Historical Articles

February, 1954 issue of Plating

 

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Radiometric Study of Supplementary Chromate Coatings for Zinc and Cadmium Plating

Stanley L. Eisler, Jodie Doss and Mary Ann Henderson
Ordinance Corps, Rock Island Arsenal, Rock Island, Ill.

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ABSTRACT
Radiosulfur and radiochromium were used to determine the amounts of sulfate and chromium contained in coatings produced from various supplementary dip solutions. The leaching of these two ions during exposure was also determined. It was found that the amount of sulfate and chromium in the coating increases as the concentration of H₂SO₄ in the dip solution is increased, although not proportionately. The loss of chromate during exposure was very slow and did not exceed 10 per cent after extended exposure periods. However, the sulfate was lost more rapidly, with the greatest loss in the first week and continuing up to as high as 70 per cent after several weeks’ exposure.

INTRODUCTION
Chromate supplementary coatings came into prominence during the last World War due to the need for improving the corrosion resistance of cadmium and zinc plated objects. The original solutions consisted of chromate salts and either sulfuric acid or nitric acid. At present, a large number of commercial preparations use-one or more of the following materials in addition to one of the above named acids and chromate salts: formic acid, soluble formate salts, ferric chloride, silver nitrate, acetic acid, sodium chloride, trifluoroacetic acid and zinc nitrate.

Chromate supplementary coatings are believed to be gelatinous films formed on the surface of metallic zinc or cadmium by the chemical reaction of the chromate solution with the metal during a short immersion period. The color of these thin coatings may be olive drab, bronze, yellow iridescent, blue bright, or clear bright. In addition, many of these coatings may be dyed black, red, blue or green.¹ The coatings are believed to be approximately 0.02 mil thick.² An analysis of the coating on zinc reveals that it is composed of chromium, traces of zinc and other occlude constituents of the acid chromate bath.³ Maxon⁴ states that the chromate supplementary coating is composed of approximately 28 per cent trivalent chromium and 8 per cent hexavalent chromium. Anderson believes the coating to be a colloidal film of the basic chromium chromate of the general formula Cr₂O₃ · CrO3 · XH₂O and that the inhibitory property is due to the hydrolysis of this film in the presence of moisture and the release of the soluble hexavalent chromium which prevents rapid corrosion of the metal. The trivalent chromium is also believed to offer protection of the metal by exclusion of water from the surface.

It had been observed that some supplementary chromate coatings on zinc and cadmium plated articles processed in a chromate-sulfuric acid bath showed corrosion products and pitting after storage in a damp area. However, coatings produced by treatment in a chromate-nitric acid bath showed no signs of corrosion, indicating that the failure of the first coating may have been due to free sulfate occluded in the supplementary chromate coating.

This investigation was undertaken to prove or disprove the effect of occluded SO₄^-- in the coating provided the presence of the occluded SO₄^-- in the coating could first be established. The use of radiometric techniques for this investigation was chosen since the amounts of chromate or sulfate are extremely small and other means of measurement would prove inadequate.

MATERIALS EMPLOYED
Six supplementary chromate dip solutions were prepared. Each solution contained 200 g/l of Na₂Cr₂O₇ and varying amounts of H₂SO₄. Solutions were numbered 1 through 6 and contained 4, 5, 6, 7, 8 and 9 ml of H₂SO₄ per liter, respectively.

The radiosulfur as H₂S³⁵O₄ and radiochromium as Cr⁵¹Cl₃ were purchased from the Oak Ridge National Laboratory by authorization of the U. S. Atomic Energy Commission, Isotopes Division. The Cr⁵¹Cl₃ was oxidized to the hexavalent state by treatment with NaOH and H₂O₂. Efficient amounts of the two tracers were added to each of the six solutions to provide an activity level of approximately 0.2 microcurie of S³⁵ and 20.0 microcuries of Cr⁵¹ per ml of solution.

Two commercial supplementary dip compounds, designated compounds A and B, were made up according to the manufacturers’ specifications for comparison in the leaching test only. Subsequently, identical solutions of A and B were prepared with radiochromium added to provide an activity level of approximately 20.0 microcuries per milliliter. The radiosulfur was omitted from these solutions since HNO₃ is used as the main constituent other than the chromate salt. The latter two solutions were used for tests A-7 through A-10 which will be mentioned later.

The coupons used in this investigation were circular discs made of SAE 1020 steel, 0.975 inch in diameter, and 1/16 inch thick. A 0.073 inch hole was drilled 1116 inch from the edge for ease of handling with Monel wire hooks.

