium are added to give a ratio of 60 to 1; 20 g./l. for a ratio of 80 to 1; and 25 For baths containing 300 obtained with 12' and'20 g./l., respectively, of dissolved suppressor calcium.

The unplated steel mayhave a Rockwell hardness, of at least 20C. The upper limit of hardness. may be'as desired, for example up to 54C. Good results .are obtainablev With steels .havinga Rockwell hardness in'the range of 28C..to 47C, particularly. 33C to-47C. Alloy or high quality: steels are quite suit-able. for plating, but theinvention is notlimited to them. Substantial thicknesses of chromium are deposited on the steel, ranging from 1 or 2 to 30 or. more mils, and preferably from 5 to. 15 mils. Plating .times may very widely, for example 1 from 1.to24 or morehours.

[Chromium 'electrodeposits exhibit the characteristioof stress, a property Which..changes with the thickness of the depositand Which,.by means of. a spiral contractometer, can be measured throughout the course of afplating operation. With .this instrumentjthe stress in a long continuous section ofchromium plateis transferredto the underlying base metal comprisingalong, helically wound strip of thin metal havingone end .free to rotate. vThe stress in the plate can be calculated from the amount. of rotation. It has been foundthat initially, the stress in a deposit increases very rapidlyduring the first'few mo- -ments ofplating until it exceeds the tensile strength. of

the deposit, at which point crack-lines form inthe chr0- 'miurn to relieve'the high tensile stress. As thethickness guished. from the. positive values previously obtained. This behaviour is illustrated in Table 1 belowwhich sets forth stress data, as obtained by the spiral contractometer,

at various plate thicknesses for a bath whose composition and operating. conditions aregiven in Example? of Table 3. It may. benotedth'at inthe plating run carried out at2 a. s. i., the stress was already dropping by the time the first readings on the contractometer were taken.

Table 1 Stress, 1,000 p.s.i. Chromium thickness, mils 1 a.s.i. i 2 a.s.i.

0.025 161 175 0.05 .166 159 0.1-- 156 157 0.2 106 0.3 74 87 0.4-- 54 64 I 0.5 42 47 0.75- 26 31 1.0 v 17 20 1.5 8 =10 2.0 3 5 2.5 1 2 3.0.- 0 o. 5 3.5.- 0.5 4.0... ?1 1 i of -15,000 or -20,000 p. s. i.

'- 1-8) and exhibited the properties listed in Table 2. iEach deposit was 2 mils thick. The three deposits were produced from baths described in Examples 7, 1 and 6.

Table 2 ,5 from measured stress, and conversely, the fewer the cracklines, the higher the stress. The correlation applies over a wide stress range, varying from a positive tensile stress of about +80,000 p. s. i. to a negative compressive stress Between about -15,000 and 20,000 p. s. i. the stress tends to level off such that further increases in thenumber of crack-lines is not accompanied by more negative stress values. The correlation holds for thicker deposits, say those of at least 1 mil thickness. By counting the number of crack-lines in a deposit it is possible to obtain an indirect semi-quantitative measure of the stress in the deposit and to compare one deposit with another.

The number of crack-lines in chromium plate deposited on hardened high strength steel is also correlated with the fatigue strength of the plated steel, the more numerous. the crack-lines, the higher being the fatigue strength. It follows that deposits exhibiting high tensile stress result in a severe lowering of the fatigue strength of the steel, whereas deposits of low stress give much less lowering of the fatigue strength. It is desirable to deposit on the steel a plate having 1000, preferably 1200, to 10,000 crack-lines per inch. In terms of stress, the plate should range from +5000 p. s. i. tensile stress, preferably p. s. i., to -20,000 p. s. i. compressive stress. Steel plated with deposits of this character exhibit a higher fatigue strength than if plated with deposits not so characterized. Fatigue strengths of about 70%, and more, of the unplated steel are obtainable, with strengths of at least 85% being preferred. Strengths of 60 to 70% of i the unplated steel are satisfactory for many purposes.

The crack pattern of a deposit'is determined early in the plating operating, say after about 0.5 mil of chromium is deposited. A consequence of this fact is that an article may be plated to a point where a desirable crack pattern, i. e., numerous crack-lines, has been established a and then transferred to another bath of the same type but operated at a higher speed, so that a desirable plate of required thickness may be obtained in a shorter time. Another characteristic of the method is that the crack pattern of a deposit maybe favorably changed during plating as by periodically reversing the current.

An incident of the chromium plating of steel in chromic acid plating baths is the embrittlement of the steel by hydrogen released during the plating operation. Where embrittlement of the chromium plated steel is a factor to be considered in connection with the end use 'of the steel, it can be prevented or substantially minimized by applying a barrier metal such as a copper lstrike between the steel and the chromium. Copper strike films may be applied by plating the steel in a suitable copper plating bath for a short time to deposit about 0.01 to 0.1 or, 0.2 mil of copper. The fatigue strength of the resulting copper plated, chromium plated steel, by comparison with a like steel similarly chromium plated but not copper plated, is not affected by the presence ofthe copper'barrier.

