Titanium alloys pitting corrosion

Enhancing Crevice Corrosion Resistance. Several effective strategies for preventing titanium alloy crevice corrosion and smeared iron pitting are alloying titanium, predous metal surface treatments, metallic coatings, thermal oxidation, noble alloy contact, and surface... 

Pitting corrosion is usually associated with active-passive-type alloys and occurs under conditions specific to each alloy and environment. This mode of localized attack is of major commercial significance since it can severely limit performance in circumstances where, otherwise, the corrosion rates are extremely low. Susceptible alloys include the stainless steels and related alloys, a wide series of alloys extending from iron-base to nickel-base, aluminum, and aluminum-base alloys, titanium alloys, and others of commercial importance but more limited in use. In all of these alloys, the polarization curves in most media show a rather sharp transition from active dissolution to a state of passivity characterized by low current density and, hence, low corrosion rate. As emphasized in Chapter 5, environments that maintain the corrosion potential in the passive potential range generally exhibit extremely low... 

The failure time, however, incorporates both the time required for crack initiation and a period of slow crack growth so that the separate effect of the environment on each of these stages cannot be ascertained. (Some of the difficulty stems from the lack of a precise definition for crack initiation.) This difficulty is underscored by the results of Brown and Beachem [1] on SCC of titanium alloys. They showed that certain of the alloys that appeared to be immune to stress corrosion cracking in the traditional (smooth specimen) tests are, in fact, highly susceptible to environment-enhanced crack growth. The apparent immunity was explained by the fact that these alloys were nearly immune to pitting corrosion, which was required for crack nucleation in the same environment [1]. 

Materials classes that were tested included ceramics, nickel-based and cobalt-based alloys, refractory metals and alloys, reactive metals and alloys, noble metals and alloys, and high-temperature polymers, a total of 26 materials. Test periods varied between 37.5 and 47.5 hours. None of the materials was found to be suitable for all test conditions, and most exhibited moderate (equivalent to between 10 and 200 mil per year) to severe (>2()0 mil per year) corrosion. Titanium and titanium alloys (Nb/Ti and Ti-21S) exhibited the best performance, showing only slight corrosion in the presence of excess sodium hydroxide. Under acidic conditions, titanium showed increased rates of corrosion, apparently from attack by sulfuric acid and hydrochloric acid. Both localized pitting and wall thinning were observed. 

Table 7.50 shows the pitting potentials of different metals and alloys observed in 0.1 M NaCl at 25°C. The particularly high values for titanium and tantalum provide an explanation for their excellent resistance to pitting corrosion in chloride-containing environments. 

A common reason for corrosion on decanters is the presence of chlorides in the process material. The chlorides can cause pitting corrosion, crevice corrosion and stress corrosion of the decanter parts in contact with the process. To avoid pitting and crevice corrosion in severe environments, more corrosion resistant materials, normally special stainless steels with a higher content of alloying elements, are used. In extremely corrosive environments, more exotic materials, such as the nickel-based Hastelloy or even titanium, may be used. 

The enhanced corrosion resistance in reducing environments from molybdenum is achieved, however, at the expense of corrosion resistance in oxidizing conditions. Therefore, pitting and repassivation potential are expected to be lower them most other titanium alloys, eilthough no data are available. 

Anodic breakdown residual potential (ABRP) of selected titanium alloys. The higherthe ABRP test value, the less susceptible the material to pitting corrosion and crevice corrosion. 

Methanol (CH3OH) appears to attack titanium nickel only when diluted with low concentrations of water and halides. This impure methanol solution leads to pitting and tunneling corrosion similar to that found in titanium alloys. 

The major corrosion problems with titanium alloys appear to be crevice corrosion, which occurs in locations where the corroding media are virtually stagnant. Pits, if formed, may progress in a similar manner. A general comparison of corrosion resistance for titanium is provided in Fig. 1(a). 

Cobalt-chromium alloys, tike titanium, are passive in the human body. They are widely in use in orthopedic applications. They do not exhibit pitting corrosion. 

For high-sulfur environments, use of galvanizing may be preferable to austenitic stainless steels such as Type 316 or Alloy 20 stainless steel because of the hazard of pitting corrosion. High-molybdenum stainless steels have been shown to be particularly resistant to long-term exposure in road tunnels, exhaust stacks, and similar environments. Titanium offers excellent resistance to corrosion but may be cost prohibitive. 

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