- Email: [email protected]
Optimized corrosion performance of a carbon steel in dilute sulfuric acid through heat treatment
Applied Surface Science 491 (2019) 460–468
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Optimized corrosion performance of a carbon steel in dilute sulfuric acid through heat treatment
T
⁎
Madjid Sarvghada, , Darwin Del Aguilab, Geoffrey Willa a b
Science and Engineering Faculty, Queensland University of Technology (QUT), Queensland 4001, Australia Aushex Pty Ltd, Brisbane, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon steel Sulfuric acid Heat treatment Corrosion Electrochemistry
Corrosion performance of pearlitic-ferritic carbon steel A179 was compared to its spheroidized condition in contact with 0.5 M H2SO4 using a combination of microstructural and electrochemical investigations. Results confirmed a more heterogeneous microstructure with fine ferrite and pearlite grains, increased grain boundary length per unit area and lattice defects (due to lattice strain) for the pearlitic metal compared to the spheroidized condition. Electrochemical measurements then showed different mechanisms for the corrosion of the metal samples. Polarization resistance was lower in the pearlitic condition with localized attack sites and high amounts of surface deposits. Spheroidization treatment was, on the other hand, found to result in 21 times reduction in the corrosion rate because of the development of a homogeneous microstructure.
1. Introduction Mild steel is a well-known and cost effective construction material for handling acids and other chemicals in the industry. However, its susceptibility to corrosion is a concern leading to the development of practical methods to mitigate corrosion [1–5]. Of the industries that demand a high compatibility between construction materials and chemicals are sulfuric acid plants, petroleum and chemical industries. H2SO4, as one of the most commonly used chemicals in the world, has applications in gasoline, automobile batteries, steel manufacturing, water treatment, fertilizers, etc. [6,7]. However, its high corrosivity requires a proper choice of materials to be considered. The fabrication process is a key factor in the performance of steel alloys especially when there are limitations regarding the costs or construction requirements. Desired changes to the microstructure or mechanical properties of steel alloys are usually achievable through the heat treatment of fabricated parts [8–13]. For example, grain refinement heat treatment has been reported to result in significant changes in the corrosion rate of an steel alloy in NaCl solution [9]. Savas et al. [12] reported changes in the corrosion mechanism of an stainless steel in HNO3 + Na2Cr2O7 solution due to air and oil quenching treatments. An investigation by Godec et al. [13] also showed that quenching, tempering and annealing can cause alteration in corrosion mechanism and subsequently lead to significant changes in the corrosion resistance of stainless steels in dilute sulfuric acid.
⁎
At room temperature, most mild steels usually come with a ferriticpearlitic structure in annealed or normalized condition. This structure could be modified to spherical carbides uniformly distributed in a matrix of ferrite, which is thermodynamically the most stable microstructure in steels. Such a spheroidization process usually leads to coarser grains compared to the original microstructure. The degree of spheroidization is then determined by the percentage of remaining lamellar pearlite [11]. Mechanisms of corrosion in sulfuric acid have been well studied in the literature [1,4,6,14–21]. In its dilute form, sulfuric acid reacts with metals (e.g. iron) through Eq. (1) producing hydrogen (gas) and metal sulfate (salt) on the metal surface [7,16,22]:
Fe(s) + H2 SO4 (aq) → H2 (g) + FeSO4 (aq)
(1)
Corrosion mechanisms, however, could be more complicated especially when significant changes are applied to the metal; e.g. through modification processes such as heat treatment. Fig. 1 shows an example of the in-service behaviors (according to Aushex Pty Ltd) of two uninhibited cold-drawn low-carbon steel ASTM-A-179 tubes, in ferriticpearlitic condition versus spheroidized condition, after 15 h exposure to 6% v/v H2SO4 at 60 °C and flow rate of 1 m/s. Significant corrosion of the pearlitic tube with 55.7 wt% mass loss is comparable to the relatively unaffected spheroidized tube with only 6.2 wt% mass loss. To investigate the above phenomena electrochemical measurement techniques, such as impedance spectroscopy and polarization
Corresponding author. E-mail address: [email protected] (M. Sarvghad).
https://doi.org/10.1016/j.apsusc.2019.06.180 Received 6 January 2019; Received in revised form 11 May 2019; Accepted 18 June 2019 Available online 18 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 1. Mass loss images of pearlitic vs. spheroidized ASTM-A-179 tubes subjected to 6% v/v H2SO4 at 60 °C and flow rate of 1 m/s for 15 h, according to Aushex Pty Ltd.
were selected and mounted into I) a conductive resin, and II) a nonconductive resin in order to provide suitable specimens for microstructural and corrosion studies. Both pairs were mechanically wet ground and polished down to 0.04 μm using standard procedures in colloidal silica, washed with ethanol and dried in the air. The coupons mounted in conductive resin were used for pre-exposure microanalysis and therefore were not subjected to corrosion. The other pair was used to monitor the impact of the corrosive medium on the as-polished surfaces by submerging them in 0.5 M H2SO4 for 10 min for static corrosion investigation. This pair was then re-polished and submerged in the acid solution for electrochemical analysis.
resistance, are accordingly considered as useful corrosion monitoring practices to track changes in mechanisms [23–25]. This paper, elaborates changes in corrosion rates and mechanisms of ferritic-pearlitic tubes of cold-drawn low-carbon steel ASTM-A-179 vs. spheroidized tubes in 0.5 M H2SO4 at room temperature. The tubes are produced seamless, with a final cold-draw to guarantee dimensional tolerances and surface finish. Finally, heat treatment is conducted at 650 °C or above [26,27]. 2. Experimental Coupons of ferritic-pearlitic and spheroidized ASTM-A-179 tubes, supplied by Hangzhou Wanyun Precision Cold-draw Steel Pipe Co., Ltd. and organized by Aushex Pty Ltd., were employed to study the effect of 0.5 M sulfuric acid on the corrosion behavior for application in alumina refineries as heat exchanger (see Fig. 1). The manufacturing processes of the studied steels were as below: I) Piercing of the billet in 1200 °C for a short time, II) first cold drawing, III) second cold drawing (final size 38.1 × 2.6 mm), IV) spheroidization treatment at 780 °C for 4 h; ferritic-pearlitic tubes were normalized at the same temperature for 20 min. The chemical compositions and microstructural characteristics of the studied alloys are provided in Table 1. Samples of each set are referred to as spheroidized (or spheroid) and pearlitic (or pearlite) hereinafter in this paper. Several cut-offs from as-received tubes of the studied alloys were prepared for microstructural and electrochemical studies as described below.
2.2. Microstructural investigation 2.2.1. Scanning electron microscopy and EBSD Pre-exposure microstructure was studied using a Field Emission Scanning Electron Microscope (FE-SEM) machine model JEOL 7001F, with automated feature detection equipped with secondary electron and Energy Dispersive X-ray Spectroscopy (EDS) analysis system, OXFORD Electron Back-Scatter Diffraction (EBSD) pattern analyzer and Channel 5 analysis software. Strain contouring component of OXFORD Channel 5 software was employed to estimate the extent of deformation, or strain, in individual grains in EBSD maps. Maps were obtained using accelerating voltage of 25 kV and step size of 0.9 μm at 700× magnification. In order to reveal carbides in the ferritic-pearlitic and spheroidized microstructures resulted from the discussed heat treatment processes, samples mounted in the conductive resin were also etched in 10 ml glycerin +6 ml HCl + 3 ml HNO3 solution for 10 s [28]. This was done as complementary to EBSD since it is not capable of revealing pearlitic structures in steels. SEM images were also obtained immediately (in the first few hours) after the exposure of electrochemical samples in secondary electron mode.
2.1. Sample preparation Two pairs of cut-off coupons of spheroidized and pearlitic samples Table 1 Chemical composition and microstructure of the studied alloys (according to Moly-Cop, NATA accreditation). Microstructure
Spheroidized Pearlitic
2.2.2. Optical microscopy analysis Corrosion attack and microstructures were immediately observed on as-polished samples after 10 min exposure to the H2SO4 solution using an optical microscope model Leica DMi8A with the magnification range of 2.5× to 50× equipped with Leica Application Suite software.
Chemical composition (wt%) C
Si
Mn
P
S
0.102 0.117
0.243 0.226
0.491 0.353
0.008 0.012
0.004 0.001
461
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
2.3. Electrochemical investigation
on the left) with similar GB attack.
