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Corrosion behaviour of copper containing low alloy steels in sulphuric acid
Corrosion Science 54 (2012) 174–182
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Corrosion behaviour of copper containing low alloy steels in sulphuric acid J.H. Hong a, S.H. Lee a, J.G. Kim a,⇑, J.B. Yoon b a b
Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea POSCO Technical Research Lab., 1 Goedong-Dong, Nam-Gu, Pohang 790-785, South Korea
a r t i c l e
i n f o
Article history: Received 21 April 2011 Accepted 10 September 2011 Available online 16 September 2011 Keywords: A. Low alloy steel A. Acid solutions B. Polarization C. Acid corrosion C. Hydrogen overpotential C. Rust
a b s t r a c t The influence of the addition of Cu on the corrosion resistance of low alloy steel in sulphuric acid was investigated using AC and DC electrochemical methods and a weight loss test, which took place in 10 wt% H2SO4 solution at room temperature. The electrochemical measurements indicated that the corrosion rate is suppressed by the addition of Cu due to a higher hydrogen overpotential and prevention of the active dissolution. Surface analyses (XPS, EPMA and SEM) of the corroded surfaces conducted after the immersion test indicated that the rust layer formed on the Cu containing steels was enriched with Cu compounds. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Being the major structural material used in the construction industry, the prevention of steel corrosion has attracted much attention. As most steels are stable in neutral or alkaline media, acidic environments are the major concern [1], with two approaches to reducing the corrosion rate in acidic media having been developed [2]. The first is based around suppressing the hydrogen evolution reaction using a surface metal which has a high overpotential for hydrogen reactions. The other is the well known barrier effect, in which a dense and continuous metal layer retards the dissolution of the anodic metal substrate in the corrosive environment. These two approaches can be accomplished simultaneously by alloying noble elements to the active substrate. In particular, many authors have reported the advantageous effects of the use of noble elements for inhibiting the anodic and cathodic reactions of the steel immersed in a corrosive environment [3–6]. The noble element constituents or impurities required to be present in the matrix may be placed onto the surface of the alloy during a few minutes [6]. In low alloy steels the addition of copper plays a major role in the improvement of corrosion resistance [2]. However, the effect which the small amount of copper in the low alloy steel has on the corrosion resistance may be questionable. Minor constituents of the corrosion media may influence the corrosion kinetics more than those of the main components, this is in most cases a
⇑ Corresponding author. Tel.: +82 31 290 7360; fax: +82 31 290 7371. E-mail address: [email protected] (J.G. Kim). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.012
direct consequence of the selective adsorption at the electrified interface [7]. The purpose of this present paper is to study the alloying effect of the addition of Cu on the corrosion properties of low alloy steels in a 10 wt% sulphuric acid solution in ambient temperature and conditions. 2. Experimental 2.1. Materials and test condition The compositions for the three low alloy steels examined in this study are given in Table 1. Square plates (1.5 by 1.5 by 0.1 cm) with a surface area of 2.25 cm2 were used, with all measurements being carried out in a 1 dm3 solution of 10 wt% H2SO4 in ambient temperature and conditions. The solution was prepared from analytical reagent grade chemicals and de-ionized water. To ensure reproducibility, at least three measurement sets were run for each specimen.
2.2. Electrochemical measurements The electrochemical test specimen was ground using 800-grit silicon carbide (SiC) paper, cleaned in an ultrasonic bath with ethanol for 5 min and then dried in hot air. A three-electrode electrochemical system was used, with a saturated calomel electrode (SCE) and two pure graphite rods, which were used as the reference electrode and counter electrodes respectively. All of the potentials reported were measured with respect to the value of
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182 Table 1 Chemical compositions of the tested steels (wt%). Specimen
2 2
Composition (wt.%)
0 wt% Cu 0.2 wt% Cu 0.35 wt% Cu
Fe
C
Si
Mn
P
S
Cu
Balance Balance Balance
0.07 0.07 0.07
0.25 0.25 0.25
0.7 0.7 0.7
0.01 0.01 0.01
0.012 0.012 0.012
– 0.2 0.35
4
+
2
4
2
4
+
2
+
2 + +
+
200 0 wt.% Cu 0.2 wt.% Cu 0.35 wt.% Cu
0
Potential mVSCE
2
4
2+
-200
3+ 3+ 2+
2+ 2+
-400
2+
2+ 2+
2+
-600
-800 -6
10
-5
-4
10
10
-3
10
-2
-1
10
10
0
10
Fig. 2. Schematic diagrams of the selective dissolution and re-deposition mechanism of the Cu containing low alloy steel in H2SO4 solution.
2
Current density A/cm
Fig. 1. Potentiodynamic polarization curves for the three steels in 10 wt% H2SO4.
the SCE. An open-circuit potential (OCP) was established from 1 h before each of the electrochemical measurements. Electrochemical impedance spectroscopy (EIS) and electrochemical DC measurements, including potentiodynamic polarization and linear polarization tests, were performed using a PARSTAT 2263 potentiostat. For the potentiodynamic polarization test, a scan rate of 0.166 mV/s was used, in the range from 250 mVOCP to +100 mVSCE. A scan rate of 0.166 mV/s was used over a range from 20 mVOCP to +20 mVOCP in the linear polarization test. EIS data was obtained over a frequency range from 100 kHz to 10 mHz, an alternating current with an amplitude of ±20 mVRMS was applied. The impedance data was analyzed using an analysis software which used non-linear least square fitting. The corrosion current density for all of the specimens was determined using the Tafel extrapolation method. The polarization resistance (Rp) value was obtained from the linear polarization and EIS data sets [8]:
RP ¼
ba bc 2:3icorr ðba þ bc Þ
ð1Þ
