Materials Transactions, Vol. 53, No. 6 (2012) pp. 1048 to 1055 ©2012 The Japan Institute of Metals Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid Jun-Seob Lee1, Soon-Tae Kim1, In-Sung Lee1, Gwang-Tae Kim2, Ji-Soo Kim2 and Yong-Soo Park1,+ 1Department of Materials Science and Engineering, Yonsei University, 134 Shinchon-dong, Seodaemun-gu, Seoul 120-749, Korea 2Stainless Steel Research Group, POSCO Technical Research Laboratories, Goedong-dong, Nam-Gu, Pohang, Gyeongbuk 790-785, Korea The effect of copper (Cu) addition on the active corrosion behavior of hyper duplex stainless steels in sulfuric acid was investigated. The addition of Cu in the base alloy enhanced the resistance to general corrosion by decreasing the critical and corrosion current densities, and increasing the polarization resistance. There are two primary reasons for the considerable enhancement of the corrosion resistance of the experimental alloys containing Cu. First, the protective surface film was enriched with the noble metallic copper (Cu) due to the selective dissolution of the active metallic Cr, Fe, and Ni, and the electrochemical dissolution of the corrosion products such as iron-sulfide (FeS2), iron sulfate (FeSO4), ferrous oxide (FeO) and hydrous iron sulfate (FeSO4·7H2O). Second, chromium oxide (Cr2O3), chromium trioxide (CrO3), nickel oxide (NiO), molybdenum dioxide (MoO2), molybdenum trioxide (MoO3), and tungsten trioxide (WO3) in an oxide state, molybdenum 2¹ 2¹ oxy-hydroxide (MoO[OH]2) and chromium hydroxide (Cr[OH]3) in a hydro-oxide state, molybdate (MoO4 ) and tungstate (WO4 )as + corrosion inhibitors in an ion state, and ammonium (NH4 ) elevating the pH in an ion state were increased and assisted in improving the corrosion resistance. [doi:10.2320/matertrans.M2012008] (Received January 6, 2012; Accepted March 15, 2012; Published May 9, 2012) Keywords: stainless steel, copper, scanning auger multi-probe (SAM), X-ray photoelectron spectroscopy (XPS), general corrosion 1. Introduction and Ni, have contributed to the corrosion resistance by being a specific chemical species on the surface film in an elevated Duplex stainless steels (DSSs) with nearly equal fractions temperature, concentrated H2SO4 environments, further of ferrite (¡) and austenite (£) phases are being increasingly in-depth research analysis of surface film is required. used for various applications such as fuel gas desulphuriza- Moreover, it is necessary to quantitatively verify the effects tion (FGD) facilities in fossil power plants, desalination of the Cu addition on the difference of the resistance to facilities, off-shore petroleum facilities, and chemical plants general corrosion between the £-phase and ¡-phase using a due to their high resistance to stress corrosion cracking, formula for the sulfuric-acid resistance equivalent (SRE = pitting corrosion, crevice corrosion, good weldability, mass% Cr + 1.5 mass% Ni + 0.5 mass% Cu + 2mass% Mo + excellent mechanical properties and relatively low cost due 2 mass% W + 20 mass% N),14) and to clarify its related stage to the addition of low Ni, as compared with austenite of corrosion initiation and propagation. stainless steels.1­3) Thus, in this work, the effect of Cu addition on the active In general, super duplex stainless steels (SDSSs), such corrosion behavior of hyper duplex stainless steels (HDSSs) as UNS S32750, UNS S32760 and UNS S32550, are in highly concentrated sulfuric acid was investigated using defined as DSSs with a pitting resistance equivalent immersion tests, electrochemical measurements, a scanning (PRE = mass% Cr + (3.3 mass% Mo + 0.5 mass% W) + 16 Auger multi-probes (SAM) analysis and an X-ray photo- mass% N)4,5) of 40 to 45. Hyper duplex stainless steels electron spectroscopy (XPS) analysis of surface film. (HDSSs) such as UNS S32707 are defined as highly alloyed DSSs with a PRE in excess of 45.4) 2. Experimental Procedures It is well known that the addition of copper (Cu) to ferritic, austenitic or duplex stainless steels improves the resistance to 2.1 Calculation of phase diagram and equilibrium general corrosion in sulfuric acid.6­10) It has been reported in fractions of each phase previous studies that the mechanism of the beneficial effect of The effects of the Cu addition on the phase diagram and the Cu addition on the steels is based on the suppression of equilibrium fractions of each phase were calculated against the anodic dissolution by the noble metallic Cu enriched in the temperature for the HDSS alloy using a commercial the surface film of austenitic stainless steels in sulfuric Thermo-Calc software package. acid.11,12) Furthermore, it has been also reported that the enhancement mechanism of the corrosion resistance by Cu 2.2 Material and heat treatment addition