Black oxide passes hydrogen-embrittlement test

Protective coating has no adverse effects on fastener life. Notched square-bar specimens are black-oxide coated and tested to failure to determine susceptibility to hydrogen embrittlement. Fastener manufacturers routinely apply a protective finish to bolts and screws that prevents rust and corrosion. However, there is some concern that plating processes can contribute to hydrogen contamination in fasteners, resulting in a potential for embrittlement. Materials undergo hydrogen embrittlement when atomic hydrogen is absorbed and diffused throughout the metal during electroplating or coating. Accepted theories of hydrogen behavior in high-strength materials stipulate that hydrogen is a mobile atom in the crystalline matrix of metals and will, over time, migrate to points of highest stress. This can eventually result in brittle failure. Generally, higher applied stress means shorter time to failure. And susceptibility to embrittlement tends to increase with the tensile strength or hardness of the steel. Thus, the very design of high-strength fasteners, with their many stress risers and notches and typical exposure to high dynamic loads, makes them ideal candidates for failure by hydrogen embrittlement. Hydrogen is an acknowledged by-product of many surface-treatment processes including black-oxide coating, and zinc and cadmium plating. Acid cleaning prior to plating is of major concern because it can generate hydrogen on the fastener surface. Plating can involve electrodeposition of protective materials, which can produce hydrogen. The processes also expose fasteners to various cleaning solutions and coatings, many of which have not been fully evaluated with respect to hydrogen contamination. One reason for a lack of information is that there has been a limited ability to measure hydrogen embrittlement. Recently, a method has been developed to measure the hydrogen-generating potential of coating and plating processes. This test gives a clean bill of health to fasteners with a black-oxide coating. Black oxiding is a dip process in which parts