Environmentally Assisted Cracking – Some types of corrosion take place as a result of chemical conditions within the environment and the mechanical condition of the metal itself. Both conditions must be present for this type of corrosion to take place. Corrosion fatigue, stress corrosion cracking, liquid metal cracking, and hydrogen embrittlement are all forms of environmentally assisted cracking.

Corrosion Fatigue – Corrosion fatigue failure takes place due to the reduction of fatigue resistance of the metal with the presence of a corrosive medium. Thus it is normally encountered not as a visible degradation of the metal but as a premature failure of a component under cyclic conditions. The stress under which the material fails would not normally be considered sufficient to cause failure.

Stress Corrosion Cracking – This refers to the cracking caused by the simultaneous presence of tensile stress and a specific corrosive medium. During this type of cracking, the metal is virtually unattacked over most of the surface while fine cracks progress through it at the attack site. Chloride stress cracking of stainless steels and ammonia stress cracking of nickel-copper alloys are examples of this type of attack.

Liquid Metal Cracking – Liquid metal cracking is a specialized form of stress corrosion. The susceptible metal (usually due to residual tensile stresses from fabrication) is attacked (cracked) by a low-melting liquid metal. An example is the cracking of aged Ni-Cr-Fe alloys by silver-base braze alloys.

Hydrogen Embrittlement – Hydrogen embrittlement takes place due to the penetration of the surface of susceptible metals by elemental hydrogen. This can result from the formation of metallic hydride compounds in some materials while in others it takes place by the interaction of dissolved hydrogen atoms. Regardless of the mechanism, the attack results in the formation and propagation of fine cracks and voids in the metallic structure.