Caustic soda (NaOH) is one of the most important chemical feed stocks, with a total annual production of 106t. NaOH is used in organic chemistry, in the production of aluminium, in the paper industry, in the food processing industry, in the manufacture of detergents, etc. Caustic soda is a co-product in the production of chlorine, 97% of which takes place by the electrolysis of sodium chloride.
Caustic soda has an aggressive impact on most metallic materials, especially at high temperatures and concentrations. It has been known for a long time, however, that nickel exhibits excellent corrosion resistance to caustic soda at all concentrations and temperatures, as Figure 1 shows. In addition, except at very high concentrations and temperatures, nickel is immune to caustic-induced stress-corrosion cracking. The nickel standard grades alloy 200 (EN 2.4066/UNS N02200) and alloy 201 (EN 2.4068/UNS N02201) are therefore used at these stages of caustic soda production, which require the highest corrosion resistance. The cathodes in the electrolysis cell used in the membrane process are made of nickel sheets as well. The downstream units for concentrating the liquor are also made of nickel. They operate according to the multi-stage evaporation principle mostly with falling film evaporators. In these units nickel is used in the form of tubes or tube sheets for the pre-evaporation heat exchangers, as sheets or clad plates for the pre-evaporation units, and in the pipes for transporting the caustic soda solution. Depending on the flow rate, the caustic soda crystals (supersaturated solution) can cause erosion on the heat exchanger tubes, which makes it necessary to replace them after an operating period of 2–5 years. The falling-film evaporator process is used to produce highly concentrated, anhydrous caustic soda. In the falling-film process developed by Bertrams, molten salt at a temperature of about 400 °C is used as the heating medium. Here tubes made of low carbon nickel alloy 201 (EN 2.4068/UNS N02201) should be used because at temperatures higher than about 315 °C (600 °F) the higher carbon content of the standard nickel grade alloy 200 (EN 2.4066/UNS N02200) can lead to graphite precipitation at the grain boundaries.
Nickel is the preferred material of construction for caustic soda evaporators where the austenitic steels cannot be used. In the presence of impurities such as chlorates or sulfur compounds – or when higher strengths are required – chromium-containing materials such as alloy 600 L (EN 2.4817/UNS N06600) are used in some cases. Also of great interest for caustic environments is the high chromium containing alloy 33 (EN 1.4591/UNS R20033). If these materials are to be used, it must be ensured that the operating conditions are not likely to cause stress-corrosion cracking.
Alloy 33 (EN 1.4591/UNS R20033) exhibits excellent corrosion resistance in 25 and 50% NaOH up to boiling point and in 70% NaOH at 170 °C. This alloy also showed excellent performance in field tests in a plant exposed to caustic soda from the diaphragm process.39 Figure 21 shows some results regarding the concentration of this diaphragm caustic liquor, which was contaminated with chlorides and chlorates. Up to a concentration of 45% NaOH, the materials alloy 33 (EN 1.4591/UNS R20033) and nickel alloy 201 (EN 2.4068/UNS N2201) show a comparable outstanding resistance. With increasing temperature and concentration alloy 33 becomes even more resistant than nickel. Thus, as a result of its high chromium content alloy 33 seems to be advantageous to handle caustic solutions with chlorides and hypochlorite from the diaphragm or mercury cell process.
Post time: Dec-21-2022