Electrochemical Behavior of Ti-Ni Alloys in Acidic Chloride Solutions

A study has been made of the electrochemical behavior of titanium, nickel, and various titanium-nickel alloys, containing 2, 3.5, 4.5, 28.5, 38, and 98 wt % nickel, in an acidified (pH = 1) 3.5% NaCl solution, both at ambient temperature and at the boiling point. The anodic and cathodic polarization behavior was studied using a Wenking potentiostat and surface film growth characteristics were examined using a Rudolph thin film ellipsometer. Cathodic polarization studies showed that the introduction of Ti₂Ni, by alloying titanium with nickel, greatly facilitated the overall cathodic reaction, both in ambient temperature and boiling solutions. Anodic polarization showed that the introduction of Ti₂Ni in the alloy generates a surface which is passive in the boiling solution. These and other results are discussed in terms of the current understanding of the electrochemical behavior of titanium alloys, and it is concluded that Ti₂Ni increases the corrosion resistance of titanium in hot acidic chloride solutions by increasing the efficiency of the cathodic process, thereby creating a mixed potential which is in the passive potential range of titanium. No susceptibility to stress corrosion cracking (SCC) was detected in the commercially available titanium-2 wt % nickel alloy on testing in the boiling acidified chloride solution for a period of 500 hours.