Effect of deformed subsurface on the corrosion resistance of biomedical CoCrMo alloy in simulated physiological solution

In most cases, the microstructure of implants is stable and changeless after implanted into the body. However, the metallic joint replacements would be worn during patients’ movements. Their subsurface microstructures were deformed and changed. That may affect the corrosion resistance of the joint. Due to the complex surface situation on the worn joints, it was difficult to distinguish and evaluate the influence of the subsurface microstructure from surface roughness, wear debris, etc. In this study, a CoCrMo alloy used for artificial joints was treated by high-energy shot peening method to obtain a similar deformed surface to the worn joints, but without introducing other influencing factors. The corrosion rate of the deformed alloy which was treated for 4 h was down by 93.3% in the phosphate-buffered saline (PBS) solution compared with the untreated alloy. However, with the addition of bovine serum albumin (BSA) into PBS, the treated alloy showed the poorer corrosion resistance when its concentration was equal to or greater than 0.1 μmol L−1. The defects (grain boundaries and stacking faults), which were introduced into the alloy surface during peening process, could accelerate anodic dissolution, improve element diffusion and increase the number of active sites. These were beneficial for forming a more compact and thicker passive film which enhanced the corrosion resistance of the alloy. However, the deformation of subsurface could promote the adsorption of protein on the alloy surface. The adsorbed protein molecules would chelate with metallic ions and weaken the passive film.