Effects of zinc coatings on the stress corrosion cracking and hydrogen embrittlement of low-alloy steel |
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Authors: | H. E. Townsend |
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Affiliation: | (1) Corrosion Prevention and Coatings Section, Homer Research Laboratories, Bethlehem Steel Corporation, 18016 Bethlehem, Pa. |
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Abstract: | The effects of electroplated and hot-dip zinc coatings on the fracture of low-alloy steel AISI 4140 bars tempered to hardnesses in the range Rc 33 to 49 were studied. Either electroplated or hot-dip zinc coatings decrease resistance to stress corrosion cracking,i.e., they reduceK sc, the threshold stress intensity for stress corrosion cracking in 3.5 wt pct NaCl solution. AboveK scelectroplated-zinc coatings do not appear to affect the crack-growth rate, although the incubation period prior to the onset of crack growth is reduced. Hot-dip zinc coatings increase stress corrosion crack growth rates slightly because of the additive effect of internal dissolved hydrogen. Hot-dip zinc coatings reduce the critical stress intensity for fracture in the absence of a corrosive environment because of embrittlement by internal hydrogen which is released from traps during hot-dip coating and confined by the inter metallic coatings which form on the steel surface in the hot dip bath. A simple fracture mechanics analysis indicates that either increasing diameter or the presence of a zinc coating lowers the critical hardness at which the stress corrosion cracking of structural bolts can occur. |
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