Electrochemical experiments were conducted (on as-polished samples) using a three-electrode cell containing 0.5 M H2SO4 at room temperature by means of a VMP3-based BioLogic instrument controlled by EC-Lab® software. Test coupons were subjected to Open Circuit Potential (OCP), Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and Potentiodynamic Polarization (PDP) measurements. Prior to experiments, specimens were mounted in a nonconductive resin (exposing 0.27 and 0.29 cm2 of the surfaces of pearlitic and spheroidized samples, respectively) and then polished as discussed in Section 2.1. A platinum sheet was used as counter electrode, a standard Ag/AgCl (VL_Ionode004) as reference electrode with the sample of interest as working electrode. Electrochemical measurements were carried out in the following order for each sample: OCP was measured for 90 min after the immersion of samples into the acid solution. EIS measurements were then obtained using a frequency range of 100 kHz to 100 mHz with the amplitude of ± 10 mV. EIS measurements were repeated 3 times at 5-min intervals for each sample. LPR measurements were run afterwards at the potential scan rate of 0.1 mV/s and potential range of −25 to +25 mV with respect to OCP. Finally, PDP was conducted at the potential scan rate of 10 mV/ min and potential range of −200 to +500 mV with respect to the open circuit potential. ZFit analysis of EC-Lab® software was used to fit successive impedance cycles. LPR plots were analyzed using the polarization resistance (RP) fit tool of EC-Lab® software to calculate Rp, corrosion current density (icorr) and corrosion potential (Ecorr) values within ± 20 mV from the corrosion potential. PDP plots were also analyzed using the software's Tafel fit tool to calculate Tafel slopes (βa and βc), icorr and Ecorr values.
3.3. Electrochemical behavior Open Circuit Potentials (OCP) of the samples in 0.5 M H2SO4 are shown in Fig. 6a. Both samples show an active behavior in the first 500 s of exposure. The spheroidized sample then tends to stabilize at around −490 mV while the pearlitic sample gains nobler potential values over time, but is not stabilized even after 90 min (5400 s) of exposure. Ecorr values in E-logi plots (PDP and LPR) shown in Fig. 6b reflect OCP values in Fig. 6a. Fig. 7 also shows LPR plots (E vs. i) of the test samples in the corrosive medium. Data extracted from PDP and LPR curves are summarized in Table 4. Polarization resistance (Rp) is defined as the slope of potential vs. current density curve, Fig. 7, at the free corrosion potential according to Faraday's law [22,24]:
dE Rp = ⎛ ⎞ ⎝ di ⎠Ecorr
(2)
where, E is the corrosion potential and i is the corrosion current. Corrosion current density (icorr) is then calculated according to the SternGeary relationship [25,29]:
icorr =
βa |βc | Rp (βa + |βc|) ln 10
(3)
where, the activation controlled Tafel slopes (βa and βc) are determined through the fitting of PDP curves, see Table 4. Fig. 7 and data in Table 4 show a much higher Rp value for the spheroidized sample with an RP drop of around 93% for the pearlitic sample. This is in agreement with the noticeable rise in icorr values (separate calculations from PDP and LPR measurements, see Table 4) and the accompanied steeper Tafel slopes in both cathodic and anodic branches of the pearlitic sample compared to the spheroidized one. This suggests that different mechanisms control corrosion of the samples here. Corrosion current density corresponds to corrosion rate (CR) according to [24,30]:
3. Results 3.1. Pre-exposure microstructure Optical and SEM images of etched samples (Fig. 2) as well as EBSD maps of polished and mirror-like samples (Fig. 3) show a texture consisting of pearlites (α + Fe3C) distributed through coarse grains of ferritic iron (α) with particles of cementite (Fe3C) mostly dispersed along grain boundaries (GB) for spheroidized sample. Remaining pearlites here point to the fact that the spheroidization process was not 100% complete. On the other hand, relatively fine grained proeutectoid ferrite (α) with a higher fraction of pearlites (α + Fe3C) is observed for the pearlitic-ferritic sample. Table 2 shows the fraction of phases extracted from EBSD mappings. GB carbide (cementite) in the pearlitic sample is around twice of that in the spheroidized sample. The fraction of zero solution areas (white areas in band contrast and black areas in phase map images of Fig. 3) in both samples could point to the lamellar pearlitic structure which is hard to detect with EBSD. EBSD strain contouring maps in Fig. 4 show that the maximum of lattice strain for pearlitic sample (≃2) is almost 10 times of that for spheroidized sample (≃0.2). This means that spheroidization has led to significant stress relieving resulted in lower amounts of residual stresses remaining in the matrix compared to the pearlitic sample.
CR = K . EW .
i corr ρ
(4)
where, K = 3.272e-3 mm.g.μA−1.cm−1.yr−1, EW is equivalent weight (= 27.92 for carbon steel [24]) and ρ is density (= 7.87 for iron) in g.cm−3. CR values extracted directly from PDP measurements as well as those from LPR are also shown in Table 4. Accordingly, the pearlitic sample is shown to corrode around 21 times faster than the spheroidized one. It is worth noting that the CR values reported here only indicate the initial corrosion rates and no long-term corrosion measurement was conducted in this study. To further understand the mechanisms of corrosion, EIS was measured immediately after OCP and repeated for 3 times at 5-min intervals. Nyquist and Bode-Phase plots in Fig. 8 and Fig. 9 confirm much higher polarization resistance (Rp) of the spheroidized sample against 0.5 M sulfuric acid compared to that of the pearlitic one. In addition, repetition of EIS confirms that the spheroidized sample has been well stabilized after 90 min exposure, whereas the pearlitic sample is still active with a continuous reduction of Rp over time (diameters of the capacitive loops in Fig. 8b). Nyquist plots in Fig. 8 were successfully fit (only the last curve, 10 min, for each sample) using the indicated equivalent circuits with the extracted data summarized in Table 3. Here Rs, Rct and RL point to solution resistance, charge transfer resistance and inductance resistance, respectively. Qdl is the double layer Constant Phase Element (CPE), which points to the non-ideal capacitance of the surface and is used to calculate the surface charge transfer capacitance during
3.2. Static corrosion behavior To investigate the impact of the corrosive solution on the material surface, as-polished mirror-like surface samples were subjected to 0.5 M H2SO4 for 10 min and then examined immediately under light microscope. Low and high magnification optical microscopy images of the spheroidized sample are shown in Fig. 5(a to c). The image shows that the acid attacks selective areas on the surface. These areas are mostly corresponding to GBs and the interfaces between cementite and ferrite (see Fig. 2 and Fig. 3). Images of the pearlitic sample in Fig. 5(d to f) show a more serious corrosion (compared to the spheroidized sample 462
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 2. Optical (left) and SEM (right) images of as-received spheroidized (a and b) and pearlitic (c and d) samples etched in 10 ml glycerin +6 ml HCl + 3 ml HNO3 solution for 10 s.
Fig. 3. EBSD band contrast image (a) and phase map (b) of spheroidized sample, EBSD band contrast image (c) and phase map (d) of pearlitic sample prior to exposure. 463
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
solubility limit in the solution which could hinder sulfate saturation, compared to the pearlitic sample [37].
Table 2 Phase fraction of samples resulted from EBSD. Phase
Phase fraction (%)
4. Discussion Fe3C Ferrite (iron bcc) Zero solutions
Spheroidized
Pearlitic
2.8 83.6 13.6
4.6 80.0 15.4
As shown in Fig. 2 and Fig. 3, the pearlitic sample contains finer grains resembling an increased GB length (per unit volume) compared to the spheroidized sample. The higher fraction of pearlites (and subsequently the interface of cementite/ferrite) and GB carbides along with increased GB length lead to a heterogeneous microstructure in the pearlitic sample. Also, the co-existence of ferrite and cementite (Fe3C) in pearlite can cause galvanic effects which accelerate the dissolution rate of the inside and surrounding ferrite [38]. Therefore, the microstructure of this metal is expected to be more susceptible to localized attack, mainly because of the formation of micro-anodes/cathodes on the surface [39–41], compared to the spheroidized structure with coarse proeutectoid ferrites. Lattice defects and dislocations have been already found to be the initials for the formation of localized attack in steel alloys [38,41]. The higher amount of lattice strain and subsequently residual stresses (lattice defects in general) in pearlitic sample implies less thermodynamic stability compared to the spheroidized sample. This could be confirmed through OCP plots and 5-min interval impedance measurements as shown in Fig. 6a and Fig. 8. Such an instability could also provide driving force for corrosion and contribute to the higher corrosion rate and/or lower polarization resistance (see Table 4) of the pearlitic sample as confirmed by PDP and LPR measurements in Fig. 6b and Fig. 7, respectively. Noticeable discrepancies between the Tafel slopes (βa and βc) and Rp values of the samples indicate a significant difference in corrosion kinetics. According to Eq. (1), an insoluble iron sulfate (FeSO4) is likely to precipitate on the surface of the metal [7,16,22]. This involves the simultaneous exchange of electrons through the oxidation of metal (anodic reaction) and reduction of hydrogen (cathodic reaction) according to the below reactions, respectively [22]:
Fig. 4. EBSD strain contouring maps of the same areas as band contrast images of the samples in Fig. 3 with their corresponding strain contouring profiles showing the amount of lattice strain.
