Dear Dr. Gustavo Tontini,

steels can be subject to a form of stress-corrosion cracking (SCC) to which the misnomer of “caustic embrittlement” has been applied. This environmental cracking occurs well below the critical corrosion parameters. The limiting parameters of temperature and concentration for SCC. In particular, the corrosion resistance of Types 304L and 316L in 50% or 73% caustic solutions, in which both alloys suffered active corrosion as well as some pitting attack, is low. It seems probable that these products contained unknown amounts of chlorates, as produced.

The effect of chromium in the austenite phase was investigated in a set of experimental duplex alloys. It was found that corrosion resistance of the duplex steels was directly proportional to their chromium content. The range of chromium contents in the experimental heats was from 26.54% to 29.04% with corrosion rates in boiling 60% NaOH of 0.39 mm/y to 0.05 mm/y (15.6 mpy to 2 mpy), respectively. SCC tests in 50% NaOH at 137 °C (279 °F) showed that S32906 was immune to cracking in this environment. Nickel 200, however, had a lower corrosion rate than any of the duplex alloys in pure caustic solutions at any test temperature.

In most caustic applications Nickel 200 and Nickel 201 are very resistant to caustic SCC at all concentrations and temperatures up to about 290 °C (550 °F). Though Nickel 200 or 201 are usually most resistant, Alloy 400 and 600 are often used for higher strength. Nickel 600 alloy also exhibits a good resistance . It is favoured for heating coils, because of higher strength at temperature than the pure nickel alloys. It may cause a small amount of contamination from Cr (VI) ions, compared to the use of Nickel 200. It is subject to caustic cracking under severe conditions in prolonged exposure, and should be used in the stress-relieved condition. A variant of Alloy 600 is Alloy 601 (N06601) which is a solid-solution strengthened, higher-chromium (23% Cr) alloy. This alloy was developed primarily for high temperature applications but shows excellent corrosion resistance in up to 98% caustic because of the high nickel content.The other chromium-containing nickel-based alloy that is sometimes used in caustic soda is Alloy 690. The corrosion rate of Nicrofer® 6030 (N06690) in 70% NaOH at 170 °C (338 °F) was 0.03 mm/y (1.18 mpy). In the same tests, Alloy 400 and Alloy 33 (R20033) had the same corrosion rate (i.e. 0.03 mm/y), while Alloy 59 (N06059) corroded at 0.48 mm/y (18.9 mpy) and Alloy C-22 (N06022) at 0.51 mm/y (20.1 mpy).

For more details, please see the source at:

https://www.nickelinstitute.org/media/4341/nickelpub10019_causticsoda_20pg.pdf

Best regards, Pierluigi Traverso

SS 316 may not suitable above 100 deg C, however high nickel alloys with low carbon like Ni201 will suitable for ur application temperature.

Another option could be Titanium, at least 20 mol/L NaOH at 250 °C was possible, but I never tried higher temperatures or higher NaOH concentrations. The titanium vessel was quite stable for a lot of experiments, but the mechanical titanium stirrer had to be replaced a few times, due to minor corrosion.

Have a look at: https://www.nickelinstitute.org/media/4341/nickelpub10019_causticsoda_20pg.pdf

Dear Gustavo Tontini,

Sodium hydroxide, solid appears as a white solid. Corrosive to metals and tissue. It is used in chemical manufacturing, petroleum refining, cleaning compounds, drain cleaners. Generally, Sodium hydroxide will not react with iron or steel, in fact the alkaline conditions will not allow rust to grow; if you make the conditions oxidising, the black magnetic oxide will grow in its stead; hence the use of alkaline solutions of sodium nitrate for hot bluing baths is quite popular (if you worked in industry earlier you will know it).

Austenitic stainless steels as well as nickel alloys generally provide the most corrosion resistance because of their high amounts of chromium. This makes grade 304 an excellent choice when corrosion resistance is important. Both steel types are resistant to a wide range of concentration and temperature. Below 800C they can be considered resistant to any concentration of sodium hydroxide, up to the limit of solubility.

Stainless steels are more resistant to general corrosion compared with carbon steel; The concern for chloride contaminations is caustic when using stainless steel equipment is not well founded. Nickel 200 exhibits outstanding corrosion resistance to sodium hydroxide even at high concentrations is, Monel 400, Inconel 600, Hastelloy C-276, and other nickel alloys are used to perform the same. However, they can suffer Caustic Stress Corrosion Cracking (CSCC) at approximately 250oF/121oC. Alloy 200 (pure nickel) is considered to be immune to all but the most severe caustic environments, including molten caustic. A nickel-base alloy, Hastelloy N, has proved to be the most corrosion resistant.

Hastelloy C and D, monel can work up to 300 degreee centigrade with minimum corrosion rate 0.02 mm/year. But nickel and stainless steel can work up to 910C and 800C Respectively. Nicel 200 can work up to 130-260 0C.

So, Nickel and hastalloy can be used at higher temperature.

For autoclave corrosion testing, design pressures upto350 bar and temperatures upto 650°C are required. In that case you cannot use, it may suffer caustic SCC or caustic embrittlement too. it is not advisable.

Otherwise upto 300 degree centigrade it(hastelloy and nickel 200 and 600) can be easily used. Temperature limitations are there.

Just refer the chemical chart: https://www.jaspereng.com/wp-content/uploads/2016/12/SiemensChemical-ResistanceChart.pdf

Hope it is helpful for you.

Ashish

I think the Cr content is a big factor here in terms of corrosion resistance, pls determine their corresponding Cr content.