Effect of compressive stress on corrosion protection performance of phytic acid conversion coating on carbon steel

As an environmentally friendly organic coating, phytic acid conversion coating (PACC) has been widely used for the protection of metals. In this study, the evolution of PACC on carbon steel under compressive stress is studied via multiple electrochemical and morphological approaches. Interestingly, electrochemical results show that a strengthening effect on PACC can be found when proper stress is applied, which is much distinct from the rule for oxide passive films. The proper compressive stress can effectively improve the corrosion protection performance of PACC, thus enhancing the corrosion resistance of carbon steel to chloride attack. As the applied stress increases, a detrimental effect is confirmed for compressive stress on the PACC layer. The protective PACC layer with the highest protection efficiency and coating adhesion is obtained at 100 MPa. In combination with the observations of SEM/XPS, a three-stage model is proposed to illustrate the evolution of PACC under compressive stress. In the natural stage, numerous cracks form in the PACC layer. In the strengthening stage, the PACC layer is compacted and the cracks in it are greatly healed. In the weakening stage, buckles and partial shedding emerge due to the overlarge stress.