INCOLOY alloy 25-6MO (UNS N08926) is a fully austenitic alloy containing 6% molybdenum and 0.2% nitrogen, which offers excellent corrosion resistance in a wide variety of aggressive, aqueous environments. It replaces conventional, austenitic stainless steels (UNS S31600 and S31700), where their capabilities are pushed to their performance limits.

One of the outstanding attributes of alloy 25-6MO is its resistance to environments containing chlorides or other halides. Alloy 25-6MO offers excellent resistance to pitting and crevice corrosion. Performance in resisting these types of corrosion is often measured using Critical Pitting Temperature (CPT), Critical Crevice Temperature (CCT), and Pitting Resistance Equivalent Number (PREN = %Cr + (3.3 x % Mo) + (30 x %N)). Corrosion properties of stainless steels are basically dependent on their chemical compositions. As a general rule, the higher the PREN, the better the resistance to pitting corrosion. However, alloys having similar values may differ considerably in actual service. Those alloys with values greater than 38 on the PREN scale offer more corrosion resistance than austenitic stainless steels such as 316 and 317 with a PREN £ 31. Alloy 25-6MO, with its PREN of 47, offers a cost-effective means to avoid aggressive chloride attack. Resistance to environments containing chloride or other halides, makes the alloy highly suitable to environments such as brackish water, seawater, caustic chlorides and pulp mill bleaching solutions.

The nickel and chromium contents of alloy 25-6MO make it resistant to a wide range of corrosive environments. The alloy is especially resistant to non-oxidizing acids such as sulfuric and phosphoric. The high molybdenum and nitrogen content provide resistance to pitting and crevice corrosion, while copper enhances resistance to sulfuric acid.

Alloy 25-6MO exhibits excellent resistance to sulfide stress cracking (hydrogen embrittlement) and chloride stress corrosion cracking in sour brine oil field environments. The alloy is included in the NACE International Standard Materials Requirement MR0175 for "Sulfide Stress Cracking Resistant Metallic Materials for Oilfield Equipment".

The alloy also exhibits excellent resistance to Microbiologically Influenced Corrosion (MIC). Because of resistance to MIC, alloy 25-6MO and other 6% molybdenum austenitic stainless steels are being used in service water piping systems of power plants.

Applications include its use in chemical and food processing, pulp and paper bleaching systems, marine and offshore platform equipment, desalination units, salt plant evaporators, air pollution control systems, as well as condenser tubing, service water piping and feedwater heaters in the power industry. Alloy 25-6MO is

approved under the Boiler and Pressure Vessel Code of the American Society of Mechanical Engineers (ASME). The ASME Code Case 2120 (Ni-Fe-Cr-Mo-Cu-low Carbon alloy N08926) defines the chemical composition, maximum allowable stress values, and the mechanical property requirement for the alloy.

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