What corrosion-resistant coatings are available for high-strength (over 150 ksi) bolts?
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?
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.
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.