, Suitable copper plating baths include conventional cyanforming, Blum and Hogaboom, revised 3rd edition, 1949, pages 295 and 290, respectively; and conventional pyrophosphate copper baths,as described in U. S. Patents 2,250,556, 2,437,865, and 2,493,092.

The invention may be illustrated by the examples which follow, showing several plating baths, some within and some outside of the invention, their operation, and crack properties of the plate produced by the baths.

EXAMPLES 1 TO 8 A number of baths, the compositions of which are listed in Table 3, were made up from chromic and sulfuric acids to provide the ClO3/S04 ratios noted. Standard steel fatigue test specimens were plated in the baths at the temperature and current density conditions set forth. The specimens were the same, comprising onehalf inch round bar stock of SAE 4140 steel. Each specimen was 3% long and had a central portion that was reduced to a diameter of A". The specimens had been heat-treated and had a hardness of Rockwell C43. The fatigue strength of the specimen steel was 109,000 p. s. i. and the tensile strength was 222,500 p. s. i. Crack-lines were determined by microscopically examining the surface of a specimen at 600K: an arbitrary straight line one inch long was selected in the microscopic field and the average number of crack-lines, including filled-in lines, which crossed such arbitrary line was counted.

Table 3 Exam. CrOa, Ratio: Temp., 0. D., Thick- Crack- No. g./l. CrOi to F. a. s. i. ness, lines per SO mils in.

250 100 130 2 2 400 250 130 2 6 to 7 1, 200 250 70 130 2 6 t0 7 1, 200 250 55 130 2 6 to 7 1, 400 250 40 130 2 6 t0 7 1, 600 250 150 2. 9 2 50 250 100 130 2 2 400 250 100 2 2 1, 200 250 40 130 2 6 to 7 1, 600 250 40 2 6 to 7 1, 600

As shown by Examples 1 to 5, as the CrOs to sulphate ratio is decreased, the number of'crack-lines per inch increases, the temperature being constant. If the ratio is held contant, and the temperature is decreased, the number of crack-lines increases, as shown by Examples 6, 1 and 7, and also by Examples 5 and 8. In these last two groups of examples, the listing of Examples 1 and 5 are repeated merely to illustrate the relationship more readily. Example 1 represents a standard or conventional sulphate bath operated at standard conditions, and as is apparent, it does not produce a plate having the desired number of crack-lines. Example 6, like Example 1, also illustrates a plate not within the invention, the temperature in both examples being too high. Fatigue strengths of specimens plated according to Examples 1 and 8, except that thicker deposits were plated, are compared in Example 9.

EXAMPLE 9 Test specimens of the kind described were plated according to Examples 1 and 8, except that thicknesses of 10 and 11.6 mils, respectively, were deposited on the specimens. Fatigue strengths of the plated specimens were determined with a rotating beam fatigue machine of the R. R. Moore type. The specimen plated according to Example 1 had a fatigue strength of 57,000 p. s. i., while that plated according to Example 8-had a strength of 67,500 p. s. i.

Unless otherwise specified, the reference in the claims to the Cl'Os content of the baths is intended to include the amount ofv CrOs added per se and the amount added in the form of a chromate or dichromate salt.

In the light of the foregoing description, the following is claimed:

1. A method for reducing the loss of fatigue strength following chromium plating of an article of hard steel comprising electrodepositing chromium on the article by passing a current of A to 5 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 90 to 110F., said bath having a CrOa content of 100 to 400 g./l. and containing dissolved sulfate, SO4=, as catalyst, the ratio of CrOs to dissolved sulfate being 100:1 to 40:1, plating chromium on the article to form a deposit at least 1 mil thick, said deposit being characterized by having at least 1000 crack-lines per inch and a stress below +5,000 p. s. i., and thereby producing a chromium plated article of hard steel having a fatigue strength of at least 60% of that of the original unplated steel.

2. A method for reducing the loss of fatigue strength following chromium plating of an article of steel having a hardness of at least 20C Rockwell and a fatigue strength of at least 50,000 p. s. i., comprising electrodepositing chromium on the article by passing a current of A to 5 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 90 to 110 F., said bath having a CrOs content of 100 to 400 g./l. and containing dissolved sulfate, SO4=, as catalyst, the ratio of C103 to dissolved sulfate being 100:1 to 40:1, plating chromium on the article to form a deposit about 1 to 30 mils thick, said deposit being characterized by having 1000 to 10,000 crack-lines per inch and a stress of +5,000 to 20,000 p. s. i., and thereby producing a chromium plated article of hard steel having a fatigue strength of at least 60% of that of the original unplated steel.

