Effect of chloride ions on 316L stainless steel in cyclic cooling water

Transcription

1 Available online at Acta Metall. Sin.(Engl. Lett.)Vol.23 No.6 pp December 2010 Effect of chloride ions on 316L stainless steel in cyclic cooling water Zuojia LIU 1), Xuequn CHENG 1), Shengjie LÜ 2) and Xiaogang LI 1) 1) Key Laboratory of Corrosion and Protection (Ministry of Education of China), University of Science and Technology Beijing, Beijing , China 2) Equipment and Power Department, Shijiazhuang Refine & Chemical Company Limited, SINOPEC, Shijiazhuang , China Manuscript received 22 April 2010; in revised form 20 August 2010 The effect of Cl on the 316L stainless steel in simulated cooling water has been studied using polarization curves, electrochemical impedance spectroscopy (EIS), Mott- Schottky plot and scanning electron microscopy (SEM) techniques. Cl concentrations vary from 200 to 900 mg/l. Results reveal that the corrosion resistance increases with the decrease of Cl concentration in simulated cooling water. The increase of Cl concentration leads to the shift of the corrosion potential towards the positive direction. Mott-Schottky curves show that in the passive film, Cr 2O 3 and FeO at ) N-type semiconductive properties. The SEM/EDX data demonstrate that elements such as Fe, O, C, Si and Cl as well as the presence of calcium and aluminum are presented on the surface of the inner layer exhibit P-type but Fe 2 O 3 and CrO 3 (CrO 2 4 the metal. KEY WORDS Polarization curve; EIS; Mott-Schottky plot; SEM/EDX; Simulated cyclic cooling water 1 Introduction The corrosion of metal equipment in contact with cyclic cooling water is one of the major failures in cooling systems [1]. The corrosion of metal equipment usually happens in cooling systems because of the presence of dissolved oxygen and aggressive ions such as chloride and sulphate [2,3]. If seawater is injected into the system to act a role as the coolant which circulates under the high concentration condition, the corrosion problem will be particularly intensified to a great extent [4]. Additionally, the cyclic cooling water is utilized widely in oil industries, so to extend the life-span of the equipments and to keep people working safely have become extremely important. In China, the metal condenser pipelines were mostly made up of brasses in the past few decades. However, in recent years, water shortage and pollution were primary reasons for that those equipments became easier to be depraved, which led to plenty of corrosion problems with increasing the utilization rate of brass alloys. Therefore, the brass alloy has been replaced by a series of good quality Corresponding author. Professor; Tel: ; Fax: address: lixiaogang99@263.net (Xiaogang LI)

2 432 stainless steels in more and more relevant industries. It is well known that stainless steel has a great corrosion resistance in the oxidizing corrosive environment because of the passive film which forms on the steel surface to protect the material from destroyed in the atmosphere. However, in many acidic corrosive mediums such as sulfuric acid, acetic acid solutions and other solutions including Cl, the passivity cannot last for a long time [5]. It is generally believed that the passive film on the stainless steel is a double-layer, which is consisted of [6] : Cr oxides in the inner layer that is the primary part during formation of the passive film forms, the Fe oxides and hydroxides which are mainly full up in the outer layer. Piao et al. [7] considered that Fe 2 O 3 and Cr 2 O 3 are two major components when the passive film forms, thus the film will be broken with the dissolution of Cr 2 O 3. Besides, many corrosive ions can be found in the cyclic cooling water, and Cl is the main anion which is responsible for the corrosion problems. The aim of this study is intended to evaluate and understand the corrosion behavior and the usability of 316L stainless steel in simulated cyclic cooling water, and attempted to provide a theoretical evidence for practical production about the influence of Cl on austenitic stainless steel AISI 316L in the situation. 2 Experimental The experiment was carried out in a standard three-electrode cell. The reference electrode was saturated calomel electrode (SCE) and the counter electrode was a platinum bar. The working electrode was a 316L stainless steel with surface area of 1 cm 2 and the chemical composition (wt pct) is as follows: C, 0.60 Si, 0.80 Mn, Cr, Ni, S and Fe balanced. The stainless steel surface was prepared by mechanical polishing with silicon carbide paper from 400 to 1200 mesh, cleaned with acetone, washed in distillated water and finally dried with hot air. All assessments were carried out at temperature of (32±1) C and ph 7.0, respectively, which are characteristic operating conditions for these systems. All potentials were expressed with respect to SCE. The composition of solution was already defined [8], and the chemical composition of synthetic cooling water system is presented in Table 1. Table 1 Chemical composition of the simulated cooling water Ion Mg 2+ Cl Ca 2+ SO 2 4 HCO 3 Na + NO 3 Concentration/(mg/L) , 400, 600, 700, 800, The working electrode was immersed in test solution for one hour until a steady state open circuit potential (E ocp ) was obtained. After that, the anodic polarization curve was recorded by polarization from the E ocp in positive direction under potentiodynamic condition of mv/s (sweep rate) and air atmosphere with different Cl concentrations. The electrochemical impedance spectroscopy (EIS) measurements were performed using a transfer function analyser (Voltalab PGZ 100), with a small amplitude a.c. signal (10 mv) over a frequency range from 100 khz to 10 mhz. The impedance diagrams were given in the Nyquist representation. In order to ensure reproducibility, all experiments were repeated three times. The evaluated inaccuracy did not exceed 10%. The electric parameters for the impedance plots were obtained by analyzing the experimental data using equivalent circuits and the non-linear regression algorithm of Boukamp [9].

