Effect of cerium and lanthanum ion implantation on corrosion behaviour of hot dip galvanised steel

Abstract

The effect of implanting La⁺ and Ce⁺ on the electrochemical behaviour of galvanised steel as well as the resulting microstructural changes have been studied. Ion implantation was carried out with doses of 1-2 x 10¹⁶ ions/ cm² Ce⁺ and 2x10¹⁶ ions/ cm² La⁺, at an energy of 150 KeV and at normal incidence. The SEM micrographs show that the greatest change on the surface appearance of galvanised steel is due to lanthanum implantation. In the galvanised steel SEM image, a segregation of lead can be observed at the grain boundaries and also inside the grains. La⁺ implantation results in à vermicular structure completely different to that of unimplanted galvanised steel, whilst in the case of Ce⁺ implantation, a similar structure of galvanised steel is maintained. The only effect of Ce⁺ implantation observed is that some of the lead precipitates disappear leaving just the holes. All the lead precipitates disappear when a high cerium dose is used. Linear polarisation in 3.56 % wt. NaCl solution indicates that the ion implantation improves the corrosion resistance of galvanised steel by one order of magnitude, being slightly better for La⁺ implantation.