HYDROGEN EMBRITTLEMENT IN PLATED PARTS 2

[KUSTA]

I am looking for the appropriate spec or web site to explain hydrogen embrittlement of plated parts. We are currently plating heat treated parts, to SAE J429 gr 8. this requires the parts to be Q&T to somewhere over the 30Rc range. Our vendor has told us the parts should be baked after zinc plating to avoid hydrogen embrittlement. Is there a specific spec which qualifies this????

 

[wroberts]

Well, this is my understanding of Hydrogen embrittlement. It's not very exact, but since no body else has chimed in I'll give it a try.
Hydrogen embrittlement is caused by the Hydroclauric acid that the parts are initially dipped in before plating. They are dipped to clean the parts from corrosion and foreign matter. However since it is acid it also "eats" and leaves behind some Hydrogen from the acid. These parts are then plated after being dipped. After plating the baking process removes some of the hydrogen left over on the parts from acid cleaning process.
You may also want to add some chemicals to the acid (PavPrep) to keep it from touching the metal. I restore lots of old stuff and acid dip parts in HCL and PavPrep and I can leave the parts sitting in the acid for days, and it only cleans the rust. It will also limit you hydrogen embrittlement.

Will

 

[GGH]

My understanding of hydrogen embrittlement (hydrogen blisters / cracking etc.) is that atomic hydrogen (H) can be intoduced by say, wet electodes in welding activities or hydrogen process steams, etc. Atomic hydrogen can diffuse easily being a small atom (the smallest) however when an atom of hydrogen meets an another atom of hydrogen molecular hydrogen (H2) is formed. This requires a much greater volume and will cause cracking, blistering etc. as it is formed. Heat treatment allows the atomic hydrogen to quickly diffuse from the metal before molecular hydrogen is formed. I imagine your plating process involves hydogen production at some point.

 

[Snork]

GGH is close. I am a metallurgical engineer and have dealt with HE in both the petroleum industry and in fasteners. Atomic H is developed during the plating process which is basically an electrochemical cell. This H is samll and migrates into the microstructure. When the matrix is loaded in tension there is energy provided that allows the H to move to 'prefered sites'. The H combines as GGH says and the result is localized overload and ultimately failure. It has been found that baking does not remove the H as originally thought. (See 'Hydrogen Embrittlement in Automotive Fastener Applications' SAE Technical paper 960312) But rather it allows the H to move to locations that are 'safe'. For parts over 30 HRC it is definitely wise to bake after plating. Another source you can look in is the American Society of Metals Handbook on plating.

 

[gdodd]

ISO standard 2081- "Electroplated coatings of zinc on iron and steel" section 7.4 suggests hydrogen de-embrittlement be carried out on hardened and tempered parts with tensile strengths greater than 1050 Mpa ( 145 000 psi). This equates to a HRC of 32 which is on the boundary for a grade 8 bolt.

 

[mcguire]

I'm a Ph.d in metallurgy and did a thesis on this topic.The threshold hardness for susceptibility to HE is Rc 22. As ScottMc correctly points out, hydrogen is generated during plating and enters the steel. Bake it, for sure.

 

[Skipn8r]

Is hydrogen embrittlement a concern with aluminum? We have a 7075-T6 aluminum pressure vessel that will be pressurized with 1,600 psig H2.

 

[gdodd]

In answer to Skipn8r
Aluminum and its alloys are not susceptible to the same type of hydrogen embrittlement that attacks steel. Its probably explained best by considering the atomic structure of each metal. Aluminum has a face centred cubic atomic structure which does not allow atomic hydrogen to diffuse as readily as in steel which, at room temperature is body centred cubic.

 

[jpf]

After welding and zinc plating the steel parts, specfically normalized 4130 .03-.13 sheet, how long and how hot is the baking process requirement to eliminate the HE?