Titanium hydrochloric acid, corrosion rates

The addition of 0-2% palladium to titanium decreases the corrosion rate in boiling 5% sulphuric acid by a factor of 500, and in boiling 5% hydrochloric acid by a factor of 1 500, in relation to the rates obtained with unalloyed titanium. The addition of palladium in these quantities thus provides an adequate measure of resistance to relatively weak concentrations of the acids mentioned. ... 

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. 

Alloy B-3 can tolerate quite high concentrations of hydrochloric acid without severe corrosion, provided the solution is free of oxidizing ions such as Fe, Cu" ", Ni" " Mo" " , and Ti" ". In contrast, hydrochloric acid solutions containing these ions can be easily handled in vessels made of titanium, which would quickly corrode in pure hydrochloric acid. Even low concentrations (<30 ppm) of ferric ion in hydrochloric acid have been demonstrated to greatly reduce the corrosion rate of titanium. ... 

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. 

The outstanding corrosion property of zirconium is its resistance to alkalies at all concentrations up to the boiling point, ft also resists fused sodium hydroxide. In this respect, it is distinguished from tantalum and, to a lesser extent, titanium, which are attacked by hot alkalies. Zirconium is resistant to hydrochloric and nitric acids at all concentrations and to <70% H2SO4 up to boiling temperatures. In HCI and similar media, the metal must be low in carbon (<0.06%) for optimum resistance. In boiling 20% HCI, a transition or breakaway point is observed in the corrosion rate (see below) after a specihc time of exposure. The... 

Titanium offers excellent resistance to corrosion by several other inorganic acids. It is not significantly attacked by boiling 10 wt.% solutions of boric or hydriodic acids. At room temperature, low corrosion rates are obtained upon exposure to 50 wt.% hydriodic and 40 wt.% hydrobromic acid solutions. But hydrochloric acid readily attacks titanium. 

The addition of nitric acid to hydrochloric or sulfuric acid significantly reduces corrosion rates. Titanium is essentially immune to corrosion by aqua regia (3 HCl 1 HNO,) at room temperature. ASTM grades 7 and 12 show respectable corrosion rates in boiling aqua regia. Corrosion rates in mixed acids will generally rise with increases in the reducing acid component concentration or temperature. 

Titanium in sulfuric and hydrochloric acids easily undergoes corrosion, but easily passivates during anodic polarization. For example, anodic protection in 40% H2SO4 at 60 °C decreases the corrosion rate of titanium by 1100 times. Also, anodic protection of this metal is applied in solutions containing chloride ions, especially in hydrochloric acid. The corrosion rate of titanium in 30% HCl at 80 °C after the application of protection decreases by approximately 800 times. More information can be found in the works of Locke (1987) and Kuzub and Novitski) (1984). 

Besides alloy composition (Table 2-19), the corrosion behavior of titanium in reducing acids is very dependent on acid concentration, temperature, and impurities in the acid (Schutz and Thomas, 1987). Anodic polarization curves of titanium in sulfuric acid solutions showed that the critical current for passivation at fixed temperature increased with the acid concentration (Levy, 1967 Peters and Myers, 1967) and with temperature at a given acid concentration (Levy, 1967). Fig. 2-29 shows the rates of corrosion of Ti Gr 2 and other alloys as a function of the concentration of pure hydrochloric acid at the boiling temperatures. As the acid concentration increases the rate of corrosion of Ti Gr 2 increases rapidly. 

General corrosion is characterized by a uniform attack over the entire exposed surface of the metal. The severity of this kind of attack can be expressed by a corrosion rate. With titanium, this type of corrosion is most frequently encoimtered in hot, reducing acid solutions. In environments where titanium would be subject to this type of corrosion, oxidizing agents and certain multivalent metal ions have the ability to passivate the titanium. Many process streams, particularly sulfuric and hydrochloric acid solutions, contain enough impurities in the form of ferric ions, cupric ions, etc., to passivate titanium and give trouble-free service. Refer to Table 20.6 for compatibility of titanium with selected corrodents. 

General Molybdemun additions improve the corrosion Corrosion resistance of titanium alloys in reducing media, and this effect is evidenced by the general corrosion rates of Beta C in reducing media such as hydrochloric and sulfuric acid (see figures). This increase in reducing environment resistance is... 

Hydrochloric acid (HCl) has a variety of effects on the corrosion of titanium-nickel alloys depending on temperature, acid concentration, and specific alloy composition. With 3% HCl at 100 C (212 F) and a range of alloy compositions, the rate of attack was as low as 0.36 mpy and as high as 3.3 mpy. At 25 °C (77 °F) and 7M solution, titanium-nickel-iron alloys can lose up to 457 mpy. 

In aqueous media where hydrochloric and hypochlorous acid and halogens are present in either vapor or liquid phase, the utility of the above materials of construction is severely lin ited and can best be determined by rates of corrosion study during pilot laboratory operation. Tantalum, zirconium, and titanium are usually resistant but expensive. The plastics are of variable resistance and are severely limited by temperature and solvent attack. Stoneware, Karbate, glass, glazed tile, carbon brick, and enameled steel all have utility within rigid limits. The other metals and alloys are usually questionable but may be desirable for replaceable piarts i... 

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