A while back, I received a message from the manager of a plating shop at a captive facility that specializes in the production of aerospace components.

Frank AltmayerFrank AltmayerHere is what he wrote to me:

We have a small shop that only plates cadmium and zinc-nickel on hardened steel parts. As more production changes from the cadmium finish to zinc-nickel, we have run into a problem that has us confused.

We operate an alkaline zinc nickel plating process that delivers a 95% zinc deposit. The additive package is purchased from a commercial supplier. The plating tanks and rectifiers are less than one year old and are in very good condition. We analyze the solutions daily, which are typically well within recommended concentrations. The brighteners are evaluated using Hull Cell tests, and we make adjustments based upon visual examination of panels plated before and after additions of various proprietary additives.

Most of our parts require hydrogen embrittlement relief baking after plating. Some parts are baked at 375°F (190°C), others require a lower temperature (275°F; 135°C). Our plating thickness is generally around 0.0005" (12.7 μm). A new solution functions well, and we seem to have no problems.

After several weeks of production, parts show very small blisters only after baking (See photo.). These usually show up within the first 60 minutes of baking. The blisters are uniformly dispersed and are unrelated to current density or part geometry. Still, they seem far more prevalent over ground or blast surfaces versus machined surfaces.

If we change to a new solution, using the same steps for part preparation, the problem will go away but return after several more weeks of operation. We are convinced that something is going on with our plating solution, but we don’t know what. We have checked the plating solutions for impurities, and we found as high as 3.5 ppm lead in one of our three plating solutions, but we experience blistering even when the lead content is at “not detected” levels. Please investigate this problem for us.

Hydrogen Effects and Steel

measles1Our first discussion must cover the general issue of hydrogen effects and steel. Any plating process less than 100% cathode current efficient will generate some hydrogen gas along with the deposit. High-strength steels will absorb hydrogen ions during the first burst of applied current for plating. As plating builds up, the deposit is a barrier to further hydrogen absorption. As a result, the hydrogen concentration profile consists of a “bump” in hydrogen concentration that tails off to “zero” concentration as we go deeper into the steel.

The more stressed the steel is from operations such as blasting and grinding, the higher the hydrogen absorption tendency. Some blast and ground finished parts need to be baked before plating to limit the hydrogen takeup during plating. Also, past experience has shown better results when thin deposits are applied and baked, and the specified deposit's balance is plated over the initial one. The hydrogen has an easier time diffusing through a thin deposit than one that is 0.0005” (12.7 μm) thick. A final bake is then used as insurance.

Studies have shown that we need to get rid of this “bump” in hydrogen concentration near the surface of the steel, and we do this by heating the part at a temperature high enough and for a time long enough to cause the hydrogen to diffuse out of the steel into the atmosphere. The bake also causes some hydrogen to diffuse into the steel and “spread out.” The result is that no spot within the steel has a hydrogen concentration high enough to initiate a crack, which would cause the part to fail when statically loaded.

Blisters Located Between Basis Metal and Initial Plated Deposit

In your case, we investigated several parts and found that the blisters were located between the basis metal and the initial plated deposit. We also found that some parts had far more plating than specified (three times too much). Further, your solution has an additive package that includes brightener(s) and other organics. We, therefore, concluded that the blistering was caused by hydrogen gas, which could not readily diffuse through the deposit during the baking because the deposit was not porous enough. Instead, the hydrogen gas expanded and “pushed” upon the deposit, creating the blisters.

We dug deeper into your process and found that organic additives were being added under less than optimum conditions. Normally, the additives are added based upon ampere-hours of operation, while you add them based on the visual interpretation of Hull Cell panels. We suspect that this visual method resulted in large amounts of additives being added. After a few weeks of operation with this visual control, the organic content of your solutions would reach a level where the deposit was too “dense” to allow for hydrogen diffusion.

Additionally, we found that the zinc-nickel plating process was operated with no control over the anode-cathode ratio and little control over the plating current density (Parts were racked without regard to the total surface area.). Further research and a few calls to some friends in the finishing community revealed some interesting information:

Hydrogen Embrittlement Zinc-Nickel Plating Process

Boeing has a patent on a low hydrogen embrittlement zinc-nickel plating process (L.M. Tran, M.P. Schriever & J.H. Jones, U.S. Appl. #20060254923, application published 11-06-2006). In the text of this patent, Boeing teaches the following:

“The plating solution is substantially free of brightening agents, which retard hydrogen bake-out, ... or at least inhibit the release of hydrogen from the article.”

“It is preferred that the coating consist of about 90% weight zinc and 10% nickel. Here, corrosion resistance is maximized, and hydrogen embrittlement is minimized.”

“Best results occurred at 45 ASF, giving a good coating distribution, good resistance to corrosion, and good resistance to hydrogen embrittlement. Specimens at 36 ASF and 68 ASF also showed good results in all categories, but the specimen at 68 ASF showed slight corrosion.”

German Patent DE 19834353A1 (02-032000) teaches the following:

“Electrolysis of alkaline zinc-nickel baths containing poly(alkyleneimines) produces amine breakdown at the anode into nitriles and cyanides if the anode is exposed to the plating bath.”

Taskem Inc., — which became Coventya and was later sold to MacDermid Enthone — has a patented method of avoiding this problem by encasing the anode in a membrane cell [R.E. Frischauf & W.E. Eckles, U.S. Patent 6,755,960 (0629-2004)].

Based on the above, I recommend the following:

  1. The anode-cathode ratio should be controlled by measuring the surface area of the parts and ensuring it is less than 2:1, preferably 1.5:1.
  2. Consider baking the parts before plating and baking twice: first after applying a thin (0.0001”; 2.5 μm) deposit and then after applying the majority of the deposit.
  3. The current density needs to be more carefully controlled.
  4. Additives should be added based on ampere-hours.
  5. Keep the brightener concentration as close to zero as possible (be careful not to infringe upon the Boeing patent).
  6. Consider increasing the nickel content to nearly 10%.

We thank Becky Zinni-Kettering, of the company formerly known as Taskem Inc., for her help with this problem.

Frank Altmayer is a Master Surface Finisher, an AESF Fellow, and the technical education director of the AESF Foundation and NASF. He owned Scientific Control Laboratories from 1986 to 2007 and has over 50 years of experience in metal finishing. He received the AESF Past Presidents Award, NAMF Award of Special Recognition, AESF Leadership Award, AESF Fellowship Award, Chicago Branch AESF Geldzahler Service Award, and NASF Award of Special Recognition.