Comparative Evaluation of Low Nickel and Nickel Free Lean Duplex Stainless Steels with 316L in a Variety of Corrosive Media

For many applications duplex stainless steels with their superior strength coupled with lower raw material cost have emerged as an attractive alternative to austenitic stainless steels. With emphasis on conservation of scarce resources like nickel and molybdenum there is continuing endeavour to develop essentially molybdenum free lean duplex stainless steels with low nickel content such as 2304 (23Cr–4Ni), 2202 (22Cr–2Ni), 2101 (21Cr–1.5Ni). This paper compares the corrosion behaviour of a low nickel duplex (21Cr–1.5Ni) and a nickel free duplex (21Cr–1.5Cu) with 316L stainless steel in several corrosive media. All the three alloys exhibit similar excellent corrosion resistance under boiling conditions in less aggressive organic acids such as 20 % acetic acid, 25 % lactic acid, 25 % citric acid. However, in stronger organic acids such as 5 % formic acid, 5 % oxalic acid, and mixture of formic and acetic acid, the duplex grades exhibit superior corrosion resistance. This edge over 316L continues on addition of chloride ions in these acids. In boiling 50 % nitric acid solution, the corrosion resistance of these nickel free and low nickel duplex is slightly better than 316L grade. Since 304L grade is generally used in nitric acid plants, tests were also conducted on 304L and these duplex grades were found to be more resistant. Similarly in 50 % phosphoric acid also, the duplex grades exhibit superior corrosion resistance compared to 316L grade. Alloying with nickel and molybdenum is known to give rise to significant improvement in corrosion resistance in this acid. However, even in the absence of these elements, the beneficial effect of higher chromium content is evident. Of all the inorganic acids, sulfuric acid is used in largest volume in the industries. Boiling tests in dilute 1 and 5 % H₂SO₄ indicate that nickel free copper bearing duplex is more resistant than low nickel duplex grade and vastly superior to 316L Thus nickel-free and low-nickel duplex stainless steels offer a very attractive combination of high corrosion resistance coupled with cost effectiveness in a wide variety of corrosive media.