Updated Corrosion Performance of Alternative Stainless Steels

For many years, high quality, long service life and durability have been associated with austenitic stainless steels such as 304 and 316 grades that account for more than 60% of the Australasian market. The popularity of these steels is due to a combination of good fabrication properties, reasonable level of corrosion resistance and satisfactory experience in many different applications. In addition to about 18% chromium, these steels also contain about 8% nickel that results in an austenitic microstructure.

In recent years, as it has also been the case in the past, the price of nickel has increased dramatically, directly affecting the cost of 304 and 316 grades. Historically, each peak in nickel prices has triggered research into low-nickel alternatives that ideally would be cheaper yet still offer qualities comparable to the established austenitic grades.

Several low-nickel alternatives to 304 and 316 can be found among ferritic stainless steels such as 430, 404GP and 445M2, which contain essentially no nickel, or austenitic stainless steels such as 200 series in which part of the nickel content has been replaced by nitrogen stabilized in manganese. The same principle is at work in lean duplex stainless steel LDX 2101 where nickel content is reduced to about 1.5%.

The majority of New Zealand stainless steel applications can be found in dairy and wine industry, food production plants and machinery as well as architectural. Many stainless steel products are located along the coastal line and therefore exposed to the marine environment. In this case, the grade selection may be governed by the need to prevent surface discoloration known as tea staining and general/local corrosion from the sea salt spray.

Most of the stainless steel is subject to welding as a key fabrication process. Welding changes surface profile and leads to segregation of some alloying elements such as molybdenum, potentially reducing corrosion resistance of welded zones. The effect of welding cannot be verified without considering previous experiences with similar products, simulated testing or testing under real conditions.

In order to evaluate the performance of different alternative stainless steel grades in the coastal environment, HERA in co-operation with the NZ Stainless Steel Development Association (NZSSDA) industry partners and overseas experts, conducted a long-term atmospheric exposure test on several grades of stainless steel in both the welded and unwelded conditions. First results of this study were presented at the 18th International Corrosion Congress 2011. To download a copy of the conference paper, please click HERE.