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Duplex stainless steels in the hydrometallurgical industry page 2

Corrosion testing of stainless steel for metal leaching applications page 7

Special issue: Materials for hydrometallurgicalapplications

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Duplex stainless steels in the hydrometallurgical industry

Sophia Ekman, Outokumpu Stainless AB, Sweden

Rachel Pettersson, Outokumpu Stainless AB, Sweden

Keywords: Duplex stainless steels, copper refining cathode plates, corrosion testing

Duplex stainless steels are characterised by a two-phase microstructure comprising interleaved austenite and ferrite with approximately a 50/50 ratio, see Figure 1. The duplex microstructure combines advantages of both phases: the ferrite provides high strength and resistance to stress corrosion cracking (SCC), while the austenite contributes good ductility and general corrosion resistance. The duplex grades have also lower levels of nickel and molybdenum than their austenitic counterparts, resulting in a more stable price level.

The workhorse duplex stainless steel grade is 2205, EN 1.4462, which was developed in the 1970’s. Subsequent developments have extended the range of duplex grades to both the high-performance superduplex steels and the more economical lean duplex grades. A pioneer in the latter category is LDX 2101®, EN 1.4162, a low-alloyed, general-purpose stainless steel, where a part of the nickel content has been replaced by manganese and nitrogen, resulting in a leaner alloy [1]. 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 lead to good austenite reformation after welding. The chemical com-position of some duplex stainless steels, and their austenitic counterparts can be seen in Table 1.

Grade EN ASTM/UNS C N Cr Ni Mo Others Microstructure

LDX 2101® 1.4162 S32101 0.03 0.22 21.5 1.5 0.3 5 Mn Duplex

4307 1.4307 304L 0.02 – 18.1 8.1 – – Austenitic

2304 1.4362 S32304 0.02 0.10 23 4.8 0.3 – Duplex

4404 1.4404 316L 0.02 – 17.2 10.1 2.1 – Austenitic

2205 1.4462 S32205 0.02 0.17 22 5.7 3.1 – Duplex

904L 1.4539 N08904 0.01 – 20 25 4.3 1.5 Cu Austenitic

Typical chemical composition (weight percent) Table 1

Fig. 1 Typical transverse section microstructure of a duplex stainless steel, showing continuous ferritic matrix, etched dark, and austenite bands running parallel to the rolling direction of the plate.

100 µm

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The minimum mechanical strength values for some different steel grades according to ASTM A 240 are shown in Table 2. Generally, the duplex grades have approximately twice the mechanical strength compared to the austenitic grades, but a lower rupture elongation. The higher strength of the duplex grades can be utilized to reduce the gauge of the sheets and plates used for construction of a number of items, such as tanks, where design is based on the proof strength of the material. This results in large cost savings since using duplex reduces the weight of the item.

Mechanical properties (minimum values) at 20°C for some steel

grades according to ASTM A 240 Table 2

Proof Tensile strength, strength, ElongationGrade EN Rp0.2 (MPa) Rm (MPa) A2” (%)

LDX 2101® 1.4162 450 650 30

4307 1.4307 170 485 40

2304 1.4362 400 600 25

4404 1.4404 170 485 40

2205 1.4462 450 655 25

904L 1.4539 220 490 35

Corrosion propertiesThe need to avoid or minimise corrosion is often the reason for using stainless steel in preference to other material alternatives. Uniform corrosion, localised corrosion, atmospheric corrosion and stress corrosion cracking can be issues in different applications, but the most widely used indicator of the corrosion resistance of different stainless grades is the pitting resistance equivalent (PRE). This is an empirical formula which predict the resistance of a grade to pitting corrosion based on the contents of key alloying elements. A higher PRE value indicates a higher resistance to pitting corrosion and one frequently-used expression is:

PRE = %Cr + 3.3%Mo + 16%N

Another common way of ranking the different steel grades is by measuring the critical pitting temperature (CPT). The ASTM standard G150 specifies a test method in 1M 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 different steel grades are shown in Table 3 and illustrate the point that for every austenitic grade there is a duplex counterpart having the approximately same resistance to pitting corrosion.

Grade EN PRE CPT (°C)

LDX 2101® 1.4162 26 17 ± 3

4307 1.4307 18 <10

2304 1.4362 26 22 ± 3

4404 1.4404 24 20 ± 2

2205 1.4462 35 52 ± 3

904L 1.4539 34 62 ± 3

Typical values of critical pitting temperature and pitting

resistance equivalent values for some stainless steels Table 3

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The duplex grades show a very good resistance to stress corrosion cracking (SCC), as a direct result of the two phase microstructure, and this is only matched by the high alloyed austenitic stainless steels such as 904L, 254 SMO®, and 1.4565.

