Performance of Nickel based super alloys in industrial chloride conditions

The commonly accepted application of commercially pure nickel is to handle the highly concentrated solutions. Nickel shows lower corrosion rates in hot caustic solutions than alloyed nickel as alloying elements like chromium and molybdenum dissolve commonly from alloy in hot caustic solutions. Nickel can also withstand cold reducing acids due to slow discharge of hydrogen on its surface. Hot reducing acids and oxidizing acids quickly attack pure nickel. 

The key application of Monel bars is in handling pure hydrofluoric acid. Although if oxidants like oxygen exist in hydrofluoric acid, Monel alloys may experience intergranular attack. Monel alloys are slightly more resistant to general corrosion than Nickel 200 in hot reducing and oxidizing acids like sulfuric acid and nitric acid. Ni-Mo alloys usually called as Hastelloy B type alloys, are made to withstand reducing HCl at all concentration and temperature limits. As costlier materials like Hastelloy alloys are also used in handling other corrosive reducing conditions like dilute sulfuric, acetic, formic and hydrofluoric acids. Alloy B2 has the minimum corrosion rate in boiling 10% sulfuric acid. Although Hastelloy alloys show poor performance in oxidizing acids, for instance, in hydrochloric acid contaminated with ferric ions.

There are several commercially available Ni-Cr-Mo alloys. They are derived from original C alloy, the advanced grade is Hastelloy C2000. Although the more common grade in industrial applications is Hastelloy C276. NiCrMo alloys are the most versatile nickel alloys as they comprise of molybdenum for protection against corrosion under reducing conditions and chromium that secures the component from corrosion in oxidizing conditions.

Hastelloy C276 has nominal corrosion rates in reducing and oxidizing conditions. One of the major applications of NiCrMo alloys is in the presence of hot chloride containing solutions. In these environments, most of steel grades receive crevice and pitting corrosion as well as stress corrosion cracking. Although NiCrMO alloys are extremely resistant it is not immune to chloride induced corrosion in major industrial applications.

Nickel based corrosion resistant alloys are NiCrFe alloys. They also contain smaller magnitudes or molybdenum and copper as in Incoloy 825. Nickel-Chromium-Iron alloys are usually less resistant to corrosion as compare to Nickel-Chromium-Molybdenum alloys, although they could be less costly and hence find a great range of industrial applications where the application of stainless steels is limited. The corrosion rate of Inconel 600 in sulfuric acid is higher than corrosion rate of Incoloy 825 as grade 825 contains nominal magnitudes of molybdenum and copper that are advantageous alloying elements for resistance to sulfuric acid. Incoloy 825 has nominal corrosion rate in nitric acid as it comprises of larger magnitudes of chromium. The common applications of Ni-Cr-Fe-Mo alloys like Hastelloy G30 is in the industrial development of phosphoric acid and in highly oxidizing conditions like nitric acid.

Cold processed Nickel 200 is resistant to cracking in NaCl and in chloride concentration of CaCl2 and MgCl2 at 121oC, 149oC, 177oC, 204oC and 232oC.