Surface Oxidation and Subsurface Microstructure Evolution of Alloy 690TT Induced by Partial Slip Fretting Corrosion in High-Temperature Pure Water

Abstract

The surface oxidation and subsurface microstructure evolution of Alloy 690TT can occur during partial slip fretting corrosion in high-temperature pure water. Detailed characterization methods such as laser scanning confocal microscopy, scanning electron microscopy, electron probe micro-analyzer, and transmission electron microscopy were used to reveal the related mechanism. The results showed that Cr2O3 oxides together with a small number of spinel oxides were formed in sticking region since a small quantity of high-temperature water could pass through the gaps between the asperities to oxidize the materials. Widespread distribution of oxides in microslip region consisted of (Ni, Fe)Cr2O4, because Ni2+ and Fe2+ ions could react with Cr2O3 to generate a small amount of non-stoichiometric spinel oxides. The oxides around micropitting in microslip region consisted of double-layer structure. The outermost layer contained (Fe, Cr)-rich oxides due to the effect of fretting leading to mechanical mixing between Cr2O3 and (Ni, Fe)(Fe, Cr)2O4. The inner layer consisted of (Fe, Ni)-rich oxides owing to the consumption of Cr2O3 by the reaction with Ni2+ and Fe2+ ions. The reciprocating motion of oxide particles in microslip region resulted in the stress–strain supporting the recrystallization for the formation and development of a tribologically transformed structure in subsurface and plowing effect by fretting in surface.