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"Unfinished” Business—Thoughts of a Senior Plater, Part I


BY JOHN (A.KA. “JACK”) HORNER, PLATING CONSULTANT

In this retrospective piece, industry veteran John “Jack” Horner reflects on the early research and development and testing that spurred innovations in modern plating.

Background. The writer has spent a lifetime in many phases of plating and metal finishing. The first years, with production plating plants in control, trouble-shooting, and process engineering. Later, he was in the employ in technical service and product development of a chemical supplier to the plating and metal finishing industry. During this era, much of the research and development of the advances in the art and science of plated coatings was performed by the suppliers to the plating Industry. In this private sector, justification for work on any project had to be made, before the Company would spend for the support of continuing any proposed development project. Over the years, there were several proposed projects that were very interesting, and seemed as if there was some possible potential, to the writer, but were turned down by the holders of the purse strings. These became part of a “something to do on a rainy-day” file that has lingered in a mental file. Now, the writer has reached the conclusion that he has run out of rainy days, and if anything may be done with any of these topics, it will have to be done by others.
 
This paper outlines some of these subjects. The writer makes no claims to any patentability, and welcomes any and all efforts to finish any of this unfinished business.
 
A list of the topics included in Part I of this paper is followed by some background and rationale for the interest in each.
 
1. [S]-free Copper Plate. It had been found that [S]-free (or at least a very low [S] content in a CuCN plating solution) improved the corrosion resistance of Cu-Ni-Cr plating systems in accelerated corrosion tests. The term, [S], in this paper, refers to certain forms of sulfur-containing compounds that co-deposit. This excludes the stable, hexavalent sulfur (sulfate) compounds. The amount of sulfur was below the limit of detection by analytical methods available at the time. The effect of [S] was determined by its results on the deposit.  
 
2. Acid-Resistant Zinc Plate. There is a considerable beneficial effect on the acid resistance of Zn plated from a [S]-free or a very low [S]-containing, reformulated cyanide Zn plating solution. Deposits from these baths were pure enough to withstand any attack when placed in 50% muriatic acid for over an hour.
 
3. Simple Test for Hex Chromium in Plating Baths. Hexavalent Cr, as a contaminant in very low quantities in several plating baths, is detected with a variation in Hull cell tests. What does this suggest? How can this be used?
 
4. Effects of Trace Amounts of [S] in Other Plating Baths. Effects in silver baths and EN solutions have been noted. Importance?
 
5. A direct Relationship Exists Between Trace Amounts of [S] in Nickel Deposits and the Amount of Ni-release of Deposits in Contact with Human Skin. What can be done?

 

DISCUSSION OF INTEREST AND RATIONALE

Item 1.  Traces of [S] in CN-Cu Plating Baths
This was put into commercial practice while with a small supplier Company (Wagner Bros.) and as such is the only unfinished project in the list that ever reached industry. It was kept on the unfinished list because it never received any credit for the benefits it provided that it deserved.There had been reports that the presence of copper as an undercoat decreased (!) the corrosion resistance of a Ni-Cr plated part. (circa 1960). When the tests were repeated, the results were inconclusive (ASTM?). Wagner Bros had developed and were selling an additive system for CuCN baths based on Se and Tl for bright copper plating, primarily of Zn-based die castings. Some customers were using this process in barrel plating of steel articles, where Cu was the final finish, and a Cu-finish was wanted that would resist tarnishing (oxidation, a form of corrosion).     

The Company had been working on an organic....non-metallic.....brightener system, and had noticed a lesser tendency for the deposits to tarnish. The tarnishing was traced to [S] and/or [Se] in the Cu-deposit. When tested in a barrel Cu (CN) plating line, where Cu was the final finish, the tarnish-resistance was improved significantly. Further work showed that the additive, an acetylenic alcohol, seemed to reduce the dulling effects of S-compounds on the brightness of a high-speed Cu CN plating system. S-bearing compounds had long been used as “brightening” agents in CuCN plating baths; but the brightening effects were in the low current density areas. There was a dulling effect in increased current density areas. It should be noted that sulfur is a common impurity in commercial CN salts. The acetylenic alcohol seemed to either inhibit the co-deposition of the [S] impurity or make it behave as a non-S-brightening system. There was no lab equipment in-house to analyze for such small amounts of [S] at the time, so the amounts of [S] were not determined. The observation of the effects was relied upon.
 
