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hot dip galvanized bolts forbidden on Stainless Steel vessel? 4

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YuJie_PV

YuJie_PV

Mechanical
Jan 19, 2017
135
Hi all experts,

we are implementing a project which locate alongside the sea. that means corrosion is serious.
recently, our corrosion engineer raise additional statement that "hot dip galvanized bolts are forbidden on Stainless Steel vessel", even the operating temperature is ambient.
the reason is that in case of fire, the zinc in HDG coating could cause LME(liquid metal embrittlement) of SS vessel. i've never heard of that before.
is it a common practice?
the project is completed for almost 50%, i don't like to introduce any new requirement.
and the project is not offshore, i think the risk of LME is overestimated.
any experience in this field?
Thanks in advance.
 
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- the “8-inch hypothesis”. The corrosion engineer is also probably sticking rigidly to API RP 571 (which some consider to be carrying on the 8-inch hypothesis as a legend)

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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I have personally seen LME in SS with Cu and Sn.
I have never tried with Zn.
But we prohibited the use of galvanized materials in out plant out of precaution.

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@Ed - was the cracking a result of welding? There’s a current paper suggesting that copper only becomes an issue at somewhere around 1000 deg C. Similarly, for Zn, peak susceptibility was seen at around 800 deg C.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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In both cases it was surface contamination prior to annealing, so we are dealing with >1000C.
If you heat in a highly oxidizing environment then the Zn might oxidize before it causes issues.
In a hydrogen furnace the metals stay around long enough to really cause problems.

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@SJones, @Ed, thanks for your points.
I agree that zinc could potentially cause LME failure. If galvanization is ruled out, it's hard to find an alternative that is both economical and obtainable, especially in highly corrosive atmospheres. It's a tradeoff, I think.
thanks
 
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'Potentially' = 800 deg C + a high stress situation; what's the flange sealing status going to be like? This is the trade off that leads to a questioning of the galvanising "fear factor." Is an LME crack the worst consequence after a fire driven 800 deg C temperature excursion?


How about a ZnNi electroplate to run past your corrosion engineer? Won't last as long as galvanising but may not give so much zinc anxiety.



Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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Have you looked into inorganic coatings such as Dacromet? These are commercially available at similar pricing to hot dip galvanized.
 
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@TugboatEng, thanks.
Actually, Dacromet is mainly comprises of Zinc flakes. For my project, the engineers have agreed to abandon the idea of universally forbidding HDP regardless of the specific conditions of each equipment after discussion.

Thanks
 
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@SJones, never heard of the coating before.
it seems impossible to be economical.
Thanks
 
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It’s been a standard bolt coating with Aramco for a few years. Of course, they’re not short of cash. Never had an idea of cost myself.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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@Louisiana Inspector,
there are some vessels subject to pressure fluctuation in the project. it's said that PTFE-coated fasteners are prone to loose themselves in case of vibration.

thanks
 
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@SJ, it's from the project specification, and my colleagues also believe that.
 
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Some recent work on bolt loosening behaviour tends to suggest the opposite.

Steve Jones
Corrosion Management Consultant


All answers are personal opinions only and are in no way connected with any employer.
 
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@SJones, never heard of that before.
if it's true, there seems a large misconception widespread in the industry.

my spec states as follows:
"As a reminder, due to low friction property of PTFE coating, in certain cases,
PTFE coated bolts and nuts show self-loosing characteristic, and this
characteristic should be considered, especially if equipment in service will face
thermal cycling and/or mechanical fatigue during lifetime."

Thanks.
 
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