CORROSION OF TITANIUM GRADE 7 IN SOLUTIONS CONTAINING FLUORIDE AND CHLORIDE SALTS

摘要

The commercial alloy Titanium Grade 7 (Ti-7) was tested for both general and localized corrosion in environments containing fluoride and chloride salts. The environment was a solution designed to simulate fluoride-containing groundwater to which chloride and fluoride were added. Tests were conducted on both disks and U-bend specimens of Ti-7 in order to determine the effects of fluoride and chloride in solution under static and stressed conditions. The experiments were run over a broad range of temperatures and pH values for periods of up to 30 days. The aggressiveness of the various environments was determined primarily by the time required to observe the formation of pits using visual inspection and cross-section metallographic imaging. It was found that Titanium Grade 7 is susceptible to intergranular attack in the test solutions at moderately elevated temperatures, leading to conspicuous localized corrosion. Pit initiation was detected within 14 days, with pit depths of the order of 0.1 mm. Surface defects appeared to promote the initiation of pitting. The same environment caused a catastrophic stress corrosion cracking failure in a statically stressed U-bend.