Corroded galvanized bolts puzzle.....

[Hercules28]

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.

 

[CoryPad]

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.

 

[Hercules28]

Hot dipped.

 

[gr2vessels]

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.

 

[emxhps06]

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?

 

[Ron]

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

 

[blacksmith37]

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.

 

[dik]

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

Dik

 

[chicopee]

Agree, these bolts were not hot dip galvanized

 

[racookpe1978]

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.

 

[TomDOT]

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

 

[Hercules28]

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

What are possible issues with that change?

Plated lasts less than HDG?

 

[TomDOT]

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.

 

[TVP]

The issue is much reduced corrosion resistance. Here are some links with additional details:

http://www.nucor-fastener.com/Files/PDFs/TechDataSheets/TDS_007_Hot_Dip_Galvanizing.pdf

http://www.galvanizeit.org/images/uploads/publicationPDFs/Galvanized_Steel_Fasteners.pdf