The hot dip galvanizing process has a detrimental effect on the strength properties of steel

True or False?

The tensile properties of structural steels are not affected by galvanizing. The immersion period is relatively short (normally 4 to 6 minutes) and the molten zinc temperature (450°C) is well below that required for normal heat treatment of steels.

Embrittlement

Embrittlement is by no means a common occurrence in hot dip galvanized steel. If correct galvanizing procedures are observed, it should not be implied that galvanizing increases the potential for embrittlement.

Hydrogen Embrittlement

This occurs due to atomic hydrogen being absorbed by steel. In the case of galvanized steel, the acid pickling reaction prior to galvanizing presents a potential source of hydrogen. The heat of the molten zinc, however, partially expels hydrogen which may have been absorbed while the use of an inhibitor in the pickling solution limits the degree of absorption in susceptible steels. Hydrogen embrittlement does not result from the galvanizing of unalloyed and low-alloyed steels.

Hardened steels (>HRC 32) can become brittle due to hydrogen absorption. This is not a common occurrence in the case of galvanized products where extensive tests show that even class 10.9 fasteners (HRC 44) are unlikely to be affected provided good galvanizing practice is adhered to.

Any possibility of hydrogen absorption can be eliminated by utilising an alternative cleaning method such as abrasive blasting followed by a brief rinse in acid.

Fatigue Failure

In the case of certain high strength materials (e.g. spring steel), thick galvanized coatings where iron / zinc alloys predominate can result in a greater propensity for failure. Micro cracks developing in the hard coating alloys would seem to result in a notch effect in the vicinity of potential stress raisers such as tool marks and surface rolling defects. This phenomenon is unlikely to occur if the galvanizing coating does not exceed 65µM in thickness.

Strain Age Embrittlement

Strain ageing results in loss of ductility in susceptible steel as a result of strains induced by cold working. At normal temperatures, ageing proceeds slowly but is accelerated at higher temperatures such as that of molten zinc. Steel made to fine grain practice using aluminium is less prone to ageing, due to the anchoring of nitrogen atoms as aluminium nitride. Steels containing low levels of nitrogen are unlikely to give rise to strain ageing.

In order to limit stress concentrations in susceptible steels, severe cold bending should be avoided. Heavy deformation from cold rolling, punching and cold forming (e.g. cold heading of bolts), requires sub critical annealing prior to galvanizing.

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