THE ROLE OF ALKALINE CLEANING SOLUTIONS AND CL-CONTAINING LUBRICANTS ON PITTING CORROSION IN NiTi ALLOYS

Alkaline cleaning agents are found to react with remnant manufacturing lubricants trapped in surface defects of a NiTi alloy to form Cl-based salts. These sub-surface “reservoirs” may act as nucleation sites for pitting corrosion either latent under ambient conditions, or on exposure to a limited range of high temperatures used to impart pseudoelastic properties in these alloys as part of normal processing. The elevated temperature range over which corrosion was found to occur correlates well with published thermodynamic data for the liquid stability range of specific titanium chlorides. No corrosion is seen on exposure to higher temperatures. The combination of sufficient cavity size, amounts and ratios of reactive species viz., Cl and K (or Na), and water are thought to be necessary for a pit to evolve to a mature size.