Abstract
Titanium alloys have become well established as choice corrosion-resistant materials for high temperature seawater service based on their superior chloride corrosion resistance. Useful pH-temperature guidelines have been developed which advocate the selection of grade 7 or 12 titanium when crevice corrosion becomes a limiting mode of attack for unalloyed titanium. However, a few unexpected cases of severe pitting of grade 12 titanium tubes in high temperature seawater-cooled oil refinery overhead exchangers have been reported. Investigation revealed that attack involved total seasalt pluggage of exchanger tubes, associated with very low (< 0.6m/sec) tubeside seawater flow rates and high process-side temperatures (> 177°C). Laboratory simulation of the tubeside salt pluggage phenomenon demonstrated that evaporative concentration of seawater within tubes produced highly concentrated MgCl2 solutions within salt plugs. Correlation of results from these simulated seasalt plug corrosion tests and laboratory immersion test results in pure concentrated MgCl2 solutions indicated that local high temperature MgCl2 hydrolysis was directly responsible for the unique form of pitting observed. Temperature limits for localized corrosion of titanium alloys as a function of NaCl and MgCl2 concentration are generated as well, revealing that grade 12 and, especially, grade 7 titanium offer useful resistance to a significantly expanded range of seawater concentrations and temperatures as compared to unalloyed titanium. Strategies for assuring successful performance of grade 12 titanium in high temperature seawater-cooled exchangers are offered.
