Stress corrosion of a coated titanium alloy plate in seawater

Abstract

The object was to determine whether painting a butt-welded titanium plate would reduce its susceptibility to stress-corrosion cracking (s.c.c.). Tests were conducted with a large, 1 in.-thick butt-welded titanium-721 plate coated with Navy vinyl paint and subjected to static and cyclic loading in seawater at a nominal bending stress of 80,000psi at the toe of the weld. Electrochemical potentials of the titanium were continuously recorded during the test. A 10 per cent increase in deflection was noted after 18,800 cycles and 40hr of static loading. In contrast, a previous report on a similar but uncoated titanium-721 plate showed catastrophic failure after 17min in seawater under static load only. Potentials of the painted titanium indicated that the coating was an effective barrier to the passage of water. A fatigue crack caused paint rupture, followed by catastrophic failure under static load 20hr after the rupture. Analysis of the potentials showed that cracking may first be in the fatigue mode followed by two s.c.c. steps. It is hypothesized that the coating retarded s.c.c. by preventing and then limiting access of water to the titanium surface, and by reducing the effective cathodic area. The Navy vinyl paint system is considered a promising candidate for the protection of susceptible titanium alloys against stress-corrosion cracking in seawater.