The hydrogen absorption tendencies of several platinum group metal (PGM) enhanced titanium alloys were investigated. The short-term (24-240 hour) tests were conducted in hydrochloric acid (HCl) solutions that were partially deaerated or naturally aerated, at subboiling or boiling temperatures. Coupons were sampled before and after each test exposure to determine hydrogen content and corrosion rate. The hydrogen uptake was used to determine the hydrogen uptake efficiency for each alloy under each set of conditions. Based on the tests conducted, the effects of temperature, dissolved oxygen content, and alloy content were evaluated and are discussed.
The following is a comparison of the hydrogen absorption tendencies of commercially pure (C.P.) titanium (Grades 1, 2), Ti-0.15Pd (Grades 7, 11), Ti-0.1Ru (Grade 27), Ti-0.5Ni-0.05Ru (Grade 13), Ti-0.05Pd (Grade 16), Ti-3Al-2.5V-0.1Ru (Grade 28) and Ti-0.4Ni-0.15Cr-0.015Pd- 0.025Ru (Grade 33). The evaluation was done by conducting weight loss tests in boiling and sub-boiling naturally aerated and partially deaerated HCl solutions, and measuring the weight loss and hydrogen content of the coupons before and after each test. The data obtained was used to determine a corrosion rate, and hydrogen uptake efficiency (HUE) for each alloy in each set of test conditions in order to determine which alloys were more prone to hydrogen absorption.
The mechanism of passivation for platinum group metal (PGM) enhanced titanium alloys is due to the modification of the cathodic hydrogen evolution reaction (HER). PGM additions are useful in this regard because they provide a high cathodic exchange current density for the HER and a lower cathodic Tafel slope. This is in contrast to C.P. titanium, which has a low cathodic exchange current density for the HER and a higher cathodic Tafel slope. The PGM additions act as cathodic depolarizers and create a mixed potential in the passive range. For these alloys, the potential is shifted in the noble direction and the cathodic (HER) reaction on the PGM enhanced Ti intersects the anodic polarization curve at a potential positive to its anodic loop or critical anodic current density and in its passive region. Evans? diagrams illustrating this effect are shown in References 1 and 3.
Several investigators have studied the effects of PGM additions on hydrogen absorption of titanium alloys in reducing acids. A review on the subject of hydrogen absorption of Ti, Ti- Grade 12 and PGM enhanced Ti as related to hydrogen induced cracking (HIC) is given by Hua et al. Fukuzuka found that Pd enhanced alloys may be more prone to hydrogen absorption than unalloyed titanium. Bishop reported that C.P. titanium and Ti-0.2Pd were similar with respect to hydrogen absorption tendencies in cathodic charging experiments. In HCl weight loss studies, the Ti-0.2Pd alloy was much less susceptible to hydrogen absorption. Sedricks has reported that Ru enhanced titanium alloys are less prone to hydrogen absorption. Also of note is Schutz and Xiao?s study of hydrogen absorption of beta titanium alloys Ti-Grade 19 (Ti-3Al-8V-6Cr-4Zr-4Mo), Ti-Grade 20 (Ti-3Al-8V-6Cr-4Zr-4Mo-Pd), and also an additional version of Grade 20 + Ru, which showed that these PGM additions did not aggravate hydrogen absorption in boiling HCl suggesting that the oxide film further presented a barrier to atomic hydrogen.
The effects of these PGM additions on the metallurgy of the binary Ti-Ru and Ti-Pd alloys have also been investigated. Palladium additions to titanium in Ti-0.15Pd alloys lead to the formation of a Ti2Pd compound. In contrast to these Pd alloys, Ti-0.1Ru alloys form a large number of beta phase particles which are much more e
