P-6 Optimization of Citric Acid Passivation for Corrosion Resistance of Stainless Steel 440C Chi Tat Kwok 1,2 and Wen He Ding 1 1 Department of Electrochemical Engineering, University of Macau 2 fstctk@umac.mo Abstract Passivation of martensitic stainless steel AISI 440C was carried out in citric acid solution under various passivation conditions such as processing time, temperature and concentration of the solution. The corrosion behavior of citric acid-passivated 440C in 0.9 wt% NaCl solution, open to air, at 37 o C was studied by open circuit potential measurement and potentiodynamic cyclic polarization technique. The optimized passivation conditions for 440C were obtained by L 9 (3 4 ) orthogonal experimental design in order to achieve the best corrosion performance. Compared with the untreated (air-passivated) 440C, significant improvement in corrosion resistance was found in some of the citric acidpassivated 440C as evidenced by a noble shift in open circuit potential and pitting potential, a wider passive range, and a reduction in corrosion current density. By orthogonal analysis, the optimized condition for passivating 440C is in 30 wt% citric acid at 50 o C for 60 minutes. Temperature and passivation time were found to be the most important factor in affecting the open circuit potential and pitting potential resepctively. X-ray photoelectron spectroscopy (XPS) analysis showed that the outer layers of the citric acid-passivated 440C mainly composed of oxides such as Cr(OH) 3, Cr 2 O 3, FeO and Fe 3 O 4. Compared with the untreated 440C, the citric acid-passivated 440C showed significant increase in the Cr/Fe ratios for forming a thicker and more tenacious oxide film for improving its corrosion resistance. Key words: Passivation, stainless steels, citric acid, corrosion, orthogonal experimental design. 1. Introduction Chemical passivation, is a process of immersing stainless steels in a solution of nitric acid or citric acid, will dissolve the embedded iron and restore the original corrosion-resistant oxide film [1]. Nitric acid, being a strong acid and a powerful oxidizing agent, is commonly used for passivating the stainless steels in industry. However, corrosive and oxidizing nature of the nitric acid cause significant personnel hazards and threaten the environment. Its risk can be intensified under heating. For the sake of protecting the environment, special disposal procedures and long-term liabilities are required for complying with federal, state, and local regulations. Citric acid, on the other hand, being a moderate, environmentalfriendly and safe oxidizer, is a prospective substitute for nitric acid in passivating the stainless steels. Citric acid provides worker and environmental safety, versatility, ease of use and lower costs. However, the effect of citric acid on passivation of stainless steels and hence their corrosion behavior are scarcely reported in the literature. Orthogonal arrays are fractioned factorial designs that allow testing multiple independent processes variables within a few experiments. Orthogonal experimental design has been applied in different areas [2,3], but it has not yet been used to optimize the passivation conditions of stainless steels. In the present study, the passivation conditions such as processing time, temperature and concentration of the solution for 440C were optimized by L 9 (3 4 ) orthogonal experimental design in order to achieve the best corrosion performance of 440C. 2. Experimental details Martensitic stainless steel 440C (Fe-17%Cr-1.1%C) for passivation was in hardened condition with hardness of 790 Hv. In the passivation experiments for 440C, passivation time, concentration and temperature of the citric acid solution serve as the three major factors, each factor containing three levels. Therefore, a L 9 (3 4 ) table of orthogonal design is used for the nine experiments for evaluating the effect of the three factors on open circuit potential (E corr ) and pitting potential (E pit ). No interaction among the three factors is assumed. Table 1 lists the coded levels of the orthogonal test factors for passivating 440C. Corrosion behavior of passivated 440C with different conditions in simulated physical saline solution (0.9% NaCl), opened to air, at 37±1 o C was evaluated by open circuit potential (E corr ) measurement and potentiodynamic polarization technique using a Princeton Applied Research VersaStat II corrosion system according to ASTM Standard G61-92 [4]. The specimens were embedded in cold-curing epoxy resin, exposing a surface EcoDesign2007/ December 10-13 in Tokyo 1
