Which alloy material is most resistant to caustic soda?

Caustic alkali generally refers to caustic soda and caustic potassium, namely sodium hydroxide and potassium hydroxide. Caustic embrittlement or caustic cracking occurs when the alloy material cracks under the action of tensile stress and corrosion medium in alkaline solution. In alkaline solutions, the concentration of hydrogen ions is usually low and the corrosion rate in chemical media usually decreases with the increase of PH value. At certain PH values, the corrosion rate of some metals hovers around the lowest value, while the PH value increases continuously as the corrosion degree increases.

Caustic soda has a wide range of applications. It can be used in papermaking, textile printing and dyeing, alumina, daily chemical, pharmaceutical, water treatment, steel and other industries. Caustic corrosion occurs when metals come into contact with caustic soda. Caustic corrosion can lead to pitting and other localized corrosion, as they tend to form cathode films that concentrate corrosion in susceptible anode areas. Austenitic stainless steels and other low Nickel materials may experience stress corrosion cracking or general corrosion in hot caustic soda.

Nickel and its alloys offer excellent alkali corrosion resistance for high temperature and concentration environments. Generally, the corrosion resistance of caustic soda usually increases with the increase of nickel content. In alkaline media, the most commonly used nickel alloys are Nickel 200/201, Monel 400, Nickel 600 and 625.

Plant tests in 23% caustic soda in tank receiving liquor from evaporator. average temperature,104℃
Material	Corrosion rate，mpy(mm/a)
Nickel 200	0.16(0.004)
Monel alloy 400	0.20(0.005)
Inconel alloy 600	0.17(0.004)

Factory and laboratory test results, some of which are shown in the table below, show that nickel 200/201 and high nickel alloys are very satisfactory for the treatment of these materials

Laboratory corrosion tests in evaporation of caustic soda from 73-96 %, with and without chlorate
Materials	Corrosion rate, mpy(mm/a)
	Without chlorate with 0.30%	Chlorate
Nickel 200	1.5(0.038)	260(6.60)
Inconel alloy 600	2.2(0.056)	380(9.65)

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