Boiling titanium alloys

Titanium does not stress-crack in environments that cause stress-cracking in other metal alloys, eg, boiling 42% MgCl2, NaOH, sulfides, etc. Some of the aluminum-rich titanium alloys are susceptible to hot-salt stress-cracking. However, this is a laboratory observation and has not been confirmed in service. Titanium stress-cracks in methanol containing acid chlorides or sulfates, red Aiming nitric acid, nitrogen tetroxide, and trichloroethylene. 

Fig. 41 Corrosion of titanium alloys in boiling, uninhibited HNO3 solutions. Acid solutions were refreshed every 24 h [43].

Titanium alloys are resistant to alkaline media [43]. In most cases, the corrosion rate is low even in boiling solutions, except in boiling potassium hydroxide. In concentrated NaOH and KOH solutions, the corrosion rate increases at temperatures above the boiling point. 

S.C.C. of 18-8 stainless steel (>200h) [27]. Similarly, Cl or acetates inhibit S.C.C. of mild steel in boiling nitrates [21], andSOi or NO3 can be used to inhibit a titanium alloy (8% Al, 1% Mo, 1% V) that otherwise cracks in 3.5% NaCl solution at room temperature [28]. 

Group IV(4) titanium and titanium alloys Acid cleaning, rough finish Etching bath boiling oxalic acid solution (10 wt.% H,C,0,), hydrochloric acid (20 wt.% HCl), or sulfuric acid (30 wt.% HjSO,) for 10-30 min. Discard solution after each operation due to the interference of inhibiting titanium(IV) cations. 

Table 2-19 shows that in the reducing acidic solutions the rate of corrosion of Ti Gr 7 is lower than that of non-alloyed titanium (Ti Gr 2). Although the corrosion resistance of Ti Gr 12 is between those of Ti Gr 2 and Ti Gr 7, that of Ti Gr 12 more resembles the corrosion behavior of Ti Gr 2 than that of Ti Gr 7. Fig. 2-28 shows the rates of corrosion of these three titanium alloys in dilute sulfuric acid solutions at the boiling point. As already observed for other reducing acidic solutions, the lowest rate... 

Table 2-19. Rates of corrosion, in mpy, of titanium alloys in boiling solutions.

Figure 2-28. Rates of corrosion of three titanium alloys in boiling sulfuric acid solutions (Covington and Schweitzer, 1989).

Corrosion Rates of Titanium Alloys in Boiling H2SO4, (mpy)... 

Repassivation potentials of as-annealed titanium alloys in boiling chloride media... 

General corrosion of aged titanium alloys in naturally aerated boiling HCI solutions. 

General corrosion of anrreaied titanium alloys in naturaly aerated boiling HCI solutions. 

Ti-15Mo-5Zr has high corrosion resistance to reducing atmospheres. It has better corrosion resistance in boiling hydrochloric acid or sulfuric add solutions than commerdally piu e titanium. Additionally, Ti-15Mo-5Zr has higher erosion resistance compared to Ti-6A1-4V or other P titanium alloys. 

Salt Solutions. Titanium alloys are highly resistant to practically all salt solutions over the pH range of 3 to 11 and to temperatures well in excess of boiling. Titanium withstands exposure to solutions of chlorides (Ref44,45), bromides, iodides, sulfites, sulfates, borates, phosphates, cyanides, carbonates, bicarbonates, and anunonium compoimds. Corrosion rate values for titanium alloys in these various salt solutions are generally less than 0.03 mm/yr (1.2 mils/yr). 

Some metals and alloys have low rates of film dissolution (low /p) even in solutions of very low pH, e.g. chromium and its alloys, and titanium. In these cases the value of /p is quite low, and although it increases as the temperature increases, a maximum is reached when the solution boils. The maximum current is below and breakdown does not occur. However, in certain alloys, e.g. Cr-Fe alloys, the protective film may change in composition on increasing the anode potential to give oxides that are more soluble at low pH and are therefore more susceptible to temperature increases. This occurs in the presence of cathode reactants such as chromic acid which allow polarisation of the anode. 

Other alloys of molybdenum which have been investigated for their corrosion resistance contain 10-50% Ta and were found to have excellent resistance to hydrochloric acid. Ti-Mo alloys were found to resist chemicals that attack titanium and Ti-Pd alloys, notably strong reducing acids such as hot concentrated hydrochloric, sulphuric, phosphoric, oxalic, formic and trichloroacetic. For example, a Ti-30Mo alloy has the following corrosion rates in boiling 20% hydrochloric acid, 0-127-0-254 mm/y in 10% oxalic acid at 100°C, 0-038 mm/y, which compares favourably with the respective rates of 19-5 and 122 mm/y for the Ti-0-2Pd alloy. 

From the corrosion-resistance aspect, one of the most effective additions to titanium is that of molybdenum. According to Yoshida and his colleagues ", the addition of 15% Mo produces an alloy fully resistant to virtually all concentrations of sulphuric and hydrochloric acid at room temperatures, while with 30% Mo, the alloy is resistant to all strengths of boiling sulphuric acid up to a concentration of 40% by weight, and to 10% boiling hydrochloric acid. 

Because titanium, unlike 18-8 stainless steel, has a low critical current density for passivity in chlorides as well as in sulfates, passivity in boiling 10% HCl is made possible by alloying titanium with 0.1% Pd or Pt [16]. Pure titanium, on the other hand, corrodes in the same acid at very high rates (see Fig. 25.2, Section 25.3). 

Figure 25.2. Corrosion of titanium in boiling 10% HCI as a function of and Cu concentration and alloyed palladium or platinum.

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