where ba and bc are the anodic and cathodic Tafel slopes respectively, and icorr is the corrosion current density for each specimen. The corrosion rate can then be determined from the corrosion current density using Faraday’s law [8]:
Corrosion rateðmm=yÞ ¼
per metal atom, F is Faraday’s constant and q is the density of the metal (g/cm3). 2.3. Weight loss test Weight loss was measured by a method which followed ASTM G1-03 and ASTM G31-72 [9,10]. The exposed surface area (5.6 cm2) of each specimen was ground using 800-grit SiC paper, then cleaned in an ultrasonic bath with ethanol for 5 min and dried in hot air. After the initial mass (mi) was measured, the specimen was immersed in the test solution for 6 h. After the test, each specimen was cleaned using distilled water, then pickled in a solution of 0.5 dm3 HCl (sp. gr. 1.19), 3.5 g of hexamethylene tetramine (C6H12N4) and balanced distilled water for 5 min to remove the products of the corrosion. These were then degreased in an ultrasonic cleaner with ethanol for 5 min, followed by a cleaning with distilled water and drying in hot air. The final mass (mf) of the tested specimen was measured for the weight loss test and the corrosion rate was determined using the following equation [8]:
Corrosion rateðmm=yÞ ¼
87; 600 W Atq
ð3Þ
where W is the weight loss (g), A is the area of exposure (cm2), t is the time of exposure (h) and q is the density (g/cm3). 2.4. Surface analyses
316icorr M zF q
ð2Þ
where icorr is the corrosion current density (lA/cm2), M is the molar mass of the metal (g/mol), z is the number of electrons transferred
To investigate the relationship between the alloying element and the corrosion products, the surface was examined using X-ray photoelectron spectroscopy (XPS, ESCA 2000 LAB MK-II
Table 2 Results of the potentiodynamic polarization test. Specimen
Ecorr (mVSCE)
icorr (lA/cm2)
-bc (V dec1)
Corrosion rate (mm/year)
gH at 0.01 A/cm2 (mV)
0 wt% Cu 0.2 wt% Cu 0.35 wt% Cu
463.6 463 475.1
695 177 137
0.065 0.115 0.118
8.06 2.05 1.58
276 425 449
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
anodic reactions, with the cathodic polarization of 0.35 wt% Cu exhibiting the highest hydrogen evolution reaction (HER) overpotential, gH. The potential for cathodic current density of 0.01 A/ cm2 and the equilibrium potential (241 mVSCE at pH 0) for hydrogen evolution reaction were used to calculate the HER overpotential. On the alloy surface this may occur through the preferential dissolution of one component and an enrichment of the other component, therefore enrichment of Cu can be expected on the alloy surface. The formation of a metallic, Cu enriched layer, on the alloy surface can be explained by the preferential dissolution and redeposition process. The dissolution of Fe (as Fe2+) into the acid solution occurs at a voltage above 858 mVSCE, whereas Cu dissolution (as Cu2+) occurs at a voltage above 76 mVSCE. Copper is normally assumed to be immune to corrosion by water (a hydrogen evolution reaction). Copper corrosion in water is based on an interpretation of the conventional potential-pH diagrams where the copper immunity range extends well above the hydrogen
spectrometer, VG Microtech, England) and an electron probe microanalysis (EPMA, EPMA1600, Shimate, Japan) after the 6 h immersion. The corroded surface morphology after the weight loss measurements was inspected using scanning electron microscopy (SEM, S-3000F, Hitachi, Japan).
3. Results and discussion 3.1. Corrosion properties The results obtained from the potentiodynamic polarization tests are presented in Fig. 1 and Table 2. All of the specimens showed active corrosion behaviour without passivation. The corrosion current density decreased with an increase of the Cu content, confirming that improved corrosion resistance can be achieved by Cu addition. The Cu addition suppressed both the cathodic and
1.0
(a)
1h 2h 3h 4h 5h 6h
0.6
2
Z" (ohm-cm )
0.8
0.4 0.2 0.0
(a)
50
1h 2h 3h 4h 5h 6h
40 30
Phase angle (degree)
(a)
20 10 0 -10 -20 -30
-0.2 0.0
0.5
1.0
1.5
-40 -2 10
2.0
-1
10
0
10
2
Z' (ohm-cm )
4
10
20 15 10 5
(b)
80
5
10
1h 2h 3h 4h 5h 6h
70 60
Phase angle (degree)
1h 2h 3h 4h 5h 6h
25
2
3
10
90
(b)
Z" (ohm-cm )
2
10
Frequency (Hz)
30
(b)
1
10
50 40 30 20 10 0 -10
0
-20
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
-2
-1
10
10
0
10
1
10
2
10
3
10
4
10
5
10
2
Z' (ohm-cm )
Frequency (Hz)
30
90
(c)
1h 2h 3h 4h 5h 6h
2
Z" (ohm-cm )
25 20 15 10 5
80
(c)
1h 2h 3h 4h 5h 6h
70 60
Phase angle (degree)
(c)
50 40 30 20 10 0 -10 -20 -30
0 0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
-40 -2 10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
2
Z' (ohm-cm )
Frequency (Hz)
Fig. 3. Nyquist and Bode plots for the EIS data of the specimens against the immersion time: (a) 0 wt%, (b) 0. 2 wt% and (c) 0.35 wt% Cu containing steels.
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
0 wt% Cu
0.2 wt% Cu
0.35 wt% Cu
Immersion time
Rs(O cm2)
1h 2h 3h 4h 5h 6h 1h 2h 3h 4h 5h 6h 1h 2h 3h 4h 5h 6h
0.2461 0.2543 0.2445 0.2375 0.2285 0.2259 0.4576 0.4448 0.4397 0.4492 0.4419 0.3052 0.2862 0.262 0.2549 0.255 0.2592 0.2636
Rct(Ocm2)
CPE1 Crust (mF/cm2)
n (01)
0.496 0.688 0.711 0.711 0.757 0.749 0.240 0.249 0.269 0.286 0.304 0.285 0.180 0.199 0.224 0.252 0.278 0.302
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
1.447 1.447 1.426 1.437 1.439 1.452 38.91 48.77 54.22 54.69 55.69 57.36 49.59 52.82 54.69 55.69 55.69 57.53
(a)
2.0
2
Specimen
before the anodic polarization is performed, and the amount of plating Cu should increase with an increase of the Cu2+ ion concentration [6]. (4) The re-deposited Cu will be precipitated as re-crystallized Cu and Cu oxides. The selective dissolution and redeposition process will repeat until the Cu enriched layer is stabilized. As a consequence of the Cu accumulation on the Cu containing steel surface, the corrosion current density decreased with an increase of the Cu content through the suppressed anodic and cathodic reactions as shown in Fig. 1. The influence of the Cu addition is reflected by an increase in the value of the Tafel slopes (b),
Rct ohm-cm
Table 3 EIS data of the specimens at various immersion intervals.