The coupons were degreased in a trichloroethylene vapor degreaser for five minutes and electrolytically cleaned in an alkaline derusting bath. After a cold water rinse, half of the coupons were plated in a bright cadmium bath at room temperature using a current density of 20 amps/sq ft. The remaining coupons were plated in a bright zinc bath at room temperature using a current density of 30 amps/sq ft. Forty coupons without holes were-cleaned and degreased in the same manner and cadmium plated for use as controls. A plated thickness of approximately 0.2 mil was obtained in all cases. The coupons were rinsed thoroughly after plating and air blown until dry.

COATING PROCEDURE
A small portion of each of the test solutions was set aside, for use in control coupon preparation, prior to coating the large number of test coupons.

The coupons were individually immersed for 15 seconds in the supplementary chromate dip solutions within 30 minutes-after plating. After removal from the solutions, each coupon was rinsed in running tap water and air blown until dry.

COUNTING PROCEDURE
The coupons were counted by placing the test coupon in the center of an aluminum slide held in a Lucite mount so that the surface of the coupon was approximately 5 millimeters from the mica window of a Geiger-Müller tube operated in conjunction with a scaling unit. The Lucite mount was enclosed in a 1 inch lead shield to reduce the background count due to cosmic radiation and other sources.- Each coupon was counted for a 3 minute period and the counting rate corrected for: background, coincidence loss and decay.

Radiometric determinations of radiochromium were carried out by use of an argon-filled TGC-3 Geiger-Müller tube. A beryllium absorber with a thickness of 150 mg/sq cm was employed with this tube to cut out all beta emissions from the radiosulfur. The absorber also prevents approximately 50 per cent of the low energy X-rays of the radiochromium from reaching the sensitive portion of the tube but this was not considered disadvantageous.

A helium-filled TGC-2 Geiger-Müller tube was used for measuring the radiosulfur in the presence of radiochromium. This was possible because this tube has a very low efficiency for -detection of the soft X-rays emitted by the radiochromium. Through the use of these two tubes, it was possible to count the radiations emitted by each isotope in the presence of the other.

PREPARATION OF CONTROLS
Five cadmium plated control coupons for each tracer containing solution were prepared by pipetting 1 milliliter of a 1-100 dilution of the original supplementary dip solution onto the surface of the coupon.

The five control coupons were placed on the table of a sample spinner which was operated at 20 rpm and the solution pipetted onto the coupons. The solution was evaporated to dryness under an infrared lamp as the table rotated in a slightly inclined position. This method of allowing the solution to be washed repeatedly over the coupon surface was employed to produce a more even distribution of the solution residue.
The average counting rates of the control coupons and the specific activity for each solution are presented in Table I. The specific activity (counts per minute per gram) was calculated for the Cr and SO₄ in each solution by dividing the average counting rate of the control coupons by the amount of Cr or SO₄ contained in the aliquot portion pipetted onto each control coupon. The amounts of Cr contained in solutions A and B were determined by analysis and were found to be 20.7 and 37.0 g/l, respectively.

EFFECT OF SULFATE CONCENTRATION
Three sets of zinc plated and three sets of cadmium plated coupons were employed for this series of tests. The three tests of zinc plated coupons are designated tests A-2, A-3 and A-6 while the cadmium tests are designated tests A-1, A and A-5. Each test included thirty coupons, five treated in each of the six supplementary dip solutions.

The average net counting rates for the six tests are presented in Table II. It will be noted that separate values have-been obtained for the S35 and Cr5t radioisotopes by use of the two types of Geiger-Müller tubes previously described.

Utilizing the counting rates of the test coupons and the specific activities of the various solutions (cf. Table I) it was possible to calculate the weight per square foot of SO₄^-- or Cr on the test coupons as follows:

———————————Counting Rate of test coupons
Weight on test coupon = ——————————————
————————————Specific activity of solution

Weight on test coupon x 192.8 = grams/sq ft
(192.8 is the conversion factor for converting the coupon area to a square foot)

The weights of sulfate and chromium expressed as grams per square foot obtained under the various test conditions are presented in Tables III and IV, respectively. Examination of these two tables reveals the following facts:

1. The amount of sulfate in the coating incased as the concentration of H₂SO₄ in the solution was increased, although not proportionately.

2. It may be noted from the values presented that the differences between solutions 1 and 2, 2 and 3, etc., become greater in nearly every case, with the exception of the difference between 4 and 5, as the H₂SO₄ concentration is increased. A possible explanation of this variation in behavior may be that as the concentration of H₂SO₄ in the solution is increased a desorption reaction begins and becomes competitive with the adsorption reaction. Therefore, at the concentration of solution No. 5 this second reaction attains its maximum, causing a decrease in the-amount of sulfate expected in the coating.

3. The amount of chromium in the coating also increased as the amount of H₂SO₄ in the solution was increased, although not proportionately.

4. The chromium results also indicate the same drop-off in adsorption at the level of solution No. 5 as noted for the sulfate in the coating.

