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Zinc Migration into B7 Bolts

2000

I am the director of a project management team at our company. Our core business is supplying auxilliary equipment to the main power generation OEM's.

Recently, we built and shipped a mechanical skid assembly to one of our customers. The customer requested that all pipe bolting hardware be zinc-plated A193 B7 bolts and zinc-plated A194 2-H nuts. The piping (304 SS) itself will experience temperatures from 850 deg F down to 225 deg F.

Question: I have heard that zinc migration into the material of this bolt is possible above 300 deg F and that a potential exists of weakening of the bolt. Is this true? Can anyone direct me to where I can read more information on this matter?

Thank you.

Ken Hosterman
- Orlando, Florida, USA

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2000

I think that there will be little diffusion of the zinc into the steel at 300F. About 300C sounds like about the threshold for significant alloying to me.

The problem with this application is the potential for liquid metal embrittlement (LME). In LME, a structural material under stress can crack at very low stress when in contact with certain liquid metals. Sometimes the cracking can occur at slightly below the melting point of the metal. For Zinc, the melting temperature is about 420C/790F.

The steel fastener material will be subject to catastrophic failure if they are heated into the range that you describe for the piping. You can find more detailed information on this phenomenon and probably specific temperature thresholds in the ASM Handbooks, specifically Volume 11 Failure Analysis. Libraries at technical universities or major metropolitan libraries will have the handbooks or you can purchase on line at www.asm-intl.org.

Good luck.

Larry Hanke
Minneapolis, Minnesota