3 433 The potential range of Mott-Schottky plot was set up between 0.3 V and 1.2 V, which was according to the passive range in polarization curve tests. The M-S test was made by measuring the frequency response at 1000 Hz [10] and scan rate of 50 mv/s. Examination of 316L stainless steel after exposure to the simulated cooling water for 7 days was carried out using scanning electron microscope (SEM; FEI QUANTA-250). 3 Results and Discussion 3.1 Polarization curves The polarization curves obtained are shown in Fig.1. The temperature (32 C) is just the temperature of circulation cooling water. In the cathodic range, the currents are less sensitive to the Cl than that in the anodic range, a significant increase of the anodic current densities leads to a shift of the corrosion potential (E corr ) towards more cathodic direction when the Cl concentration Fig.1 Polarization curves for 316L stainless increases. This suggests that variation of Cl steel in simulated cooling water with acts only on the anodic sites. This can be explained on the basis that specific adsorption different Cl concentrations at 32 C and de/dt =0.167 mv/s. of the Cl ions is on the electrode surface, and thus retards the Cl diffusive action [11,12]. Apparently, the bond between the surface atoms of metal and the adsorbed Cl ions is so strong as to destroy passive film on the surface of stainless steel [12]. In Fig.1, the parameters can be viewed on the zoom in the inset enumerated in Table 2. It shows that the corrosion current has a beneficial uptrend with increasing Cl concentration. Pitting stabilized at potentials between V and V, which can be seen that the 316 L stainless steel has an outstanding ability to inhibit corrosion in the cooling water situation. SO 2 4 and NO 3 have also serious destructive action, while coexistence of HCO 3 and Ca2+ are able to inhibit corrosion to a great extent. On the other hand, according to the analysis of Valeria et al. [13], there is a protective film on the specimen. Most corrosion products would be deposited at inner surface of the steel, which led to the pitting problem seriously. 3.2 Electrochemical impedance spectra This part is not to give a thorough modeling of the interface metal/electrolyte, but to Table 2 Electrochemical parameters for 316L stainless steel in simulated cooling water with different Cl concentrations Cl concentration/(mg/l) E corr /V I corr /(A/cm 2 ) E b /V