Storage tanksDuplex stainless steels have proved a successful choice for many types of tanks and vessels, where the higher strength can be utilised to reduce gauge thicknesses and thus costs. The lean duplex grade LDX 2101® is suitable as long as the corrosivity of the environment is moderate, in more aggressive conditions higher alloyed duplex grades may fit the bill. A design example for construction of a storage tank for a water-based liquid in LDX 2101®, compared to its austenitic counterpart 4301, is shown in Figure 2 and highlights the esti-mated weight saving. If the tank is 20 m high and 20 m in diameter 115.2 tonnes of 4301 are needed to build the shell of the tank compared to 73.5 tonnes of LDX 2101®.

Fig. 2 Example of a design for a storage tank in LDX 2101® compared to 304 [2]

Fig. 3 Use of duplex grades for storage tanks: marble slurry tanks in LDX 2101® (left) and gold leaching tanks in 2205 (right)

Another positive feature of the duplex grades is that they have a higher surface hardness compared to the austenitic grades, which makes them more resistant to abrasion. In Figure 3 two examples of storage tanks in duplex stainless steels are given.

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Electrowinning and electrorefining Stainless steel has found extensive use within the hydrometallurgical industry [3, 4]. The first use as permanent cathode plates for copper refining was in Townsville Australia in the late 1970’s. The ISA process 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 4404 for copper refining. More recently, joint development work has focussed on the use of LDX 2101® as an alternative to 4404 [5], see Figure 4. Initially driven as a lower cost alternative, the use of 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.

Fig. 4 Cathode plates in LDX 2101® (left) and 4404 (right). Courtesy of Xstrata Technology

In order to be able to make the step from 4404 to LDX 2101® an extensive laboratory programme of corrosion testing was first 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 Table 4, and the experimental set-up shown in Figure 5. After 30 days of immersion, none of the tested samples of LDX 2101® or 4404 suffered from any type of corrosion.

The switch to the duplex LDX 2101® with superior mechanical strength compared to 4404, allowed Xstrata Technology to redefine cathode plate parameters, using thinner material and increasing the operating range in stripping machines. The joint development work also lead 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 Australia, and in third party plants in Australia and overseas [5]. Several years on from the first tests, LDX 2101® continues to perform at least as well as 4404 in full operations. The success of this development project has now led to a full scale project utilising LDX 2101®, the Tenke Fungurume tankhouse in DRC Africa.

Test number 1 2 3 4

Temperature (°C) 45 45 45 45

Chlorides (ppm) 20 50 80 110

Sulphuric acid (g.l-1) 180 180 180 180

Copper sulphate (g.l-1) 113 113 113 113

Solutions for corrosion tests Table 4

Fig. 5 Experimental set-up

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References[1] P. Johansson, M Liljas, 4th European Stainless Steel – Science and Market

Congress, 10–13 June 2002, Paris, Vol. 2, 153[2] Storage tank shell thickness calculation, available at Outokumpu

website : http://www.outokumpu.com/applications/documents/start.asp[3] Alfonsson, E., Coates, G. and J. Olsson. Stainless steels for the hydrometallurgical

industry. Proc. 14th ICC congress. Cape Town. South Africa, 1999[4] S. Ekman, A. Bergquist: Suitable stainless steel grades for hydrometallurgical

applications, Hydrometallurgy 2008 conference, Phoenix, AZ USA, 17–21 August 2008

[5] K.L. Eastwood and G.W. Whebell, Developments in permanent stainless steel cathodes within the copper industry, Proc. Cu2007 Volume V, Copper Electrorefining and Electrowinning, p.p. 35– 46, Toronto, Canada, 2007.

Reproduced with permission from IOM Communications Ltd for the Instituteof Materials, Minerals and Mining. This article was originally published with the title “Desirable duplex” in Materials World, 17 (9), 2009, pp. 28–30.

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Fig. 1 Iso-corrosion diagram in pure sulphuric acid for austenitic stainless steels

Fig. 2 Iso-corrosion diagram in pure sulphuric acid for duplex stainless steels

Corrosion testing of stainless steel for metal leaching applications

Sophia Ekman, Outokumpu Stainless AB, Sweden

Arne Bergquist, Outokumpu Stainless AB, Sweden

Elisabeth Torsner, Outokumpu Stainless Inc, USA

Keywords: Mining, corrosion testing, sulphuric acid, sodium hydroxide, pitting corrosion, crevice corrosion, field testing

IntroductionThe earth’s resources are finite. High-grade traditional ores are rapidly approaching depletion and the extractive metallurgical industry is the target of ever-tougher environmental legislation. In the light of these developments, hydrometallurgy is fast emerging as a viable process for the recovery of a variety of metals. It is less energy-consuming, with far lower emissions than conventional pyrometallurgical processes. To select a cost-efficient construction material for each step of a hydrometallurgical operation is difficult, as conditions are hostile and there are several competing corrosion mechanisms.

Sulphuric acid is the most commonly used acid in hydrometallurgical processes for treating zinc, nickel and copper containing ores, as it is cheap and can be produced on site as a by-product at facilities that treat sulfide ores. Alkaline media, such as sodium hydroxide, are also used for the production of aluminum through leaching of bauxite via the Bayer process. These environments place high demands on the materials used for construction. Stainless steels generally show good resistance in both oxidizing acids as well as alkaline media.