corrosion [31], while n represents the CPE parameter. The difference in mechanisms is illustrated in the EIS clearly by the spheroidized sample detected fitting a Randles-like equivalent circuit with a semicircle and one time constant while an inductive loop appears for the pearlitic sample with a modified inductance (La) at low frequencies. It is noteworthy that modified inductance could be used as an element with positive values in an equivalent circuit to avoid interpreting negative values associated with some inductive loops [32]. Impedance of an inductance (ZL) is expressed as [33–36]:
ZL (jω) = Ljω
(7)
2H+
(8)
+
2e−
→ H2
The anodic dissociation of Fe in aqueous solutions also involves the formation of adsorbed intermediate (FeOH)ads [42,43]. Consequently, the below reactions contribute to Fe dissociation [43]:
(5)
Fe + (FeOH )ads → Fe (FeOH )ads
where L indicates inductance, j = −1 , angular frequency ω = 2πf (f is frequency in Hz). The impedance of a modified inductance (La) is then expressed as [32]:
ZLa (jω) = La (jω)aL
Fe → Fe 2 + + 2e−
(6)
(9)
Fe (FeOH )ads + OH− → FeOH+ + (FeOH )ads + 2e−
(10)
FeOH+ + H+ → Fe 2 + + H2 O
(11)
Accordingly, through reducing the defects such as GBs and ferrite/ cementite interfaces in pearlite, spheroidization seems to shift the equilibrium of Eq. (9) to the left [43] leading to significant reductions in both metal dissolution and hydrogen evolution rates of the steel, see Tafel slopes in Table 4. Such a transition is actually because of a change in the mechanism of corrosion as shown by Nyquist plots and their corresponding equivalent circuits in Fig. 8. Impedance spectra show that the corrosion of spheroidized sample is under charge transfer (activation) control with a large capacitive loop (high Rp value). However, the diameter of the semicircle (impedance) significantly reduces for the pearlitic sample (low Rp value) while an inductive loop appears in its low-frequency region which indicates adsorption (precipitation) following the Volmer-Heyrovský mechanism [44–47]. The continuous ohmic drop of the double layer (reduction of the diameter of semicircle with time) also indicates that the corrosion rate of the pearlitic sample increases with time [48]. SEM-EDS images in Fig. 10 further approve the above discussion. Discrepancy in mechanism appears as different precipitates on the samples' surface after the electrochemical measurements. So the much
where aL represents the La parameter (if aL = 1 then La ≡ L). Polarization resistance Rp could be also calculated from data in Table 3 where for the spheroidized sample Rp = Rct and for the pearlitic sample Rp = Rct + RL. These values (which are also shown in Table 4) are correspondingly in good agreement with those of the LPR measurement in Table 4. Electrochemical samples were analyzed with SEM-EDS just a few hours after exposure to visualize the results seen in the electrochemical experiments. Fig. 10 shows comparative images of the samples' surfaces with the same magnification, including their corresponding EDS analysis maps. Different corrosion mechanisms with different corrosion precipitates formed on the samples' surfaces are obvious. Amount and density of precipitates developed on the surface of pearlitic sample are higher than those of the spheroidized sample. EDS analysis maps clearly show oxygen and sulfur containing products on the surface of the pearlitic sample, while S was not detected for the spheroidized sample, probably because of its low concentration on the surface. It is possible that the ferrous sulfates have been produced at a rate lower than their 464
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 5. Low and high magnification optical images of spheroidized sample (a) to (c), and pearlitic sample (d) to (f) subjected to 0.5 M H2SO4 at room temperature for 10 min.
investigations was carried out to analyze the impact of 0.5 M H2SO4 on carbon steel A179 with a pearlitic-ferritic structure compared to when heat treated to a spheroidized structure. Electron Back-Scatter Diffraction, Optical and Scanning Electron Microscopy observations showed a more heterogeneous microstructure containing fine ferrite and pearlite grains with an increased grain boundary length and lattice defects (lattice strain) for the pearlitic metal compared to the spheroidized condition. Electrochemical Impedance Spectroscopy and Linear Polarization Resistance measurements showed different mechanisms for the corrosion of the samples with a much lower polarization resistance in the absence of stabilization for the pearlitic sample. Accordingly, polarization showed that spheroidization has led to 21 times reduction in the corrosion rate. This was further shown to result from reduction in the kinetics of both anodic and cathodic reactions. Further microscopical observations showed higher amounts of precipitates and higher amounts of corrosion on the surface of pearlitic sample. Finally, spheroidization treatment was found to lead to a more homogeneous microstructure which de-risks the formation of micro-
faster the corrosion rate of pearlitic sample is quite obvious with large amounts of porous sulfate precipitates. In the absence of an inhibitor, adsorption here could be referred to as surface contamination or precipitation [49]. Thermodynamically, this could be facilitated when the surface energy of the metal increases. Heterogeneity of microstructure, due to the presence of either polycrystalline metal surfaces (various crystals and grains) or crystal defects (such as GBs, dislocations, point defects, etc.), can increase the level of surface energy stored in the material [50–52]. Here, the pearlitic sample contains both the polycrystalline surface (increased number of grains with high fractions of pearlites and carbides) and crystal defects (due to the presence of residual stresses resulted from lattice strain together with the increased GB length). These are the areas which are susceptible to be preferentially attacked by the corrosive solution. Optical microscopy images in Fig. 5 confirm the above. 5. Conclusion A
combination
of
microstructural
and
electrochemical 465
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 6. (a) Open Circuit Potential (OCP), and (b) Potentiodynamic Polarization (PDP) curves of the test samples in 0.5 M H2SO4. Dash-dot lines in (b) show corresponding Evans diagrams (E-logi) resulted from LPR measurement.
Fig. 9. Bode-phase plots of the samples corresponding to the third (10 min) Nyquist plots in Fig. 8.
Fig. 7. Linear Polarization Resistance (LPR) curves of the samples in 0.5 M H2SO4.
Fig. 8. Nyquist plots resulting from EIS measurements of (a) spheroidized vs. pearlitic and (b) pearlitic samples along with calculated equivalent circuits and fit data points for the third (10 min) plots. 466
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Table 3 Data extracted from equivalent circuit models used to fit the curves in Fig. 8. Rs
Rct 2
Spheroidized Pearlitic
Qdl 2
ndl n-1
[Ω.cm ]
[Ω.cm ]
[F.s
2.4 3.4
350 20
1.5e-04 5.9e-04
−2
.cm
La
aL a-1
]
[H.s
[Ω.cm2]
.cm ]
– 1.4
0.77 0.86
RL
2
– 0.7
– 8.4
Table 4 Data extracted from PDP, LPR and EIS curves in Fig. 6b, Fig. 7 and Fig. 8, respectively. βa
Ecorr [mV vs. Ag/AgCl]
|βc|
icorr
Rp −2
CR 2
]
[Ω.cm ]
[mm/year]
[mV/dec]
[μA.cm PDP
LPR
LPR
EIS
PDP
LPR
67 1390
65 1420
313 23
350 28
0.78 16.2
0.76 16.6
Source
PDP
LPR
PDP
Spheroidized Pearlitic
−483 −458
−484 −462
69 131
141 180
Fig. 10. SEM and corresponding EDS map analysis images of samples after exposure for electrochemical investigation.
anodic/cathodic sites on the surface of the metal and significantly improves the corrosion performance.
Chem. 2013 (2013) 9. [2] L. Herrag, B. Hammouti, S. Elkadiri, A. Aouniti, C. Jama, H. Vezin, F. Bentiss, Adsorption properties and inhibition of mild steel corrosion in hydrochloric solution by some newly synthesized diamine derivatives: experimental and theoretical investigations, Corros. Sci. 52 (2010) 3042–3051. [3] S. Bilgic, N. Caliskan, An investigation of some Schiff bases as corrosion inhibitors for austenitic chromium–nickel steel in H2SO4, J. Appl. Electrochem. 31 (2001) 79–83. [4] M.A. Quraishi, S. Khan, Inhibition of mild steel corrosion in sulfuric acid solution by thiadiazoles, J. Appl. Electrochem. 36 (2006) 539–544. [5] F. Bentiss, M. Bouanis, B. Mernari, M. Traisnel, M. Lagrenee, Effect of iodide ions on corrosion inhibition of mild steel by 3,5-bis(4-methylthiophenyl)-4H-1,2,4-triazole in sulfuric acid solution, J. Appl. Electrochem. 32 (2002) 671–678. [6] Industrial Applications of Sulfuric Acid, © WOC Article, (2014). [7] M.G. Fontana, Corrosion Engineering, 3 ed., McGraw-Hill Book Company, 1985. [8] L.L. Huang, H.M. Meng, L.K. Liang, S. Li, J.H. Shi, Effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 thermal barrier coatings, Int. J. Miner. Metall. Mater. 22 (2015) 1050–1059. [9] A.H. Seikh, Influence of heat treatment on the corrosion of microalloyed steel in sodium chloride solution, J. Chem. 2013 (2013) 1–7. [10] A.N. Isfahany, H. Saghafian, G. Borhani, The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel, J. Alloys Compd. 509 (2011) 3931–3936.
Acknowledgement The data reported in the paper were obtained at the Central Analytical Research Facility (CARF) operated by the Institute for Future Environments (IFE) at Queensland University of Technology (QUT). Access to CARF was supported by funding from the Science and Engineering Faculty, QUT. The commercial tube materials were supplied by Aushex Pty Ltd. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] P. Thiraviyam, K. Kannan, P. Premkumar, A characteristic investigation of aminocyclohexane-N′-methylurea as a corrosion inhibition for mild steel in 1 N H2SO4, J.
