A nonlinear ultrasonic nondestructive testing method for microdefects of medical magnesium alloy coatings

The microdefects of polylactic acid coating on medical magnesium alloy significantly reduce the corrosion resistance of magnesium alloy components, which is difficult to detect by conventional non-destructive testing methods. To address this issue, a nonlinear ultrasonic detection technique is proposed to detect polylactic acid coated specimens with different microdefects. Through electrochemical corrosion tests and nonlinear ultrasonic tests, it has been observed that the ultrasonic nonlinear coefficient β increases with the decrease of corrosion resistance. Furthermore, the microscopic test results show that the bubble microdefects caused by the uneven solidification of polylactic acid coating are the main source of ultrasonic nonlinearity. With the increase of microdefects, the β value of the specimen will increase. The correlation analysis reveals a strong relationship between β and parameters related to coating uniform coverage and electrochemical corrosion resistance, and the lowest correlation coefficient is 0.78. Therefore, using nonlinear ultrasonic non-destructive testing technology can effectively detect the microdefects of polylactic acid coating on magnesium alloys, and it also has a certain guiding role in the biological application of magnesium alloys.