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?

Protective Coatings
ASTM recommends that if a protective coating is required for A490 bolts, one of two approved coatings is recommended. First is a Zinc/Aluminum Corrosion Protective Coating  in accordance with 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.  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.

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13 comments

    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.

    Hi, Recently we were informed that Dacromet coating is being phased out. Since we are using A490 bolts in our structure, we have to propose a replacement., like Magni565. But there are some doubts.
    The bolts and nuts are already coated with Dacromet and the proposal is to coat the DTI washers with Magni 565. Since the Dacromet and Magni 565 will be in contact with one another, Are they compatible with one another? is it possible to use separate coatings for one assembly.
    Since the bolts are pre-tensioned, will there be any issues with Magni 565 with regard to pre-tensioning (i.e. coating being damaged during pre-tensioning)?

    @Bandana- There are no issues mixing coatings with DTI washers. It is required that the bolt and nut are coated by the same process, but the washer can be an alternate coating. We also see no issues with pre-tensioning.

    Is fluorocarbon coating acceptable to A490 bolts for corrosion prevention? I suppose they are similar to painting. Thanks.

    @Aaron- Per ASTM A490 and F3125, no other coatings are permitted other than F1136 and F2833. That said, I do not know much about fluorocarbon coatings and how they are applied. A coatings engineer may choose to allow an exception in some cases, but ASTM will not officially sanction any coatings that are not approved by the F16 committee.

    What will be the impact of Magni 565 on A325 (F1852) and A490 (F2280)TC Bolts ? We assumed that the properties on the bolts is not affected but the final tension might be change ? If so, did the tension will be higher ?

    Thanks!

    @Pierre-Paul – The properties of the bolt and the final tension should not be impacted. The tightening torque may vary from plain finish or other coatings, but the bolt should still function the same. I do not have any friction coefficient information on Magni 565, so I do not know how much the torque will be affected.

    Although hot-dip galvanize provides high thickness level barrier protection (2 mils) for substrates, it is not approved for use on high strength bolts because hot dip galvanize is susceptible of hydrogen embrittlement and cracking of fasteners that can result out in the field.

    Coatings approved for ASTM F1136 are very thin layer coatings that offer the best performance/thickness ratio for corrosion protection that do not induce hydrogen (internal/environmental hydrogen embrittlement) for metal components.

    Is 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.

    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.

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