1.5
1.0 1
2
3
4
5
6
5
6
5
6
Immersion time (h)
(b)
60
Linear fit (Y = 7.66X + 31.99)
55
50
45
40
1
2
3
4
Immersion time (h)
(c)
2
60
Rct ohm-cm
electrode potential. Thus, copper corrosion in aqueous solutions has been found to involve electron or charge transfer between the copper dissolution and the dissolved oxygen reduction reactions. In an acid solution which contains dissolved oxygen, both Fe and Cu dissolve, however the dissolved Cu re-deposits as a precipitate of re-crystallized Cu and Cu oxides [11]. During the potentiodynamic polarization test, the process of Cu re-deposition is thought to be accelerated by the cathodic polarization [6]. Eventually, the cathodic overpotential in the hydrogen evolution is increased through Cu enrichment [2]. The mechanisms of the selective dissolution and re-deposition processes of Cu containing low alloy steel are shown in Fig. 2. The process is split into four steps; (1) Under ambient conditions, dissolved oxygen and hydrogen cations coexist in the sulphuric acid solution. (2) Near to the solution/metal interface, the composition of the Cu containing low alloy steel mainly consists of Fe with a small amount of added Cu. (3) When the Fe dissolves, the Cu may subsequently become detached from the metal surface as the Cu containing low alloy steel does not have a continuous Cu network and preferential dissolution occurs along the continuous paths of less noble atoms [12]. As soon as the Cu is separated from the surrounding Fe, the Cu dissolves to Cu2+ ions by reduction of the dissolved oxygen. When Cu is associated with Fe, the Fe will be preferentially dissolved by galvanic corrosion, however Cu2+ in a Fe, H2O and O2 system acts as an oxidant to the Fe. Therefore, a proportion of the Cu2+ will reduce and then re-deposit onto the substrate through a reduction reaction (consumption of the electrons produced by oxidation of the Fe), this is especially true near to the attached Cu. The cathodic polarization causes a reduction of both the Cu2+ and H+ ions on the steel surface. Cu plating should therefore be emplaced
Rct ohm-cm
2
Fig. 4. Equivalent circuit for interpretation of the impedance spectra.
Linear fit (Y = 2.55X + 47.27)
55
50
45
1
2
3
4
Immersion time (h) Fig. 5. Charge transfer resistance (Rct) as a function of the exposure time: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35wt.% Cu containing steels.
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Corrosion rate [mm/year]
1,000 EIS LPR Potentiodynamic Weight Loss
100
10
1 0wt.% Cu
0.2wt.% Cu
Table 3 summarizes the best fitting values of the capacitive semicircles, these impedance results can be fitted to the circuit model seen in Fig. 4. The equivalent circuit of a one time constant consists of the following elements: the solution resistance (Rs), capacitance (Crust) and the charge transfer resistance (Rct). During the fitting process, the capacitance was represented by a constant phase element (CPE) to allow for the depressed semicircles. The corrosion rate in the Cu containing steels was much lower than that in the Cu-free steel, however there were no significant differences between the corrosion rates of the 0.2 wt% Cu and 0.35 wt% Cu steels. For the 0 wt% Cu specimen, the charge transfer resistance remained constant regardless of the immersion time. However, for the two specimens with added Cu, the charge transfer resistance was markedly increased with an increase in the time, as shown
0.35wt.% Cu
Fig. 6. Comparison of corrosion rates by test methods.
which provides a clear explanation as to why the corrosion potential of all the low alloy steels in the potentiodynamic results was the same. When only the anodic Tafel slope (ba) increases, the corrosion potential also increases and the corrosion current density decreases. On the other hand, when only the cathodic Tafel slope (bc) increases, both the corrosion potential and the corrosion current density decrease. In this case, as both the anodic and cathodic Tafel slopes increase, the corrosion current density decreases and the corrosion potential remains almost the same. Nyquist and Bode plots for the low alloy steels are shown in Fig. 3. The spectra of the Cu containing steels exhibited a single semicircle, in contrast, the spectra of Cu-free steels exhibit two time constants. These are a capacitive semicircle in the high -medium frequency range and an inductive loop in the low frequency range. The capacitive semicircle characterizes the active state of the interface when the steel is exposed to the sulphuric acid solution. The inductive loop observed in Fig. 3(a), which occurs at low frequency, is attributed to the adsorption of a species which promotes the corrosion rate [13,14]. The inductive signal present at the low frequency disappears and the amplitude of the capacitive semicircle increases with Cu addition, as shown in Fig. 3(b) and (c). The phase angle maxima for the Cu-free steel are far removed from 90° which is what is required to exhibit pure capacitive behaviour. This is due to the effect of porous corrosion products [15]. However, the phase angle maxima for the Cu containing steels were close to 90° indicating the improved structural properties of the corrosion products. This improvement is a result of the Cu containing steels having developed a dense rust layer. The continuous formation of the Cu enrichment layer resulted in a higher number of sites which were favourable for the adsorption reaction of the protective corrosion products and less favourable for the overall charge transfer process. The copper layer which formed on the steel surface decreases the rate of anodic dissolution by a blocking mechanism and a screening mechanism [6]. The copper present in the solid solution diminishes the iron dissolution as a result of blocking the active points in the lattice. The accumulation of copper on the surface of the alloy leads to further deceleration of the anodic dissolution, which is caused by partial screening of the alloy surface with the deposited copper. In addition, the Cu enrichment layer serves as an electronic conductor where the reduction of hydrogen ions takes place following the intermediate reactions at the interface. This leads to microgalvanic cells being formed between the Cu enrichment layer and substrate. As a result the corrosion rate decreased with the addition of Cu and immersion time due to the higher overpotential of Cu in the HER resulting in a depressed cathodic reaction rate.
(a)
(b)
(c) Fig. 7. SEM micrographs after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
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in Fig. 5. This confirms that the Cu content in the steel improved the corrosion resistance due to the accumulation of the Cu corrosion product. The average corrosion rates found from the weight loss test and the three electrochemical tests are compared in Fig. 6. The corrosion resistance was improved as a result of the Cu addition. Cu is considered to be a relatively noble metal due to its accumulation on the alloy surface [16]. The optimum quantities for alloy addition
were available for the design of alloys with an improved corrosion resistance [17]. When a specimen with Cu is exposed to a corrosive environment, the relatively active metals are preferentially dissolved prior to the Cu. Therefore, a Cu enriched layer is composed of copper and iron oxides and this protects the steel by suppressing the active dissolution of the substrate [6,18–22]. This positive effect of the Cu content in the steel is not washed out by the high activity of the acid solution [23]. The Cu accumulation on the Cu
(a)
(a) 50000
FeOOH Fe2(SO4)3
Fe2p
8000
Cu2p
40000
C/S
C/S
6000 30000
4000
20000
10000
2000
0
0 700
710
720
730
740
920
Binding Energy (eV)
950
(b)
FeOOH Fe2(SO4)3
Fe2p
940
960
970
Binding Energy (eV)
(b) 60000 50000
930
8000
40000
Cu, Cu2O, CuO
Cu2p
C/S
C/S
6000
30000
4000 20000
2000
10000
0
0 700
710
720
730
740
750
920
930
Binding Energy (eV)
(c) 60000 50000
8000
40000
960
970
Cu, Cu2O, CuO
Cu2p
6000
C/S
C/S
950
(c)
FeOOH Fe2(SO4)3
Fe2p
940
Binding Energy (eV)
30000
4000 20000
2000
10000 0
0 700
710
720
730
740
750
Binding Energy (eV) Fig. 8. XPS spectra of the Fe after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
920
930
940
950
960
970
Binding Energy (eV) Fig. 9. XPS spectra of the Cu after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
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Table 4 Binding energies of the components as found from the XPS data. Analyses of the XPS spectra
Standard chemicals
Binding energy (eV)
Spectrum of Fe 2p
FeO FeOOH* Fe2O3 Fe2(SO4)3 CuO* Cu2O* Cu*
710.0 711.5, 724.3 711.5, 724.0 713.5 933.49, 953.6 932.15, 952.3 932.0932.8, 952
Spectrum of Cu 2p
*
Compound with the closest binding energy to the main peaks of the XPS spectra.
