zinc on austenitic stainless 3

[weldmete]

I've had the understanding that austenitic stainless must not be coated with zinc containing products because it will cause hot cracking if the stainless is heated or welded. Is this correct? If so does anyone know of any info. location on this topic? Are there other problems that can occur like galvinic corrosion?

Thanks,
weldmete

 

[mcguire]

Molten zinc is corrosive to stainless steel. I can't give you data, but this refers to something more than just a coating which is what you have in mind.
A coating of zinc will corrode before the stainless and cathodically protect it, so no harm there.
I guess the question is, "why have zinc coating on stainless to begin with" since it serves no useful purpose. Small amounts in a coating aren't going to ruin a weld. They will volatilize because of zinc's high vapor pressure.

 

[grampi1]

Issue is more focused on not letting galvanized rub against SS or using Al weather jacketing over SS pipe in critical plant locations.

 

[PAN]

This will be liquid metal (zinc) embrittlement. Try google search.

 

[Metalguy]

Look up the Flixbourough disaster in a rocket-fuel plant that was caused by molten zinc dripping off a galvanized roof onto red hot SS piping during a fire.

 

[Rich2001]

Two main type of interactions of zinc and austentic stainless steel have been observed.

TYPE I - Embrittlement. Zinc slowly erodes unstressed 18-8 austentic stainless steel at 419 to 570 C and penetrates the steel. Formations of intermettallic beta nickel-zinc coumpound occurs between 570 to 750C. At higher temperatures, penetration along the grain boundaries occurs, with a diffusion of the nickel into the zinc-rich zones. This diffusion results in a nickel-exposed zone adjacent to the grain boundaries, hence reducing the stability of the gamma phase, causing a tranformation to an alpha-ferrite phase. This tranformation to the alpha-ferrite phase results in a volume change producing internal stresses that facilates fracture alog the grain boundaries.

TYPE II Embrittlement - occurs in stainless steel above 750C and is characterized by an extremely fast rate of crack propagation (several order faster than TyPE I crack propagation. The cracks are perpendicular to the appliesd stress.

Liquid metal embrittlement may be produced by welding of austentic stainless steel in the presence of zinc or zinc-based paints. Intercrystalline cracking in the HAZ is common.

Note that galvanizing is generally 98% zinc, with the remain being lead, tin, aluminum and bismuth.