Improving corrosion resistance of Al‐11mass%Si alloy through a large number of ECAP passes

Abstract

Ultrafine-grained (UFG) Al-11mass%Si alloy, processed by multi-pass equalchannel angular pressing (ECAP) at 573 K, was investigated on corrosion behavior in 0.6 M NaCl solution. Potentiodynamic polarization tests and scanning electron microscopy observation showed that a large number of ECAP passes resulted in lower corrosion current density, more positive corrosion potential, and rather smooth corroded surface with shallow corrosion pits. The uniform distribution of fine secondary-phase particles on UFG Al matrix weakened the susceptibility to pitting corrosion while inhibited general microgalvanic reactions. The present results indicate that grain refinement of aluminum matrix to the UFG state and uniform redistribution of broken particles (including eutectic silicon and secondary phases), via severe plastic deformation at elevated temperature undergoing dynamic recrystallization, can significantly improve the corrosion resistance of Al alloys, besides the known exceptional mechanical advantages. The simple and effective ECAP procedure makes UFG Al alloys more attractive for high strength structural application in corrosive environment.

Corrosion of Ultrafine Grained Materials by Severe Plastic Deformation, an Overview

“…Indeed, Cu-depleted zone in the vicinity of grain boundaries become much smaller 87) or disappeared in UFG Al-Cu alloys, 150) and this may lead to higher resistance to intergranular corrosion of Cu-Al alloy systems. 75,150) In passivated materials such as aluminum alloy, the passive lm may be different in its stability 151) or even absent 152) on second phase. Therefore, the distribution of the precipitation strongly affect the resistance to pitting corrosion, and in some cases, its impact is stronger than grain size.…”

“…Processing materials with second phase such as eutectic Al-Si, 151,153) and two-phase (α+β) AZ91, 154) (α+β) Ti-13Nb-13Zr alloy 155) by SPD may impact the corrosion behavior because the dissolution potential between the two phases are generally different such as high-potential Al rich β-phase in AZ91 and Si-phase in Al. Most of commercially pure aluminum have also impurity precipitation, and can be regarded as this type.…”

“…When the second phase is nobler than the matrix by large potential difference as in the case of Al 2 Cu intermetallics in aluminum and Al 3 Fe in magnesium alloys, the change of size and distribution of the second phase may impact on the corrosion behavior or stability of passivity. Indeed, ne fragmentation of Al 2 Cu, FeAl in aluminum alloys 87,120,158) and Al-Si alloy 151) raise passivity breakdown potential.…”

“…Corrosion behavior has been investigated on UFG materials fabricated mainly by processing of SPD such as ECAP, ARB, HPT for aluminum and its alloy, [87][88][89][90][91][92][93]120,121,143,[150][151][152][153]156,[158][159][160][161][162][163][164][165][166][167][168][169][170][171] magnesium and its alloy, 34,72,[115][116][117]122,123,154,157,[172][173][174][175][176][177][178][179] copper and its alloy, 6,[96][97][98]…”

“…Eizadjouet et al 93) attributed lower corrosion resistance to re nement of Al 3 Fe, Al 6 Fe intermetallics and related defective passive lms. Pitting corrosion resistance of aluminum alloys with precipitation or the second phase (type D) is mostly improved by SPD, 45,75,87,92,120,121,143,[150][151][152][153]156,158,167) , and this improvement is mainly due to physical breakup of the second phase or precipitation which differs in dissolution potential from the matrix and causes defective structure of passive lm. For example, pitting occurs at the interface between the matrix and nobler second phase.…”