Research of electrochemical corrosion behavior under coatings with defects by scanning Kelvin probe

The corrosion behavior under coatings with a point defect was investigated in situ by means of scanning Kelvin probe in both salt spray test and cyclic wet-dry immersion test. The results showed that the Kelvin potential distribution was identical in both the tests due to the same division of coating surfaces: defect area with the highest potential, delaminated area, and intact area with the lowest potential. Through monitoring the augment of diameter of Kelvin potential peaks with time, the expansion of the delaminated area was revealed. In cyclic wet-dry immersion test, the highest Kelvin potential V_max did not change obviously with time while the lowest potential V_min increased after reaching at the valley. On the other hand, in salt spray test, different from the rule in cyclic wet-dry test, V_min was minimized at the first stage of the test resulted from the continued diffusion of the electrolyte and the corrosive ability of slat spray particles. Based on the changing pattern of V_max-V_min, it was concluded that in cyclic wet-dry immersion test, the tendentiousness of corrosion got the highest level when the test kept going for 5h. However, in salt spray test, it achieved the highest level just at the beginning of the test and was more intensive than that in cyclic wet-dry immersion test.