containing steel surface is a kind of electroless plating, in which metallic coatings are formed as a result of a chemical reaction between the reducing agent present in the solution and metal ions. The metallic phase that appears in such reactions may be obtained in the bulk of the solution or as a precipitate in the form of a film on a solid surface. Localization of the chemical process on a particular surface requires that the surface must serve as a catalyst. Therefore, the process will last until almost all of the iron substrate is covered with copper where the stabilization of the charge transfer resistance with time characterizes the dynamic equilibrium of the atoms of copper that escape the alloy lattice, emerge and get crystallized onto the surface [24]. This imposes a limit on the
maximum film thicknesses obtained using electroless deposition [25]. The charge transfer resistance of the Cu containing steels increased with the Cu content and time during the early stage of the immersion test but levelled off and became independent of the Cu content due to the limited film thickness. Therefore, it is expected that the same charge transfer resistance is expected after the 6 h immersion. The addition of Cu causes an immediate reduction (linear section of the curves before the inflection) in the anodic dissolution of Fe, as shown in Fig. 5(b) and (c), where intercepts with yaxis, charge transfer resistance of as-immersed Cu containing steels, are corresponding to the blocking mechanism. Values of the charge transfer resistance at the intercept (31.99 and 47.27 ohm-cm2 for 0.2 and 0.35 wt% Cu containing steels, respectively) increased with the Cu content, but converged to 57.5 ohm-cm2 after 6 h immersion regardless of the Cu content. Tomashov et al. concluded that the blocking mechanism makes the most significant contribution to the deceleration of the active dissolution of chromium [25]. However, in this case, it is expected that the decelerated dissolution of the Cu added steels can be governed by either the blocking effect or the screening effect depending on the Cu content. Platinum group metals (Ru, Pd, Ir and Pt) are immune or well passivated in ambient solutions [26], and surface diffusion of these noble adatoms is responsible for the accumulation on the alloy surface. However, the Cu accumulation occurs
Fig. 10. EPMA mapping data before the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
181
Fig. 11. EPMA mapping data after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
by simultaneous dissolution and re-deposition, with solid diffusion also present but not rate controlling. This is considered why the charge transfer resistance of the Cu containing steels levelled off and became independent of the Cu content after 6 h immersion. 3.2. Surface analyses Fig. 7 shows the SEM images after the removal of the corrosion product from the weight loss test. The specimen without Cu suffered much more damage than those with Cu. The greatest surface heterogeneity was observed on the surface of the 0 wt% Cu specimen, with the surface morphology of the Cu containing steels being different from that of the Cu-free steel. The corrosion was more severe on the Cu-free steel when compared with the Cu containing steels. The grain boundaries of the 0.2 wt% Cu specimen were corroded slightly more than those of the 0.35 wt% Cu specimen and a considerable amount of debris (corrosion products) remained on the surface of the 0.35 wt% Cu specimen. This result indicates that the corrosion products on the 0.35 wt% Cu specimen are more adhesive and protective than those found on the 0.2 wt% Cu specimen. Figs. 8 and 9 show the XPS spectra of the specimens after immersion testing for 6 h. The XPS analysis further explains the improvement in the corrosion resistance gained through Cu
addition. Table 4 lists the surface products which were obtained by an analysis of the XPS peaks. In Fig. 8, chemical compounds such as FeOOH and Fe2(SO4)3 were detected in all of the specimens. In Fig. 9, Cu, Cu2O and CuO were detected in the two specimens with added Cu. These results indicate that the precipitation of Cu and Cu oxides and a higher amount of FeOOH on the surface of the specimens improve the corrosion resistance of the Cu containing steel. FeOOH is formed by galvanic corrosion of the Fe particles to Fe2+, oxidation of the Fe2+ to Fe3+ by the dissolved oxygen, hydrolysis to Fe(OH)3, which is positively charged and in a colloidal form, and then electrophoretically deposited as FeOOH onto the Cu matrix [27]. Although Fe2(SO4)3 was detected in all of the specimens, its effect on the corrosion resistance of the low alloy steels was not corroborated. To determine changes in the distribution and concentration of the alloying elements on the specimen surface before and after the 6 h immersion test, an EPMA mapping was conducted. Figs. 10 and 11 present the EPMA mapping data before and after the test respectively. Before the immersion test, Cu was uniformly distributed on the surface of the specimen and the MnS was partially observed. After the immersion test, the Cu concentration on the surface was increased, indicating that the presence of the Cu enrichment layer on the surface protected the steel by suppressing active dissolution of the substrate.
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A comparison of the EPMA results with the XPS results was made because each depth resolution of the two methods is different (XPS 5–50 Å, EPMA 100–5 um). The surface analysis (XPS) shows that Cu and Cu oxides are precipitated onto the Cu containing low alloy steels and that FeOOH increased with the addition of Cu, however the sub-surface analysis (EPMA) exhibited only Cu accumulation. This means that (1) the Cu enrichment layer resulted in a higher number of sites favourable for the adsorption reaction of protective corrosion products (FeOOH) and (2) this process occurred at the electrolyte/corrosion product interface. 4. Conclusions The Cu containing steel presents much lower corrosion rates than Cu-free steel in a sulphuric acid solution. Gradual enrichment of the Cu on the alloy surface by re-deposition of Cu was confirmed through the use of EIS and surface analyses. The accumulated Cu compound on the surface, which was composed mainly of Cu and Cu oxides associated with FeOOH, improved the corrosion resistance of the low alloy steel by reducing the active dissolution of the substrate. Additionally, the improved corrosion resistance of the Cu containing steel was attributed to the high hydrogen overpotential suppressing the cathodic hydrogen evolution reaction. Acknowledgement This work was supported by the Korea Ministry of Knowledge Economy through the Strategic Technology Development Program. References [1] R.H. Perry, D.W. Green and J.O. Maloney, Perry’s Chemical Engineer’s Handbook, seventh ed., McGraw-Hill, New York, 1997. [2] C. Kato, H.J. Grabke, B. Egert, G. Panzner, Electrochemical and surface analytical studies on hydrogen permeation with Fe-Cu alloys in sulfuric acid with and without H2S, Corros. Sci. 24 (1984) 591–611. [3] N.D. Greene, C.R. Bishop, M. Stern, Corrosion and electrochemical behavior of chromium-noble metal alloys, J. electrochem. Soc. 108 (1961) 836–841. [4] J. Guo, M. Seo, Y. Sato, G. Hultquist, C. Leygraf and N. Sato, Electrochemical behavior and surface composition of copper containing ferritic stainless steel in sulfuric acid solution, Boshoku Gijutsu (J. Corros. Engng of Japan) 35 (1986) 283-288. [5] A. Higginson, R.C. Newman, R.P.M. Procter, The passivation of Fe-Cr-Ru alloys in acidic solutions, Corros. Sci. 29 (1989) 1293–1318.
