Abstract
The effect of the deformation structure and annealing temperature on the corrosion of ultrafine-grain (UFG) Fe-Cr alloys with 8 to 12% Cr prepared by equal channel angular pressing (ECAP) was investigated with particular emphasis on the stability of the passivation layer. Fe-Cr alloys were processed by ECAP using up to eight passes at 423 K by the Bc route, followed by annealing at temperatures of 473 to 1173 K for 1 h. Passivity appeared in all alloys as a result of ECAP, and the stability of the passivation layer was evaluated by anodic polarization measurements in a 1000 mol·m−3NaCl solution. The stability of the passivation layer increased as the degree of deformation became more extensive with successive ECAP passes, and distinct escalation occurred with the formation of a UFG microstructure. In the early stages of annealing at moderate temperatures, the stability of the passivation layer deteriorated, although no visible grain growth occurred, and this effect increased monotonically with increasing annealing temperature. The high degree of stability of the passivation layer on UFG alloys following ECAP can be attributed to the large number of high-angle nonequilibrium grain boundaries, which may lead to Cr enrichment of the surface region. The deterioration of the passivation layer in the early stages of annealing may be attributed to a change in the grain boundaries to an equilibrium state. The present results show that the superiority of as-ECAPed materials of the Fe-Cr alloy to recovered ones by heat treatment can be achieved with 8–10% Cr as observed in 20% Cr.
Highlights
Metal forming processes such as equal channel angular pressing (ECAP) have received considerable attention over the past two decades as methods for producing ultrafine-grain (UFG) bulk materials for structural applications [1, 2]
Annealing at moderate temperatures leads to annihilation and rearrangement of these dislocations and causes the grain boundaries to change to an equilibrium state [4]
It was found that UFG Fe-Cr alloys with 20% Cr processed by ECAP, with a large fraction of nonequilibrium high-angle grain boundaries (HAGBs), exhibited higher corrosion resistance than coarse-grained (CG) material, and this was attributed to enhanced passivation [7,8,9,10,11]
Summary
Metal forming processes such as equal channel angular pressing (ECAP) have received considerable attention over the past two decades as methods for producing ultrafine-grain (UFG) bulk materials for structural applications [1, 2]. Annealing at moderate temperatures leads to annihilation and rearrangement of these dislocations and causes the grain boundaries to change to an equilibrium state [4]. Such annealing is likely to impact the corrosion behavior of UFG materials because the grain boundary state influences the mechanical and electrochemical properties. It was found that UFG Fe-Cr alloys with 20% Cr processed by ECAP, with a large fraction of nonequilibrium high-angle grain boundaries (HAGBs), exhibited higher corrosion resistance than coarse-grained (CG) material, and this was attributed to enhanced passivation [7,8,9,10,11]. (1) To examine the microstructure, hardness, and anodic polarization of UFG Fe-Cr alloys with different Cr content and the effect of annealing
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