Hydrogen embrittlement is caused by breakdown of molecular hydrogen (H2) into atomic hydrogen (H) that diffuses into the grain boundaries of the contacting metal. Cd or Zn reduces the breakdown of hydrogen molecules into atomic hydrogen and, hence, would reduce the hydrogen embrittlement phenomenon.

Professor Yehia Khalil, Yale University, USA

Fellow of the University of Oxford, UK

Thank you for your answer!

According to the ISO standard the Cd or Zn coating can also be used to "contain" the hydrogen inside of an hydrogen charged sample.

Your explanation would explain why hydrogen would not enter the samples, but not explain why it can't leave.

Could it be that the ISO standard is incorrect in its statement and assumed that Cd and Zn will hinder outward realease of hydrogen even if it doesn't?

Thanks again for your attention

//

Erik

I serve on many Standards' Writing Committees and Working Tasks / Groups and I find that many of their discussions are more qualitative than quantitative. Maybe they meant to say that adding Cd or Zn atoms could plug the pores and grain boundaries in the substrate metal and hence would hinder hydrogen diffusion out. The more rationale explanation is that Cd or Zn hinder hydrogen diffusion into the pores and the grain boundaries as I explained before.

Dear Erik,

the key for understanding is that there are hydride forming materials which easily react with hydrogen = absorb, devide into two atoms (decompose) on the metal surface and transport into the bulk (grain boundaries are the fastest way) to form hydrides (under certain conditions). And there are another materials which do not easily react with H. If I look at the table available in internet - there are a lot of them not only Zn and Cd, but Zn as I think is considered to be the cheapest alternative to the other expensive or rare materials for protecting against hydrogen from environment when it is a gas molecule. Low ability to react = much lower degree of H2 decomposition. As a molecule it is almost impossible to diffuse into material - no driving force for that because molecule is neutrally charged.

concerning when the hydrogen was inside and you put the coating above. It is already decomposed into atoms. It means high diffusion ability and high reaction ability. I suppose if someone tries to let it go out by means of temperature treatment it can form some hydrogen enriched layer on the interface between hydride forming material which you want to protect and Zn coating, But still you have some grain boundaries even in coatings and nothing disturbs hydrogen atoms to diffuse out of the sample along this paths. Only the time is much longer. The diffusion rate difference between two materials can also create a hydrogen enriched layer. I suppose here increasing of treatment time can help to let maximum amount of hydrogen to come out of the sample though it is harder than without coating and some hydrogen will still remain inside. The main idea is you cannot "seal" the hydrogen in a sample. Temperature increase and time will let it out of there.

Good luck in your research,

Anton