The difference between titanium and stainless steel is that there is no over-passivation in haloacetic acid with strong oxidation, and pitting and SCC will not occur. It has excellent corrosion resistance in high-temperature acetic acid containing various concentrations of Cl-, Br- and formic acid, etc. For example, it is widely used in equipment for producing acetic acid by oxidation of PTA, PV A and acetaldehyde.
However, in reductive acetic acid containing I- and Cl-, such as methanol carbonylation to acetic acid equipment, titanium will be seriously corroded and hydrogen embrittlement, so it cannot be used. In concentrated acetic acid evaporation, water content less than 0.1% and the presence of acetic anhydride will aggravate titanium corrosion, pure iron is particularly sensitive to crevice corrosion.
Because titanium materials can be supplied in bulk in China, the manufacturing technology of large-scale titanium equipment has made breakthroughs. Titanium has low density and high specific strength, which is more economical than nickel-based alloy.
In order to resist the strong reducing medium containing I-acetic acid, and the reducing and oxidizing alternate medium, as well as resist the acetic acid erosion of catalyst containing solid particles, the titanium and high-performance stainless steel must use nickel-based alloy.
Molybdenum in nickel-based alloys is particularly effective against reducing acetic acid, forming protective film and effectively inhibiting the destructive effects of I-, Br- and Cl-. Chromium is conducive to the formation of passivation film, mainly used in the antioxidant acetic acid. Nickel base alloy is austenite structure, nickel in acetic acid will be passivated, has the role of anti-reducing medium, but nickel will reduce the solid solubility of carbon, due to the metal interphase and carbide precipitation at the grain boundary, produce poor molybdenum, so there is a tendency of intercrystal corrosion, and when the alloy contains more than 35% nickel, almost no SCC.
The corrosion resistance of G-3 in the "G" head alloy of Ni-Cr-Mo-Fe is higher than that of Inconel 625 and Inconel 825, and it can withstand the erosion of alternating oxidation and reduction environment. But g-30 welding performance is better, and improve the weld heat affected zone corrosion resistance.
Ni-mo's "B" head alloy, B-2 has outstanding anti-reducing acid corrosion ability, of course, it is also good against methanol carbonyl synthetic acetic acid corrosion, but the oxidation impurity promotes the corrosion. B-3 and B-4 perform better than B-2, mainly by reducing some harmful mesophase.
Zirconium in the reducing environment, such as hydrochloric acid corrosion resistance is excellent, in the oxidizing environment, such as nitric acid corrosion resistance is very good, but in containing Fe3 +, Cu2 + oxidizing impurities medium corrosion, pitting and produce, because of the expensive, applies only to titanium and stainless steel shall not apply to the environment of high performance, such as the reaction of methanol carbonyl synthesis of acetic acid equipment, etc.
