Is the wearing away or alteration of metal by Galvanic (electro-Chemical) reaction, or by a direct chemical attack, such as the rusting of Iron & Steel. Corrosion can be thought of as an electro chemical reaction in which one metal is changed into a chemical, or simply eaten away.
Is the combination of two dissimilar metals with an electrolyte is all that is needed to form a corrosive reaction. The use of dissimilar metals in structural design is common, especially where the fastener is a different material from the structures being joined. The necessary ingredient to induce corrosion, the electrolyte, may be present in the form of rain, snow dew, high humidity, ocean salt spray or even air pollution. Cell corrosion and pitting are similar types of corrosion because each requires only one metal and electrolyte to set up a corrosive attack system.
A term representing particular condition where cracks are induced and propagated in a fastener under combined effects of stress and corrosive environments .The initial corrosion may occur at a point of high stress that contributes to crack initiation, which can be either intergranular or transgranular. Continued exposure to the corrosion environment will propagate the crack and can result in serious, and possibly catastrophic, failure.
It is associated with fasteners made of carbon and alloys steels. It is a type of deterioration that can be linked to corrosion –control process. There are three main ways to fight to fight hydrogen embrittlement:
||Use the proper plating procedures and bake fasteners correctly.
||Use fastener coatings that do not involve electroplating.
||Use a softer bolt material. As with stress corrosion, harder, stronger materials are more susceptible to this type of failure than weaker, softer ones.
Methods for Fighting Hydrogen Embrittlement:
There are three methods used to fight hydrogen embrittlement, Hardness is a major contributor to hydrogen embrittlement. Harder, stronger materials are more susceptible to failure than weaker, softer ones. If the hardness is less than 35 HRC, there will probably be difficulty with hydrogen embrittlement and if the hardness is above 40 HRC, problems are more likely to occurs.
While coating process can also induce hydrogen embrittlement. Use a coating process that does not introduce hydrogen into material. If electroplating is still desired, ensure that plater uses the proper procedures and bakes the fasteners correctly based on its hardness. ASTM F 1941 has a hydrogen embrittlement relief requirement for coated fasteners made from steel heat treated to a hardness of HRC 40 or above, case- hardened
Fasteners made from hardened steel .The extract time and temperature of the bake is not specified ,but times is between 2 and 24 hours at temperature between 350 to 450 F are listed as suitable depending on type, size, geometry etc.
The proper selection of the material for the services environment can reduce the risk of embrittlement. The potential for hydrogen embrittlement cracking is accelerated if the fasteners is acting as the cathode in a galvanic couple . Caustic or sour environments may require much lower hardness levels to lower the susceptibility to hydrogen embrittlement.