Fatigue crack tip corrosion processes and oxide induced closure

Abstract

This paper reviews the corrosion and hydrolysis reactions that occur inside a fatigue crack at ambient temperatures and pressures in humid air and aqueous solutions. This study examines the individual reaction steps, intermediate phases formed, and the time required for the metal ions to transition into hard oxide phases. This study also finds that when a fatigue crack closes at ambient temperatures, the corrosion products inside the crack should consist of thin (≈2 nm) hard oxide films on the fracture surfaces and viscous layers of water with metal ions in various states of the transition from aqueous ions to stable end-product phases, but with insufficient strengths to resist load transfer at the crack tip. This analysis assumes that metal ion complexes form at the crack tip and transition toward the end-product phases in the wake of the crack in a manner analogous to that reported for plane surfaces exposed to humid air and aqueous solutions. It is concluded that the contribution of the viscous layers to an oxide induced crack closure (OICC) mechanism should be negligible. The impact of this finding on the interpretation of R-ratio effects on fatigue crack threshold and oxide thickness measurements is discussed. The OICC mechanism should be possible at high temperatures and in the near threshold fatigue crack growth region where hard oxides can grow to sizes relevant to the cyclic crack opening displacement (COD).