3. A method for reducing the loss of fatigue strength following chromium plating of an article of steel having a hardness of 33C to 47C Rockwell and a fatigue strength of 70,000 to 110,000 p. s. i., comprising electrodepositing chromium on the article by passing a current of A to 3 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 90 to 130 F., said bath having a C1'O3 content of 150 to 300 g./l. and containing dissolved sulfate, SO4=, as catalyst, the ratio of CrOz to dissolved sulfate being 85:1 to 40:1, plating chromium on the article to form a deposit about 2 to 15 mils thick, said deposit being characterized by having 1200 to 10,000 crack-lines per inch and a stress of to 20,000 p. s. i., and thereby producing a chromium plated article of hard steel having a fatigue strength of at least 70% of that of the original unplated steel.

4. A method for reducing the loss of fatigue strength following chromium plating of an article of hard steel comprising electrodepositing chromium on the article by passing a current of A to 5 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of 90 to 110 F., said bath having a CIOs content of 300 to 400 g./l. and containing dissolved sulfate, SO4=, as catalyst, said bath containing calcium sulfate as the source of said dissolved sulfate, the amount of calcium sulfate being sutficient to saturate the bath and to provide an undissolvcd excess of calcium sulfate, said bath also containing a soluble calcium compound in an amount sufficient to suppress the concentration of dissolved sulfate from the unsuppressed saturation concentration of the latter to a lower value corresponding to a ratio of CrOs to dissolved sulfate of 100:1 to 40:1, plating chromium on the article to form a deposit at least 1 mil thick, said deposit being characterized by having at least 1000 crack-lines per inch and a stress below +5,000 p. s. i., and thereby producing a chromium plated article of hard steel having a fatigue strength of at least of that of the original unplated steel.

5. The method of claim 1 wherein, prior to chromium plating, a copper strike is deposited on said steel article to minimize any embrittlement of the steel article in said chromium plating bath.

6. The method of claim 1 wherein the steel of said upplated article has a hardness of at least 33C Rockwell.

7. The method of claim 1 wherein the fatigue strength of the steel of said unplated article is 60,000 to 110,000 p. s. 1.

8. The method of claim 1 wherein said chromium deposit is at least 5 mils thick.

9. The method of claim 1 wherein said chromium plated article of hard steel has a fatigue strength of at least of that of the original unplated steel.

10. The method of claim 1 wherein said sulfate, SOE, comprises substantially the entire catalyst in said bath.

11. A method for reducing the loss of fatigue strength following chromium plating of an article of hard steel comprising electrodepositing chromium on the article by passing a current of A to 5 a. s. i. to said article as cathode in an aqueous chromium plating bath at a temperature of to F., said bath having a CrOz content of 300 to 400 g./l. and containing dissolved sulfate, SO4=, as catalyst, said bath containing calcium sulfate as the source of said dissolved sulfate, the amount of calcium sulfate being sufficient to saturate the bath and to provide an undissolved excess of calcium sulfate, said bath also containing a soluble calcium compound in an amount sufficient to suppress the concentration of dissolved sulfate from the unsuppressed saturation concentration of the latter to a lower value corresponding to a ratio of CrOa to dissolved sulfate of 85:1 to 40:1, plating chromium on the article to form a deposit at least 1 mil thick, said deposit being characterized by having at least 1000 crack-lines per inch and a stress below +5,000 p. s. i., and thereby producing a chromium plated article of hard steel having a fatigue strength of at least 60% of that of the original unplated steel.

References Cited in the file of this patent

Claims (1)

1. A METHOD OF REDUCING THE LOSS OF FATIGUE STRENGTH FOLLOWING CHROMIUM PLATING OF AN ARTICLE OF HARD STEEL COMPRISING ELECTRODEPOSITING CHROMIUM ON THE ARTICLE BY PASSING A CURRENT OF 1/4 TO 5 A.S.I.TO SAID ARTICLE AS CATHODE IN AN AQUEOUS CHROMIUM PLATING BATH AT A TEMPERATURE OF 90 TO 110* F., SAID BATH HAVING A CRO3 CONTENT OF 100 TO 400 G./L. AND CONTAINING DISSOLVED SULFATE, SO4=, AS CATALYST, THE RATIO OF CRO3 TO DISSOLVED SULFATE BEING 100:1 TO 40:1 PLATING CHROMIUM ON THE ARTICLE TO FORM A DEPOSIT AT LEAST 1 MIL THICK, SAID DEPOSIT BEING CHARACTERIZED BY HAVING AT LEAST 1000 CRACK-LINES PER INCH AND A STRESS BELOW +5,000 P.S.I., AND THEREBY PRODUCING A CHROMIUM PLATED ARTICLE OF HARD STEEL HAVING A FATIGUE STRENGTH OF AT LEAST 60% OF THAT OF THE ORIGINAL UNPLATED STEEL.