Corrosion behavior and failure mechanisms of phosphating/Cd-Ti composite coating on A100 steel under acidic salt fog conditions

Abstract

This study focuses on the corrosion behavior and failure mechanisms of the phosphating/Cd-Ti composite coating widely used in the surface treatment of aviation fasteners under acidic salt spray conditions. The results indicate that the phosphating treatment of the CdTi coating can effectively reduce its self-corrosion rate by approximately 2.5 times. During the 42-day exposure to acidic salt spray, the shape of the polarization curve for the specimen remains unchanged. The self-corrosion potential (E_corr) exhibits only minor changes, and the self-corrosion current density (i_corr) increases by less than an order of magnitude. Acidic salt spray induces a rapid reduction in the unit cell size of Cd and Zn₃(PO₄)₂·4H₂O, leading to cracks in the coating. These cracks provide pathways for aggressive ions, such as chloride ions (Cl⁻), which instigate pitting corrosion. The failure process of the phosphating/Cd-Ti composite coating can be divided into three stages: 1) Under the action of acidic salt spray, cracks form in the coating film of the phosphating/Cd-Ti composite coating, allowing Cl⁻ to infiltrate and initiate pitting corrosion; 2) The pitting corrosion expands laterally, resulting in “under-film corrosion” with corrosion products overflowing to the surface along the cracks in the coating; 3) The CdTi coating is extensively consumed, exposing the substrate to the corrosive environment, and the corrosion gradually shifts from pitting to uniform corrosion. These research findings can provide valuable insights for accurately determining maintenance cycles for aviation fasteners.