Why Nickel-Chromium-Molybdenum (NiCrMo) alloys are preferred over stainless steels?

The major development of chemical products and processes demand materials with enhanced mechanical characteristics, metallurgical stability and supreme corrosion resistance in severe conditions. Over the time, highly corrosion resistant alloys made of nickel, chromium and molybdenum have been proven to prevent corrosion in vigorous environments in the various industries such as chemical, pollution control, marine, oil and gas units.

Hastelloy alloy C-276 and Hastelloy C-22 offer excellent resistance to general and localized corrosion. The service of the material in preventing acid corrosion, mixed acid and localized corrosion in acid and marine media. These alloys have supreme metallurgical stability and can be easily formed and welded.

Plant operators are widely using Inconel-Weld filler metal as filler metal for a great range of nickel alloy based compositions. The versatility of the welded product and high alloying concentration has improved the use of the material in extremely vigorous environments.

Widely corrosion resistant materials like Inconel alloy 625 and Hastelloy C276 wire offer supreme corrosion resistance for use in chemical, power production, pollution control, marine and major applications. The reason for the use of these alloys is the development of new chemicals and improved processes. The latest challenge has been met by Hastelloy C-2000 that is increasingly used to prevent corrosion and safeguard product purity in media, that was not feasible for stainless steels or other materials. These materials can offer several benefits such as enhanced life cycle cost service, enhanced reliability, nominal maintenance and decreased downtime costs.

Significant inclusions of nickel, chromium, molybdenum and other elements are required for several applications that offer supreme corrosion resistance. Chromium offers resistance to oxidizing media while molybdenum enhances resistance to reducing media. Chromium and molybdenum prevent localized attack. Inclusion of tungsten also increases to localized corrosion. However nickel prevents attack in caustic and mild reducing media, its main advantage in alloys consisting of chromium and molybdenum is to retain stable austenitic single-phase structure. It is significant in offering adequate corrosion resistance in an alloy to make it economically manufactured and fabricated.

Inconel 625, Hastelloy C276, Hastelloy C22 and Hastelloy C2000 offer enhanced corrosion resistance with better flexibility and versatility in preventing attack in diverse conditions. The Nickel – chromium – molybdenum alloys are used in corrodents and fluctuating environments including oxidizing and reducing media. While comparing alloys for a specific service environment it is often recommended to test the alloy in the service media or in a lab test that most closely approximates it.

Localized corrosion attack that is most often  occurring failure mechanisms of stainless steels and Ni-Cr-Mo alloys, it is less predictable as compare to general corrosion.  

The high alloyed materials such as Hastelloy C22 and Hastelloy C2000 are adequately alloyed to prevent pitting and crevice corrosion to the test temperature of 85oC. The highly aggressive oxidizing chloride conditions differentiate the influence of chemical composition on localized corrosion resistance of the advanced nickel alloys. At 125oC, Hastelloy grades C22, C276 and C2000 receive some corrosion however these alloys are fully resistant to stress corrosion cracking in waters including chlorides.