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Corrosion of B7 Studs

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FastenerDude

FastenerDude

Industrial
Jan 3, 2013
17
Good Afternoon,

I have attached a picture for reference. We supply ASTM A193-B7 studs that are Zinc Yellow plated to a customer who uses them to build a compressor. The studs are corroding quicker than expected. The flanges that these studs bolt up, are made of stainless steel so I suspect that galvanic corrosion is taking place. Would this be correct? The unit is located in-doors so it is away from outside elements. I asked my customer how long the studs have been in service and he guessed a couple of years, but if the unit is in-doors, corrosion on a plated stud should not happen this fast. We have B7, Zinc Plated studs that sit on our shelves for years that don't show any corrosion. Should I try to convince my customer to move to stainless steel studs? Any insight would be greatly appreciated!

Greg
System 22, Inc.
 
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Galvanic corrosion is suspected. Stainless fasteners are recommended, not just for galvanic corrosion but also compatibility for thermal expansion.
 
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I'd like to see inside the bolt hole for contamination of stainless steel...B7 studs should have never been used.
However, the picture is not quite clear, how could any moisture get to close-up a galvanic circuit, to initiate the galvanic corrosion. In some cases, the poor electrolytic galvanizing could start a separate corrosion process on the thread, weather the galvanic corrosion is normally localized to the contact area with the large mass of stainless steel (inside the bolt hole). Nevertheless, I would also look for the source of moisture and signs of condensation on metallic surfaces.
Cheers,
gr2vessels
 
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Agree with the others. IF one were to use a zinc coating, a thick (> 50 micrometers) hot-dip galvanizing should have been used rather than thin (~ 10 micrometers) electrolytic layer. Either way, expect continuous corrosion of the fasteners. Convince your customer to switch to stainless steel fasteners.
 
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Are the studs from China ? Where were they coated ? Are you sure that this is a Zinc coating ?

Could the bolts be corroding even before they are installed ?

Also, I disagree with gr2vessels who claims the general rule stated above that "B7 studs should never have been used"

According to both NORSOK guidelines and PIP piping standards, B7 studs are acceptable for use on stainless steel flanges.

 
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Then obviously disagree also with the embrittlement of stainless steel in contact with zinc in case of fire...
 
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NORSOK requires hot dip galvanizing not electroplating.
 
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Have got real world (not cleaned up and washed) pictures of the actual flange in its actual location? Cleaning fluids, residues, trash, sand, grit, process fluid residues, inside-outside contamination (when doors are opened or equipment loaded) can all catch or deposit ON the stainless flange that would otherwise resist corrosion just fine.

Consider somebody washing the floor every nightshift with chlorox or a bleach solution. A spill three months ago of sulfuric acid that was wiped off, but got in and around the bolts.
 
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