[6] A.A. Hermas, K. Ogura, T. Adachi, Accumulation of copper layer on a surface in the anodic polarization of stainless steel containing Cu at different temperatures, Electrochemica Acta 40 (1995) 837–844. [7] P. Sury, Similarities in the corrosion behavior of iron, cobalt and nickel in acid solutions. A review with special reference to the sulphide adsorption, Corros. Sci. 16 (1976) 879–901. [8] D.A. Jonse, Principles and Prevention of Corrosion, Prentice-Hall, NJ, 1996. [9] ASTM G 1-03, Standard practice for preparing, cleaning, and evaluating corrosion test specimens, ASTM, West Conshohocken, PA, 2009, p. 16. [10] ASTM G 31-72, Standard practice for laboratory immersion corrosion testing of metals, ASTM, West Conshohocken, PA, 2002, p. 101. [11] S.S. El-Egamy, Corrosion and corrosion inhibition of Cu-20%Fe alloy in sodium chloride solution, Corros. Sci. 50 (2008) 928–937. [12] H. Kaiser, Selective dissolution of high and low diffusivity alloys–A comparison of kinetical and micromorphological aspects, Corros. Sci. 34 (1993) 683–699. [13] F. Farelas, M. Galicia, B. Brown, S. Nesic, H. Castaneda, Evolution of dissolution processes at the interface of carbon steel corroding in a CO2 environment studied by EIS, Corros. Sci. 52 (2010) 509–517. [14] R. Cottis and S. Turgoose, Electrochemical Impedance and Noise, NACE International, Manchester, 1999. [15] G. Moretti, F. Guidi, G. Grion, Tryptamine as a green iron corrosion inhibitor in 0.5 M deaerated sulphuric acid, Corros. Sci. 46 (2004) 387–403. [16] P. Peled, D. Itzhak, The surface composition of sintered stainless steel containing noble alloying elements exposed to a H2SO4 envionment, Corros. Sci. 32 (1991) 83–90. [17] H.J. Cleary, N.D. Greene, Corrosion properties of iron and steel, Corros. Sci. 7 (1967) 821–831. [18] M. Seo, G. Hultquist, C. Leygraf, N. Sato, The influence of minor alloying elements (Nb, Ti and Cu) on the corrosion resistivity of ferritic stainless steel in sulfuric acid solution, Corros. Sci. 26 (1986) 949–960. [19] Y.S. Choi, J.J. Shim, J.G. Kim, Effects of Cr, Cu, Ni and Ca on the corrosion behavior of low carbon steel in synthetic tap water, J. Alloy. Compd. 391 (2005) 162–169. [20] S.T. Kim, Y.S. Park, H.J. Kim, Effects of copper and sulfur additions on corrosion resistance and machinability of austenitic stainless steel, Corrosion Science and Technology 31 (2002) 122–129. [21] J.K. Kim, An electrochemical study on the effect of pitting inhibition in weakly alkaline solution by copper addition in pure iron, Metals and Materials International 9 (2003) 47–51. [22] E. Williams and F. G. Komp, Effect of copper content of carbon steel on corrosion in sulphuric acid, Corrosion 21 (1965): p. 9-16. [23] A. Pardo, M.C. Merino, M. Carboneras, F. Viejo, R. Arrabal, J. Munoz, Influence of Cu and Sn content in the corrosion of AISI 304 and 416 stainless steels in H2SO4, Corros. Sci. 48 (2006) 1075–1092. [24] B.D. Barker, Electroless deposition of metals, Surface Technology 12 (1981) 77–88. [25] N.D. Tomashov, G.P. Chernova, E.N. Ustinsky, Effect of platinum elements additions on the active dissolution of plastic chromium in sulphuric acid, Corrosion 40 (1984) 134–138. [26] M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966. [27] R.F. North, M.J. Pryor, The nature of protective films formed on a Cu-Fe alloy, Corros. Sci. 9 (1969) 509–517.
Contents lists available at SciVerse ScienceDirect
Corrosion Science journal homepage: www.elsevier.com/locate/corsci
Corrosion behaviour of copper containing low alloy steels in sulphuric acid J.H. Hong a, S.H. Lee a, J.G. Kim a,⇑, J.B. Yoon b a b
Department of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Chunchun-Dong, Jangan-Gu, Suwon 440-746, South Korea POSCO Technical Research Lab., 1 Goedong-Dong, Nam-Gu, Pohang 790-785, South Korea
a r t i c l e
i n f o
Article history: Received 21 April 2011 Accepted 10 September 2011 Available online 16 September 2011 Keywords: A. Low alloy steel A. Acid solutions B. Polarization C. Acid corrosion C. Hydrogen overpotential C. Rust
a b s t r a c t The influence of the addition of Cu on the corrosion resistance of low alloy steel in sulphuric acid was investigated using AC and DC electrochemical methods and a weight loss test, which took place in 10 wt% H2SO4 solution at room temperature. The electrochemical measurements indicated that the corrosion rate is suppressed by the addition of Cu due to a higher hydrogen overpotential and prevention of the active dissolution. Surface analyses (XPS, EPMA and SEM) of the corroded surfaces conducted after the immersion test indicated that the rust layer formed on the Cu containing steels was enriched with Cu compounds. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Being the major structural material used in the construction industry, the prevention of steel corrosion has attracted much attention. As most steels are stable in neutral or alkaline media, acidic environments are the major concern [1], with two approaches to reducing the corrosion rate in acidic media having been developed [2]. The first is based around suppressing the hydrogen evolution reaction using a surface metal which has a high overpotential for hydrogen reactions. The other is the well known barrier effect, in which a dense and continuous metal layer retards the dissolution of the anodic metal substrate in the corrosive environment. These two approaches can be accomplished simultaneously by alloying noble elements to the active substrate. In particular, many authors have reported the advantageous effects of the use of noble elements for inhibiting the anodic and cathodic reactions of the steel immersed in a corrosive environment [3–6]. The noble element constituents or impurities required to be present in the matrix may be placed onto the surface of the alloy during a few minutes [6]. In low alloy steels the addition of copper plays a major role in the improvement of corrosion resistance [2]. However, the effect which the small amount of copper in the low alloy steel has on the corrosion resistance may be questionable. Minor constituents of the corrosion media may influence the corrosion kinetics more than those of the main components, this is in most cases a
⇑ Corresponding author. Tel.: +82 31 290 7360; fax: +82 31 290 7371. E-mail address: [email protected] (J.G. Kim). 0010-938X/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2011.09.012
direct consequence of the selective adsorption at the electrified interface [7]. The purpose of this present paper is to study the alloying effect of the addition of Cu on the corrosion properties of low alloy steels in a 10 wt% sulphuric acid solution in ambient temperature and conditions. 2. Experimental 2.1. Materials and test condition The compositions for the three low alloy steels examined in this study are given in Table 1. Square plates (1.5 by 1.5 by 0.1 cm) with a surface area of 2.25 cm2 were used, with all measurements being carried out in a 1 dm3 solution of 10 wt% H2SO4 in ambient temperature and conditions. The solution was prepared from analytical reagent grade chemicals and de-ionized water. To ensure reproducibility, at least three measurement sets were run for each specimen.
