Abstract
Although recognized for their exceptional corrosion resistance in a wide range of chemical environments, titanium and its alloys can be susceptible to corrosion in reducing acid media, crevice attack in hot chlorides and other halide solutions, and/or stress corrosion in certain higher-strength alloys. These serious limitations may be practically and cost-effectively overcome to a great extent by the addition of very minor (<0.25 wt%) amounts of platinum group metals (PGMs) to titanium and its alloys; while imparting little or no effect on alloy mechanical/physical properties or metallurgy. Through the catalytic facilitation of cathodic reduction reactions (primarily the hydrogen evolution reaction [HER]) in acidic aqueous media, these PGM alloy additions shift titanium toward noble (positive) potentials where the protective titanium oxide surface film is stable. This radically decreases alloy corrosion rates in dilute inorganic and organic acids with no detrimental influence in oxidizing media or to hydrogen absorption resistance; and dramatically elevates alloy crevice and stress corrosion temperature threshold limits in aqueous chloride (halide) environments. Although any PGM may be effective, titanium alloys based on minimal Pd and/or Ru alloying additions have been commercialized and proven to be most cost-effective to date. This paper presents an overview of the commercial PGM-enhanced titanium alloys available today, their historical evolution and metallurgy, insight into corrosion-resistance enhancement mechanisms, the scope of expanded alloy corrosion performance limits achieved, relative cost impact, and relevant current and candidate applications.