467
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
88–97. [32] The Modified Inductance Element La, Application Note 42, Bio-Logic Science Instruments, (2012). [33] S. Ramanathan, Negative resistances and inductances in equivalent circuits for adsorption-reaction kinetics, ECS Trans. 33 (2011) 21–35. [34] A. Lasia, Impedance spectroscopy applied to the study of electrocatalytic processes, in: K. Wandelt (Ed.), Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, 2018, pp. 241–263. [35] A. Lasia, Electrochemical Impedance Spectroscopy and its Applications, Springer, New York, NY, 2014. [36] C. Gabrielli, M. Keddam, H. Takenouti, The Use of a.c. Techniques in the Study of Corrosion and Passivity, vol. 23, (1983), pp. 395–451. [37] S. Mischler, E. Rosset, G.W. Stachowiak, D. Landolt, Effect of sulphuric acid concentration on the rate of tribocorrosion of iron, Wear 167 (1993) 101–108. [38] C. Liu, X. Cheng, Z. Dai, R. Liu, Z. Li, L. Cui, M. Chen, L. Ke, Synergistic effect of Al (2)O(3) inclusion and pearlite on the localized corrosion evolution process of carbon steel in marine environment, Materials (Basel) 11 (2018). [39] D. Clover, B. Kinsella, B. Pejcic, R. De Marco, The influence of microstructure on the corrosion rate of various carbon steels, J. Appl. Electrochem. 35 (2005) 139–149. [40] Q.Y. Pan, W.D. Huang, R.G. Song, Y.H. Zhou, G.L. Zhang, The improvement of localized corrosion resistance in sensitized stainless steel by laser surface remelting, Surf. Coat. Technol. 102 (1998) 245–255. [41] C. Liu, R.I. Revilla, D. Zhang, Z. Liu, A. Lutz, F. Zhang, T. Zhao, H. Ma, X. Li, H. Terryn, Role of Al2O3 inclusions on the localized corrosion of Q460NH weathering steel in marine environment, Corros. Sci. 138 (2018) 96–104. [42] J.O.M. Bockris, D. Drazic, A.R. Despic, The electrode kinetics of the deposition and dissolution of iron, Electrochim. Acta 4 (1961) 325–361. [43] E.E. Oguzie, S.G. Wang, Y. Li, F.H. Wang, Corrosion and corrosion inhibition characteristics of bulk nanocrystalline ingot iron in sulphuric acid, J. Solid State Electrochem. 12 (2007) 721–728. [44] J.P. Diard, P. Landaud, B. Le Gorrec, C. Montella, Calculation, simulation and interpretation of electrochemical impedance. Part II. Interpretation of VolmerHeyrovsky impedance diagrams, J. Electroanal. Chem. Interfacial Electrochem. 255 (1988) 1–20. [45] J.P. Diard, B. Le Gorrec, C. Montella, Calculation, simulation and interpretation of electrochemical impedances. Part 3. Conditions for observation of low frequency inductive diagrams for a two-step electron transfer reaction with an adsorbed intermediate species, J. Electroanal. Chem. 326 (1992) 13–36. [46] J.P. Diard, B. Le Gorrec, C. Montella, Non-linear impedance for a two-step electrode reaction with an intermediate adsorbed species, Electrochim. Acta 42 (1997) 1053–1072. [47] F. Mansfeld, Analysis and Interpretation of EIS Data for Metals and Alloys, Technical Report No. 26, University of Southern California, Los Angeles, CA, USA, 1999. [48] J. Wei, J.H. Dong, W. Ke, X.Y. He, Influence of inclusions on early corrosion development of ultra-low carbon bainitic steel in NaCl solution, Corrosion 71 (2015) 1467–1480. [49] E. McCafferty, Introduction to Corrosion Science, (2010). [50] H.P. Stüwe, A.F. Padilha, F. Siciliano, Competition between recovery and recrystallization, Mater. Sci. Eng. A 333 (2002) 361–367. [51] R.A. Vandermeer, B.B. Rath, Interface migration during recrystallization: the role of recovery and stored energy gradients, Metall. Trans. A. 21 (1990) 1143–1149. [52] D. Raabe, Recovery and recrystallization: phenomena, physics, models, simulation, in: D.E.L. Hono (Ed.), Physical Metallurgy, Fifth edition, Elsevier, Oxford, 2014, pp. 2291–2397.
[11] G.E. Totten, Steel Heat Treatment: Metallurgy and Technologies, 2nd ed., CRC Press, Boca Raton, 2006. [12] T.P. Savas, A.Y.-L. Wang, J.C. Earthman, The effect of heat treatment on the corrosion resistance of 440C stainless steel in 20% HNO3 + 2.5% Na2Cr2O7 solution, J. Mater. Eng. Perform. 12 (2003) 165–171. [13] R. Godec, V. Dolecek, Influence of different heat-treatments on corrosion-resistance of martensitic stainless-steels, Werkst. Korros. 45 (1994) 517–522. [14] M. Itagaki, H. Nakazawa, K. Watanabe, K. Noda, Study of dissolution mechanisms of nickel in sulfuric acid solution by electrochemical quartz crystal microbalance, Corros. Sci. 39 (1997) 901–911. [15] M.J. Kim, J.G. Kim, Effect of manganese on the corrosion behavior of low carbon steel in 10 wt.% sulfuric acid, Int. J. Electrochem. Sci. 10 (2015) 6872–6885. [16] M.J. King, W.G. Davenport, M.S. Moats, Materials of Construction, Sulfuric Acid Manufacture - Analysis, Control, and Optimization, 2nd edition, Elsevier, San Diego, Calif, 2013, pp. 349–356. [17] E. Lazarova, G. Petkova, R. Raicheff, G. Neykov, Electrochemical study of the adsorption and inhibiting properties of halogen derivatives of aniline on iron in sulfuric acid, J. Appl. Electrochem. 32 (2002) 1355–1361. [18] V.V. Mehmeti, A.R. Berisha, Corrosion study of mild steel in aqueous sulfuric acid solution using 4-methyl-4H-1,2,4-triazole-3-thiol and 2-mercaptonicotinic acid-an experimental and theoretical study, Front. Chem. 5 (2017) 61. [19] O. Olivares-Xometl, E. Álvarez-Álvarez, N.V. Likhanova, I.V. Lijanova, R.E. Hernández-Ramírez, P. Arellanes-Lozada, J.L. Varela-Caselis, Synthesis and corrosion inhibition mechanism of ammonium-based ionic liquids on API 5L X60 steel in sulfuric acid solution, J. Adhes. Sci. Technol. 32 (2017) 1092–1113. [20] A. Rafsanjani-Abbasi, A. Davoodi, Electrochemical characterization of natural chalcopyrite dissolution in sulfuric acid solution in presence of peroxydisulfate, Electrochim. Acta 212 (2016) 921–928. [21] N. Sato, G. Okamoto, Anodic passivation of nickel in sulfuric acid solutions, J. Electrochem. Soc. 110 (1963). [22] S. Lyon, Overview of corrosion engineering, science and technology, Nuclear Corrosion Science and Engineering, Elsevier Ltd, 2012, pp. 3–30. [23] F. Mansfeld, Tafel slopes and corrosion rates obtained in the pre-Tafel region of polarization curves, Corros. Sci. 47 (2005) 3178–3186. [24] ASTM, G102-89(2015)e1, Standard Practice for Calculation of Corrosion Rates and Related Information From Electrochemical Measurements, ASTM International, West Conshohocken, PA, 2015. [25] ASTM, G59-97(2014), Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements, ASTM International, West Conshohocken, PA, 2014. [26] ASTM, A179/A179M-90a(2012), Standard Specification for Seamless Cold-Drawn Low-Carbon Steel Heat-Exchanger and Condenser Tubes, ASTM International, West Conshohocken, PA, 2012. [27] S.A. Al-Zubail, An Evaluation of Efficiency of Phenylenediamines as Corrosion Inhibitors for ASTM-A-179 Steel in 1.0 N Hydrochloric Acid at Room Temperature, Ohio University, 1987. [28] R.I. Olivares, D.J. Young, P. Marvig, W. Stein, Alloys SS316 and Hastelloy-C276 in supercritical CO2 at high temperature, Oxid. Met. 84 (2015) 585–606. [29] ASTM, G3-14, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, ASTM International, West Conshohocken, PA, 2014. [30] K. Vignarooban, P. Pugazhendhi, C. Tucker, D. Gervasio, A.M. Kannan, Corrosion resistance of Hastelloys in molten metal-chloride heat-transfer fluids for concentrating solar power applications, Sol. Energy 103 (2014) 62–69. [31] M. Sarvghad, T. Chenu, G. Will, Comparative interaction of cold-worked versus annealed inconel 601 with molten carbonate salt at 450°C, Corros. Sci. 116 (2017)
468
Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full length article
Optimized corrosion performance of a carbon steel in dilute sulfuric acid through heat treatment
T
⁎
Madjid Sarvghada, , Darwin Del Aguilab, Geoffrey Willa a b
Science and Engineering Faculty, Queensland University of Technology (QUT), Queensland 4001, Australia Aushex Pty Ltd, Brisbane, Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Carbon steel Sulfuric acid Heat treatment Corrosion Electrochemistry
Corrosion performance of pearlitic-ferritic carbon steel A179 was compared to its spheroidized condition in contact with 0.5 M H2SO4 using a combination of microstructural and electrochemical investigations. Results confirmed a more heterogeneous microstructure with fine ferrite and pearlite grains, increased grain boundary length per unit area and lattice defects (due to lattice strain) for the pearlitic metal compared to the spheroidized condition. Electrochemical measurements then showed different mechanisms for the corrosion of the metal samples. Polarization resistance was lower in the pearlitic condition with localized attack sites and high amounts of surface deposits. Spheroidization treatment was, on the other hand, found to result in 21 times reduction in the corrosion rate because of the development of a homogeneous microstructure.
