Articles 

Embrittlement Issues with Fasteners

2011-02-23

It is generally agreed that hydrogen, in atomic form will enter and diffuse through a metal surface whether at elevated temperatures or ambient temperature. Once absorbed, dissolved hydrogen may be present as either as atomic or molecular hydrogen or in combined molecular form (e.g. methane). Since these molecules are too large to diffuse through the metal, pressure builds at crystallographic defects (dislocations and vacancies) or discontinuities (voids, inclusion/matrix interfaces) causing minute cracks to form. Whether this absorbed hydrogen causes cracking or not is a complex interaction of material strength, external stresses and temperature. Sources of hydrogen include: heat treating atmospheres; breakdown of organic lubricants; the steelmaking process (e.g. electric arc melting of damp scrap); the working environment; arc welding (with damp electrodes); dissociation of high pressure hydrogen gas; and even grinding (in a wet environment). Parts that are undergoing electrochemical surface treatments such as etching, pickling, phosphate coating, corrosion removal, paint stripping, and electroplating are especially susceptible.