Mechanochemical reactivity and destruction of metallic system under well-defined stress conditions

Well-defined stress conditions usually exist when a metal is loaded and deformed as a whole without comminution. The most important example of such conditions is loading metallic constructions in active environment and their catastrophic destruction due to the synergetic interaction between mechanical and chemical processes described as mechanochemical phenomena. The consequences of these interactions are especially focused in local places on the interface "metal-environment" where stress concentration can develop. Selective dissolution of such active sites on the surface is the main reason of all the forms of local corrosion, including stress corrosion (stress corrosion cracking, corrosion fatigue, corrosion creep, etc.). A simple SCC model represented in this paper, which is based on the difference in dissolution rates of local sites on the metal surface due to the existence of stress tensor gradient, may satisfactorily describe stress corrosion cracking (SCC) both in case of active state of the metal and in the presence of passivating films. A calculation of free enthalpy due to dislocations explains 10⁴-fold acceleration of anodic dissolution of metal in the crack tip. Joint synergetic action of mechanochemical and chemomechanical effects in the crack tip accelerates crack propagation according to the proposed autocatalytic mechanism. The feasibility of stainless steel SCC in the active state against a high rate of general corrosion (35 mm/year) rejects film conception of SCC as a universal one and requires an account for mechanochemical phenomena in the crack tip. As an example of mechanoelectrochemical behavior of light alloys, the data on Mg-Al alloys are presented. The correlation between mechanoelectrochemical behavior and stress corrosion resistance (corrosion fatigue and creep corrosion) gives a possibility to predict and to improve mechanical stability and durability of metallic constructions subjected to simultaneous action of mechanical stress and active environment.