2.2. Electrochemical measurements The electrochemical test specimen was ground using 800-grit silicon carbide (SiC) paper, cleaned in an ultrasonic bath with ethanol for 5 min and then dried in hot air. A three-electrode electrochemical system was used, with a saturated calomel electrode (SCE) and two pure graphite rods, which were used as the reference electrode and counter electrodes respectively. All of the potentials reported were measured with respect to the value of
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182 Table 1 Chemical compositions of the tested steels (wt%). Specimen
2 2
Composition (wt.%)
0 wt% Cu 0.2 wt% Cu 0.35 wt% Cu
Fe
C
Si
Mn
P
S
Cu
Balance Balance Balance
0.07 0.07 0.07
0.25 0.25 0.25
0.7 0.7 0.7
0.01 0.01 0.01
0.012 0.012 0.012
– 0.2 0.35
4
+
2
4
2
4
+
2
+
2 + +
+
200 0 wt.% Cu 0.2 wt.% Cu 0.35 wt.% Cu
0
Potential mVSCE
2
4
2+
-200
3+ 3+ 2+
2+ 2+
-400
2+
2+ 2+
2+
-600
-800 -6
10
-5
-4
10
10
-3
10
-2
-1
10
10
0
10
Fig. 2. Schematic diagrams of the selective dissolution and re-deposition mechanism of the Cu containing low alloy steel in H2SO4 solution.
2
Current density A/cm
Fig. 1. Potentiodynamic polarization curves for the three steels in 10 wt% H2SO4.
the SCE. An open-circuit potential (OCP) was established from 1 h before each of the electrochemical measurements. Electrochemical impedance spectroscopy (EIS) and electrochemical DC measurements, including potentiodynamic polarization and linear polarization tests, were performed using a PARSTAT 2263 potentiostat. For the potentiodynamic polarization test, a scan rate of 0.166 mV/s was used, in the range from 250 mVOCP to +100 mVSCE. A scan rate of 0.166 mV/s was used over a range from 20 mVOCP to +20 mVOCP in the linear polarization test. EIS data was obtained over a frequency range from 100 kHz to 10 mHz, an alternating current with an amplitude of ±20 mVRMS was applied. The impedance data was analyzed using an analysis software which used non-linear least square fitting. The corrosion current density for all of the specimens was determined using the Tafel extrapolation method. The polarization resistance (Rp) value was obtained from the linear polarization and EIS data sets [8]:
RP ¼
ba bc 2:3icorr ðba þ bc Þ
ð1Þ
where ba and bc are the anodic and cathodic Tafel slopes respectively, and icorr is the corrosion current density for each specimen. The corrosion rate can then be determined from the corrosion current density using Faraday’s law [8]:
Corrosion rateðmm=yÞ ¼
per metal atom, F is Faraday’s constant and q is the density of the metal (g/cm3). 2.3. Weight loss test Weight loss was measured by a method which followed ASTM G1-03 and ASTM G31-72 [9,10]. The exposed surface area (5.6 cm2) of each specimen was ground using 800-grit SiC paper, then cleaned in an ultrasonic bath with ethanol for 5 min and dried in hot air. After the initial mass (mi) was measured, the specimen was immersed in the test solution for 6 h. After the test, each specimen was cleaned using distilled water, then pickled in a solution of 0.5 dm3 HCl (sp. gr. 1.19), 3.5 g of hexamethylene tetramine (C6H12N4) and balanced distilled water for 5 min to remove the products of the corrosion. These were then degreased in an ultrasonic cleaner with ethanol for 5 min, followed by a cleaning with distilled water and drying in hot air. The final mass (mf) of the tested specimen was measured for the weight loss test and the corrosion rate was determined using the following equation [8]:
Corrosion rateðmm=yÞ ¼
87; 600 W Atq
ð3Þ
where W is the weight loss (g), A is the area of exposure (cm2), t is the time of exposure (h) and q is the density (g/cm3). 2.4. Surface analyses
316icorr M zF q
ð2Þ
where icorr is the corrosion current density (lA/cm2), M is the molar mass of the metal (g/mol), z is the number of electrons transferred
To investigate the relationship between the alloying element and the corrosion products, the surface was examined using X-ray photoelectron spectroscopy (XPS, ESCA 2000 LAB MK-II
Table 2 Results of the potentiodynamic polarization test. Specimen
Ecorr (mVSCE)
icorr (lA/cm2)
-bc (V dec1)
Corrosion rate (mm/year)
gH at 0.01 A/cm2 (mV)
0 wt% Cu 0.2 wt% Cu 0.35 wt% Cu
463.6 463 475.1
695 177 137
0.065 0.115 0.118
8.06 2.05 1.58
276 425 449
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
anodic reactions, with the cathodic polarization of 0.35 wt% Cu exhibiting the highest hydrogen evolution reaction (HER) overpotential, gH. The potential for cathodic current density of 0.01 A/ cm2 and the equilibrium potential (241 mVSCE at pH 0) for hydrogen evolution reaction were used to calculate the HER overpotential. On the alloy surface this may occur through the preferential dissolution of one component and an enrichment of the other component, therefore enrichment of Cu can be expected on the alloy surface. The formation of a metallic, Cu enriched layer, on the alloy surface can be explained by the preferential dissolution and redeposition process. The dissolution of Fe (as Fe2+) into the acid solution occurs at a voltage above 858 mVSCE, whereas Cu dissolution (as Cu2+) occurs at a voltage above 76 mVSCE. Copper is normally assumed to be immune to corrosion by water (a hydrogen evolution reaction). Copper corrosion in water is based on an interpretation of the conventional potential-pH diagrams where the copper immunity range extends well above the hydrogen
spectrometer, VG Microtech, England) and an electron probe microanalysis (EPMA, EPMA1600, Shimate, Japan) after the 6 h immersion. The corroded surface morphology after the weight loss measurements was inspected using scanning electron microscopy (SEM, S-3000F, Hitachi, Japan).