Sulphuric acid has dualistic properties. It is reducing and very corrosive to stainless steels at concentrations between 40–90%, above 90% it becomes oxidising. Uniform corrosion of stainless steel in sulphuric acid increases with acid concentration and temperature in

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pure acid conditions as shown in Figure 1 and 2. The corrosion causes thinning of a tank or pipe wall thickness until the remaining section is unable to carry the mechanical load.

As early as 1971 it was established that oxidizing ions from dissolved metals as Cu2+, Fe2+ and Fe3+ have a beneficial influence on the uniform corrosion rate of stainless steel in sulphuric acid [1]. Electrochemical investigations show that oxidizing ions push the corrosion potential into the passive range and thus protect the surface from uniform corrosion. It is therefore possible to use standard grades at acid concentrations and temperatures which otherwise would be far too aggressive, see Figure 3. [2, 3]

The chloride content can be very high in hydrometallurgical environments. Chlorides may originate from the ore itself, ground-water contaminated by geological breakdown of the ore or neighbouring ore bodies, or even by using seawater as make-up water. The chloride content can have three negative effects. The first is to increase the uniform corrosion rate above that in pure acids, see Figure 3. This effect can be dramatic even at low chloride contents [3]. Another effect is to initiate localized corrosion, such as pitting and crevice corrosion. The third consequence is an increase in the risk for stress corrosion cracking. All types of localized corrosion may

Fig. 4 Examples of chloride-induced localized corrosion, pitting corrosion (left), crevice corrosion (middle) and stress corrosion cracking (right)

Fig. 5 Iso-corrosion diagram in pure sodium hydroxide for some stainless steel grades

Fig. 3 Iso-corrosion diagram for 4404 in sulphuric acid with additions of chlorides, cupric, ferric and ferrous ions

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cause penetration or cracking of a tank or pipe wall and subsequent risk of leakage of solution to the outside, Figure 4.

The presence of metal ions and chloride ions in hydrometallurgical processes complicates the prediction of the corrosion of stainless steel. The beneficial influence from the dissolved metal ions on the uniform corrosion properties of stainless steel in sulphuric acid also increases the risk for localized corrosion, when the corrosion potential is pushed to more noble values.

In the absence of impurities, sodium hydroxide is only slightly corrosive towards stainless steel up to a temperature of 100°C, regardless of the concentration. At higher temperatures, austenitic grades containing less than 20% chromium are more susceptible to uniform corrosion and can be prone to stress corrosion cracking. The iso-corrosion diagram for some stainless steel grades in pure sodium hydroxide is shown in Figure 5. Sodium hydroxide that is used in industrial applications is often contaminated with chlorides. Below 150°C the contamination usually has little effect on uniform corrosion, at least for low concentrations. In contrast, contamination of chlorides may increase the risk for stress corrosion cracking, especially for austenitic grades with low nickel content.

Sodium hydroxide

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Stainless steels have several advantages from a design and process flexibility point of view in hydrometallurgical environments. Under the most severe conditions, the highest-alloyed superaustenitic and superduplex stainless steel grades are necessary, while other steel grades find use in less severe environments. By performing laboratory corrosion tests and field exposures to gather information about the performance of stainless steel in environments used within the hydrometallurgical industry, it is possible to choose an appropriate grade.

This paper is the most recent in a series detailing the corrosion performance of Outokumpu stainless steel grades in hydrometallurgical applications. A number of previous works are given in the reference list [4, 5, 6].

Stainless steelsStainless steel is by definition an iron-base alloy with at least 11–12 weight percent chromium. Such an alloy will resist a humid atmosphere in the absence of significant contaminants and fresh water without chlorides. For harsher environments it is necessary to increase the amount of relevant alloying elements, i.e. mainly chromium, molybdenum, nitrogen and, for certain applications, copper. Nickel is also commonly added to achieve an austenitic structure that can easily be fabricated, i.e. formed, bent, machined and above all, welded. The most common types of stainless steel that fulfills these requirements are the austenitic grades according to the American Society for Testing and Materials (ASTM) standard 300-series. These include 304 (Outokumpu 4301, EN 1.4301), 316L (Outokumpu 4404, EN 1.4404), and further developments along this route with even higher contents of mainly chromium, molybdenum and nitrogen, for example 904L (N08904, EN 1.4539) and the 6%Mo Outokumpu grade 254 SMO® (UNS S31254, EN 1.4547). By the introduction of new metallurgical processes, above all the Argon Oxygen Decarburization (AOD) process, it has been possible to re-vitalize another type of stainless steel, the ferritic-austenitic or duplex grades. This type was originally developed during the late 1920’s, but with available processes at that time it was not possible to fully utilize their properties as engineering materials. However, addition of nitrogen via the AOD-converter to get a better phase balance has meant that this group of stainless steels is nowadays the fastest-growing type of stainless steel. They have approximately twice the strength of the austenitic grades, a higher surface hardness, and they have a very good resistance to stress corrosion cracking (SCC), as a result of their two-phase microstructure. The duplex stainless steels have a wide range of corrosion resistance depending on the alloying level and there is a duplex counterpart to most of the standard austenitic grades.

