Effect of variable intensity ultraviolet radiation on passivity breakdown of AISI Type 304 stainless steel

The effect of ultraviolet illumination on passivity breakdown of a commercial grade (CG) of AISI Type 304 stainless steel and of a high purity heat (HPH) of the same material in neutral 0.5 M NaCl solution and in 0.025 M NaCl + 0.15 M H 3BO 3/0.007 M Na 2B 4O 7, pH = 7.5, solution was experimentally studied. Passivity breakdown on the CG, as indicated by the measured breakdown potential and the intensity of the noise observed in the current under potentiostatic conditions, was inhibited at low illumination intensities but was enhanced at high illumination intensities. The low intensity photo-inhibition of passivity breakdown (PIPB) is consistent with previous observations on Fe, Ni, Cu/Ni alloys, and stainless steels, and has been attributed to quenching of the electric field and a subsequent modification of the vacancy structure in the barrier layer. These processes inhibit the formation of a critical cation vacancy condensate at the metal/barrier layer interface. The breakdown sites are envisioned to be regions of inherently high cation vacancy flux, and to include the points of intersection of the barrier layer with inclusions (e.g., MnS) and second phase particles (e.g., Cr 2O 3). Thus, at low intensities (power densities less than 100 mW/cm 2, λ=325 nm), cation vacancy condensation/cap dissolution at the periphery of the inclusion is considered to be the fundamental breakdown event, even though, MnS dissolution eventually occurs, as an acidic crevice develops adjacent to the inclusion. On the other hand, under high intensity illumination conditions (power density > 100 mW/cm 2), enhancement of passivity breakdown is attributed to photoactivation and dissolution of the MnS inclusions, which occur in parallel with vacancy condensation. The HPH exhibited breakdown voltages that were about 0.5 V more positive than that for the commercial grade of AISI Type 304 SS, under identical experimental conditions. The breakdown voltage, V b was shifted in the positive direction, upon irradiation with super band-gap light at a high power density in 0.5 M NaCl, but no change in V b was noted when irradiated in 1.0 M NaCl.