It seemed logical to assume that, since the acetylenic alcohol aided in delaying the tarnishing effect, that it could have a beneficial effect in increasing the corrosion resistance, when used in the Cu as an undercoat in bright Ni-Cr plating processes. The in-house tests verified this effect. Being a small supplier, the funding was not available to get support from the major auto companies. The Company did manage to get a large automotive plating plant to run a comparative accelerated corrosion test (CASS) on Zn-based die castings between the [S]-free CuCN process and the most popular bright acid Cu of the time. As near as possible, only the Cu deposits were different, and the parts were Ni-Cr plated at the same time in an automatic machine. It must be mentioned that this comparative test was not “authorized” by the plating plant management at the time. The tests were done only for the personal edification of the parties involved. The results? The CuCN plated parts outperformed the acid Cu-plated parts! Similarly, a second automotive company was approached. This Company had an adequate pilot plating facility, capable of running the comparison, but decided not to run the tests. Had they already made up their minds? There was no way to get around this barrier, except through the auto company’s plated goods suppliers. (Our customers). This was during the time that the auto company was demanding their suppliers go to duplex-Ni as a means to improve the corrosion resistance results as measured by the Corrodkote test. Our approach? The existing customers, the plating shops, were started on a conversion to the organic brightened, non-[S], Se-free CuCN systems. As Se and the [S] were depleted, the results of the accelerated corrosion tests improved. The Cu plate thickness was set at 0.0004 inch minimum and maintained. The Cu-plate thickness was found important.
 
Something should be said of the era. It was the “Hey-Day of CuCN.”  It had its problems, but when treated properly, it was the best to be offered. Even with the small chemical supplier company this writer was involved with, at one time, there were nearly 500,000 gals of CuCN solutions utilizing the Company’s additives.
 
Why the interest, now? There are other non-CN copper plating systems on the market today. Would these behave like the CN systems with respect to the trace amounts of [S]. It should be known that most activated carbons have trace quantities of [S].          
 
Item 2. Acid-Resistant Zinc Plate
This project started out to be to labeled “Hot Zinc.” As waste restrictions were inevitable in the early 1960's, one method of reducing the costs was to recover the plating solution by using the drag-out rinse, following the plating bath, as a source for maintaining the solution level in the plating tank. This wasn’t effective with the cyanide Zn plating solutions of the era since the baths were not just run at ambient temperatures, they were often run with chillers. Warm cyanide Zn solutions would consume more brightener. It was found that reformulating the basic bath chemistry, we were able to run the exploratory bath at 140°F. There were some advantages with the higher temperature. The amount of caustic could be reduced, as well as the Zn content. With a reduced alkalinity, the use of glucoheptonate, a complexing agent that was more effective at higher pH than the amines in common usage in other baths. This would avoid the use of the polysulfide “purifiers” in use in alkaline cyanide Zn baths at the time. A word about the polysulfide use: It would precipitate some heavy metal impurities and allow good deposits to continue, but wait!—the heavy metal sulfides that precipitated are slowly soluble in excess cyanide, so the addition of polysulfide would have to continue. This was great business for the supplier, but not much fun for the plater. The Zn cyanide plating solution became a Zn thiocyanate plating bath. With the experience of the effects of very low amounts of [S] in the Cyanide Cu deposits, The exploratory work set out to remove as much of the [S] as possible in the Hot Zn experimental formulation. It was not well-known that most, if not all of the Zn mined in the U.S at the time was in the form of ZnS ores (more Zn per ton of ore than from the oxide ores). It was assumed from this that in the commercial production of Zn metal and Zn salts, [S] was probably a common impurity. Certainly, when large adds of polysulfides were added to the cyanide Zn baths, a slight S-impurity would be of no concern. To put the exploratory [S]- free Zn bath together, each of the ingredients was treated to remove as much of the traces of [S] possible. The bath was activated with carbon that had also been treated to remove any traces of [S]. The Zn anode that was to be used in the test cell was electroformed from an acid Zn sulfur-free plating solution. 99.99% Zn anodes were in commercial use at the time, but the 0.01% of impurities could still contain an excess of [S] for the exploratory tests. It was not determined if all these treatments were necessary, but the results would be helpful in determining which treatments were really required.
 