1. Introduction Mild steel is a well-known and cost effective construction material for handling acids and other chemicals in the industry. However, its susceptibility to corrosion is a concern leading to the development of practical methods to mitigate corrosion [1–5]. Of the industries that demand a high compatibility between construction materials and chemicals are sulfuric acid plants, petroleum and chemical industries. H2SO4, as one of the most commonly used chemicals in the world, has applications in gasoline, automobile batteries, steel manufacturing, water treatment, fertilizers, etc. [6,7]. However, its high corrosivity requires a proper choice of materials to be considered. The fabrication process is a key factor in the performance of steel alloys especially when there are limitations regarding the costs or construction requirements. Desired changes to the microstructure or mechanical properties of steel alloys are usually achievable through the heat treatment of fabricated parts [8–13]. For example, grain refinement heat treatment has been reported to result in significant changes in the corrosion rate of an steel alloy in NaCl solution [9]. Savas et al. [12] reported changes in the corrosion mechanism of an stainless steel in HNO3 + Na2Cr2O7 solution due to air and oil quenching treatments. An investigation by Godec et al. [13] also showed that quenching, tempering and annealing can cause alteration in corrosion mechanism and subsequently lead to significant changes in the corrosion resistance of stainless steels in dilute sulfuric acid.
⁎
At room temperature, most mild steels usually come with a ferriticpearlitic structure in annealed or normalized condition. This structure could be modified to spherical carbides uniformly distributed in a matrix of ferrite, which is thermodynamically the most stable microstructure in steels. Such a spheroidization process usually leads to coarser grains compared to the original microstructure. The degree of spheroidization is then determined by the percentage of remaining lamellar pearlite [11]. Mechanisms of corrosion in sulfuric acid have been well studied in the literature [1,4,6,14–21]. In its dilute form, sulfuric acid reacts with metals (e.g. iron) through Eq. (1) producing hydrogen (gas) and metal sulfate (salt) on the metal surface [7,16,22]:
Fe(s) + H2 SO4 (aq) → H2 (g) + FeSO4 (aq)
(1)
Corrosion mechanisms, however, could be more complicated especially when significant changes are applied to the metal; e.g. through modification processes such as heat treatment. Fig. 1 shows an example of the in-service behaviors (according to Aushex Pty Ltd) of two uninhibited cold-drawn low-carbon steel ASTM-A-179 tubes, in ferriticpearlitic condition versus spheroidized condition, after 15 h exposure to 6% v/v H2SO4 at 60 °C and flow rate of 1 m/s. Significant corrosion of the pearlitic tube with 55.7 wt% mass loss is comparable to the relatively unaffected spheroidized tube with only 6.2 wt% mass loss. To investigate the above phenomena electrochemical measurement techniques, such as impedance spectroscopy and polarization
Corresponding author. E-mail address: [email protected] (M. Sarvghad).
https://doi.org/10.1016/j.apsusc.2019.06.180 Received 6 January 2019; Received in revised form 11 May 2019; Accepted 18 June 2019 Available online 18 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 1. Mass loss images of pearlitic vs. spheroidized ASTM-A-179 tubes subjected to 6% v/v H2SO4 at 60 °C and flow rate of 1 m/s for 15 h, according to Aushex Pty Ltd.
were selected and mounted into I) a conductive resin, and II) a nonconductive resin in order to provide suitable specimens for microstructural and corrosion studies. Both pairs were mechanically wet ground and polished down to 0.04 μm using standard procedures in colloidal silica, washed with ethanol and dried in the air. The coupons mounted in conductive resin were used for pre-exposure microanalysis and therefore were not subjected to corrosion. The other pair was used to monitor the impact of the corrosive medium on the as-polished surfaces by submerging them in 0.5 M H2SO4 for 10 min for static corrosion investigation. This pair was then re-polished and submerged in the acid solution for electrochemical analysis.
resistance, are accordingly considered as useful corrosion monitoring practices to track changes in mechanisms [23–25]. This paper, elaborates changes in corrosion rates and mechanisms of ferritic-pearlitic tubes of cold-drawn low-carbon steel ASTM-A-179 vs. spheroidized tubes in 0.5 M H2SO4 at room temperature. The tubes are produced seamless, with a final cold-draw to guarantee dimensional tolerances and surface finish. Finally, heat treatment is conducted at 650 °C or above [26,27]. 2. Experimental Coupons of ferritic-pearlitic and spheroidized ASTM-A-179 tubes, supplied by Hangzhou Wanyun Precision Cold-draw Steel Pipe Co., Ltd. and organized by Aushex Pty Ltd., were employed to study the effect of 0.5 M sulfuric acid on the corrosion behavior for application in alumina refineries as heat exchanger (see Fig. 1). The manufacturing processes of the studied steels were as below: I) Piercing of the billet in 1200 °C for a short time, II) first cold drawing, III) second cold drawing (final size 38.1 × 2.6 mm), IV) spheroidization treatment at 780 °C for 4 h; ferritic-pearlitic tubes were normalized at the same temperature for 20 min. The chemical compositions and microstructural characteristics of the studied alloys are provided in Table 1. Samples of each set are referred to as spheroidized (or spheroid) and pearlitic (or pearlite) hereinafter in this paper. Several cut-offs from as-received tubes of the studied alloys were prepared for microstructural and electrochemical studies as described below.
2.2. Microstructural investigation 2.2.1. Scanning electron microscopy and EBSD Pre-exposure microstructure was studied using a Field Emission Scanning Electron Microscope (FE-SEM) machine model JEOL 7001F, with automated feature detection equipped with secondary electron and Energy Dispersive X-ray Spectroscopy (EDS) analysis system, OXFORD Electron Back-Scatter Diffraction (EBSD) pattern analyzer and Channel 5 analysis software. Strain contouring component of OXFORD Channel 5 software was employed to estimate the extent of deformation, or strain, in individual grains in EBSD maps. Maps were obtained using accelerating voltage of 25 kV and step size of 0.9 μm at 700× magnification. In order to reveal carbides in the ferritic-pearlitic and spheroidized microstructures resulted from the discussed heat treatment processes, samples mounted in the conductive resin were also etched in 10 ml glycerin +6 ml HCl + 3 ml HNO3 solution for 10 s [28]. This was done as complementary to EBSD since it is not capable of revealing pearlitic structures in steels. SEM images were also obtained immediately (in the first few hours) after the exposure of electrochemical samples in secondary electron mode.
2.1. Sample preparation Two pairs of cut-off coupons of spheroidized and pearlitic samples Table 1 Chemical composition and microstructure of the studied alloys (according to Moly-Cop, NATA accreditation). Microstructure
Spheroidized Pearlitic
2.2.2. Optical microscopy analysis Corrosion attack and microstructures were immediately observed on as-polished samples after 10 min exposure to the H2SO4 solution using an optical microscope model Leica DMi8A with the magnification range of 2.5× to 50× equipped with Leica Application Suite software.
Chemical composition (wt%) C
Si
Mn
P
S
0.102 0.117
0.243 0.226
0.491 0.353
0.008 0.012
0.004 0.001
461
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
2.3. Electrochemical investigation
on the left) with similar GB attack.
Electrochemical experiments were conducted (on as-polished samples) using a three-electrode cell containing 0.5 M H2SO4 at room temperature by means of a VMP3-based BioLogic instrument controlled by EC-Lab® software. Test coupons were subjected to Open Circuit Potential (OCP), Electrochemical Impedance Spectroscopy (EIS), Linear Polarization Resistance (LPR) and Potentiodynamic Polarization (PDP) measurements. Prior to experiments, specimens were mounted in a nonconductive resin (exposing 0.27 and 0.29 cm2 of the surfaces of pearlitic and spheroidized samples, respectively) and then polished as discussed in Section 2.1. A platinum sheet was used as counter electrode, a standard Ag/AgCl (VL_Ionode004) as reference electrode with the sample of interest as working electrode. Electrochemical measurements were carried out in the following order for each sample: OCP was measured for 90 min after the immersion of samples into the acid solution. EIS measurements were then obtained using a frequency range of 100 kHz to 100 mHz with the amplitude of ± 10 mV. EIS measurements were repeated 3 times at 5-min intervals for each sample. LPR measurements were run afterwards at the potential scan rate of 0.1 mV/s and potential range of −25 to +25 mV with respect to OCP. Finally, PDP was conducted at the potential scan rate of 10 mV/ min and potential range of −200 to +500 mV with respect to the open circuit potential. ZFit analysis of EC-Lab® software was used to fit successive impedance cycles. LPR plots were analyzed using the polarization resistance (RP) fit tool of EC-Lab® software to calculate Rp, corrosion current density (icorr) and corrosion potential (Ecorr) values within ± 20 mV from the corrosion potential. PDP plots were also analyzed using the software's Tafel fit tool to calculate Tafel slopes (βa and βc), icorr and Ecorr values.