5. In all cases, the amounts of sulfate and chromium deposited on the cadmium plated coupons was greater than that deposited on similarly treated zinc plated coupons. This may be attributable, at least in part, to the lower solubility of cadmium salts as compared to zinc salts.

6. It may be noted that generally there was a drop in the amount of sulfate in the coatings prepared from the same solutions in later tests. This was due to a slight depletion of the H₂SO₄ in the supplementary dip solutions. This was not noted with chromium, no doubt, because of the greater concentration of Na₂Cr₂O₇ in the solution.

The results of these tests, presented above, indicate that the sulfate in the supplementary dip solution becomes an integral part of the coating, which was the primary question to be answered by this investigation. There also appears to be evidence that the colloidal film mentioned by Andersons as consisting of Cr₂O₃ · CrO₃ · XH₂O may have a more complex formula containing the sulfate ion or a complex sulfate salt. However, there is also the possibility that some, if not all, of the sulfate may be held in the film merely by occlusion. Evidence to be presented in a later section dealing with leaching tests performed on the various coatings seems to substantiate the latter hypothesis.

EVALUATION OF NITRATE CONTAINING SOLUTIONS
Duplicate runs of cadmium plated coupons (tests A-7 and A-8) and zinc plated coupons (tests A-9 and A-10) were used for this phase of- the investigation. Each test included ten coupons, five processed in solution A and five in solution B. Both solutions contained Cr⁵¹ as the tracer.

The average net counting rates based on Cr⁵¹ determinations are presented in Table V as are the weights of chromium-per square foot calculated from the counting rates and the specific activities (cf Table I).

Comparison of the results for the amount of chromium contained in the coating reveals the following:

1. Coatings produced from compound A contained one and one-half times more chromium than coatings produced from compound B.

2. Both compounds produce approximately 60 per cent more chromium in the coating on cadmium plate than they do on zinc plate.

3. Both nitrate containing compounds produced coatings having a chromium content from 2 to 4 times that found in coatings produced from sulfate containing baths. This may, of course, be attributed to the presence of buffering agents, wetting agents, or metallic additives, which may be included in the two commercial products.

LEACHING STUDIES
This series of tests was conducted to endeavor to determine the following:

1. The rate of dissolution of the chromium and sulfate from the coating under various exposure conditions.

2. The effect of the sulfate coating on the corrosion resistance.

3. The corrosion resistance of coatings prepared from nitrate containing dip solutions as compared to those prepared from sulfate containing solutions.

The first phase of this series of tests involved a cold water immersion test of the coupons prepared for tests A-1 and A-2 previously mentioned. These coupons, control coupons- (with no supplementary dip) and coupons processed in compounds A and B (without tracer) were suspended 2 to 3 inches below the surface of the water from Monel wire hooks in a rectangular copper tank through which tap water (approx. 19° C) flowed continually. Both cadmium plated and zinc plated coupons were processed in the compound A and B solutions.

The second phase of this series of tests involved the exposure of similar groups of coupons in the Rock Island Arsenal type humidity cabinet. This test was conducted as it was thought that the 95-100 per cent relative humidity maintained at a temperature of 100° 17 would provide a more corrosive atmosphere so that results could be obtained in a much shorter time. This phase included the coupons of tests A-3, 4, 5, 6, 7, 8, 9 and 10 previously mentioned with controls and coupons processed in compounds A and B containing no radiochromium.

The coupons exposed in the two environments were removed periodically, to be counted, as previously noted, to determine the amount of chromium or sulfate lost from the coating during the exposure period. After correction of the counting rates for decay, the per cent of residual sulfate or chromium on the coupons was determined by dividing the original counting rate by the corrected final counting rate and multiplying by 100.

The percentages of residual chromium for test A-1 are presented in Table VI along with the percentages of residual sulfate for tests A-1 and A-2, the-two tests conducted in the cold water. The results for per cent residual chromium for test A-1 are the only chromium results presented as in all other tests it was found that the amount of chromium lost from the coupons, even after an extended exposure period, was less than 10 per cent. Table VII contains the per cent residual sulfate results for the humidity cabinet tests. The results of similar tests are presented as an average of the two tests.

The following salient points maybe noted from the data presented in Tables VI and VII:

1. The per cent of residual chromium on supplementary chromate dipped cadmium plated coupons after six weeks exposure was found to be independent of the total amount of chromium on the specimens prior to exposure. This would seem to indicate that the chromium is all in the same form although larger amounts are deposited on the coupons from solutions of higher H₂SO₄ concentration.

2. All tests indicated that the largest percentage of either chromium or sulfate was lost during the first week of exposure. This leads to the assumption that the original quantities of either ion leached from the surface are present in the-coating merely as occlusions and not as true components of the coatings.