The chemistry of the exploratory bath:

NaOH- 4 opg
NaCN- 10 opg
4% v/v of a proprietary solution of sodium heptonate
1/4% v/v of a proprietary solution of butyne1,4-diol
Steel test panels were run at 140F for 10-minutes at 2 amp
     
So, what were the results? The Hull cell deposits were semi-bright (!). Then came the surprise—the zinc plate wasn’t stripped in the 50% muriatic acid stripper. A new stripper was made, but with the same results. Panels were immersed in the acid for over two hours and not a bubble of gas appeared. The deposit was very noble. At this point, all lab work on the project ceased. It was years later that a reference was found in a Metal Data text book that claimed that pure Zn was not attacked by hydrochloric acid. The reference was to thermally produced pure Zn and didn’t mention electroplated Zn.
 
Questions:
(1) Can the same results be obtained with non-cyanide zinc baths, using the [S]-free approach?
(2) Are there commercial applications where the more noble zinc would be useful?
(3) Can the removal of the traces of [S] be practical?
(4) Can analytical methods be developed for this very low amount of [S].
(5) Can electro-potential measurements be used for indirect analysis?.
(6) Is there a direct relationship between the [S]content and the electrode potential of the Zn?
(7) What other properties may be affected by a [S]-free Zn deposit?    
(8).....and a hundred more questions.
 
Item 3- A Simple Test for Hex Cr as an Impurity in Plating Baths
A simple Hull cell test was found several years ago to detect the presence of hexavalent Cr in Cu Cyanide plating solutions, which is applicable to other electroplating solutions, but it appears not to have been publicized. The effects of even a very small amount of Cr-6 can be considerable. It can cause blistering, skip plate (no plate) for visible effects, and raise the tensile stress, dramatically—enough to cause cracking of the deposit.
 
The test: is done in a Hull cell, but could be conducted in any container with an anode and DC current supply. For a Hull cell test in an alkaline Cu plating solution, the brass test panel is cleaned and prepped for plating, immersed in the test cell, but no current is applied for a set time, usually one-minute. Then 2-amps is applied for a minute or two. The presence of Cr-6 is shown if there are any areas of “skip” (no plate!). The test is accurate enough to detect 5-ppm in an alkaline Cu bath. It may be even more sensitive wiyh longer immersion periods. The test is also partly quantitative in that higher amounts of Cr-6 will passivate a brass panel surface in a shorter immersion (no current) time, and show a greater amount of the area of skip. The same effect happens in a plating tank with parts. It was never determined if the passivation left a film of Cr-6 on the metal surface, or, if the film would provide a method of removing small amounts of Cr-6 contaminants. Brass panels seemed to be more sensitive in copper solutions than steel. Very little work was done in testing the technique on Ni-electroplating solutions.
 
Unfinished work:
(a) Standardize the test
(b) Could this be developed into a semi-quantitative test?
(c) Could the principle be used for the removal of Cr-6 impurities from plating solutions? At one time, Zn-electroplating solutions were treated with Zn “dust” to remove metallic contaminants. It wasn’t a popular treatment. The Zn dust re-dissolved rapidly. Would a filter, packed with copper or brass shavings, turnings, or powders, effectively absorb hex Cr from a contaminated Cu plating solution?
 