3.3. Electrochemical behavior Open Circuit Potentials (OCP) of the samples in 0.5 M H2SO4 are shown in Fig. 6a. Both samples show an active behavior in the first 500 s of exposure. The spheroidized sample then tends to stabilize at around −490 mV while the pearlitic sample gains nobler potential values over time, but is not stabilized even after 90 min (5400 s) of exposure. Ecorr values in E-logi plots (PDP and LPR) shown in Fig. 6b reflect OCP values in Fig. 6a. Fig. 7 also shows LPR plots (E vs. i) of the test samples in the corrosive medium. Data extracted from PDP and LPR curves are summarized in Table 4. Polarization resistance (Rp) is defined as the slope of potential vs. current density curve, Fig. 7, at the free corrosion potential according to Faraday's law [22,24]:
dE Rp = ⎛ ⎞ ⎝ di ⎠Ecorr
(2)
where, E is the corrosion potential and i is the corrosion current. Corrosion current density (icorr) is then calculated according to the SternGeary relationship [25,29]:
icorr =
βa |βc | Rp (βa + |βc|) ln 10
(3)
where, the activation controlled Tafel slopes (βa and βc) are determined through the fitting of PDP curves, see Table 4. Fig. 7 and data in Table 4 show a much higher Rp value for the spheroidized sample with an RP drop of around 93% for the pearlitic sample. This is in agreement with the noticeable rise in icorr values (separate calculations from PDP and LPR measurements, see Table 4) and the accompanied steeper Tafel slopes in both cathodic and anodic branches of the pearlitic sample compared to the spheroidized one. This suggests that different mechanisms control corrosion of the samples here. Corrosion current density corresponds to corrosion rate (CR) according to [24,30]:
3. Results 3.1. Pre-exposure microstructure Optical and SEM images of etched samples (Fig. 2) as well as EBSD maps of polished and mirror-like samples (Fig. 3) show a texture consisting of pearlites (α + Fe3C) distributed through coarse grains of ferritic iron (α) with particles of cementite (Fe3C) mostly dispersed along grain boundaries (GB) for spheroidized sample. Remaining pearlites here point to the fact that the spheroidization process was not 100% complete. On the other hand, relatively fine grained proeutectoid ferrite (α) with a higher fraction of pearlites (α + Fe3C) is observed for the pearlitic-ferritic sample. Table 2 shows the fraction of phases extracted from EBSD mappings. GB carbide (cementite) in the pearlitic sample is around twice of that in the spheroidized sample. The fraction of zero solution areas (white areas in band contrast and black areas in phase map images of Fig. 3) in both samples could point to the lamellar pearlitic structure which is hard to detect with EBSD. EBSD strain contouring maps in Fig. 4 show that the maximum of lattice strain for pearlitic sample (≃2) is almost 10 times of that for spheroidized sample (≃0.2). This means that spheroidization has led to significant stress relieving resulted in lower amounts of residual stresses remaining in the matrix compared to the pearlitic sample.
CR = K . EW .
i corr ρ
(4)
where, K = 3.272e-3 mm.g.μA−1.cm−1.yr−1, EW is equivalent weight (= 27.92 for carbon steel [24]) and ρ is density (= 7.87 for iron) in g.cm−3. CR values extracted directly from PDP measurements as well as those from LPR are also shown in Table 4. Accordingly, the pearlitic sample is shown to corrode around 21 times faster than the spheroidized one. It is worth noting that the CR values reported here only indicate the initial corrosion rates and no long-term corrosion measurement was conducted in this study. To further understand the mechanisms of corrosion, EIS was measured immediately after OCP and repeated for 3 times at 5-min intervals. Nyquist and Bode-Phase plots in Fig. 8 and Fig. 9 confirm much higher polarization resistance (Rp) of the spheroidized sample against 0.5 M sulfuric acid compared to that of the pearlitic one. In addition, repetition of EIS confirms that the spheroidized sample has been well stabilized after 90 min exposure, whereas the pearlitic sample is still active with a continuous reduction of Rp over time (diameters of the capacitive loops in Fig. 8b). Nyquist plots in Fig. 8 were successfully fit (only the last curve, 10 min, for each sample) using the indicated equivalent circuits with the extracted data summarized in Table 3. Here Rs, Rct and RL point to solution resistance, charge transfer resistance and inductance resistance, respectively. Qdl is the double layer Constant Phase Element (CPE), which points to the non-ideal capacitance of the surface and is used to calculate the surface charge transfer capacitance during
3.2. Static corrosion behavior To investigate the impact of the corrosive solution on the material surface, as-polished mirror-like surface samples were subjected to 0.5 M H2SO4 for 10 min and then examined immediately under light microscope. Low and high magnification optical microscopy images of the spheroidized sample are shown in Fig. 5(a to c). The image shows that the acid attacks selective areas on the surface. These areas are mostly corresponding to GBs and the interfaces between cementite and ferrite (see Fig. 2 and Fig. 3). Images of the pearlitic sample in Fig. 5(d to f) show a more serious corrosion (compared to the spheroidized sample 462
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 2. Optical (left) and SEM (right) images of as-received spheroidized (a and b) and pearlitic (c and d) samples etched in 10 ml glycerin +6 ml HCl + 3 ml HNO3 solution for 10 s.
Fig. 3. EBSD band contrast image (a) and phase map (b) of spheroidized sample, EBSD band contrast image (c) and phase map (d) of pearlitic sample prior to exposure. 463
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
solubility limit in the solution which could hinder sulfate saturation, compared to the pearlitic sample [37].
Table 2 Phase fraction of samples resulted from EBSD. Phase
Phase fraction (%)
4. Discussion Fe3C Ferrite (iron bcc) Zero solutions
Spheroidized
Pearlitic
2.8 83.6 13.6
4.6 80.0 15.4
As shown in Fig. 2 and Fig. 3, the pearlitic sample contains finer grains resembling an increased GB length (per unit volume) compared to the spheroidized sample. The higher fraction of pearlites (and subsequently the interface of cementite/ferrite) and GB carbides along with increased GB length lead to a heterogeneous microstructure in the pearlitic sample. Also, the co-existence of ferrite and cementite (Fe3C) in pearlite can cause galvanic effects which accelerate the dissolution rate of the inside and surrounding ferrite [38]. Therefore, the microstructure of this metal is expected to be more susceptible to localized attack, mainly because of the formation of micro-anodes/cathodes on the surface [39–41], compared to the spheroidized structure with coarse proeutectoid ferrites. Lattice defects and dislocations have been already found to be the initials for the formation of localized attack in steel alloys [38,41]. The higher amount of lattice strain and subsequently residual stresses (lattice defects in general) in pearlitic sample implies less thermodynamic stability compared to the spheroidized sample. This could be confirmed through OCP plots and 5-min interval impedance measurements as shown in Fig. 6a and Fig. 8. Such an instability could also provide driving force for corrosion and contribute to the higher corrosion rate and/or lower polarization resistance (see Table 4) of the pearlitic sample as confirmed by PDP and LPR measurements in Fig. 6b and Fig. 7, respectively. Noticeable discrepancies between the Tafel slopes (βa and βc) and Rp values of the samples indicate a significant difference in corrosion kinetics. According to Eq. (1), an insoluble iron sulfate (FeSO4) is likely to precipitate on the surface of the metal [7,16,22]. This involves the simultaneous exchange of electrons through the oxidation of metal (anodic reaction) and reduction of hydrogen (cathodic reaction) according to the below reactions, respectively [22]:
Fig. 4. EBSD strain contouring maps of the same areas as band contrast images of the samples in Fig. 3 with their corresponding strain contouring profiles showing the amount of lattice strain.
