Atmospheric Corrosion

Corrosion of alloy 400 is negligible in all types of atmospheres, although a thin gray-green patina will develop. In sulfurous atmospheres, a brown patina may be produced. Because of its low corrosion rate and pleasing patina, alloy 400 has been used for architectural service, such as roofs, gutters, and flashings, and for outdoor sculpture.

Corrosion in Waters

Freshwater

Monel 400 has very low corrosion rates and are used in freshwater systems for valve seats and other fittings.

In steam/hot water systems

Monel 400 may occur appreciable corrosion if noncondensables (CO2 and air) in the steam exist in certain proportions.

Seawater

Alloy 400 and nickel-base alloys containing chromium and iron are very resistant to flowing seawater, but in stagnant or very low velocity seawater pitting or crevice corrosion can occur, especially under fouling organisms or other deposits. In moderate- and high-velocity seawater or brackish water, alloy 400 is frequently used for pump and valve trim and transfer piping. It has excellent resistance to cavitation erosion and exhibits corrosion rates less than 0.025 mm/year (1 mil/yr). Alloy 400 sheathing also provides economical seawater splash zone protection to steel offshore oil and gas platforms, pilings, and other structures. Although pitting can occur in alloy 400 under stagnant conditions, such pitting tends to slow down after fairly rapid initial attack and rarely exceeds 1.3 mm (50 mils) in depth.

Corrosion in Sulfuric Acid

Monel 400 is used for handling sulfuric acid solutions under reducing conditions.

Corrosion in Hydrochloric Acid

Alloy 400 have room-temperature corrosion rates below 0.25 mm/yr (10 mils/yr) in air-free HCl at concentrations up to 10%.

Organic Acids

Monel 400 is generally not as resistant as the other nickel-based alloys to most organic acids. In air-free acetic acid at room temperature, rates are less than 4 mpy (0.10 mm/a) for all concentrations. Aeration and temperature tend to increase rates, however, and the gradual accretion of cupric ions as corrosion products aggravates attack in an autocatalytic manner. In acetic acid of more than 50% concentration, the rates begin to fall and in a test in glacial acid at 230°F (110°C) the rate was only 13 mpy (0.33 mm/a). Monel 400 has been used for distillation of fatty acids at temperatures up to 500°F (260°C).

Corrosion in Hydrofluoric Acid

Hydrofluoric acid is commercially available in concentrations ranging from 30 to 70% and as anhydrous HF. Plain carbon steel can withstand the anhydrous acid and is extensively used in rail cars carrying anhydrous HF. Of the nickel-base alloys, alloy 400 has been the most extensively examined. The alloy possesses good corrosion resistance in all concentrations of HF up to a temperature of about 120 °C (250 °F).

Corrosion in Alkalies

Because of its high nickel content, Monel 400 is nearly as resistant as Nickel 200 to caustic soda through most of the concentration range, although subject to stress-corrosion cracking in strong alkalis at elevated temperatures. The corrosion rates of Monel 400 are higher in highly concentrated caustic soda and caustic potash at high temperatures. Monel 400 is resistant to anhydrous ammonia and to ammonium hydroxide solutions of up to 3% concentration in the absence of dissolved oxygen.