Item 4- Effects of Small Amounts of [S] in Silver and EN Baths
(a) Silver Electroplating baths are sensitive to small amounts of [S], which promote tarnishing. The Company didn’t market any silver plating systems, but did make and sell DC Rectifiers, The Cu-buss joints were silver-plated in an in-house plating tank. Tests were run using the same acetylenic alcohol to check the tarnish reduction. When the writer moved to a new Lab some 10 years later, test panels that had been stored in a desk drawer, exposed to Lab “air,” were interesting. The panels showed no sign of tarnishing. Do silver platers and their suppliers know this?
 
(b) EN, electroless nickel plating solutions are sensitive to small amounts of [S]. Some suppliers and some published formulations have used small amounts of certain (S)-containing materials, such as thiourea, as stabilizers. Many may still do. (S)-containing EN deposits tend to have good plating rates, but be more porous, lower in phosphorus, less corrosion-resistant, harder as-plated. Internal stress may be affected, directly or indirectly, through the effect on the phosphorus reduction in the alloy deposit. Using non (S)-bearing stabilizers, the intent of the unfinished business of a study with the lowest [S] in an EN was to test the effect of the lowest [S] or [S]- free formulation, with regards to stability, stress, P-content, rate, as well as the effect on fatigue strength, etc. Time ran out. Anyone is welcome to join in the fun!
 
Item 5- A direct Relationship Exists Between Trace Amounts of [S] in Nickel Deposits and the Amount of Ni-release in tests of Deposits in Contact with Human Skin. 
There has been increased pressure to eliminate the use of Ni in those applications where the Ni is in contact with the human skin. Some people are sensitive to trace amounts of Ni that are released. It produces a reaction of what has been called, “Nickel Itch.” This has been going on for years. It has now reached a point where the Ni is being withdrawn from those applications. There is a test being used to detect the tendency of the Ni-deposit to dissolve in a test solution that simulates the chemicals found on human skin (a sweat-test). At the time of this writing, the test has not been in wide usage in the U.S. The findings of the effects of [S] in trace amounts on the corrosion resistance of other plated metals suggest that Ni plated deposits would be similarly affected, and suggests that this should be considered with respect to satisfying the “sweat test” requirements. Initial results from one producer have shown a direct relationship exists between the [S] content of a Ni deposit and the amount of Ni “released” in the tests (corrosion).
 
Needs:
     (a) Local testing of the Sweat Test
     (b) Reliable methods for [S] in Ni deposits
     (c) Verification of the [S] to Ni-release test results relationship
     (d) Work to reduce the [S] in Ni deposits. Note that [S] in the Ni deposit will vary with the current density used;  [S] content is generally higher at lower current densities.
     (e) It is expected that S-containing additives to the Ni plating solutions be avoided.
     (f) It is also expected that an improved Ni plate that comes from this project would be useful for other applications.
 
(Topics to be covered in Part II of “Unfinished Business”: Results obtained in Ni electroplating from a sulfamate plating solution when plated using a superimposed AC power source over convention DC power; Anodized Ni; Zinc as an undercoating for CU-Ni-Cr on steel; Unreported effects of arsenic on Cu from CN-plating baths; Selenium/copper in EN baths; and chloride-free EN and sulfamate Ni.)   
 
About the Author.

John “Jack” Horner earned a BS in Chemical Engineering from Michigan Technical University (1950) and worked in many phases of metal finishing throughout his career—much of it in technical service and product development. This included many years in training and working management. He had been very active in the AESF and ASTM. Since retirement, he had been consulting, on a limited basis—most recently for The Nickel Institute in the area of nickel electroplating and nickel electroforming. Horner had received the Frederick Lowenheim Award from ASTM for his work in Committee B08. (1993). He also wrote the chapter   on “Electroplating” found in Kirk Othmer’s Encyclopedia of Chemical Technology (1991) and its Fourth Edition of its Concise Encyclopedia (1998).

 

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