corrosion [31], while n represents the CPE parameter. The difference in mechanisms is illustrated in the EIS clearly by the spheroidized sample detected fitting a Randles-like equivalent circuit with a semicircle and one time constant while an inductive loop appears for the pearlitic sample with a modified inductance (La) at low frequencies. It is noteworthy that modified inductance could be used as an element with positive values in an equivalent circuit to avoid interpreting negative values associated with some inductive loops [32]. Impedance of an inductance (ZL) is expressed as [33–36]:
ZL (jω) = Ljω
(7)
2H+
(8)
+
2e−
→ H2
The anodic dissociation of Fe in aqueous solutions also involves the formation of adsorbed intermediate (FeOH)ads [42,43]. Consequently, the below reactions contribute to Fe dissociation [43]:
(5)
Fe + (FeOH )ads → Fe (FeOH )ads
where L indicates inductance, j = −1 , angular frequency ω = 2πf (f is frequency in Hz). The impedance of a modified inductance (La) is then expressed as [32]:
ZLa (jω) = La (jω)aL
Fe → Fe 2 + + 2e−
(6)
(9)
Fe (FeOH )ads + OH− → FeOH+ + (FeOH )ads + 2e−
(10)
FeOH+ + H+ → Fe 2 + + H2 O
(11)
Accordingly, through reducing the defects such as GBs and ferrite/ cementite interfaces in pearlite, spheroidization seems to shift the equilibrium of Eq. (9) to the left [43] leading to significant reductions in both metal dissolution and hydrogen evolution rates of the steel, see Tafel slopes in Table 4. Such a transition is actually because of a change in the mechanism of corrosion as shown by Nyquist plots and their corresponding equivalent circuits in Fig. 8. Impedance spectra show that the corrosion of spheroidized sample is under charge transfer (activation) control with a large capacitive loop (high Rp value). However, the diameter of the semicircle (impedance) significantly reduces for the pearlitic sample (low Rp value) while an inductive loop appears in its low-frequency region which indicates adsorption (precipitation) following the Volmer-Heyrovský mechanism [44–47]. The continuous ohmic drop of the double layer (reduction of the diameter of semicircle with time) also indicates that the corrosion rate of the pearlitic sample increases with time [48]. SEM-EDS images in Fig. 10 further approve the above discussion. Discrepancy in mechanism appears as different precipitates on the samples' surface after the electrochemical measurements. So the much
where aL represents the La parameter (if aL = 1 then La ≡ L). Polarization resistance Rp could be also calculated from data in Table 3 where for the spheroidized sample Rp = Rct and for the pearlitic sample Rp = Rct + RL. These values (which are also shown in Table 4) are correspondingly in good agreement with those of the LPR measurement in Table 4. Electrochemical samples were analyzed with SEM-EDS just a few hours after exposure to visualize the results seen in the electrochemical experiments. Fig. 10 shows comparative images of the samples' surfaces with the same magnification, including their corresponding EDS analysis maps. Different corrosion mechanisms with different corrosion precipitates formed on the samples' surfaces are obvious. Amount and density of precipitates developed on the surface of pearlitic sample are higher than those of the spheroidized sample. EDS analysis maps clearly show oxygen and sulfur containing products on the surface of the pearlitic sample, while S was not detected for the spheroidized sample, probably because of its low concentration on the surface. It is possible that the ferrous sulfates have been produced at a rate lower than their 464
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 5. Low and high magnification optical images of spheroidized sample (a) to (c), and pearlitic sample (d) to (f) subjected to 0.5 M H2SO4 at room temperature for 10 min.
investigations was carried out to analyze the impact of 0.5 M H2SO4 on carbon steel A179 with a pearlitic-ferritic structure compared to when heat treated to a spheroidized structure. Electron Back-Scatter Diffraction, Optical and Scanning Electron Microscopy observations showed a more heterogeneous microstructure containing fine ferrite and pearlite grains with an increased grain boundary length and lattice defects (lattice strain) for the pearlitic metal compared to the spheroidized condition. Electrochemical Impedance Spectroscopy and Linear Polarization Resistance measurements showed different mechanisms for the corrosion of the samples with a much lower polarization resistance in the absence of stabilization for the pearlitic sample. Accordingly, polarization showed that spheroidization has led to 21 times reduction in the corrosion rate. This was further shown to result from reduction in the kinetics of both anodic and cathodic reactions. Further microscopical observations showed higher amounts of precipitates and higher amounts of corrosion on the surface of pearlitic sample. Finally, spheroidization treatment was found to lead to a more homogeneous microstructure which de-risks the formation of micro-
faster the corrosion rate of pearlitic sample is quite obvious with large amounts of porous sulfate precipitates. In the absence of an inhibitor, adsorption here could be referred to as surface contamination or precipitation [49]. Thermodynamically, this could be facilitated when the surface energy of the metal increases. Heterogeneity of microstructure, due to the presence of either polycrystalline metal surfaces (various crystals and grains) or crystal defects (such as GBs, dislocations, point defects, etc.), can increase the level of surface energy stored in the material [50–52]. Here, the pearlitic sample contains both the polycrystalline surface (increased number of grains with high fractions of pearlites and carbides) and crystal defects (due to the presence of residual stresses resulted from lattice strain together with the increased GB length). These are the areas which are susceptible to be preferentially attacked by the corrosive solution. Optical microscopy images in Fig. 5 confirm the above. 5. Conclusion A
combination
of
microstructural
and
electrochemical 465
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Fig. 6. (a) Open Circuit Potential (OCP), and (b) Potentiodynamic Polarization (PDP) curves of the test samples in 0.5 M H2SO4. Dash-dot lines in (b) show corresponding Evans diagrams (E-logi) resulted from LPR measurement.
Fig. 9. Bode-phase plots of the samples corresponding to the third (10 min) Nyquist plots in Fig. 8.
Fig. 7. Linear Polarization Resistance (LPR) curves of the samples in 0.5 M H2SO4.
Fig. 8. Nyquist plots resulting from EIS measurements of (a) spheroidized vs. pearlitic and (b) pearlitic samples along with calculated equivalent circuits and fit data points for the third (10 min) plots. 466
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
Table 3 Data extracted from equivalent circuit models used to fit the curves in Fig. 8. Rs
Rct 2
Spheroidized Pearlitic
Qdl 2
ndl n-1
[Ω.cm ]
[Ω.cm ]
[F.s
2.4 3.4
350 20
1.5e-04 5.9e-04
−2
.cm
La
aL a-1
]
[H.s
[Ω.cm2]
.cm ]
– 1.4
0.77 0.86
RL
2
– 0.7
– 8.4
Table 4 Data extracted from PDP, LPR and EIS curves in Fig. 6b, Fig. 7 and Fig. 8, respectively. βa
Ecorr [mV vs. Ag/AgCl]
|βc|
icorr
Rp −2
CR 2
]
[Ω.cm ]
[mm/year]
[mV/dec]
[μA.cm PDP
LPR
LPR
EIS
PDP
LPR
67 1390
65 1420
313 23
350 28
0.78 16.2
0.76 16.6
Source
PDP
LPR
PDP
Spheroidized Pearlitic
−483 −458
−484 −462
69 131
141 180
Fig. 10. SEM and corresponding EDS map analysis images of samples after exposure for electrochemical investigation.
anodic/cathodic sites on the surface of the metal and significantly improves the corrosion performance.
Chem. 2013 (2013) 9. [2] L. Herrag, B. Hammouti, S. Elkadiri, A. Aouniti, C. Jama, H. Vezin, F. Bentiss, Adsorption properties and inhibition of mild steel corrosion in hydrochloric solution by some newly synthesized diamine derivatives: experimental and theoretical investigations, Corros. Sci. 52 (2010) 3042–3051. [3] S. Bilgic, N. Caliskan, An investigation of some Schiff bases as corrosion inhibitors for austenitic chromium–nickel steel in H2SO4, J. Appl. Electrochem. 31 (2001) 79–83. [4] M.A. Quraishi, S. Khan, Inhibition of mild steel corrosion in sulfuric acid solution by thiadiazoles, J. Appl. Electrochem. 36 (2006) 539–544. [5] F. Bentiss, M. Bouanis, B. Mernari, M. Traisnel, M. Lagrenee, Effect of iodide ions on corrosion inhibition of mild steel by 3,5-bis(4-methylthiophenyl)-4H-1,2,4-triazole in sulfuric acid solution, J. Appl. Electrochem. 32 (2002) 671–678. [6] Industrial Applications of Sulfuric Acid, © WOC Article, (2014). [7] M.G. Fontana, Corrosion Engineering, 3 ed., McGraw-Hill Book Company, 1985. [8] L.L. Huang, H.M. Meng, L.K. Liang, S. Li, J.H. Shi, Effects of heat treatment on the corrosion resistance of carbon steel coated with LaMgAl11O19 thermal barrier coatings, Int. J. Miner. Metall. Mater. 22 (2015) 1050–1059. [9] A.H. Seikh, Influence of heat treatment on the corrosion of microalloyed steel in sodium chloride solution, J. Chem. 2013 (2013) 1–7. [10] A.N. Isfahany, H. Saghafian, G. Borhani, The effect of heat treatment on mechanical properties and corrosion behavior of AISI420 martensitic stainless steel, J. Alloys Compd. 509 (2011) 3931–3936.
Acknowledgement The data reported in the paper were obtained at the Central Analytical Research Facility (CARF) operated by the Institute for Future Environments (IFE) at Queensland University of Technology (QUT). Access to CARF was supported by funding from the Science and Engineering Faculty, QUT. The commercial tube materials were supplied by Aushex Pty Ltd. This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors. References [1] P. Thiraviyam, K. Kannan, P. Premkumar, A characteristic investigation of aminocyclohexane-N′-methylurea as a corrosion inhibition for mild steel in 1 N H2SO4, J.
