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Corroded galvanized bolts puzzle.....

Corroded galvanized bolts puzzle.....

Corroded galvanized bolts puzzle.....

(OP)
Dear all,

I got these three bolts that corroded after a few months of service.
You can see white rust on them. What's puzzling is that there were other galvanized  products on site but only these failed. So that rules out the environment

what could have gone wrong with the galvanizing?
Coating thickness, moisture??

Thanks,
Herc.

RE: Corroded galvanized bolts puzzle.....

Do you mean hot dip galvanizing?  Or zinc plating (electro or mechanical)?

Thickness, top coatings, cracks/adhesion problems, localized variation in corrodents... there are several variables.

RE: Corroded galvanized bolts puzzle.....

(OP)
Hot dipped.
 

RE: Corroded galvanized bolts puzzle.....

It looks to me that the the surface preparation of the bolts was poor and the base metal started corroding before the environment got the zinc coating.

RE: Corroded galvanized bolts puzzle.....

Cathodic protection doesn't mean it is full-proof.

Where have these bolts been installed? Would it be somewhere humid or somewhere that moisture could get in easily?

RE: Corroded galvanized bolts puzzle.....

Those bolts look like they've been electroplated, not hot dipped.

RE: Corroded galvanized bolts puzzle.....

The zinc is supposed to corrode to protect the steel (sacrifical). Hot dipping is pretty tricky to use on fasteners;it messes up thread dimensions.

RE: Corroded galvanized bolts puzzle.....

There are different internal and external thread tolerances to accommodate HDG fasteners. As Ron noted, the fasteners appear to be electroplated... not HDG.

Dik

RE: Corroded galvanized bolts puzzle.....

Agree, these bolts were not hot dip galvanized

RE: Corroded galvanized bolts puzzle.....

The shininess on the shank, and texture on the sides of the bolt head and on the threads are suspicious ... These "look" more like common cad-plated, not galvanized bolts.   

RE: Corroded galvanized bolts puzzle.....

I have a project right now where the contractor "ran out" of the specified HDG bolts, and substituted plated bolts without telling anyone.

RE: Corroded galvanized bolts puzzle.....

(OP)
Well, upon a closer look, they were plated. not HDG.

What are possible issues with that change?

Plated lasts less than HDG?

RE: Corroded galvanized bolts puzzle.....

Protection of steel by zinc is directly proportional to thickness (with some variation in effectiveness by application method.) The major factor is that structural HDG typically has a zinc layer much thicker than zinc plated fasteners. See ASTM A153 for more information on HDG on fasteners.

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