4 434 schematize the electrochemical system coarsely in order to extract the values from the parameters allowing evaluating the effectiveness of the changed Cl concentration. Nyquist plot obtained in the various concentrations of Cl is given in Fig.2. In Fig.2, two capacitive loops are observed. The high frequency (HF) loop (which can be viewed on the HF zoom in the inset) shows approximate linear portion, suggesting the existence of a porous behavior, which is in agreement with de Levie theory [14]. However, the low frequency loop is generally assigned to a slow transport process like diffusion. Also, it can be seen that radius of the impedance arc increases with increasing Cl concentration at neutral ph value of 7.0. The equivalent circuit proposed is similar to the stainless steel surface corrosion in the simulated cooling water [15 17] (Fig.3). Where R s indicates the electrolyte resistance, R f is the resistance associated with the layer of products, CPE f is the capacitance of the same layer, R ct is charge transfer resistance, CPE dl is double-layer capacitance, and Z dif is the diffusion impedance through the layer of products. Fig.2 Nyquist diagrams for 316L stainless steel in simulated cooling solution at various Cl concentrations. Fig.3 Equivalent circuit proposed for fitting the impedance spectra obtained on stainless steel surface in simulated cooling water. Electrochemical parameters established with the Nyquist diagrams obtained with various Cl concentrations are listed in Table 3. The double-layer capacitance is obtained according to the equation C dl =(2πf max R t ) 1, in which f max is the frequency at the top of the capacity arc. The result shows that the increase of Cl concentrations involves the decrease of R ct and R f values, but the C dl values have an increasing trend. The increase of C dl is due to the adsorption of corrosion anions on the metal surface. This adsorption occurs directly on the basis of donor and acceptor interactions between the electrons of oxygen compound and the vacant d-orbital of stainless steel surface atoms, which already adsorbed chloride or sulfide ions. Also, it is found that the solution resistance R s slightly decreased with Table 3 Impedance parameters for 316L stainless steel in simulated cooling water at different Cl concentrations Cl concentration/(mg/l) R s /(Ω cm 2 ) R ct /(kω cm 2 ) R f /(kω cm 2 ) C dl /(µf cm 2 ) C f /(µf cm 2 )

5 435 increasing Cl concentration. This decrease may be attributed to the slight increase of the ionic concentration of the solution by iron dissolution. On the other hand, these results demonstrate that the formation of corrosion and scale products on the electrode surface is reinforced by increasing Cl concentration. The C f values are reasonable for a thin surface film formed with corrosion/scale products. The AC impedance study also confirms the corrosion character of different Cl concentrations obtained with polarization curves. It is shown clearly that the corrosion susceptibility is significantly reduced as concentration of Cl decreases. The variation of Cl concentration usually acts on the solid surface by forming an adsorption film or inducting the formation of corrosion products layer which can prevent corrosion anions from diffusing on the surface of metal. This adsorption depends on the charge of the metallic surface and the adsorption of other ionic species in the solution [18,19]. 3.3 M-S (Mott-Schottky) test Mott-Schottky analysis is related to the capacitance of the space charge layer of the passive film. The capacitance is as a function of electrode potential as follows [20] : 1 C 2 = 2 ( E E fb KT ) ξξ 0 en D e 1 C 2 = 2 ( E E fb KT ) ξξ 0 en A e where ε is the dielectric constant of the oxide film (ε=15.6 [21] ), ε 0 is the vacuum permittivity ( F/m), e is the charge of the electron ( C). N D and N A are donor density and acceptor density (m 3 ) respectively, E fb is the flat-band potential (V vs. SCE). KT/e can be negligible as it is only about 25 mv at the test temperature. According to the Mott-Schottky dependence, the values of charge space capacity were recalculated to obtain the relationship C 2 =f(e), which is presented in Fig.4. For stainless steels and other Fe-Cr or Ni-Fe-Cr-based alloys, it is often assumed that the semiconducting behavior reflects the duplex character of their surface films, with an inner region essentially formed of Cr 2 O 3 and an outer region mainly composed of Fe 2 O 3. Fig.4 shows that each line is similar in shape, but two types of slopes are found in the M-S diagram, perhaps it is related to the existence of various donor sites in the energy band [22]. Literature [23] considered that this passive film is ambipolar and polycrystal film. According to the trend of M-S curves under different Cl concentrations, the diagram is divided into two areas. The positive slope with the potential range from 0.3 V to 0.5 V (region I) suggests N-type semiconductivity of the passive film, and the donor density decreases with increasing Cl concentration. Fe 2 O 3 and CrO 3 (CrO 2 4 ) are two elements in the passive film, which exhibit N-type semiconducting property. 1/C 2 / F -2 cm 4 7x10 9 Cl - concentration / (mg/l) 6x10 9 I II x x x10 9 2x10 9 1x E / V (1) (2) Fig.4 Mott-Schottky plots of 316L SS in simulated cooling water with various Cl concentrations.