3. Results and discussion 3.1. Corrosion properties The results obtained from the potentiodynamic polarization tests are presented in Fig. 1 and Table 2. All of the specimens showed active corrosion behaviour without passivation. The corrosion current density decreased with an increase of the Cu content, confirming that improved corrosion resistance can be achieved by Cu addition. The Cu addition suppressed both the cathodic and
1.0
(a)
1h 2h 3h 4h 5h 6h
0.6
2
Z" (ohm-cm )
0.8
0.4 0.2 0.0
(a)
50
1h 2h 3h 4h 5h 6h
40 30
Phase angle (degree)
(a)
20 10 0 -10 -20 -30
-0.2 0.0
0.5
1.0
1.5
-40 -2 10
2.0
-1
10
0
10
2
Z' (ohm-cm )
4
10
20 15 10 5
(b)
80
5
10
1h 2h 3h 4h 5h 6h
70 60
Phase angle (degree)
1h 2h 3h 4h 5h 6h
25
2
3
10
90
(b)
Z" (ohm-cm )
2
10
Frequency (Hz)
30
(b)
1
10
50 40 30 20 10 0 -10
0
-20
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
-2
-1
10
10
0
10
1
10
2
10
3
10
4
10
5
10
2
Z' (ohm-cm )
Frequency (Hz)
30
90
(c)
1h 2h 3h 4h 5h 6h
2
Z" (ohm-cm )
25 20 15 10 5
80
(c)
1h 2h 3h 4h 5h 6h
70 60
Phase angle (degree)
(c)
50 40 30 20 10 0 -10 -20 -30
0 0
5
10 15 20 25 30 35 40 45 50 55 60 65 70
-40 -2 10
-1
10
0
10
1
10
2
10
3
10
4
10
5
10
2
Z' (ohm-cm )
Frequency (Hz)
Fig. 3. Nyquist and Bode plots for the EIS data of the specimens against the immersion time: (a) 0 wt%, (b) 0. 2 wt% and (c) 0.35 wt% Cu containing steels.
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
0 wt% Cu
0.2 wt% Cu
0.35 wt% Cu
Immersion time
Rs(O cm2)
1h 2h 3h 4h 5h 6h 1h 2h 3h 4h 5h 6h 1h 2h 3h 4h 5h 6h
0.2461 0.2543 0.2445 0.2375 0.2285 0.2259 0.4576 0.4448 0.4397 0.4492 0.4419 0.3052 0.2862 0.262 0.2549 0.255 0.2592 0.2636
Rct(Ocm2)
CPE1 Crust (mF/cm2)
n (01)
0.496 0.688 0.711 0.711 0.757 0.749 0.240 0.249 0.269 0.286 0.304 0.285 0.180 0.199 0.224 0.252 0.278 0.302
0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8
1.447 1.447 1.426 1.437 1.439 1.452 38.91 48.77 54.22 54.69 55.69 57.36 49.59 52.82 54.69 55.69 55.69 57.53
(a)
2.0
2
Specimen
before the anodic polarization is performed, and the amount of plating Cu should increase with an increase of the Cu2+ ion concentration [6]. (4) The re-deposited Cu will be precipitated as re-crystallized Cu and Cu oxides. The selective dissolution and redeposition process will repeat until the Cu enriched layer is stabilized. As a consequence of the Cu accumulation on the Cu containing steel surface, the corrosion current density decreased with an increase of the Cu content through the suppressed anodic and cathodic reactions as shown in Fig. 1. The influence of the Cu addition is reflected by an increase in the value of the Tafel slopes (b),
Rct ohm-cm
Table 3 EIS data of the specimens at various immersion intervals.
1.5
1.0 1
2
3
4
5
6
5
6
5
6
Immersion time (h)
(b)
60
Linear fit (Y = 7.66X + 31.99)
55
50
45
40
1
2
3
4
Immersion time (h)
(c)
2
60
Rct ohm-cm
electrode potential. Thus, copper corrosion in aqueous solutions has been found to involve electron or charge transfer between the copper dissolution and the dissolved oxygen reduction reactions. In an acid solution which contains dissolved oxygen, both Fe and Cu dissolve, however the dissolved Cu re-deposits as a precipitate of re-crystallized Cu and Cu oxides [11]. During the potentiodynamic polarization test, the process of Cu re-deposition is thought to be accelerated by the cathodic polarization [6]. Eventually, the cathodic overpotential in the hydrogen evolution is increased through Cu enrichment [2]. The mechanisms of the selective dissolution and re-deposition processes of Cu containing low alloy steel are shown in Fig. 2. The process is split into four steps; (1) Under ambient conditions, dissolved oxygen and hydrogen cations coexist in the sulphuric acid solution. (2) Near to the solution/metal interface, the composition of the Cu containing low alloy steel mainly consists of Fe with a small amount of added Cu. (3) When the Fe dissolves, the Cu may subsequently become detached from the metal surface as the Cu containing low alloy steel does not have a continuous Cu network and preferential dissolution occurs along the continuous paths of less noble atoms [12]. As soon as the Cu is separated from the surrounding Fe, the Cu dissolves to Cu2+ ions by reduction of the dissolved oxygen. When Cu is associated with Fe, the Fe will be preferentially dissolved by galvanic corrosion, however Cu2+ in a Fe, H2O and O2 system acts as an oxidant to the Fe. Therefore, a proportion of the Cu2+ will reduce and then re-deposit onto the substrate through a reduction reaction (consumption of the electrons produced by oxidation of the Fe), this is especially true near to the attached Cu. The cathodic polarization causes a reduction of both the Cu2+ and H+ ions on the steel surface. Cu plating should therefore be emplaced
Rct ohm-cm
2
Fig. 4. Equivalent circuit for interpretation of the impedance spectra.
Linear fit (Y = 2.55X + 47.27)
55
50
45
1
2
3
4
Immersion time (h) Fig. 5. Charge transfer resistance (Rct) as a function of the exposure time: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35wt.% Cu containing steels.