The different stainless steel grades can be ranked according to their corrosion resistance. The pitting resistance equivalent (PRE) value can be used to roughly quantify the composition effects on the resistance to pitting corrosion. A frequently-used expression for calculating the PRE takes the chromium, molybdenum and nitrogen content into account according to (1) where a higher PRE value indicates a higher resistance against pitting corrosion [7].

PRE = %Cr + 3.3 %Mo+16 %N (1)

Another common way of ranking the stainless steel grades is by measuring the critical pitting temperature (CPT), according to the ASTM G-150 standard. This temperature is defined as the lowest temperature where stable pitting occurs. A higher CPT value denotes a higher corrosion resistance. The chemical compositions, PRE and CPT values of the most common duplex grades, i.e. LDX 2101® (S32101, EN 1.4162), 2304 (S32304, EN 1.4362), 2205 (S31803/S32205, EN 1.4462) and 2507 (S32750, EN 1.4410), together with some common austenitic grades are given in Table 1.

Another merit of the duplex grades is the low nickel content, which implies a more stable price. The modern versions of duplex grades have been used for a number of applications where their strength, cost benefits and corrosion resistance have been utilized. These include pressure vessels in process industries, storage tanks in tanks farms, mobile vessels for chemical tankers and road tankers, and civil engineering applications such as bridges. The mechanical properties of some stainless steel grades are given in Table 2.

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The higher strength of the duplex grades can be utilized to reduce the gauge of the sheet and plates used for construction where design is based on the proof strength of the material. The consequence is that appreciable cost savings are possible by weight reductions. The European design code is more favorable in this respect than the design code by the American Society of Mechanical Engineers (ASME).

The two drawbacks of the duplex grades are the lower rupture elongation, which implies that complicated stretching and forming operations can be difficult, and the lower impact toughness at very low temperatures, i.e., they are excluded from cryogenic applications.

Experimental resultsA series of laboratory corrosion tests have been performed in environments that simulate those that exists in different leaching processes, sulphuric acid for simulation of leaching of Cu-, Zn- and Ni- containing ores and sodium hydroxide for treatment of bauxite. The acidic test environments contain dilute sulphuric acid, oxidizing metal ions and chloride ions of various content, at various temperatures, while the alkaline test environment contains sodium hydroxide with additional chlorides, tested at the boiling point.

Typical chemical composition, PRE and critical pitting temperature of some stainless steel grades Table 1

Micro- Typical TypicalGrade EN ASTM/UNS C N Cr Ni Mo Others structure* PRE CPT

4301 1.4301 304 0.04 – 18.1 8.1 – – A 18 <10

LDX 2101® 1.4162 S32101 0.03 0.22 21.5 1.5 0.3 5 Mn D 26 17±3

2304 1.4362 S32304 0.02 0.10 23 4.8 0.3 – D 26 22±3

4404 1.4404 316L 0.02 – 17.2 10.1 2.1 – A 24 20±2

2205 1.4462 S31803/S32205 0.02 0.17 22 5.7 3.1 – D 35 52±3

904L 1.4539 N08904 0.01 – 20 25 4.3 1.5 Cu A 34 62±3

254 SMO® 1.4547 S31254 0.01 0.20 20 18 6.1 Cu A 43 87±3

2507 1.4410 S32750 0.01 0.27 25 7 4 – D 43 84±2

4565 1.4565 S34565 0.02 0.45 24 17 4.5 5.5 Mn A 46 >90

*A = austenitic structure, D = Duplex (austenitic-ferritic) structure

PRE = %Cr+3.3%Mo+16%N, CPT according to ASTM G150

Mechanical properties according to ASTM and design stresses according to EN and ASME Table 2

Rp0.2 Rm A5 Design stress (MPa) at 100°C

Grade EN ASTM/UNS MPa MPa % EN ASME

4301 1.4301 304 200 520 45 150 137

LDX 2101® 1.4162 S32101 450 650 30 253 184

2304 1.4362 S32304 400 630 25 220 164

4404 1.4404 316L 170 485 40 143 115

2205 1.4462 S31803/S32205 450 655 25 240 177

904L 1.4539 N08904 220 490 35 157 137

254 SMO® 1.4547 S31254 310 655 35 205 187

2507 1.4410 S32750 550 795 15 300 227

4565 1.4565 S34565 415 795 35 – –

Rp0.2 = Proof strength Rm = Tensile strength A5 = Elongation

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Acidic laboratory corrosion tests

Two types of specimens were used: flat coupons, 60 x 30 mm, and welded coupons of the same size, equipped with crevice washers made of polytetrafluoroethylene (PTFE). The crevice washers were tightened with a torque of 1.58 Nm. The test samples were immersed in the solution for a time period of 30 days. The weight of the samples was noted before and after the exposure to calculate the corrosion rate. If the corrosion rate is below 0.1 mm.year-1 the material is considered to be corrosion resistant. The samples and the laboratory set-up are shown in Figure 6.