467
Applied Surface Science 491 (2019) 460–468
M. Sarvghad, et al.
88–97. [32] The Modified Inductance Element La, Application Note 42, Bio-Logic Science Instruments, (2012). [33] S. Ramanathan, Negative resistances and inductances in equivalent circuits for adsorption-reaction kinetics, ECS Trans. 33 (2011) 21–35. [34] A. Lasia, Impedance spectroscopy applied to the study of electrocatalytic processes, in: K. Wandelt (Ed.), Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry, Elsevier, 2018, pp. 241–263. [35] A. Lasia, Electrochemical Impedance Spectroscopy and its Applications, Springer, New York, NY, 2014. [36] C. Gabrielli, M. Keddam, H. Takenouti, The Use of a.c. Techniques in the Study of Corrosion and Passivity, vol. 23, (1983), pp. 395–451. [37] S. Mischler, E. Rosset, G.W. Stachowiak, D. Landolt, Effect of sulphuric acid concentration on the rate of tribocorrosion of iron, Wear 167 (1993) 101–108. [38] C. Liu, X. Cheng, Z. Dai, R. Liu, Z. Li, L. Cui, M. Chen, L. Ke, Synergistic effect of Al (2)O(3) inclusion and pearlite on the localized corrosion evolution process of carbon steel in marine environment, Materials (Basel) 11 (2018). [39] D. Clover, B. Kinsella, B. Pejcic, R. De Marco, The influence of microstructure on the corrosion rate of various carbon steels, J. Appl. Electrochem. 35 (2005) 139–149. [40] Q.Y. Pan, W.D. Huang, R.G. Song, Y.H. Zhou, G.L. Zhang, The improvement of localized corrosion resistance in sensitized stainless steel by laser surface remelting, Surf. Coat. Technol. 102 (1998) 245–255. [41] C. Liu, R.I. Revilla, D. Zhang, Z. Liu, A. Lutz, F. Zhang, T. Zhao, H. Ma, X. Li, H. Terryn, Role of Al2O3 inclusions on the localized corrosion of Q460NH weathering steel in marine environment, Corros. Sci. 138 (2018) 96–104. [42] J.O.M. Bockris, D. Drazic, A.R. Despic, The electrode kinetics of the deposition and dissolution of iron, Electrochim. Acta 4 (1961) 325–361. [43] E.E. Oguzie, S.G. Wang, Y. Li, F.H. Wang, Corrosion and corrosion inhibition characteristics of bulk nanocrystalline ingot iron in sulphuric acid, J. Solid State Electrochem. 12 (2007) 721–728. [44] J.P. Diard, P. Landaud, B. Le Gorrec, C. Montella, Calculation, simulation and interpretation of electrochemical impedance. Part II. Interpretation of VolmerHeyrovsky impedance diagrams, J. Electroanal. Chem. Interfacial Electrochem. 255 (1988) 1–20. [45] J.P. Diard, B. Le Gorrec, C. Montella, Calculation, simulation and interpretation of electrochemical impedances. Part 3. Conditions for observation of low frequency inductive diagrams for a two-step electron transfer reaction with an adsorbed intermediate species, J. Electroanal. Chem. 326 (1992) 13–36. [46] J.P. Diard, B. Le Gorrec, C. Montella, Non-linear impedance for a two-step electrode reaction with an intermediate adsorbed species, Electrochim. Acta 42 (1997) 1053–1072. [47] F. Mansfeld, Analysis and Interpretation of EIS Data for Metals and Alloys, Technical Report No. 26, University of Southern California, Los Angeles, CA, USA, 1999. [48] J. Wei, J.H. Dong, W. Ke, X.Y. He, Influence of inclusions on early corrosion development of ultra-low carbon bainitic steel in NaCl solution, Corrosion 71 (2015) 1467–1480. [49] E. McCafferty, Introduction to Corrosion Science, (2010). [50] H.P. Stüwe, A.F. Padilha, F. Siciliano, Competition between recovery and recrystallization, Mater. Sci. Eng. A 333 (2002) 361–367. [51] R.A. Vandermeer, B.B. Rath, Interface migration during recrystallization: the role of recovery and stored energy gradients, Metall. Trans. A. 21 (1990) 1143–1149. [52] D. Raabe, Recovery and recrystallization: phenomena, physics, models, simulation, in: D.E.L. Hono (Ed.), Physical Metallurgy, Fifth edition, Elsevier, Oxford, 2014, pp. 2291–2397.
[11] G.E. Totten, Steel Heat Treatment: Metallurgy and Technologies, 2nd ed., CRC Press, Boca Raton, 2006. [12] T.P. Savas, A.Y.-L. Wang, J.C. Earthman, The effect of heat treatment on the corrosion resistance of 440C stainless steel in 20% HNO3 + 2.5% Na2Cr2O7 solution, J. Mater. Eng. Perform. 12 (2003) 165–171. [13] R. Godec, V. Dolecek, Influence of different heat-treatments on corrosion-resistance of martensitic stainless-steels, Werkst. Korros. 45 (1994) 517–522. [14] M. Itagaki, H. Nakazawa, K. Watanabe, K. Noda, Study of dissolution mechanisms of nickel in sulfuric acid solution by electrochemical quartz crystal microbalance, Corros. Sci. 39 (1997) 901–911. [15] M.J. Kim, J.G. Kim, Effect of manganese on the corrosion behavior of low carbon steel in 10 wt.% sulfuric acid, Int. J. Electrochem. Sci. 10 (2015) 6872–6885. [16] M.J. King, W.G. Davenport, M.S. Moats, Materials of Construction, Sulfuric Acid Manufacture - Analysis, Control, and Optimization, 2nd edition, Elsevier, San Diego, Calif, 2013, pp. 349–356. [17] E. Lazarova, G. Petkova, R. Raicheff, G. Neykov, Electrochemical study of the adsorption and inhibiting properties of halogen derivatives of aniline on iron in sulfuric acid, J. Appl. Electrochem. 32 (2002) 1355–1361. [18] V.V. Mehmeti, A.R. Berisha, Corrosion study of mild steel in aqueous sulfuric acid solution using 4-methyl-4H-1,2,4-triazole-3-thiol and 2-mercaptonicotinic acid-an experimental and theoretical study, Front. Chem. 5 (2017) 61. [19] O. Olivares-Xometl, E. Álvarez-Álvarez, N.V. Likhanova, I.V. Lijanova, R.E. Hernández-Ramírez, P. Arellanes-Lozada, J.L. Varela-Caselis, Synthesis and corrosion inhibition mechanism of ammonium-based ionic liquids on API 5L X60 steel in sulfuric acid solution, J. Adhes. Sci. Technol. 32 (2017) 1092–1113. [20] A. Rafsanjani-Abbasi, A. Davoodi, Electrochemical characterization of natural chalcopyrite dissolution in sulfuric acid solution in presence of peroxydisulfate, Electrochim. Acta 212 (2016) 921–928. [21] N. Sato, G. Okamoto, Anodic passivation of nickel in sulfuric acid solutions, J. Electrochem. Soc. 110 (1963). [22] S. Lyon, Overview of corrosion engineering, science and technology, Nuclear Corrosion Science and Engineering, Elsevier Ltd, 2012, pp. 3–30. [23] F. Mansfeld, Tafel slopes and corrosion rates obtained in the pre-Tafel region of polarization curves, Corros. Sci. 47 (2005) 3178–3186. [24] ASTM, G102-89(2015)e1, Standard Practice for Calculation of Corrosion Rates and Related Information From Electrochemical Measurements, ASTM International, West Conshohocken, PA, 2015. [25] ASTM, G59-97(2014), Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements, ASTM International, West Conshohocken, PA, 2014. [26] ASTM, A179/A179M-90a(2012), Standard Specification for Seamless Cold-Drawn Low-Carbon Steel Heat-Exchanger and Condenser Tubes, ASTM International, West Conshohocken, PA, 2012. [27] S.A. Al-Zubail, An Evaluation of Efficiency of Phenylenediamines as Corrosion Inhibitors for ASTM-A-179 Steel in 1.0 N Hydrochloric Acid at Room Temperature, Ohio University, 1987. [28] R.I. Olivares, D.J. Young, P. Marvig, W. Stein, Alloys SS316 and Hastelloy-C276 in supercritical CO2 at high temperature, Oxid. Met. 84 (2015) 585–606. [29] ASTM, G3-14, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing, ASTM International, West Conshohocken, PA, 2014. [30] K. Vignarooban, P. Pugazhendhi, C. Tucker, D. Gervasio, A.M. Kannan, Corrosion resistance of Hastelloys in molten metal-chloride heat-transfer fluids for concentrating solar power applications, Sol. Energy 103 (2014) 62–69. [31] M. Sarvghad, T. Chenu, G. Will, Comparative interaction of cold-worked versus annealed inconel 601 with molten carbonate salt at 450°C, Corros. Sci. 116 (2017)
468