Hydrogen Embrittlement in Plating

A. Once the material is found to have a H/E problem, the micro-structure has already sustained damage due to the "migration" of the trapped hydrogen (very small cracks). Re-working the parts will not correct the damage. This is only my opinion, but one based upon "ACTUAL" experiences related to H/E failures. My definitive answer would be "SCRAP" the affected product, try to define and correct the "ROOT CAUSE" and try again. This has been my experience.

A. There are many good books on this subject that you can research for your project, Nina. If hydrogen embrittlement related to metal finishing is your issue, Jack Dini's "Electrodeposition - The Materials Science of Coating and Substrates" has a very interesting chapter on the mechanism.

A. The basic situation is one little hydrogen atom lost inside the metal matrix (crystal), it takes up almost no room, can move easily through the crystal and causes very little stress to the crystal structure.

Then it meets another hydrogen atom, they're dead happy to meet each other and join to become one molecule of hydrogen gas. Gas takes up about 1000 times the volume, so the molecule puts a lot of localised stress on the crystal, a few more molecules in a few more places and then the trouble starts.

A. Hydrogen embrittlement under plating does not relieve itself ever, because the problem is not the presence of hydrogen (which I suppose might dissipate over time), it's the damage that the hydrogen did to the metal. Baking must be done very shortly after the embrittling process, preferably immediately.

A. Do you mean chromium plating?

Hydrogen enters steel during casting, cleaning by most common acids (except nitric), cathodic electrocleaning, electroplating, heat treating in reducing atmospheres and some welding processes. It can be a serious problem in highly stressed, high strength steels. There are several embrittlement mechanisms, but basically, the hydrogen adds to internal stresses and accumulates at defects, leading to failure at even low loads. The hydrogen embrittlement danger increases with increasing hardness and decreasing part thickness for a given type and amount of plating.

Hydrogen build-up is especially high in hard chromium plating from Cr+6 solutions due to the low cathode efficiency (large amperage required). The amount of hydrogen liberated at the cathode (workpiece) surface is proportional to the plating thickness. Steel of ultimate tensile strength 150 ksi or greater having residual stresses from machining, grinding, hardening, etc., should be stress relieved or shot peened prior to plating. Steel having a tensile strength of at least 160 ksi or HRC 36 shall be given a hydrogen bake-out at 191 °C (375 °F) or higher for 3 or more hours as soon as possible after plating. See QQ-C-320B or AMSQQC320 CHROMIUM PLATING (ELECTRODEPOSITED), for more details.

A. Hi, Richard. We appended your inquiry to a thread which answered it.

It's not the hydrogen that is the problem, it's the damage that the hydrogen has already done to the steel. Good luck.

A. Hi, Chris. Although failure of a nut sounds less likely than failure of a screw, I do not accept the implication that failure won't happen in the nut. Picture the screw being made of high strength steel and the nut being made of glass :-)

Electroplating has no effect on soft mild steel, but does cause hydrogen embrittlement of high strength steel. Baking immediately after plating may solve the problem, but it may not -- a statistically valid testing regimen is necessary in order to know.

Regards,

A. Previous entries in this thread have alluded to the phenomenon which I will describe forthwith: In electroplating, the article being plated is cathodic; and at this cathode, metal is deposited. But there is also cathodic inefficiency in which hydrogen gas is generated; the electroplating seals the hydrogen into the article being plated. So even if the parts never see a pH below 7, there can still be hydrogen embrittlement.

P.S.: After reviewing this thread, I should add this postscript: A few inquiries asked how one can be sure that failure is due to hydrogen embrittlement. One telling characteristic of hydrogen embrittlement is intergranular failure (metallurgists call it "rock candy"), which is clearly evident under strong magnification (roughly 500x or 600x). More rounded asperities in the fracture face would contraindicate hydrogen embrittlement. I have seen respected testing laboratories conclude that plating + breaking = hydrogen embrittlement. That is sometimes the case, but sometimes not. You have to look at the fracture face.

Very interesting postscript, Tom, which I had never heard before.

Regards,

A. There is a phenomenon known as caustic embrittlement and is commonly associated with things that use very concentrated forms of caustic solution, or the solution is used under forcing conditions. I haven't seen anything that says pH 10-11 would be a problem unless you are using the solution as a cathodic cleaner, in which case you can generate hydrogen for embrittlement.