Can high strength bolts be galvanised?
Bolts can be galvanised to good effect to prevent corrosion but one should be aware of the effects of hydrogen embrittlement when galvanising high strength bolts.
As detailed in previous technical tips hydrogen embrittlement is defined as a permanent loss of ductility in a metal or alloy caused by hydrogen in combination with local tensile stress (either externally applied or arising from internal residual stress). In the case of bolts large magnitude tensile stresses related to preload are almost always present. As hydrogen embrittlement normally only affects high tensile carbon and alloy steel components with hardness values exceeding 380HV (which includes ISO 898 property class 10.9 bolts which have a specified hardness of 320-390HV), bolts with property classes of 10.9 and 12.9 are susceptible to hydrogen embrittlement. Lower grade bolts (with hardness values in the range of 240-380HV) are normally not susceptible.
Although some standards conservatively apply an upper limit of hardness of 320HV, property class 10.9 and 12.9 bolts can be galvanised if i) the material composition is appropriate, ii) the heat treatment/microstructure is well controlled, and iii) suitable precautions are taken during plating (as specified by ISO 10684:2004 or the German Deutscher Schraubenverband E.V.). Wherever possible, acid pickling should not be used in higher grade bolts where alkaline descaling or mechanical cleaning processes should be employed. Baking at temperatures of between 190 and 220°C for periods of up to 24 hours (dependent on strength level and size of the fastener) should be used to eliminate the deleterious effects of hydrogen absorbed during processing.
In hot dipped galvanising it has also been postulated that the thermal shock associated with the galvanising process releases hydrogen trapped in the material and that the coating prevents it from diffusing from the material resulting in it accumulating at grain boundaries. Hydrogen is also evolved during the electroplating processes – the amount of hydrogen being absorbed is dependent on the coating type and the process parameters. The susceptibility of the cracking is dependent on how much hydrogen gets trapped by the coating and hence its permeability.
Remember environmental hydrogen associated with the sacrificial corrosion of the zinc layer can also penetrate the material when the coating is cracked, damaged or partially consumed, and may result in hydrogen induced stress corrosion cracking.
If in doubt, sustained load tests, slow strain rate or incremental step load tests described in various ASTM and ISO standards should be undertaken to ascertain any potential loss of mechanical strength when higher grade bolts have been galvanised.
Published in Technical Tips by Origen Engineering Solutions on 1 February 2016