Application of an austenitic nitrogen-alloyed 6MO stainless steel for seawater desalination (RO) and waste water treatment

Abstract

In reverse osmosis plants, water from different sources and a variety of compositions has to be converted into fresh water. Water which contains chlorides and certain contaminants can be highly aggressive, particularly at elevated temperatures. Consequently, the piping material to be used has to show good corrosion resistance. In addition, when the R.O. process is run under increased pressure, a material with suitably improved mechanical properties is called for. The above requirements under specific R.O. conditions are met by a 6% Mo-containing austenitic stainless steel called Cronifer 1925 hMo - UNS N08926. Because of its high pitting resistance equivalent PRE N (% Cr + 3.3 × % Mo + 30 × % N) ⪖ 45 Cronifer 1925 hMo exhibits excellent resistance, notably in neutral and acidic chloride solutions. In addition, this special stainless steel possesses high resistance to general corrosion in a variety of process media. Specific investigations were carried out for the different types of components used in reverse osmosis installations, including seamless pipe, forged parts, plate and induction pipe bends. Furthermore, both longitudinal (PAW, SAW) and girth welds (GTAW, MMAW) were carried out and tested. The paper discussed the corrosion behaviour, the mechanical and metallurgical characteristics, and the high and low nickel versions of this alloy and compares them with other high-alloy stainless steels. In addition to Cronifer 1925 hMo*, a number of further materials suitable for use under various conditions in waste water treatment plants are briefly presented. These are Nicrofer 3127 hMo* - alloy 31 and Nicrofer 5923 hMo* - alloy 59.