Effect of Silicon on the Corrosion of Ti–xSi Alloys in Seawater

Abstract

The effect of silicon content on the structure and corrosion resistance of Ti–xSi alloys in a 3 wt.% NaCl solution simulating seawater was examined. Arc melting in an argon atmosphere was employed to produce Ti–1.2Si, Ti–3.6Si, Ti–5.7Si, and Ti–2Si–3Al alloys. Silicon was partly retained in solid solution in the Ti–xSi alloys and, when its solubility limit was exceeded, formed an excess Ti₅Si₃ silicide phase. At silicon contents above 1.5–2.1 wt.% Si, the amount of the Ti₅Si₃ phase as a eutectic component increased substantially. Electrochemical studies were conducted by plotting potentiodynamic curves for anodic dissolution of the initial surfaces and cathodic reduction curves in forward and reverse scans and then anodic curves for the reduced surfaces. The Ti–xSi alloys showed adequate corrosion resistance. With increasing contents of silicon and the strengthening silicide phase, and with the associated transition from polycrystalline to eutectic structure, the anodic stability of the alloys improved. The addition of 3 wt.% aluminum to the Ti–2Si alloy enhanced corrosion resistance over the range from the open-circuit potential to the onset of anodic dissolution, after which the corrosion resistance of the Ti–2Si–3Al alloy became comparable to that of the other Ti–xSi alloys. Preliminary cathodic treatment of the Ti–xSi alloys demonstrated significant hysteresis between the forward and reverse cathodic curves, resulting from the removal of surface impurities by atomic cathodic hydrogen and involving substantial improvement in the anodic stability of the alloys. The relative content of the Ti₅Si₃ silicide phase on the Ti–xSi alloys and the oxide nanofilms formed in exposure to aggressive environments were found to have the decisive role in the increase of their corrosion resistance.