Hydrogen stress cracking in power generator M42 galvanized martensitic carbon steel screws

Abstract Screws are used to fasten parts of an assembly together, and can be used to join most classes of materials. The use of sufficient initial tension is very advantageous in reducing the fatigue effects when cyclic loads are present. In general, there is high applied stresses to preserve the joining, and in many practical applications the electrical contact of dissimilar metallic materials is unavoidable. On specific environmental condition, a potential difference produces electron flow between dissimilar materials, corroding the less corrosion-resistant metal. Thus, a better understanding of this joining process and environmental interactions are needed to predict failures in design, avoiding disasters and financial losses. An unexpected fracture in power generator M42 screws was investigated by failure analysis including visual, fractographic, microstructural, chemical and mechanical characterization. It was found that the fracture has no fatigue or plastic deformation signs, and the screws are free of microstructural or mechanical failures arising from raw material or manufacturing process. Intergranular fracture and grain boundary gaps were evidenced, due to this fact, localized fracture area and high-stress level in mounting condition, it was determined that failure cause was Hydrogen Stress Cracking (HSC). Hydrogen source was a galvanic couple between the screw material and cadmium coating. As recommendations, in acid medium presence, martensitic carbon steel screws should never be cathode in a galvanic couple, they must be replaced to obtain minimum levels of uniform corrosion and should not be painted.