Aya Chiba

Cut Edge Corrosion Inhibition by Chromate in Primer of Prepainted 55% Al–Zn Alloy Coated Steel

The effect of chromate in primer layers on the cut edge corrosion of prepainted 55% Al―Zn alloy coated (galvalume) steels has been examined under simulated atmospheric corrosion conditions in a marine environment. The chromate released from the primer layers in effect prolonged and expanded the sacrificial protection of the 55% Al―Zn layers. The corrosion potentials, measured by a Kelvin probe, indicated that the cut edges were maintained at low potentials. The galvanic current measured for the 55% Al―Zn/steel couple specimen with the chromate-containing primer was lower than that of the specimen with the chromate-free primer. This concurs with the results of conventional polarization measurements, indicating that the cathodic oxygen reduction on steel substrates is inhibited by the presence of chromate. The corrosion potential variations across the interface between 55% Al―Zn layers and steel substrates were measured using Kelvin probe force microscopy (KPFM). Almost negligible variations in the KPFM potential were found at the interface in the cut edges with the chromate-containing primer. Our findings indicate that the prolonged and expanded sacrificial protection effect of the 55% Al―Zn layers is likely due to the release of chromate from the primer layers, which makes it possible for the 55% Al―Zn layers to dissolve slowly.

Microelectrochemistry of Sulfide Inclusions and Pit Initiation Mechanisms of Stainless Steels

Stainless steels suffer from pitting corrosion in chloride-containing environments. Sulfide inclusions, such as MnS, are known to act as the initiation sites of pitting. The pit initiation mechanism at MnS inclusions in chloride-containing environments is as follows : 1) dissolution of MnS inclusions leads to the deposition of elemental sulfur on and around the inclusions ; 2) the coexistence of elemental sulfur and chloride ions results in the dissolution of the steel matrix, forming the trenches at the boundaries between the inclusions and the steel matrix, 3) rapid active dissolution occurs locally at the bottom of the trenches, because of the decrease of pH due to the hydrolysis reaction of Cr and the electrode potential decrease at the bottom of the trench due to the solution resistance. Dissolution behavior of sulfide inclusions is of key importance in pit initiation process. Insoluble sulfide inclusions, such as CrS, TiS, and Ti4C2S2, are unlikely to act as the pit initiation sites. Cr-or Ti-oxide films on the inclusions act as a barrier against inclusion dissolution. An applied stress causes micro-cracks on MnS inclusions, which promotes pit initiation of stainless steels. The micro-crack initiation is closely related to oxide film formation on MnS inclusions.