How do Nickel and Inconel alloys perform in caustic media
In alkaline solutions, hydrogen ion is present in nominal concentrations. Usually, the alkali corrosion causes pitting and other localized attack due to the production of cathodic layers and the corrosion is more severe in anodic regions.
Austenitic stainless steels and other low nickel materials may experience stress corrosion cracking or general corrosion in hot concentrated caustic. Increase in nickel concentration in the functional material generally enhances the alkali corrosion resistance. It is found that high nickel alloys such as Nickel 200, Inconel 600 and 625 offer good performance in alkali process media.
Excellent corrosion resistance by Nickel 200 to caustic soda and other alkalies makes it a suitable material for this application. Nickel 200 is resistant to anhydrous ammonia in concentrations about 1%. It shows suitable resistance to all concentrations of caustic soda up to melting temperature. It is due to the layer developed during exposure to the caustic. In various conditions, a security layer of nickel oxide is developed that helps in significantly reducing the corrosion rate for long duration. For example Nickel 201 is exposed to caustic solution of 500 cc water to 2kg caustic flake by heating in Nickel 201 up to 790oF or 412oC. Initially the corrosion rate was 21mpy and thereafter it reaches to 2.8mpy.
Use of Inconel 600 in high temperature caustic applications is more recommended than Nickel 201. Although it can be susceptible to stress corrosion cracking in some alkali conditions. Therefore alloy 600 components should be stress relieved before use and application stresses should be kept minimum. The alloy prevents corrosion in sodium hydroxide at boiling temperatures in concentrations about 80%.
As using Nickel 200, the presence of chlorates in caustic soda, the corrosion increases. The presence of chlorates in caustic soda solutions enhances the corrosion rates of alloy 200 significantly. Considering these negative effects, it is recommended to eliminate chlorates fully as much as possible before the evaporation in the high temperature while using nickel.
It is also found that in the presence of oxidizable sulfur compounds the corrosiveness of caustic for nickel. This effect is mainly found to occur with sulfides like hydrogen sulfide or sodium sulfide and to a lesser level with slightly oxidized compounds like sulfites and thiosulfites. The influence of oxidizable sulfur compounds can be prevented by using caustic solutions comprising adequate sodium peroxide for their oxidation to sulfates. large concentration of peroxide is not found to affect the corrosiveness of caustic soda to nickel.
In other conditions, when nickel is required to be used at temperatures over 600oF or 315oC, it is recommended to use Nickel 201.
Due to the presence of chromium, Inconel 600 is fully resistant to corrosion by solution of ammonia over a full range of concentrations and temperatures.
Inconel 625 and C276
Although they provide supreme resistance to corrosion, these alloys are not usually used in pure caustic conditions. They provide some benefits in halide containing conditions.