The first test series was performed in solutions containing 10% sulphuric acid with additions of 5 g.l-1 ferric ions and 4 g.l-1 cupric ions and 200, 500 and 700 ppm chloride ions at 50, 70 and 90°C. The results in Table 3 are presented as uniform corrosion rates, in mm.year-1, with a remark about the type and/or location of any localized corrosion attack (pitting and/or crevice corrosion).

The results from the first series of tests show that all the tested steel grades are corrosion resistant at 50°C, with chloride contents up to 700 ppm. At higher temperatures the risk for corrosion increases with increased chloride content and only the highest alloyed grades i.e. 254 SMO® and 2507 are corrosion resistant in the most aggressive environment, 700 ppm chlorides and 90°C. The steel grade 2205 suffered from pitting at the weld in the solution containing 500 ppm chlorides at 90°C, but was corrosion-free at 700 ppm chlorides at the same temperature. The explanation for this discrepancy is that there is nevertheless a risk for pitting even at 700 ppm at 90°C, alternatively that the sample immersed in the solution containing 500 ppm chlorides had defects in the weld that caused the pitting corrosion.

The second test series was performed in the same solutions as the first series but at higher temperatures. The tests were carried out in an autoclave at three different temperatures, 110, 125 and 150°C. The same type of specimens was used as in the first series and the exposure time was 30 days. The results are shown in Table 4.

None of the tested steel grades were corrosion resistant at 150°C in any of the tested environments. However steel grades 2205, 254 SMO® and 2507 only suffered from a low level of corrosion at 125°C in the solution containing 500 ppm chlorides. At the highest chloride level, 700 ppm, the test results showed that all tested grades were corrosion resistant at 110°C. Since 4404 suffered from corrosion at 500 ppm at 90°C it was expected to suffer from corrosion at 110°C and was excluded from this test.

A third series of laboratory corrosion tests was performed at 98°C in an environment containing 1% sulphuric acid, 4 g.l-1 cupric ions, 5 g.l-1 ferric ions and 100, 500, 1000, and 5000 ppm of chlorides. Both flat coupons and welded/creviced specimens were used. The results are displayed in Table 5. The results show that all grades are corrosion resistant at the lowest chloride level while 500 ppm seems to be a critical level for several steel grades.

Fig. 6 Examples of test specimens and experimental set-up for laboratory corrosion tests in acidic environments

Results from corrosion tests in 10% H2SO4, 5 g.l-1 Fe3+ and 4 g.l-1 Cu2+ in mm.year-1 Table 3

Temp. (°C) Cl- in ppm 4404 904L 2205 254 SMO® 2507

50 700 0.00 0.00 0.00 0.00 0.00

70 200 0.00 0.00 0.00 0.00 0.00 500 0.77 p 0.00 0.00 0.00 0.00 700 2.52 p 0.00 0.00 0.00 0.00

90 200 0.00 0.00 0.00 0.01 0.00 500 p/c 1.2 mm 0.00 0.00 p in HAZ 0.01 0.00 700 Weld attack 0.00 p in HAZ 0.00 0.01 0.00

0.77 p = uniform corrosion rate of 0.77 mm.year-1 and pittingp/c 1.2 mm = pitting and crevice corrosion with a maximum depth of 1.2 mmHAZ = heat affected zone

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A fourth series of laboratory corrosion tests was performed at 150°C in autoclaves to further investigate the influence of chloride ions at higher temperatures. Flat coupons and welded/creviced specimens (60 x 30 mm) of steel grades 2205, 254 SMO® and 2507, were immersed in a solution containing 0.5% sulphuric acid, 4 g.l-1 Cu2+ at two different levels of Fe3+ and chloride ions for a time period of 30 days. The results are shown in Table 6 as corrosion rates in mm.year-1. The highest alloyed grades 254 SMO® and 2507 showed corrosion resistance in the autoclave test at 100 ppm of chlorides while none of the tested steel grades were resistant at 1000 ppm chlorides.

Results from corrosion tests in 10% H2SO4, 5 g.l-1 Fe3+ and 4 g.l-1 Cu2+ in autoclaves in mm.year-1 Table 4

Temp. (°C) Cl- in ppm 4401 904L 2205 254 SMO® 2507

110 500 NT 0.01 p in HAZ 0.02 0.02 0.01 700 NT 0.01 0.03 0.03 0.02

125 200 2.4 p 0.13 0.06 0.13 0.05 500 NT 0.04 p in HAZ/weld 0.06 0.06 0.05 700 NT 0.13 p 1.7 p 0.12 0.04 p weld

150 200 NT 0.47 0.41 0.70 0.31 500 NT 3.8 p 3.02 p 0.75 1.3 700 NT 4.52 p weld 14.34 1.28 p 3.93 p weld

NT = not tested0.13 p = uniform corrosion rate of 0.13 mm.year -1 and pittingHAZ = heat affected zoneWeld = pitting attack in weld