is explained by protective, insoluble salt films such The experimental alloys were manufactured using a high as cuprous chloride (CuCl) or cupric chloride (CuCl2) formed frequency vacuum induction furnace and then hot rolled into on the surface of stainless steels in chloride (Cl¹) environ- plates of 6 mm thickness. The experimental alloys were cut ments.13) and solution heat-treated for 5 min per 1 mm of thickness at However, because it is difficult to locate studies that have 1363 K and then quenched in water. The chemical compo- quantitatively elucidated which elements of the HDSS, with sition of the experimental alloys is shown in Table 1. high concentrations of not only Cu but also N, Mo, W, Cr 2.3 Microstructure characterization +Corresponding author, E-mail: [email protected] In order to observe the optical microstructures of the ¡- Effect of Copper Addition on the Active Corrosion Behavior of Hyper Duplex Stainless Steels in Sulfuric Acid 1049 Table 1 The chemical compositions of the experimental alloys (mass%). 3. Results and Discussion Alloys C Cr Ni Mo W N Mn Si S Cu 3.1 Calculation of the phase diagram and equilibrium BASE 0.020 27.36 7.11 2.59 3.38 0.31 1.45 0.31 0.004 0.19 fractions of each phase 1.5Cu 0.017 26.91 6.59 2.50 3.30 0.38 0.94 0.34 0.005 1.45 Figure 1 shows the effects of the Cu addition on the phase 3Cu 0.018 26.84 6.42 2.51 3.30 0.36 0.96 0.33 0.005 3.06 diagram and equilibrium fractions of each phase for the HDSSs calculated using the Thermo-Calc software package. The sectional view at the 27 mass% Cr illustrates that the phases and £-phases in the HDSS, they were ground to 2000 alloys solidify primarily as an ¡-phase and some of the grit using SiC abrasive papers, polished with a 1 µm diamond this ¡-phase, transforms of £-phase with a decrease in the paste, and then electrochemically etched using 10 mass% temperature (Figs. 1(a) and 1(c)), irrespective of the Cu KOH. The ¡-phase volume fractions were calculated using addition in the alloy. As the temperature decreases further, the the method of manual point count according to ¡-phase decomposes into a sigma phase (·) and a secondary 15) ASTM E562. The chemical compositions of the ¡-phase austenite (£2) according to the eutectoid reaction. and £-phase were analyzed using a scanning electron L ! L þ ¡ ! L þ ¡ þ £ microscope (SEM)­energy dispersive spectroscope (EDS). ! ¡ þ £ ! ¡ þ £ þ · ! £ þ · ð2Þ The nitrogen content was analyzed using a SAM. 2 2 It is well known that Cu as a substitution element stabilizes 2.4 Corrosion tests the £-phase and provides a solid solution strengthening.17) As In order to analyze the effect of the Cu addition on the the temperature of the solution heat-treatment decreases in resistance to general corrosion of the experimental alloys, the region with the dual £/¡-phases, the volume fraction of both electrochemical measurements and immersion test were the ¡-phase decreases and that of the £-phase increases. It is made. Measurements of potentiodynamic anodic polarization predicted that the optimum temperature of the solution heat- curves were performed in a deaerated 6.34 N H2SO4 solution treatment to obtain the desired microstructure of approx- at 353 K according to the ASTM G 5.16) These electro- imately 50 vol% £-phase and 50 vol% ¡-phase is approx- chemical characteristics: the critical current density (Ic), imately 1353 to 1363 K (Figs. 1(b) and 1(d)). corrosion current density (Icorr), and polarization resistance (Rp) were measured from the potentiodynamic anodic 3.2 Effects of copper addition and solution temperature polarization curves. The test was performed at a potential on corrosion rate through immersion test range of ¹0.4 VSCE to +1.1 VSCE at a scanning rate of After the base, 1.5 Cu, and 3 Cu experimental alloys were 1mV/min using a saturated calomel electrode (SCE) as a immersed in a 18.4 N H2SO4 solution at 313, 333 and 353 K reference electrode. A potentiostatic polarization test were for 6 h, the corrosion rate was measured (Fig. 2). As the performed to measure current transients in a deaerated 6.34 N addition of copper to the base alloy increased, the corrosion H2SO4 solution at 353 K with an applied potential of rate decreased. Hence, it is concluded that the Cu addition ¹0.2 VSCE in the active region of the potentiodynamic anodic has a positive effect on the resistance to general corrosion of polarization curves. The current transients were recorded for HDSS. 3600 s. The SEM was used to observe the corrosion sites on Figure 3 shows the optical microstructure of the exper- the specimen after the potentiostatic test. After immersion of imental alloys observed after 10 min and 6 h immersion in the same specimens at 20 K intervals from 313 to 353 K for 18.4 N H2SO4 at 353 K.