Due to hydrogen embrittlement concerns ASTM recommends against hot-dip galvanizing , mechanical deposition, or electroplating with zinc or other metallic coatings on high strength bolts ultimate tensile strengths that exceed 150 ksi. So what are the alternatives to provide corrosion resistance for A490 structural bolts?
Protective Coatings
ASTM states that if a protective coating is required for A490 bolts, one of three approved coatings is recommended. First is a Zinc/Aluminum Corrosion Protective Coating in accordance with F 1136 Grade 3, commonly known as Geomet. Second is a Zinc Rich Base Coat and Aluminum Organic/Inorganic coating system in accordance with F2833 Grade 1, commonly known as Magni 565. Third is a chromium free zinc flake composite per ASTM F3019. (Update: The ASTM F16 committee combined all three of these coating into one umbrella standard, F3393. The coatings themselves have not changed, the combination will make the standards easier to maintain and add/subtract as needed). All three of these coatings require a small nut overtap and an additional proofload test to assure the oversized threads will properly function with the A490 bolts. These recommendations are based on findings from research conducted by the Industrial Fastener Institute (IFI 144) and the ASTM F16 metal coatings/bolting technology committee. No other metallic coating is allowed unless authorized by ASTM’s Fastener Committee F16.
Weathering Steel
Another option is to specify ASTM A490 Type 3. This fastener is made from weathering steel which is inherently corrosion resistant. However, these fasteners are not as readily available in the marketplace as A490 Type 1.
Consulting a structural engineer involved on the project is always the best solution.
Thanks, Dane!
No issues mixing Geomet-coated A490s with galvanized nuts and washers? Would seem cheaper to not have to coat the washers and nuts, considering they will be tentioned against galvanized steel.
@Brad- Yes, there is a huge issue with mixing those coatings. Hot dip galvanized nuts are overtapped significantly more than their Geomet coated counterparts. The galvanized nut will be quite loose when assembling onto the Geomet A490 and will be much more likely to strip during tensioning. ASTM in many of their standards does not allow mixed coatings for exactly this reason.
F 1136 Grade 3, commonly known as Dacromet. Second is a Zinc Rich Base Coat and Aluminum Organic/Inorganic coating system in accordance with F2833 Grade 1, commonly known as Magni 565?
how much does it cost if i purchased F 1136 Grade 3, commonly known as Dacromet. Second is a Zinc Rich Base Coat and Aluminum Organic/Inorganic coating system in accordance with F2833 Grade 1, commonly known as Magni 565? and be delivered here in the philippines?
@Jovito- We would be happy to quote your specific bolting needs if you want to send us a list of the bolts you are looking for.
Hello,
We have specified A490 Dracomet coated bolts for a telecommunications monopole in Canada. They were used to in flange type splices of two tubular monopole sections. One year after installation the bolts are rusting. Is there any records of these types of coated bolts loosing their protection due to installation and/or manipulation (scratching of the coating)? Is the coating that fragile?
Thank you Serge
@Serge- We have very little experience with Dacromet coating. We are not sure how fragile the coating is during installation. Apologies.
hi
recently we installed A490 plain bolts in a structural connection that are in a wet area, my question is how can we protect them against corrosion in case they do not have dacromet finishes? is there any soloution? should we change them?
how about protecting them by painting? and if we can paint them, what is specification of the paint in astm or rcsc.
thank you.
@Parham- We cannot make specific recommendations, but I can say that painting A490 bolts is quite common. However, we are not familiar with any ASTM or RCSC standards for painting.