Results from corrosion tests in 1% H2SO4. 5 g.l-1 Fe3+

and 4 g.l-1 Cu2+ at 98°C in mm.year-1 Table 5

Cl- 4404 904L 2205 254 SMO® 2507

100 0.00 0.00 0.00 0.00 0.00

500 >3 p/c 0.9 p/c 0.6 p/c 0.00 0.00 0.7 mm 1.1 mm 0.1 mm

1000 >4 p/c >2 p/c >1 p/c 0.00 p/c 0.11 p 0.8 mm 0.5 mm 0.7 mm 0.3 mm

5000 >10 >6 p/c >1 p/c 0.06 p/c 0.2 p/c 1.5 mm 0.3 mm 0.8 mm 0.2 mm

0.9 = uniform corrosion rate of 0.9 mm.year -1

p/c 0.7 mm = pitting and crevice corrosion with a maximum depth of 0.7 mm

Results from exposure in 0.5% H2SO4 and 4 g.l-1 Cu2+

at 150°C in autoclaves in mm.year-1 Table 6

Solution 2205 254 SMO® 2507

5 g.l-1 Fe3+ + 100 ppm Cl- 0.14 p/c 0.02 0.02

10 g.l-1 Fe3+ + 1000 ppm Cl- >2 p/c 0.4 p/c 0.6 p

0.9 = uniform corrosion rate of 0.9 mm.year -1

p/c 0.7 mm = pitting and crevice corrosion with a maximum depth of 0.7 mm

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Alkaline laboratory corrosion tests

Welded and creviced coupons with the same dimensions as for the acidic corrosion tests were immersed in an alkaline environment to investigate the uniform and localized corrosion behavior. U-bend specimens were also included in the test to investigate the risk for stress corrosion cracking. U-bend samples were produced according to ASTM G30-97, with dimensions 127 x 13 x t mm, cut transverse to the rolling direction. The specimen edges were dry ground with 320 grit paper. The specimens were stressed using a two-stage method around a mandrel with a 24 mm diameter, and secured with nuts and bolts. Insulators of Teflon were used between the screws and the specimens. Before the final stressing stage the specimens were degreased in acetone. The time between the two stressing stages and between the final stressing stage and the start of the test was kept as short as possible. The specimens were then immersed in 20% sodium hydroxide with an addition of 5% chlorides at a temperature of 110°C for 32 days.

The results showed that the welded and creviced coupons of 4301 and LDX 2101® performed similarly, with low corrosion rates as shown in Table 7. A corrosion rate lower than 0.1 mm.year-1 indicates that the material is corrosion resistant. No pitting or crevice corrosion could be found on any of the samples. The U-bend samples of 4301 showed a higher corrosion rate than for the welded coupons whereas for LDX 2101® the corrosion rate was similar or slightly lower for the U-bend samples. However, no stress corrosion cracking was observed for any of the samples. The appearance of the specimens after the testing is shown in Figure 7a-d.

At high temperatures and sodium hydroxide concentrations above 20%, the risk for stress corrosion cracking increases, especially for austenitic grades. The duplex stainless steels are known to show a higher resistance to stress corrosion cracking in more aggressive caustic environments as a result of their two-phase microstructure [3].

Fig. 7 Appearance of specimens after testing in sodium hydroxide, a–b) Creviced specimens of LDX 2101®

and 4301 respectively, c–d) U-bend specimens of LDX 2101®

and 4301 respectively

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b c d

Results from corrosion testing of 4301 and LDX 2101®

in 20% NaOH and 5% chlorides Table 7

Corrosion rate Steel grade Specimen (g.m-2.h-1) (mm.year-1) Remarks

4301 Coupon 0.055 0.061 Discolouration 0.047 0.052

U-bend 0.076 0.083 0.088 0.096

LDX 2101® Coupon 0.052 0.059 Discolouration 0.048 0.054

U-bend 0.039 0.043 0.027 0.029

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Field testsOver the years a number of test racks have been sent out and installed in different types of hydrometallurgical plants for evaluation of the most appropriate grades. The results from exposure at a zinc plant and a copper plant are shown in Table 8, with the results displayed as the corrosion rate in mm.year-1. At the zinc plant the test rack was placed in the liquid phase at the top of a ferrite leaching tank containing ~5% sulphuric acid and ~200 ppm of chlorides and with various contents of cupric, ferric and zinc ions at a temperature of 100°C. At the copper plant two test racks were installed, one in the circulation tank at 60°C containing 15% sulphuric acid, 60 ppm of chlorides and ~60 g.l-1 cupric ions. The second test rack was installed in the electrorefining tank at 32°C where the environment contained 35% sulphuric acid, 15 g.l-1 cupric ions and 60 ppm chloride ions. The exposure time for all racks was one year.

None of the samples suffered from uniform corrosion or showed any signs of pitting or crevice corrosion after one year of exposure. The content of metal ions seems to be high enough to enable passivation of the stainless steel even in hot diluted sulphuric acid. According to the results from the laboratory corrosion tests in Table 3 and 4 the chloride levels of 60 ppm and 200 ppm are low enough not to cause localized corrosion, which is also confirmed by the in-plant exposure tests.

