Hydrogen embrittlement of grade 2 and grade 3 titanium has been investigated in 6%NaCl (pH=l) under potential control at 70C. Grade 2 titanium was, at most, slightly susceptible to hydrogen embrittlement under the testing conditions employed, whereas grade 3 titanium was very susceptible to hydrogen embrittlement at an electrochemical potential below -800 mVsm and at a crosshead speed below 7X104 mm/min. Hydride phases began to form on the surface of the grade 3 titanium at a potential of-700 mVsm. These phases could be observed in the interior of grade 3 titanium at lower potentials, for example, -1400 mVsa, after extended cathodic charging. The hydrides in the interior of grade 3 titanium were responsible for its susceptibility to hydrogen embrittlement. Hydrides were never observed in the interior of grade 2 titanium. The formation of these hydrides in the interior of grade 3 titanium and lack thereof in grade 2 could be attributed to the higher oxygen content of grade 3.
The superior corrosion resistance of commercial purity titanium, even in chloride-containing environments over a wide range of pH values, is well known. 1As a result, titanium has been used for piping in desalination plants and for heat exchangers and condensers in marine environments.2-8 However, hydrogen embrittlement (HE) of titanium has been reported for conditions where the metal is exposed to hydrogen-bearing environments. Titanium can absorb hydrogen when it is charged at cathodic potentials or when it is galvanically coupled with another metal and becomes a cathode.2-5 Hydrides can be formed when hydrogen absorption reaches a critical level. The hydrides, which tend to be brittle and have very different elastic properties from the parent lattice, can degrade the mechanical properties to some extent1°-13.Titanium susceptibility to HE is also apparently related to its composition. Recent experiments by Simbi and Scully in room temperature seawater without potential control indicated that the cracking susceptibility of titanium increased as the purity of the titanium decreased. 14This trend needs to be examined further and developed under varied testing conditions such as high temperature and cathodic charging under potential control, Until now, the mechanism by which chemical composition affects HE susceptibility has not been well understood. This paper attempts to clarify this question.
We report in this paper the results of mechanical tests for grade 2 and grade 3 titanium in 6%NaCl solution (pH=l) at 70C carried out under potential control. It was found that grade 3 titanium was much more susceptible to HE than grade 2. The hydride thickness on the surface was measured and the fracture surface was observed. The effect of chemical compositions on the HE susceptibility is discussed in detail.
EXPERIMENTAL PROCEDURE
The materials used in this study were commercially prepared grade 2 and grade 3 titanium. They were supplied as rolled sheets 1.12 mm and 0.62 mm thick, respectively, and used in the as-received condition (749C, 3 minutes and air cooling). Their chemical compositions and mechanical properties, as reported by the manufacturer, are shown in Table 1.
The mechanical experiments were carried out in 6%NaCl solution with a pH value of 1 at 70C. Sheet tensile samples with a gauge section 11.43 mm wide were used for the tests. The gauge section contained a notch 2.54 mm deep subtending an angle of 60° filed into the center of one edge. The specimens were cut from an as-received sheet with the length perpendicular to the rolling direction and machined to the required size. The center part of the specimen was polished wi