6 436 Therefore, it is reasonable for the result of M-S test in region I. In region II ( V), negative slope suggests P-type semiconductivity of the passive film, and the acceptor density increases with increasing Cl concentration. In this region, the passive film is mainly composed of Cr 2 O 3, FeO and NiO [24] and it exhibits P-type semiconductivity property. Values of N D, N A and E fb estimated by Eqs.(1) and (2) are listed in Table 4 for different Cl concentrations. Table 4 indicates that a part of low valence oxide and hydroxide of the metal such as Fe(OH) 2 and Cr 2 O 3 which are oxidized to Fe 2 O 3 and CrO 3 (CrO 2 4 ) in the passive film, leads to the raised valence; therefore, the acceptor and donor densities of passive film increase with Cl concentration, and cause the anion concentration in electric double-layer moved up. In other words, corrosion rate accelerates due to the invaded Cl on the passive film surface. Table 4 M-S plot parameters for 316L SS in cooling water with different Cl concentrations Cl concentration/(mg/l) Slope k N D /m 3 N A /m Additionally, the flat band of passive film in the I and II regions are V and V, respectively, which shows that the flat band turns to more positive value with increasing Cl concentration. The flat-band potential of the semiconductor calculated by Nerstain formula [22] is as following: E fb = Ef 0 q + φ h (3) where E f 0 /q is potential drop of space charge layer, φ h is potential difference between electrode surface and Helmholtz layer. Flat-band potential reflects the variety of potential distribution in Helmholtz layer. The adsorbance of anions leads to negative charges in the surface of passive film and results in the variations of φ h and flat-band potential finally. Hence, the flat band of passive film increases with increasing Cl concentration. 3.3 Scanning electron microscopy observation Fig.5 shows the scanning electron microscopy (SEM) images of 316L stainless steel surface exposed to the simulated cooling medium for 7 days in two different Cl concentrations, 200 and 900 mg/l respectively, and ph of 7.0. The SEM image in the 200 mg/l Cl concentration solution (Fig.5a) shows a few areas free of corrosion and scale products, it reveals the protection of the low Cl concentration for the metal. Indeed an EDX analysis (Fig.6) further identified the characteristic corrosion products elements (Fe, O, C, Si and Cl) as well as the presence of calcium and aluminum on the layer. Therefore, the existence of some corrosion and scale products can retard the passive film dissolving and breaking. Furthermore, the image of the surface exposed to the high Cl concentration

7 437 Fig.5 SEM figures of 316L stainless steel electrode surface after 7 days immersion in simulated cooling water with Cl concentrations of 200 mg/l (a) and 900 mg/l (b). Fig.6 EDX analysis of 316L stainless steel electrode surface after 7 days immersion in simulated cooling water with Cl concentrations of 200 mg/l. solution (Fig.5b), shows a great many of pitting corrosion holes and heterogeneous scale products. In this case, EDX analysis and SEM show again the destroyed metal surface by increasing of Cl concentration. It is in accordance with electrochemical measurements, in which the increasing of Cl concentration in cooling water plays role of corrosion accelerator. 4 Conclusions (1) For 316L stainless steel in simulated cyclic cooling water with different Cl concentrations the corrosion resistance decreases with increasing Cl concentrations. (2) The variation of Cl concentration usually acts on the solid surface by forming an adsorption film or inducting formation of a corrosion products layer, which has ability to prevent corrosion anions from being diffused, and causes more and more Cl accumulated on the surface of metal. Ultimately, the stainless steel is destroyed. (3) In this system, the passive film is a structure of double-layer, so the M-S curves show duplex characters. (4) SEM/EDX data indicate that a few corrosion products elements (Fe, O, C, Si and Cl) and calcium as well as aluminum present on the metal surface. Acknowledgements This work was supported by the National Natural Science Foundation of China (No ), and Science & Technology Program of Beijing (No.D ).

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