Aqueous corrosion of metal matrix composites

Corrosion is the deterioration of a material due to dissolution, leaching, and chemical or electrochemical reaction with the environment.¹ Corrosion occurs locally or globally depending upon the environmental conditions and material microstructure. For homogeneous single-phase materials, corrosion generally occurs uniformly. However, the segregation of reactive phases and impurities at grain boundaries and dendrite boundaries leads to selective or localized corrosion. Corrosion of an alloy can be greater in a crevice created by contact with another material. Localized attack on an otherwise resistant surface produces localized pitting corrosion. In addition, environmentally induced cracking, including stress corrosion cracking (SCC), corrosion fatigue cracking, and hydrogen induced cracking, can also occur when stress and a corrosive environment are present together. When two dissimilar materials are coupled in the presence of a corrosive medium, one of them preferentially corrodes to protect the other one from corrosion. Metal matrix composites (MMCs) contain reinforcements in the forms of fibers, particles, and whiskers dispersed in a metal matrix, and they combine two materials which have different electrochemical corrosion potentials and corrosion characteristics. The reinforcement may interact electrochemically, chemically, or physically with the matrix leading to accelerated corrosion. In addition, galvanic interactions between the reinforcement and matrix can also accelerate corrosion. Preferential corrosion along a fiber–matrix interface can lead to rapid penetration of fluid along the large interfacial area in composites. This can result in enhanced corrosion of MMCs in comparison to the corrosion of the respective monolithic matrix alloys.^2–4 Crevice attack at the metal-reinforcement interfaces and....