Effect of ultrasonic shot peening and intermediate cerium salt conversion bath treatment controlling corrosion of Mg-Y-Zn-based alloy in salt water

Ultrasonic shot peening (USP) is often regarded as beneficial in mitigating corrosion of Mg alloys. However, increasing surface roughness induced by USP promotes localized corrosion pitting. Herein, we show that an intermediate phosphate bath treatment (CePT) before USP can minimize the surface roughness and ameliorate corrosion resistance of Mg-4.0Y-4.0Zn-0.5Zr-0.2Ca alloy (wt.%) in 0.1 M NaCl solution. In this regard, WZ44 alloy was subjected to Ce(NO3)3 + NaH2PO4 bath treatment followed by USP for 30 s using AISI52100 steel balls and vice versa. The sub-surface features of specimens analyzed using XRD, FESEM, surface topography, and Vickers hardness reveal that CePT coating before USP increases microhardness (-142-149 HV0.05 vs - 125-135 HV0.05) and decreases surface roughness (Ra = 1.91-2.38 mu m vs Ra = 2.58-2.76 mu m). Potentiodynamic polarization and electrochemical impedance spectroscopy demonstrate that corrosion current decreased (Icorr 9.81-13.69 mu A/cm2 vs. - 23.82-26.22 mu A/cm2), coating (R1), and polarization resistance (R2) increased to 1704-1907 omega.cm2, and - 1834-1902 omega.cm2, respectively, for specimens subjected to USP + CePT treatment together indicating superior corrosion resistance. Overall, an intermediate CePT before USP can effectively minimize corrosion pitting phenomena of WZ44 alloy in chlorinated solution by developing a ceramic coating interlocked with surface acting as a barrier between the metal/solution interface.