Unmasking the delamination mechanisms of a defective coating under the co-existence of alternating stress and corrosion

The coating delamination occurring in the conditions where alternating stress and corrosion co-exist threatens the coating integrity and operating life. Therefore, the corresponding investigation is highly significant. In this work, the delamination behavior of a defective coating in the coexistence of alternating stress and electrochemical corrosion was studied in a novel experimental setup in which the electrochemical measurement and fatigue test could be conducted simultaneously. The electrochemical impedance spectroscopy and scanning electron microscope coupled with X-ray energy dispersive spectroscopy were employed to analyze the degree and mechanism of coating delamination, as well as the ABAQUS software was used to simulate the stress distribution on the substrate surface. The results show that in a corrosive environment, the rate of the coating delamination increases dramatically once the alternating stress is applied. The coating delamination occurs first at the stress concentration and expands rapidly to the surrounding area. When the alternating stress is 460 MPa, the impedance attenuation coefficient, delamination width and delamination length are even 4.7 times, 8.1 times and 5.5 times bigger than the case under the static condition without alternating stress, respectively. The interfacial adhesion of the coating-substrate is weakened by the joint action from accumulated fatigue and local alkalization. Accumulated fatigue and local alkalization are resulted from alternating stress and corrosion, respectively. As a result, the coating delamination occurs rapidly. Meanwhile, alternating stress and corrosion show a synergistic effect on coating delamination, i.e., alternating stress accelerates corrosion process, while corrosion exacerbates the mechanical damage of alternating stress. Therefore, it is necessary to pay high attention to the risk of coating delamination in application scenarios, where alternating stress and corrosion are simultaneously present.