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.
Commercially pure titanium and its alloys are commonly used in industries where corrosion is of special concern because they are resistant to environments which are corrosive toward other metal, such as solution of oxidizing halides, reducing acids, 1 contaminated seawaters 2-4 and geothermal brines. 5 Titanium's corrosion resistance is due to a thin film of passive titanium oxide that protects metallic titanium from the environment. This oxide layer is composed primarily of the rutile form of TiO2, although other oxides may be present, depending on the environment in immediate contact with the surface.6, 7 Although the oxide layer is on the order of only 10 nm thick 8, it is stable over a wide range of temperatures and pH 9 and is chemically resistant to aggressive environments, notably concentrated mineral acids such as HC1 and H2SO4.
Of the titanium alloys, Ti-7, Ti alloyed with 0.15% palladium, is considered the most resistant to corrosion. Coupling the more noble Pd to titanium cathodically polarizes the bulk metal, making the alloy more resistant to general corrosion in the usually aggressive hot brines and acids. 1° Since the accumulation of hot acids in crevices is the primary cause of local corrosion, the acid-resistant Ti-7 has been considered immune to crevice and pitting corrosion 11 in saturated salt solutions 12 for a wide range of pH's, and at temperatures as high as 200°C. 12'13 Ti-7 also resists embrittlement 14 and stress corrosion cracking 15-16 resulting from hydrogen incorporation. It is believed that the oxide layer is enriched with islands of metallic palladium that serve as concentration and recombination sites for atomic hydrogen, preventing further sorption of hydrogen into the bulk titanium metal. 7,17-18
Although considered immune to most environments, most alloys of titanium are susceptible to environments containing the fluoride ion, especially at acidic pH's. 19,20 Cragnolino21 electrochemically examined the behavior of Ti-7 both in acidic chloride media, and in solutions containing both chloride and fluoride at 95°C, to determine its corrosion resistance to simulated hot groundwater environments. It was observed that the relatively low corrosion rates were not significantly affected by chloride concentration or pH. However, in solutions containing fluoride, a significant lowering of Ecorr and rise in current density was observed in solutions containing 1M fluoride. These results were further confirmed 22 when Ti-7 was studied electrochemically in the sam
