Passivity breakdown on 300 M and S280 ultra-high strength steels in borate buffer solutions containing chloride ion

Passivity breakdown and the pitting behavior of ultra-high strength 300 M and S280 steels, which are commonly employed for aircraft landing gear, in the presence of chloride has been explored and compared, and the data were interpreted using the point defect model (PDM). The breakdown behavior was studied using cyclic potentiodynamic polarization while SEM and digital confocal microscopy were employed to explore the micro morphology and depth distribution of pits. The cumulative distributions in the breakdown voltage are near-normal, which are consistent with the PDM, assuming the distributions of potential breakdown sites in terms of the cation vacancy diffusivity for different concentrations of chloride are normal. Compared to 300 M, S280 steel exhibits shallower pitting and is more resistant to breakdown. From the potential scan rate dependence of the breakdown potential, the critical areal cation vacancy concentration, ξ, resulting in passivity breakdown, is 8.6 × 10¹³ No.cm⁻² for 300 M and 2.4 × 10¹³ No.cm⁻² for S280. These values are structurally in close agreement with the calculated results, considering the vacancy condensation is either on the cation sublattice of the barrier layer or on the steels lattice.