Corrosion behaviour of Nickel based super alloys in Alkalies

Corrosion rates in chemical processing media often reduce with increase in pH. In alkaline solutions, the hydrogen ion is present in nominal concentrations. Although various metals pass through a minimum corrosion rate at some pH, often basic in nature and then increased corrosion with increase in pH. Uusually corrosion by alkalies result into pitting and other localized corrosion they develop cathodic layers and corrosion occurred at susceptive anodic regions. Austenitic stainless steels and other low nickel alloys may experience stress corrosion cracking or general corrosion in hot concentrated caustic media.

Resistance to corrosion by usually enhances with increase in Nickel concentration. It is found that Nickel 200 and high nickel alloys are successful in handling variety of alkalies. The most commonly used nickel alloys in alkali process media are Nickel 200, Monel 400 and Inconel 600 and Inconel 625.

Excellent corrosion properties of Nickel 200 is its resistance to caustic soda and other alkalis. Nickel 200 remains intact anhydrous ammonia or ammonium hydroxide in concentrations of 1%. Stronger concentrations can result into quick corrosion in availability of dissolved oxygen.

Nickel 200 shows outstanding resistance to all concentrations of caustic soda at temperatures up to melting point. Less than 50% concentrations, corrosion rates are nominal less than 0.1 mpy even at the boiling temperatures. With increase in temperature and concentration, corrosion rates slowly increases.

The major factor influencing the performance of nickel in highly concentrated caustic soda is the type of layer developed during exposure to caustic. In many conditions, a security layer of black nickel oxide is developed that results in a remarkable reduction in corrosion rates over long exposure. For instance, samples of Nickel 201 were subjected to a caustic solution prepared by the inclusion of 500 cc of water to kg of technical flake caustic after heating in a Nickel 201 pot at a temperature of 790oF to 830oF. The corrosion rate in the initial 24 hours averaged 21 mpy. The samples were put back into analysis without eliminating their oxide layer. After seven days, the overall corrosion rates had decreased to 2.8 mpy.

The availability of chlorates in caustic soda solutions increases the corrosion rates of Nickel 200 significantly. Considering the deleterious effects, it is recommended to eliminate chlorates completely prior to evaporation in the high temperature limit in the presence of nickel.

Considering the composition of Monel 400 wire, its high nickel concentration makes it similarly resistant as of Nickel 200 to caustic soda in the most of its concentration range, however receives stress corrosion cracking in strong alkalis at high temperatures. The corrosion rates of Monel 400 are higher in concentrated caustic soda and caustic potash at high temperatures. Monel 400 prevents corrosion in anhydrous ammonia and to ammonium hydroxide solutions of about 3% concentrations in the absence of dissolved oxygen.

Incoloy 800 and 825 offer great resistance to alkaline solutions with a corrosion rate of 0.5 mpy in boiling 50% sodium hydroxide, they are less resistant as compare to Nickel 200 and are rarely used in alkaline conditions.