area of 1 cm 2 so as to avoid crevice corrosion. All potentials were measured with respect to a saturated calomel electrode (SCE, 0.244V versus SHE at 25 o C) as the reference electrode. Two parallel graphite rods served as the counter electrodes for current measurement. Firstly, E corr scan was conducted for 2 hours to allow the potential of the specimens to reach a steady state. It was then followed by potentiondynamic polarization scan. The potential was increased from 200 mv below the E corr in the anodic direction at a scan rate of 1 mv/s. Table 1 Orthogonal experimental design and analysis for passivation of 440C Number Untreated 440C Time (min.) A Conc. (wt%) B Temp ( o C) C E corr (mv) E pit (mv) - - - -365 192 1 30 10 30-306 177 2 30 20 50-270 101 3 30 30 70-226 195 4 60 10 50-91 355 5 60 20 70-198 396 6 60 30 30-256 136 7 120 10 70-284 64 8 120 20 30-300 272 9 120 30 50-129 492 I (E corr) -267.3-227.0-287.3 II (E corr) -181.7-256.0-163.3 III (E corr) -237.7-203.7-236.0 R (E pit) 85.7 52.3 124.0 I (E pit) 157.7 198.7 195.0 II (E pit) 295.7 256.3 316.0 III (E pit) 276.0 274.3 218.3 R (E pit) 138.0 75.7 121.0 Prior to XPS study, the passivated specimens were degreased in acetone in ultrasonic bath. XPS analysis was carried out using a PHI Quantum 2000 Scanning ESCA Microprobe. The monochromatic X-ray, Al Kα (hν = 1486.6 ev), was used as the exciting source of photoelectrons, which were collected by hemispherical analyzer kept at 0.4 ev resolution capability. Depth profiling was done by in-situ Ar ion sputtering gun, working at 1.0 kev beam energy and speed 3.49 nm/min. for the standard material SiO 2. The energy spectra are reported in terms of binding energy versus intensity. The binding energy range from 0 to 1000 ev, which contains the core levels of metallic components (Fe and Cr) and non-metallic components (C and O). Additionally, O 1s, Fe 2p and Cr 2p levels were analyzed for determining the chemical state of the elements. The oxidation states of these elemental species are also determined. Element concentrations were evaluated from integrated peak heights after linear background subtraction using theoretical cross-sections [5]. The atomic percentage of the elements (A) can be determined by dividing the peak area by the sensitivity factor and expressing it as a fraction of the summation of all normalized intensities: A = {(I A /F A )/ (I/F)} x 100% where I A = Intensity for element A F A = Atomic sensitivity factor (I/F) = Summation over all elements observed in the spectrum from the specimen. 3. Results and discussion 3.1. Electrochemical measurements E corr represents the corrosion tendency whereas the pitting potential (E pit ) corresponds to the potential at which the current starts to rapidly increase in the anodic scan. It implies the lowest starting potential of localized corrosion on the stainless steel surface and reflects the capability against pitting corrosion. The higher E pit, the more difficult for stainless steel forming pits. Fig. 1 shows the typical plot of E corr versus time and the polarization curve in 0.9% NaCl solution at 37 C for 440C passivated with condition 9 (120 min., 30 wt% and 50 C). Passivity is observed in this specimen by maintaining at a low current density as the potential is increased. It is due to the more stable and thicker protective film of the passivated specimen. The passive region shows a wide range of potential values, until the passive film breaks down, causing a dramatic increase in the current density at relatively high potential. The repassivation capability of the 440C specimens with and without citric acid passivation is poor as indicated by the low protection potential (below the E corr ). The corrosion resistances of EcoDesign2007/ December 10-13 in Tokyo 2
some passivated specimens (conditions 4, 5 and 9) are improved as evidenced by noble shift in E corr and E pit, a wider passive range, and a lower passive current density. For all passivated 440C, their E corr are found to be nobler than that of untreated 440C (-365 mv), however, E pit of some passivated 440C are reduced (conditions 1, 2, 6, 7) (Table 1). values among I, II or III. The higher R, the greater effect on the results. The ranking of significance for the three factors is then: E corr : R C > R A > R B (Temperature > Time > Concentration) E pit : R A > R C > R B (Time > Temperature > Concentration) Temperature and time were found to be the most important factor for E corr and E pit respectively. Based on the value of R for different factors, the optimum