A novel strategy to control degradation rate via interface solute segregation in a surface-modified AT42-xCa alloy by friction stir processing

Abstract

Degradable Mg alloys with controllable corrosion rate and enhanced mechanical properties show promise for emerging applications such as bridge plugs in horizontal wells. We developed a novel strategy to control the degradation rate via interface solute segregation in surface-modified AT42-xCa alloys by friction stir processing (FSP). The effects of Ca contents on the microstructure evolution and corrosion behavior of the FSP surface-modified Mg-4Al-2Sn-xCa (x = 0, 0.5, 1.0 and 1.5 wt%) (AT42-xCa) alloys have been investigated based on hydrogen evolution tests and electrochemical measurements. Coarse grains and secondary phases (CaMgSn and Al₂Ca) of the base material (BM) in AT42-xCa alloy lead to rapid corrosion, with corrosion rates reaching ∼86 mm/y for AT42–1.0Ca alloy and ∼149 mm/y for AT42–1.5Ca alloy. The surface modification through FSP refines the grain size and secondary phases, improving corrosion uniformity and reducing corrosion rates. The stirring zone (SZ) of FSPed AT42–1.0Ca alloy exhibits improved corrosion resistance, with a corrosion rate of ∼3.9 mm/y. The reducing corrosion rate is mainly attributed to the segregation of solute Al at interfaces of well-dispersed nano-sized CaMgSn particles and the Al-Ca co-segregation at grain boundaries of refined grains (∼2.2 μm), which accelerates the formation of Al^3 + and induces the formation of a uniform and dense Al-rich corrosion product film on the corroded surface. Especially, the corrosion rate in the SZ of FSPed AT42–1.0Ca alloy decreases by ∼96 % compared to the BM counterpart. These findings provide a valuable reference for tailoring degrading rates in Mg alloys through FSP and Ca alloying.