Titanium is resistant to stress-corrosion-cracking (SSC) in the sea or brackish water. Both smooth and notched metals will resist SSC. The type of titanium, Ti-6Al-4V ELI (Extra Low Interstitial Alloy Content), is used for thick-walled parts of undersea submersible devices. This variety of titanium has also been used for diving bell hatch doors, sonar devices, encapsulation for deep sea cameras and electronics packages. No reports of failure due to inadequate sturdiness have been published about items made with this material.
Some varieties of titanium are vulnerable in seawater. When the oxygen level in unalloyed metals is 0.2 percent or less, the metal is prone to SSC. Pre-existing cracks will also compromise the integrity of titanium, despite the oxygen levels in the metal. The lower the levels of oxygen in titanium, the weaker the metal is. Nitrogen also strengthens and regulates the tensile strength. Grades 1 to 4 of titanium are unalloyed and their resistance to SSC increases with their grade. Titanium will also weaken in environments where there is no oxygen or water available as its protective film will not regenerate if it's weakened.
Titanium grade 2 is used for heat exchangers in aquariums as this grade has high thermal conductivity relative to other grades of titanium. Prior to the use of titanium for heat exchange, it was difficult to keep saltwater aquariums cold enough. Large municipal aquariums once used anodized aluminum, copper or plastic tubes for heat transfer, but those materials were susceptible to corrosion. Some aquariums still use copper in their exchangers due to the cost of titanium.
Titanium is protected by an oxide film that is as resistant to corrosion as platinum. The film is initially 12 to 16 angstroms thick, but in 70 days it will reach 50 angstroms, and then at 545 days, it peaks at 250 angstroms. The integrity of the metal is dependent on this layer, which will regenerate if there is moisture present in its environment. As little moisture as 50 ppm (parts per million) is necessary for the film to regenerate and again protect the metal.