condition for achieving high E corr and E pit are A 2 B 3 C 2 (60 min., 30 wt%, 50 C). The corrosion performance of 440C passivated with the optimized condition was verified by electrochemical experiments and the E corr and E pit were found to be -91 mv and 558 mv respectively (Fig. 2). The verification results reconfirm the optimized passivation condition for enhancing E corr and E pit and also show that the orthogonal design can reduce the experimental time and cost. Figure 1 Typical plot of E corr vs time polarization curves for citric acid-passivated 440C (120 min., 30 wt%, 50 C) compared with untreated 440C in 0.9 wt% NaCl solution at 37 o C 3.2. Optimization of passivation conditions The sum of E corr under the same level is used as an estimation index for determining the effect of different factors. The factors at different levels in the nine experiments and the statistical analysis results are shown in Table 1. I, II, III represent the average of the sum for E corr or E pit under the same level. The range R is defined by the difference between the highest and the lowest Figure 2 Plot of E corr vs time polarization curves for citric acid-passivated 440C (60 min., 30 wt%, 50 C) for verifying its E corr and E pit EcoDesign2007/ December 10-13 in Tokyo 3
3.3. XPS analysis Fig. 3 shows the spectrum of passivated 440C with the optimized condition (60 min., 30 wt%, 50 C). The peaks for C and O are observed in the outermost layer due to contamination of oil. The major peaks for Fe and Cr are observed and the depth profile shows the variation of Cr, Fe and O [Fig. 3]. Deconvolution of the XPS peaks provides evidence for the presence of Cr(OH) 3, Cr 2 O 3, FeO and Fe 3 O 4 in the passivated layer. From Fig. 3, an increase in the overall contents of Cr and O is observed in the passivated layer. The maximum Cr/Fe achieved in citric acid-passivated 440C is 1.2 while the Cr/Fe ratio is 0.28 for the untreated (air-passivated) 440C. (1) The untreated (air-passivated) 440C shows narrow passive region with pitting potential of 192 mv but also is not capable of repassivating in 0.9% NaCl solution at 37 o C. In addition, its overall current density is higher because of its high carbon content. (2) The corrosion resistances of some citric acidpassivated 440C are improved to a certain degree as evidenced by a noble shift in the open circuit potential and pitting potential, a wider passive range and a lower passive current density. (4) By orthogonal analysis, temperature and time are found to be the most significant factor in affecting the open circuit potential and pitting potential respectively. In addition, the optimized condition for achieving high open circuit potential and pitting potential is A 2 B 3 C 2 (60 min., 30 wt%, 50 C). (5) By XPS analysis, the outer layers of the passivated 440C compose of the oxide compounds such as Cr(OH) 3, Cr 2 O 3, FeO and Fe 3 O 4. Cr(OH) 3 and Cr 2 O 3 which have the positive effects in enhancing corrosion resistance of the passive layers. (6) Compared with the untreated 440C, the citric acid passivated 440C show significant increase in the Cr/Fe ratio (from 0.28 to 1.2) for forming a thicker and more tenacious passive film for improving their corrosion resistance in 0.9% NaCl solution at 37 o C. 5. Acknowledgements The authors wish to acknowledge the support from the infrastructure of the University of Macau. References Figure 3 Evolution of the XPS spectra of passivated 440C during depth profiling at various sputtering times and XPS depth profile of passivated 440C 4. Conclusions [1] ASTM A380-99, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems, ASTM Standard, PA, 2005. [2] M. Abud-Archila, D.G. Vázquez-Mandujano, M.A. Ruiz-Cabrera, A. Grajales-Lagunes, M. Moscosa- Santillán, L.M.C. Ventura-Canseco, F.A. Gutiérrez-Miceli, L. Dendooven, Optimization of Osmotic Dehydration of Yam Bean (Pachyrhizus Erosus) Using an Orthogonal Experimental Design, Journal of Food Engineering, 84 (2008) pp. 413-419. [3] Y. Chen, J. Zhang, C. Yang, B. Niu, The Workspace Mapping with Deficient-DOF Space for the PUMA 560 Robot and Its Exoskeleton Arm EcoDesign2007/ December 10-13 in Tokyo 4
by Using Orthogonal Experiment Design Method, Robotics & Computer-Integrated Manufacturing, 23 (2007) pp. 478-487. [4] ASTM Standard G61-92, Conducting Cyclic Potentiodynamic Polarization Measurements for Localized Corrosion Susceptibility of Iron-, Nickel-, or Cobalt-Based Alloys, ASTM Standard, PA, 2003. [5] J.J. Yeh, I. Landau, At. Data Nucl. Data Tables, 32 (1985) 1. EcoDesign2007/ December 10-13 in Tokyo 5