Test racks have also been installed at a nickel leaching plant under pressurized conditions at elevated temperatures. Samples of steel grades 904L, 2205, 254 SMO®, 2507 and 4565 were placed in autoclaves with environmental conditions described in Table 9. None of the exposed samples suffered from any type of corrosion, neither uniform corrosion nor localized corrosion i.e. pitting or crevice corrosion. This is in line with the results from the laboratory corrosion test presented earlier. The chloride level is just too low to cause any harm.

Results from the exposure of stainless steel samples

at a zinc plant and a copper plant in mm.year-1 Table 8

Grade Zinc- Copper- Copper- ferrite leach circulation electrorefining

4401 0.02 0.00 0.00

4404 0.02 0.00 0.00

904L 0.01 0.00 0.00

254 SMO® 0.01 0.00 0.00

4565 0.01 0.00 0.00

2304 0.01 0.00 0.00

2205 0.01 0.00 0.00

2507 0.01 0.00 0.00

Environmental conditions at the exposure sites in nickel

leaching autoclaves Table 9

Placement H2SO4 Ni2+ Cu2+ Cl- Temperature (g.l-1) (g.l-1) (g.l-1) (ppm) (°C)

Autoclave A 2–5 105–130 90–110 <5 125

Autoclave B 12–15 140–150 5–8 <5 135–140

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In-plant exposures have also been carried out at six different locations in a copper-nickel flotation plant. The main constituents of the ore slurry, before the copper rougher flotation, are described in Table 10. The test racks were installed in the nickel rougher circuit and pyrite flotation, a bit later in the process, where the pH is increased to around 11 using CaO milk. Also other compounds like sulphur dioxide and hydrogen sulphide can be produced during the process. The presence of oxygen may cause an oxidation of SO2 to SO3 and sulphuric acid, and evaporation of water may change the concentrations of the components. The actual service conditions are consequently not well defined, which makes the prediction of the performance of the stainless steel grades almost impossible and necessitates in-plant exposure testing

The results for the stainless steel grades 4404, LDX 2101®, 2304, 2205 and 2507 after one-year exposure in the nickel rougher circuit and pyrite flotation are shown in Table 11. The results are displayed as corrosion rates in mm.year-1. The austenitic grade 4404 suffered from shallow pitting corrosion when exposed in the air in two of the nickel flotation cells but was corrosion resistant when exposed to the froth and pulp. The duplex grades 2304 and 2205 suffered some pitting and crevice corrosion when exposed in the air above the

Chemical compounds and process parameters

in a nickel-copper ore flotation plant Table 10

Raw material, Ore Pentlandite, Chalcopyrite, Pyrite

pH 7–9

Temperature (°C) 5–20 (changing with season)

Calcium, Ca2+ (ppm) 580–630

Thiosulphate, S2O32- (ppm) 250–300

Sulphate, SO42- (ppm) 1500–1700

Chloride, Cl- (ppm) 200–400

Sodium thiosulphate , Na2S2O3 (ppm) 1–2

Flotation agents Na-butylxanthate, Na-dibutylthiosulphate, Na-dimethyldithiocarbamate

Frothing agents Fatty acids, alcohols, ethers

Results after one year exposure in a nickel-copper ore flotation plant in mm.year-1 Table 11

Location 316L LDX 2101® 2304 2205 2507

Above liquid in nickel flotation 0.00 p in HAZ 0.00 p 0.03 mm 0.00 p 0.07 mm 0.00 pe 0.00cells 0.14 mm

Top stand, nickel flotation cells 0.00 p 0.17 mm Selective corrosion 0.00 p/c 0.15 mm 0.00 pe 0.00

Upper dart valve box, 0.00 Selective corrosion 0.00 0.00 0.00nickel flotation cells

Nickel flotation tank 0.00 Selective corrosion 0.00 c 0.00 0.00

Upper dart valve box, pyrite 0.00 Selective corrosion 0.00 0.00 0.00flotation cells

Pyrite flotation tank 0.00 Selective corrosion 0.00 0.00 0.00

0.00 = uniform corrosion rate of 0.00 mm.year-1 c = crevice corrosionp = pitting corrosion HAZ = heat affected zonepe = pitting corrosion on cut edges 0.14 mm = maximum pit depth of 0.11 mm

15acom | 1 - 2010

nickel-cells. However the pits were small and shallow and hard to see without a magnifier. The duplex grade 2507 was the only grade that was completely corrosion resistant. The LDX 2101® was not corrosion resistant at any location. All specimens suffered more or less severe selective corrosion of the ferrite phase, except for one that was attacked by shallow pitting, see Figure 8.

