A490 Bolts and 90% Zinc Rich Primer. 1

[csd72]

I have a tower of exposed steelwork which is to be galvanised.

My 1.5" diameter A490 bolts obviously cannot be galvanised, so I am proposing coating these in 90% zinc rich primer after installation.

Are there any issues with this such as embrittlement?

 

[evelrod]

Cannot be galvanized? Why not? I've seen large dia A-490 galvanized. I used 1 1/4" galvanized bolts on the KNX radio towers in Hawthorn but, in all honesty, since it was in 1966 I cannot remember what grade the bolts were. Zinc rich primer works where repairs of galvanized joints are welded in the field (properly cleaned and prepped). A bolted connection might present problems in that only the exposed head and nut would receive the coating.

Rod

 

[hokie66]

Agree with Rod. Why not galvanize the bolts?

 

[gr2vessels]

Alloy bolts cannot be galvanised or coated with any kind of zinc rich paint or electroplating with zinc, due to the risk of hydrogen embrittlement. These bolts are supplied only black, plain with an organic protective coating only.
cheers,
gr2vessels

 

[hokie66]

If you are in the US, gr2vessels is probably right. There is a lot of disagreement about galvanizing A490 or 10.9 bolts, but in many countries, they are available. If your code does not allow galvanizing the bolts, I think you should use more bolts of a grade which can be galvanised. Otherwise the corrosion resisting properties of your structure will be compromised.

 

[dik]

From RCSC, "Specification for Structural Joints Using ASTM A325 or A490 Bolts"...

ASTM Specifications permit the galvanizing of ASTM A325 bolts but not ASTM A490 bolts. Similarly, the application of zinc to ASTM A490 bolts by metallizing or mechanical coating is not permitted because the effect of mechanical galvanizing on embrittlement and delayed cracking of ASTM A490 bolts has not been fully investigated to date.

Dik

 

[moltenmetal]

Hmm: you have a choice between (likely) rotting and (possible) cracking? Choose the rotting. Go with the cold galvanizing primer. At least you get a warning before failure that way!

 

[ajh1]

There is a coating called Dacromet that has now been approved by ASTM and RCSC for coating of A490 bolts. It's weathering characteristics are similar to hot-dip galvaninzing.

 

[csd72]

Thanks ajh1,

that is exactly what I was after.

 

[CoryPad]

In addition to Dacromet (or Geomet) from Metal Coatings International, you can try similar coatings from Magni and Doerken:

http://www.themagnigroup.com

http://www.doerkenusa.com

Regards,

Cory

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[HgTX]

ASTM A 490 prohibits zinc-coating of bolts by galvanizing or mechanical means (i.e., the means by which A 325 bolts may be zinc-coated by the manufacturer). However, there is NOT a prohibition on coating the bolts with zinc-rich paint.

Hg

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[evelrod]

Hg, I can see where "paint" would work out here in the Socal desert, but any damage to the coating during assembly in places like central and coastal Texas... Moisture would surely result in early corrosion and possible failure of the fastener. I'm still thinking along the lines of bridges, towers...exposed stuff.

Rod

 

[HgTX]

Bridges in Texas are painted with zinc-rich primer.

Hg

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