In the previous installment, the author reflected on the early research and development and testing that spurred innovations in modern plating. A list of subjects covered included: [S]-free Copper Plate; Acid Resistant Zinc Plate; a Simple Test for Hex Chromium in Plating Baths; Effects of Trace Amounts of [S] in Other Plating Baths; and the direct Relationship Exists Between Trace Amounts of [S] in Nickel Deposits and the Amount of ANi-release@ of Deposits in Contact with Human Skin.
Following is a continuation of that discussion, which delves into the following areas: Ni electroplating from a sulfamate plating solution when plated using a superimposed AC power source over the conventional DC power; anodized Ni; zinc, as an undercoating for Cu-Ni-Cr on Steel; the unreported effects of arsenic on Cu from CN-plating baths; selenium/copper in EN baths; and chloride-free EN and sulfamate Ni.
Ni electroplating from a sulfamate plating solution when plated using a superimposed AC power source over the conventional DC power. During the Vietnam War, one of the manufacturers of airplanes was considering replacing just the engines of the aircraft with rebuilt engines rather than new engines, but wanted reassurances that the rebuilt engines would be as reliable as the new ones. Other jet engine manufacturers had long been rebuilding engines with electroplating processes. There was no room for error. We (The Company) were contacted. The requirements for the Ni that was to be used to rebuild the worn areas was that the Ni had to have a compressive internal stress, be able to withstand temperatures of up to 1600°F, and the plating was to have a minor (?) effect on fatigue strength. At first, it appeared to be conflicting properties.
The method to produce an engineering Ni deposit with a compressive internal stress was to use a S-bearing additive in the Ni plating solution—but this included sulfur in the deposit. The co-deposited sulfur lowered the heat resistance of the Ni deposit drastically. We were about to “throw in the towel,” when a literature search revealed a little-known study of the use of AC current, superimposed over the DC current produced some unexpected results with respect to the internal stress of a nickel sulfamate electroplating bath. The study did not identify any other property changes, nor the effects on heat resistance of the deposits. The amount of AC current used was 10% of the DC. A Brenner-Senderoff Contractometer was set-up with separate electrical controls and meters for the AC and the DC, and the internal stress measured; first with all DC power and then using the AC superimposed. The first test produced an internal stress of about 4,000 psi, tensile, when plated to a thickness of 0.0006 inches at about 25 asf. The second test, using a fresh sample from the same batch of Ni sulfamate, with the AC superimposed, produced a compressive internal stress of about 8,000 psi.
(Now, the reader may logically question the details after so many years from an 85-year-old man’s memory). With the stress in the compressive state, without any S-bearing additives, the temperature-resistance was checked. Note: If there is the presence of even a very small amount of co-deposited sulfur in sulfamate Ni deposits, there is a very visible degradation of the deposits even at temperatures much lower than 1,600°F. On the AC/DC panels, there was no noticeable change after exposure to the 1,600°F level. The airplane maker’s engineers were contacted, and they sent us several Krause test bars to plate with the same method. These were done and submitted to the customer for testing. (Note: this tests the loss of fatigue strength, an important property!). We were later informed that the test results were acceptable. Then, what happened? Nothing! It was near the end of the conflict and, for whatever reason, the airplane engine maker decided against any change. Why the continued interest in this project? Presently, there seems to be no way to obtain a compressively stressed Ni deposit without using some sort of S-bearing additive in the plating bath. The generation of a breakdown product of the sulfamate ion through anodic oxidation, whether intentional or accidental, by the use of inactive anodes still ends up with a co-deposited form of sulfur.
What is needed? (a) A [S]-free additive or method to produce a S-free deposit with compressive stress in Ni- electrodeposits. Compressively stressed EN deposits have advantages, as well, especially when the nickel, electroplated or auto-catalytically plated, is used on “critical” parts. A critical part, in this reference, being one that— if it fails—the whole structure fails.
