Acetic acid with titanium, C-276, B2, zirconium

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Methanol is carbonylated to produce acetic acid using methanol and carbon monoxide as raw materials, a rhodium catalyst (with a certain substance as the active component) as the main catalyst, and methyl iodide (CH3I) as a co-catalyst, at a temperature of about 180°C and a pressure of about 30 atmospheres (3 MPa). It is worth noting that a small amount of iodide ions (hydroiodic acid) are present in this system; furthermore, carbon monoxide may also react with water to produce carbon dioxide and hydrogen. •Titanium has excellent corrosion resistance in \"pure\" acetic acid, as well as in high-temperature acetic acid containing oxygen and oxidizing metal ions (trivalent iron, divalent copper), which is why it is widely used. However, under the highly reducing conditions of the methanol low-pressure carbonylation process for acetic acid production, titanium’s oxide film is damaged, and a new oxide film cannot form a dense layer over the surface of titanium; this results in significant uniform corrosion. What’s more, the side reactions mentioned above generate hydrogen, and under such high temperatures and pressures, titanium can absorb hydrogen and become brittle (hydrogen embrittlement). Therefore, titanium is also not a suitable material for these conditions. •Under such harsh conditions, tests have shown that the corrosion rate of Hastelloy C-276 is approximately 0.2–0.6 mm per year ； •Hastelloy B2 is one of the few alloy materials capable of withstanding highly reducing environments such as boiling concentrated salt acids at atmospheric pressure at any concentration, hydrobromic acid, and boiling sulfuric acid below 70% concentration (suitable for all concentrations of sulfuric acid at temperatures below 100°C). In high-temperature and high-pressure iodide-acetic acid environments, its corrosion rate under normal conditions is only one-tenth that of C-276; it is therefore a highly corrosion-resistant material. Additionally, as it belongs to a nickel-based alloy, it is less prone to hydrogen embrittlement in hydrogen-rich environments. However, due to the lack of chromium, the corrosion rate of Hastelloy B series increases by hundreds of times in oxygen-containing environments (above 50 ppm) or in reducing acids containing oxidants; for example, experiments conducted at 150°C with a large amount of air introduced into acetic acid containing iodide ions showed a corrosion rate as high as 9 mm/year or more ； •The corrosion rate of zirconium is less than 0.025 mm/year, which is only 1/3 or even lower than that of Hastelloy B2; it thus possesses excellent corrosion resistance. Zirconium is unafraid either of strongly reducing iodoacetic acid containing carbon monoxide or of acetic acid with high dissolved oxygen levels under \"abnormal conditions.\" Additionally, although both zirconium and titanium are active metals, zirconium’s oxide film provides a much better barrier to hydrogen penetration than titanium’s, so hydrogen embrittlement is less likely to occur under these operating conditions