A study on the evolution of surface and subsurface wear of UNS S31603 during erosion–corrosion

Abstract

Abstract This paper studies the material response of UNS S31603 to incremental particle impact and evolution of surface and subsurface wear with time during erosion–corrosion. Multiple tests were performed at increasing time duration from 0.5 min to 2 h using a slurry pot erosion tester with 3.5% NaCl and 1 wt.% silica sand at a test velocity of 7 m s−1. SEM, FIB and TEM were used to investigate the mechanisms and microstructural changes that arise during this process. Between 0.5 min and 20 min of testing, when the particles are impacting the fresh uneroded surface, material removal occurs through the formation of prominent lips and deep craters. After a duration of 20 min, when the surface has been completely covered with a layer of lips and craters, a second layer starts forming. Between 0.5 min and 20 min the depth of the nanocrystalline region formed subsurface increases with direct particle impact on the surface. As the top surface layer becomes work hardened, load is transmitted by particle impact to the bulk grains leading to the formation of nano and micro sized grains. TEM investigation on the single particle impact crater revealed that deformed nanograins and twinning are formed immediately beneath the impact crater. TEM analysis of the specimen exposed to erosion–corrosion for 5 min also revealed the formation of deformed nanograins and twinning due to the high strain rates. It is believed that the compact fine grained microstructure makes it difficult for anodic dissolution to occur. However, the depassivation of the oxide film and the formation of micro galvanic cells on the deformed metal will enhance corrosion. A graph of mass loss rate versus time plotted gives good correlation with surface and subsurface features observed. Physical models are developed based on these observations.