Anodized Ni? It was found that Ni-plated (EN) steel salt spray test panels showed improved salt spray resistance on the reverse side of the tested panels, when turned over and re-tested after failure of the front side. Can this effect be duplicated electrolytically or chemically?
What was done? In the course of salt spray testing a series of EN-plated steel test panels for another project, as each panel reached its failure point, rather than remove it from the salt spray cabinet, the failure date was noted, the panel was simply turned over and exposure continued until the panels had failed on both sides or reached 1000 hours of exposure for that side. (1,000 hours should no longer qualify to be labeled as an accelerated test).
There were more than100 test panels involved with several variations; the identifying numbers were on the panel backs. The panels were evaluated daily and finally removed when the reverse side showed any sign of corrosion, The edges of all the panels had been given a protective wax coating. The results were not statistically analyzed, because there was 100% agreement; in all cases, the reverse side out-lasted the front side by several days, before the first sign of rust showed. Surely, there is an explanation. Could there be fact to claims that some forms of Ni oxides are less reactive, less soluble, than others? Some lab work was done which showed that intentionally oxidizing the Ni electrolytically accelerated its failure in salt spray testing. Several variations were attempted to vary the degree of oxidation without any success, but much remained to explore. It still seems possible that some form of oxide was formed during the long exposure to the warm, humid salt spray cabinet that somehow acted to reduce or plug any minute porosity of the deposit.
(a) Verify results
(b) Study forms of Ni Oxides and methods of producing these on Ni surfaces
(c) Is anodized Ni possible? Either chemically or electrochemically?
Zinc, as an undercoating for Cu-Ni-Cr on Steel? Using a thin acid Zn strike, as an undercoating on Cu-Ni-Cr plated steel shelving, radically improved the salt spray resistance. What about other base materials? What are the problems? What needs to be done?
Background: A manufacturer of refrigerators was having premature corrosion failures of shelving in domestic units in Southern coastal areas. Steel shelves were Cu-Ni-Cr plated to conventional specs of the era and would show considerable rusting in 24-hrs of salt spray testing. We received several raw shelves for testing. Plating with 0.0004 inches Cu, 0.0008 inches Ni, and 10 millionths Cr in a pilot plant produced similar results. Bruno Leonelli, of our Company, suggested trying a thin acid Zn strike. This had been in limited use in Sweden. The first shelf was plated with the thin Zn strike under the same Cu-Ni-Cr specifications was removed from the salt spray test after 240 hours of salt spray with no signs of rust. The customer was contacted and he asked for 300 shelves plated with the Zn strike for field test in refrigerators to be sold along the Gulf Coast. Nobody believes a small company? The shelves were plated in a small pilot room and shipped.
What happened next? .....Nothing! Not one word was ever heard of any results. The customer’s engineer disappeared, retired or passed on....maybe, the shelving outlasted the refrigerators? In any event, the whole project died.
What to Do? The benefits of the Zn strike were substantial! The Zn formulation was a simple “handbook” bath with no brightener additives. No exploratory work was done on the variations in the operating conditions or chemistry, nor the Zn-thickness. It was the amount of Zn that could be deposited in 45-seconds. It did show a way to improve the corrosion resistance of plated coatings. So what was wrong? It was found if one “folded” a shelf over on itself, the deposit may crack or peel. The part would have to be considered as failed in that condition, let alone the plating! Work to be done would include optimizing the Zn strike. There may be several applications that extreme ductility is not a prime requirement when compared to corrosion resistance.
Unreported Effects of Arsenic in Cu from CN-plating Baths. This was an experience that needs to be known. A large automotive company was in the process of designing and building a new plating machine for Cu-Ni-Cr on Zn-based die castings. Their experience with the cyanide Cu had taught them that to increase the speed they needed to increase the agitation. Further, that air agitation was not an option; air contained lots of carbon dioxide and alkaline solutions were very efficient at reacting with this and forming carbonates which had to be removed eventually. Air was also known to move itself more than the solution. So the copper tank was designed with “hydraulic” agitation. Lots of lightning mixers” in tubes positioned under the work stations where the plating racks would be stationed during the rest stop between indexing. Racks were specially built to hold parts very tightly in this vigorous agitation.