3. In the cold water test environment the sulfate was leached from the cadmium plated coupons to a greater extent than from the zinc plated coupons. However, the reverse was true for the tests conducted ‘in the humidity cabinet. Based upon the relative solubilities of the two metallic sulfates, the latter condition would be expected, if it can be assumed that the metallic sulfates are actually formed and are a constituent of the coating. Another possible reason for this reversal may have been due to the speed of reaction since only about half as much sulfate was removed from the coating after a six-week exposure in the cold water as compared to a similar humidity cabinet exposure.

4. Comparing the, amount of sulfate leached from the coating under the two test environments employed, it may be noted that in every case a much greater amount of sulfate was lost durin’ the humidity cabinet exposure.

5. The amount of sulfate retained in the coating was found to be independent of the amount of H₂SO₄ in the supplementary dip solution.

 

CORROSION RESISTANCE
Inspection of the various test coupons used for the leaching tests at the end of each exposure period was made to determine the time at which red rust was first visible. The evaluation of the above was found to be extremely difficult in the case of zinc plated coupons due to the formation of heavy zinc salt deposits. A statistical comparison was further complicated by the fact that generally there were very wide discrepancies between the times of failure of coupons within the same group.

The following general statements may be made concerning the results of the corrosion resistance tests:

1. The corrosion resistance of all treated coupons was greater in the humidity cabinet tests than in the circulating water tests. This was contrary to the expectation previously mentioned. This contradiction may have been due to the action ofcontinuous washing the circulating water.

2. It was also found that very little advantage was provided by the use of the supplementary coatings when the coupons were exposed in the circulating water. This, of course, is a severe condition which ordinarily would not be encountered in service.

3. In all of the humidity cabinet tests it was found that the supplementary coatings provided about the same amount of additional protection. This was true of the sulfate containing baths as well as the nitrate containing baths. It was also found that the amount of H₂SO₄ in the sulfate baths had no effect on the corrosion resistance of the coating.

4. The supplementary dip coatings were found to increase the corrosion resistance of the cadmium plated coupons to a greater extent than that-of the zinc plated coupons.

SUMMARY
Although it was proved that the coatings prepared from sulfate containing solutions contained sulfate, no definite relationship could be determined as to the effect of the sulfate on the corrosion resistance of the coating. The fact that the sulfate is leached from the coating to a considerable extent prior to the beginning of corrosion may indicate that this phenomenon is contributory to the failure of the coating. It is possible that the sulfate, present either as the metallic sulfate or occluded sulfate ion, upon being leached from the coating produces voids in the coating which then become focal points for corrosion of the metal beneath.

The role of the sulfuric acid appears to be one reacting with the base metal and at the same time releasing hydrogen which reduces some of the hexavalent chromium to the trivalent state thus providing the two chromium ions necessary for formation of the colloidal film, Cr₂O₃ · CrO₃ · XH₂O. This also explains why an increase in sulfuric acid concentration tends to increase the amount of chromium in the coating because the greater amount of Cr⁺⁺⁺ formed the greater the possibility for the formation of the above mentioned colloidal film.
Since the corrosion resistance is not enhanced by the additional amount of coating, as evidenced by chromium content on the coupon, it is reasonable to presume that the sulfate ion or metallic sulfate formed as a by-product of the oxidation-reduction reaction proves deleterious by weakening-the colloidal film. It also appears that the larger the amount of sulfate in the coating the more easily it is removed since the percentage removed is approximately the same, indicating that a greater amount is removed from the coatings containing more sulfate.

Based on the results of the work reported it has been found that there is no advantage of nitrate containing solutions over sulfate containing solutions as regards corrosion resistance. This answers the question of paramount interest mentioned at the beginning of this article.

It is believed that the information obtained and the theories evolved therefrom should provide for a better understanding of the supplementary dip treatment for zinc and cadmium plated work.

ACKNOWLEDGMENT
The authors wish to express their appreciation to their co-workers at the Rock Island Arsenal Laboratory for their assistance and to the Ordinance Corps, Research and Development of the Department of the Army and the Supervisory Staff of the Laboratory for permission to publish the information in this paper.

LITERATURE REFERENCES
1. H.C. Irvin, Metal Finishing 49, 109-113, July, 1951.
2. A.G. Taylor, Proc. Am. Electroplaters’ Soc. 32, 6 (1944).
3. W.M. Peirce, Private communication from N.J. Zinc Co. to Frankford Arsenal dated 12 June 1942, Research Item No. 101.4.
4. S.E. Maxon, Metal Finishing 43, 148-149, April 1945.
5. E.A. Anderson, Corrosion Handbook, Edited by H.H. Uhlig, John Wiley and Sons Inc., 1948, p. 862.

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