Desirable Duplex stainless steels in the hydrometallurgical industry
The use of duplex stainless steels in the hydrometallurgical industry
is explored by Sophia Ekman and Rachel Pettersson from Outokumpu Stainless in Sweden.
Duplex stainless steels are characterised by a two-phase microstructure comprising interleaved austenite and ferrite with approximately a 50/50 ratio (see image, below). The ferrite provides high strength and resistance to stress corrosion cracking, while the austenite contributes good ductility and general corrosion resistance. Duplex grades also have lower levels of nickel and molybdenum than their austenitic counterparts, resulting in a more stable price.
The workhorse duplex stainless steel grade is 2205, EN 1.4462, developed in the 1970s. Subsequent developments have extended the range to both high-performance superduplex steels and more economical lean duplex grades. A pioneer in the latter category is LDX 2101 (S32101), EN 1.4162, a low-alloyed, general-purpose stainless steel where a part of the nickel content is replaced by manganese and nitrogen, resulting in a leaner material. The relatively low alloying content makes LDX 2101 less prone to precipitation of intermetallic phases than other duplex stainless steels, and the high nitrogen content leads to good austenite reformation after welding. The chemical composition of some duplex stainless steels and their austenitic counterparts can be seen in the table, top right.
The minimum mechanical strength values for different steel grades according to ASTM A240 are shown in the table, bottom right. Generally, the duplex grades have approximately twice the mechanical strength as the austenitic grades, but a lower rupture elongation. The higher strength of the duplex grades can be used to reduce the gauge of sheets and plates used in items such as tanks, where design is based on the proof strength of the material. This results in large cost savings due to lower weight products.
The need to avoid or minimise corrosion is often the reason for selecting stainless steel over other materials. The most widely used indicator of its corrosion resistance is the pitting resistance equivalent (PRE), an empirical formula based on the contents of key alloying elements. A higher PRE value indicates a higher resistance and one frequently-used expression is that given below –
PRE = %Cr + 3.3.%Mo + 16.%N
Another common way of ranking different steel grades is by measuring the critical pitting temperature (CPT). The ASTM standard G150 specifies a test method in one molar NaCl at a constant applied potential and defines the critical temperature as the lowest temperature where stable pitting corrosion occurs under defined experimental conditions. The PRE and CPT for some steel grades are shown in the table on page 30 (top) and illustrate that for every austenitic grade, there is a duplex counterpart that has approximately the same resistance to pitting corrosion.
Duplex stainless steels have proved a successful choice for many types of tanks and vessels where the higher strength reduces gauge thicknesses and costs. The lean duplex grade LDX 2101 is suitable as long as the environmental corrosivity is moderate. In more aggressive conditions, higher alloyed duplex grades may fit the bill. A storage tank for a water-based liquid made from LDX 2101 is compared to its austenitic counterpart 304 in the image above, highlighting the estimated weight saving. If the tank is 20m high and 20m in diameter, 106.2t of 304 are needed to build the shell of the tank, compared to 73.5t of LDX 2101.
Another positive feature is that the duplex grades have a higher surface hardness compared to the austenitic varieties, which makes them more resistant to abrasion.
Making the switch
Stainless steel has found extensive use within the hydrometallurgical industry. Its first use as permanent cathode plates for copper refining was in Townsville, Australia, in the late 1970s. The ISA process, developed by a team at Mount Isa Mines Ltd using permanent stainless steel cathode technology, led the world in stainless steel technology in tankhouses, and Outokumpu Stainless and Xstrata Technology (previously MIM) have worked together since 1994 to enhance the austenitic grade 316L for copper refining. More recently, joint development work has focused on the use of LDX 2101 as an alternative to 316L grade (see image right). Initially driven as a lower cost option, LDX 2101 has delivered additional benefits in terms of mechanical strength and durability in what can be a harsh physical, as well as chemical, environment.
To switch from 316L to LDX 2101, an extensive laboratory programme of corrosion testing was carried out in environments simulating the electrowinning process used in copper refining. It was particularly important to simulate the conditions during maintenance stops, where the cathode plates are exposed to the electrolyte without the current that effectively provides cathodic protection. Some 30-day immersion tests were performed on stainless steel coupons, including variants with artificial crevice formers and semi-immersed specimens to investigate waterline attack. The environments are specified in the table below (Solutions for corrosion tests), and the experimental set-up shown in the image bottom, left. After 30 days of immersion, none of the tested samples of LDX 2101 or 316L suffered from any type of corrosion.
This enabled Xstrata Technology to redefine cathode plate parameters, using thinner material and increasing the operating range in stripping machines. The work also led to a patent covering the use of LDX 2101 in tankhouses. Xstrata Technology has undertaken extensive in-plant trials, initially at its own refinery in Townsville and in third party plants in Australia and overseas. Several years on, LDX 2101 continues to perform at least as well as 316L in full operations. The success has now led to a full-scale trial applying LDX 2101, in the Tenke Fungurume tankhouse in the Democratic Republic of Congo, Africa.
Dr Rachel Pettersson and Sophia Ekman