Electrochemical Corrosion Behavior of Ion-Implanted HSLA-80 Steel

A high-strength low-alloy steel (HSLA-80), designated for naval structural application and containing copper as a major precipitation-hardening element, was implanted with ⁵²Cr⁺, ⁴⁸Ti⁺, ²⁷Al⁺, and ⁶³Cu⁺ ions at 125 keV to different doses and subsequently investigated by the potentiodynamic technique for electrochemical corrosion behavior in electrolytes, viz., sodium acetate I acetic acid, pH 5.0, and 0.01 N sodium chloride, pH 6.0. Implanted samples tested in sodium acetate/acetic acid solution confirmed that the resistance was enhanced by reducing the critical corrosion current (i_crit) by several order of magnitude over an unimplanted surface. Of these four ions, chromium was found to have the most beneficial effect. On the other hand, samples implanted with the four different ions responded very differently in 0.01 N NaCI solution, which is often used as a pitting medium. In this case, chromium and aluminum ions were found to improve the corrosion properties by reducing the general corrosion current and shifting the corrosion potential towards more noble values. Surface analysis by x-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) confirmed the passivation of chromium and its enrichment on the surface in sodium acetate/ acetic solution, while in NaCI it is the copper and iron that enriches and may take part in passivation. Presence of chlorine deep inside an etch pit was also observed.