Alas! What the engineers didn’t know was that this was a Se-brightened CuCN system. One of the faults of Se is that at high current and high agitation, it produces a phenomenon called “step plate.” It is just that: a veritable step. It occurs at about 20-30 asf and severe enough to feel with one’s finger nail. Some changes can be made to minimize the effect but they all reduce the plating rate as well. In the course of lab work it was found that a small amount of arsenic, in the low ppm range, would remove the step. Hallelujah! ’Twas the week before Christmas, it is remembered well, mostly for the lesson it taught the writer. It has taken the writer years to tell! So what happened? Five pounds of arsenious oxide were added to 12,000 gallons of cyanide copper plating solution, and the machine was loaded up for a trial run. As the work came out of the copper tank, with the current set at the desired amperage, the crowd gathered at the exit of the Cu tank issued an audible sigh of relief.....No step! The machine was loaded up! Maybe a bit too early because about that time the work began coming out of the nickel tank and it was dull, a satin nickel.
The first comment by the Plater was that the Ni tank was still not up to temperature. This writer got a queasy feeling, grabbed samples of the copper and the nickel, and went to the lab. The first Hull cell of the Ni produced, as expected, a normal bright panel (!). The second panel of the Cu showed, as expected, a normal, bright panel. The third panel, the copper panel plated in the nickel solution produced, by this time, as expected, (!) a satin Ni deposit!!!! In all the panels that had been plated previously, testing the arsenic additions, not one had been plated with nickel! A big lesson was learned! Now you have to know where the writer spent the Christmas week that year. Luckily, this was during the period when the automotive companies were shut down between the holidays. It was learned that it isn’t easy to remove arsenic. Subsequently, it was found that the copper surface, though still visibly bright, when viewed under a scope, the deposit was full of microscopic pits. The leveling nickel was not able to cover it with the thickness normally used. Pretty embarrassing!
Question: Is there any application for this as a means of producing a satin-appearing finish? Say using a Cu-strike with arsenic and a Ni-strike, all over a plated base? (Don’t tell the EPA?)
Selenium/Copper in EN Baths. With all the work that has been done on EN in the past several years, certainly somewhere there has been some done in this area. Why the interest? There was a big customer who used a “home-brewed” EN bath and bought an “off-brand” nickel sulfamate of questionable purity for the nickel source. The customer also had a practice where a new bath was made up each day and plated “down” during the day. For this customer, any other source of nickel just didn’t work as well. Finally, it was determined that trace amounts of selenium and copper were responsible for the difference. When these were added to purer nickel sulfamate, the homebrew EN bath maintained an acceptable plating rate at low concentrations of Ni. Lately, this writer has been told that Se can have some stress problems. Again, it appears a non [S] stress reducer is needed. The underlying principle was in plating the bath down to a more dilute bath before waste treating—still a commendable goal!
Chloride-free EN and sulfamate Ni have been found to have improved adhesion bonds directly to Al alloys, without zincate strikes. A second benefit has been seen in less porosity. Pilot plant work looked promising using chloride-free nickel sulfamate directly over aluminum alloys. This was supported in a paper by George DiBari (retired) of Inco, that mentioned this, but it seems to be abandoned. It has taken years to get long salt spray test requirement tests removed from specifications for nickel plating. Salt spray is a test for porosity for nickel plate which will be detected in much less than 1,000 hours. With the use of SD Ni anodes, chloride is not necessary for anode corrosion in a nickel sulfamate plating solution. Proper preparation of the alloys, without zincate treatments, has shown promise. Why hasn’t this been pursued?
This is my “swan song.” I have spent a lifetime in tech service and product development and tried to teach as I learned. It is hoped that this “unfinished business” contains an idea or two that is useful. (One last thought: Don’t forget that time is still an underrated factor in rinsing!)
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).