Protection of Steel from Hydrogen Cracking by Thin Metallic Coatings

Abstract

Cold rolled and stress relieved 0.5% carbon steel specimens electroplated with Ni, 0.50–2.5μ thick and bent to the test span after plating are especially resistant to hydrogen cracking when polarized cathodically in dilute sulfuric acid saturated with . Arsenic or an As compound deposits cathodically on the Ni coating which supplements protection by Ni alone. The dual coating is also effective in the presence of additives to , such as thiourea, which normally accelerate occlusion of hydrogen by steel. The critical hardness value of steel below which some degree of protection is achieved by Ni + As coatings is about Rockwell C 35. Ni coatings alone are protective against sulfide cracking when cathodically protected in acetic acid, saturated with . The mechanism of protection is apparently one of altering the kinetics of H+ discharge, resulting in less occlusion of hydrogen by steel. Accordingly, such coatings to be protective need not be continuous.