Cerium: a suitable green corrosion inhibitor for tinplate

In chloride solutions, tinplate exhibits localised corrosion processes due to the defects or imperfections of the outer Sn layer, which leaves the Sn–Fe alloyed layer exposed to aggressive solutions. The anodic character of the external Sn with respect to the internal alloyed layer, will lead to its dissolution acting as a sacrificial anode. The addition of cerium salts to the aggressive media decreases the pitting susceptibility of tinplate by means of cerium precipitation on the cathodic sites (bare Sn–Fe areas). This induces to a change of the controlling mechanism suggesting that cerium is a cathodic inhibitor for tinplate, which could be corroborated by removal selectively the outer Sn layer and leaving the Sn–Fe layer directly exposed to the solution. In these conditions, the corrosion process of the steel base will take place through the defects of the Sn–Fe layer, which presents a cathodic character in comparison to the base metal, inducing the precipitation of cerium on the alloyed layer. Scanning electron microscopy and energy dispersive spectroscopy analysis revealed that the location of cerium precipitates depend on the distribution of the cathodic areas that could lead either to a continuous layer covering the surface or to precipitates inside the mechanical failures.