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Coating Options for High Strength 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?
Protective Coatings
ASTM recommends that if a protective coating is required for A490 bolts a Zinc/Aluminum Corrosion Protective Coating should be used in accordance with F 1136 Grade 3. This recommendation is based on findings from research conducted by the Industrial Fastener Institute (IFI 144). No other metallic coating is allowed unless authorized by ASTM’s fastener Committee F16. One trade name of this coating is Dacromet.
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.
Your first question, is it appropriate to specify ASTM F1136 Grade 3, would be best answered by an engineer familiar with the coating and the application. I do not believe that F1136 has the same corrosion resistance properties as hot-dip galvanizing, I think that hot-dip galvanizing continues to be a superior coating, but it does have it’s drawbacks, such as a lack of smooth finish and some surface irregularities. Depending on the amount of corrosive forces at work in the application, F1136 may be sufficient, but it may not. Normally where I see F1136 specified is in applications with higher strength fasteners that cannot be hot-dip galvanized, such as ASTM A490 heavy hex bolts. The corrosion protection offered by F1136 in those cases would be superior to nothing at all.
As for the thickness of both coatings, F1136 grade 3 specified a coating thickness of 0.24 – 0.47 mils, whereas hot dip galvanizing has a minimum of 1.7 mils, but can fall anywhere between that and 5.0 mils depending of the fastener and the galvanizer. Hot-dip galvanized hex nuts are therefore tapped oversize to accommodate the thick coating, whereas F1136 specifically states that the thread fit shall not be adversely affected.
August 4, 2009 at 2:49 pmIs it appropriate to specify “zinc/aluminum corrosion protective coating in accordance with ASTM F1136 Grade 3″ for protecting a steel fastener in lieu of galvanizing? I am using A354 grade BC and need a smooth finish with minimal irregularities. would hot dip galvanizing be appropriate or the zinc/alum coating? also, what increase in member thickness should be anticipated with these protective applications? Thanks.
August 4, 2009 at 10:51 am