DiscussionThe laboratory corrosion tests emphasize the influence of oxidizing ions, i.e., mainly cupric and ferric ions, on the ability of stainless steel to passivate in dilute sulphuric acid. These oxidizing ions serve as a chemical potentiostat that force the corrosion potential of the stainless steel into the passive region. The laboratory corrosion tests have not established a limit in this respect, but it is quite clear that common molybdenum-alloyed standard grades e.g., 4404 are resistant in a wide concentration and temperature range, at least up to 10% sulphuric acid at 100°C with concentrations of 5 g.l-1 ferric ions and 4 g.l-1 cupric ions. The duplex grades LDX 2101® and 2304 were not included in the laboratory corrosion tests, but there is no reason to believe they should perform differently. At higher temperatures it is necessary to use higher alloyed grades with increased corrosion resistance e.g., the duplex grades 2205 or 2507, or their austenitic counterparts 904L, 254 SMO® and 4565.

The required level of oxidizing ions like cupric and ferric ions has not been established. Nevertheless, the amount of oxidizing ions present in the electrolytes, in the range of several thousands of ppm, is high enough to create this chemical potentiostat ensuring passivity of the stainless steel surface. However, at high chloride levels, the high potentials achieved by the oxidizing ions can exceed the critical potentials for pitting and crevice corrosion, implying risks for these corrosion types instead.

The major importance of the chloride content of the electrolyte in influencing the selection of an appropriate steel grades is underlined by the corrosion tests. At temperatures up to 50°C there seems to be no problems with chloride levels approaching 1 000 ppm for the conventional grades, e.g., 4404, but above 50°C the chloride level should not exceed 200 ppm. The higher alloyed grades 2205 and 904L tolerate 90°C at this level, while the most resistant grades i.e., the superduplex 2507 and its superaustenitic counterpart 254 SMO® shows a limit of 1 000 ppm close to 100°C.

At even higher temperatures in pressurised autoclave conditions there is a pronounced risk of corrosion also on the most highly alloyed grades at chloride levels exceeding 200 ppm. The higher the temperature the less chloride content they can tolerate. The results from the in-plant exposures in real process environments do not in any way contradict the results from the laboratory corrosion tests.

The in-plant exposure at the flotation plant has been executed under very severe conditions and the results can probably not be extrapolated to other plants. In this particular plant all grades, except LDX 2101®, can be used when the stainless steel is fully immersed or at least most of the time exposed to the liquid phase. Components exposed to possible wet-dry conditions face more hostile environments, due to an evaporative effect, and should be made of either 2507 or 254 SMO®. The latter was not included in the test but should not be inferior to 2507.

In hot alkaline solutions there are risks for stress corrosion cracking of conventional austenitic stainless steels (e.g. 4301, 4401), and the risk increases if the environment is contaminated with chlorides. Duplex stainless steels are known for their excellent resistance to stress corrosion cracking. The corrosion test performed in sodium hydroxide with additional chlorides showed that LDX 2101® has a good resistance to caustic corrosion and caustic stress corrosion cracking. Further investigations are required but the results indicate that LDX 2101® could be a suitable, and cost efficient material for construction of, for example, large tanks and vessels in the alumina industry.

Fig. 8 Appearance of LDX 2101®

samples after field testing in a nickel-copper ore flotation plant

50 µm

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Conclusions• Dilutesulphuricacidusedinhydrometallurgicalprocessesi.e.containingoxidizing

metal ions, does not cause uniform corrosion on stainless steel•Thelevelofchlorideionsandthetemperatureoftheacidsolutiondeterminethe

corrosivity, regardless of which metal recovery process is involved• LDX2101®showedexcellentcorrosionresistanceinthecausticenvironmenttested•Thereisapossibilitytofindanappropriatestainlesssteelgradefordifferenthydromet-

allurgical environments, i.e. a highly corrosion resistant super duplex or super austenitic grade for aggressive leaching environments and lower alloyed duplex and austenitic grades for less aggressive process steps

•Theuseofaduplexgradeimpliescostsavingsbecauseoflowercostperweightand the possibility to reduce dimensions due to the higher mechanical strength.

References[1] Corrosion Tables, published by Avesta Jernverk, Sweden, 1971. Later republished

by B. Wallen and J. Olsson, Handbook of Stainless Steel, McGraw-Hill Company, New York, NY, USA, 1977

[2] E. Alfonsson, G.Coates and J. Olsson, “Stainless for the hydrometallurgical industry, Proc. 14th IC Congress, Cape Town, SA, 1999.

[3] Outokumpu Corrosion Handbook, tenth edition, Outokumpu Stainless AB, Sweden, 2009

[4] S. Ekman and A. Bergquist, “Suitable stainless steel grades for hydrometallurgical applications” Proc. Hydrometallurgy 2008, Phoenix, AZ, USA, 2008

[5] E. Torsner and S. Ekman, “Stainless steels for hydrometallurgical applications”, Proc. Stainless Steel World America, Huston, TX, USA, 2008

[6] E. Torsner and S.Ekman, “Stainless steels for nickel hydrometallurgical processes”, Proc. COM 2009, Sudbury, Canada, 2009

[7] Z. Szklarska – Smialowska, “Pitting and crevice corrosion”, NACE International, Houston, TX, USA, 2005

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