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
Corrosion rate [mm/year]
1,000 EIS LPR Potentiodynamic Weight Loss
100
10
1 0wt.% Cu
0.2wt.% Cu
Table 3 summarizes the best fitting values of the capacitive semicircles, these impedance results can be fitted to the circuit model seen in Fig. 4. The equivalent circuit of a one time constant consists of the following elements: the solution resistance (Rs), capacitance (Crust) and the charge transfer resistance (Rct). During the fitting process, the capacitance was represented by a constant phase element (CPE) to allow for the depressed semicircles. The corrosion rate in the Cu containing steels was much lower than that in the Cu-free steel, however there were no significant differences between the corrosion rates of the 0.2 wt% Cu and 0.35 wt% Cu steels. For the 0 wt% Cu specimen, the charge transfer resistance remained constant regardless of the immersion time. However, for the two specimens with added Cu, the charge transfer resistance was markedly increased with an increase in the time, as shown
0.35wt.% Cu
Fig. 6. Comparison of corrosion rates by test methods.
which provides a clear explanation as to why the corrosion potential of all the low alloy steels in the potentiodynamic results was the same. When only the anodic Tafel slope (ba) increases, the corrosion potential also increases and the corrosion current density decreases. On the other hand, when only the cathodic Tafel slope (bc) increases, both the corrosion potential and the corrosion current density decrease. In this case, as both the anodic and cathodic Tafel slopes increase, the corrosion current density decreases and the corrosion potential remains almost the same. Nyquist and Bode plots for the low alloy steels are shown in Fig. 3. The spectra of the Cu containing steels exhibited a single semicircle, in contrast, the spectra of Cu-free steels exhibit two time constants. These are a capacitive semicircle in the high -medium frequency range and an inductive loop in the low frequency range. The capacitive semicircle characterizes the active state of the interface when the steel is exposed to the sulphuric acid solution. The inductive loop observed in Fig. 3(a), which occurs at low frequency, is attributed to the adsorption of a species which promotes the corrosion rate [13,14]. The inductive signal present at the low frequency disappears and the amplitude of the capacitive semicircle increases with Cu addition, as shown in Fig. 3(b) and (c). The phase angle maxima for the Cu-free steel are far removed from 90° which is what is required to exhibit pure capacitive behaviour. This is due to the effect of porous corrosion products [15]. However, the phase angle maxima for the Cu containing steels were close to 90° indicating the improved structural properties of the corrosion products. This improvement is a result of the Cu containing steels having developed a dense rust layer. The continuous formation of the Cu enrichment layer resulted in a higher number of sites which were favourable for the adsorption reaction of the protective corrosion products and less favourable for the overall charge transfer process. The copper layer which formed on the steel surface decreases the rate of anodic dissolution by a blocking mechanism and a screening mechanism [6]. The copper present in the solid solution diminishes the iron dissolution as a result of blocking the active points in the lattice. The accumulation of copper on the surface of the alloy leads to further deceleration of the anodic dissolution, which is caused by partial screening of the alloy surface with the deposited copper. In addition, the Cu enrichment layer serves as an electronic conductor where the reduction of hydrogen ions takes place following the intermediate reactions at the interface. This leads to microgalvanic cells being formed between the Cu enrichment layer and substrate. As a result the corrosion rate decreased with the addition of Cu and immersion time due to the higher overpotential of Cu in the HER resulting in a depressed cathodic reaction rate.
(a)
(b)
(c) Fig. 7. SEM micrographs after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
in Fig. 5. This confirms that the Cu content in the steel improved the corrosion resistance due to the accumulation of the Cu corrosion product. The average corrosion rates found from the weight loss test and the three electrochemical tests are compared in Fig. 6. The corrosion resistance was improved as a result of the Cu addition. Cu is considered to be a relatively noble metal due to its accumulation on the alloy surface [16]. The optimum quantities for alloy addition
were available for the design of alloys with an improved corrosion resistance [17]. When a specimen with Cu is exposed to a corrosive environment, the relatively active metals are preferentially dissolved prior to the Cu. Therefore, a Cu enriched layer is composed of copper and iron oxides and this protects the steel by suppressing the active dissolution of the substrate [6,18–22]. This positive effect of the Cu content in the steel is not washed out by the high activity of the acid solution [23]. The Cu accumulation on the Cu
(a)
(a) 50000
FeOOH Fe2(SO4)3
Fe2p
8000
Cu2p
40000
C/S
C/S
6000 30000
4000
20000
10000
2000
0
0 700
710
720
730
740
920
Binding Energy (eV)
950
(b)
FeOOH Fe2(SO4)3
Fe2p
940
960
970
Binding Energy (eV)
(b) 60000 50000
930
8000
40000
Cu, Cu2O, CuO
Cu2p
C/S
C/S
6000
30000
4000 20000
2000
10000
0
0 700
710
720
730
740
750
920
930
Binding Energy (eV)
(c) 60000 50000
8000
40000
960
970
Cu, Cu2O, CuO
Cu2p
6000
C/S
C/S
950
(c)
FeOOH Fe2(SO4)3
Fe2p
940
Binding Energy (eV)
30000
4000 20000
2000
10000 0
0 700
710
720
730
740
750
Binding Energy (eV) Fig. 8. XPS spectra of the Fe after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
920
930
940
950
960
970
Binding Energy (eV) Fig. 9. XPS spectra of the Cu after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
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J.H. Hong et al. / Corrosion Science 54 (2012) 174–182
Table 4 Binding energies of the components as found from the XPS data. Analyses of the XPS spectra
Standard chemicals
Binding energy (eV)
Spectrum of Fe 2p
FeO FeOOH* Fe2O3 Fe2(SO4)3 CuO* Cu2O* Cu*
710.0 711.5, 724.3 711.5, 724.0 713.5 933.49, 953.6 932.15, 952.3 932.0932.8, 952
Spectrum of Cu 2p
*
Compound with the closest binding energy to the main peaks of the XPS spectra.
containing steel surface is a kind of electroless plating, in which metallic coatings are formed as a result of a chemical reaction between the reducing agent present in the solution and metal ions. The metallic phase that appears in such reactions may be obtained in the bulk of the solution or as a precipitate in the form of a film on a solid surface. Localization of the chemical process on a particular surface requires that the surface must serve as a catalyst. Therefore, the process will last until almost all of the iron substrate is covered with copper where the stabilization of the charge transfer resistance with time characterizes the dynamic equilibrium of the atoms of copper that escape the alloy lattice, emerge and get crystallized onto the surface [24]. This imposes a limit on the
maximum film thicknesses obtained using electroless deposition [25]. The charge transfer resistance of the Cu containing steels increased with the Cu content and time during the early stage of the immersion test but levelled off and became independent of the Cu content due to the limited film thickness. Therefore, it is expected that the same charge transfer resistance is expected after the 6 h immersion. The addition of Cu causes an immediate reduction (linear section of the curves before the inflection) in the anodic dissolution of Fe, as shown in Fig. 5(b) and (c), where intercepts with yaxis, charge transfer resistance of as-immersed Cu containing steels, are corresponding to the blocking mechanism. Values of the charge transfer resistance at the intercept (31.99 and 47.27 ohm-cm2 for 0.2 and 0.35 wt% Cu containing steels, respectively) increased with the Cu content, but converged to 57.5 ohm-cm2 after 6 h immersion regardless of the Cu content. Tomashov et al. concluded that the blocking mechanism makes the most significant contribution to the deceleration of the active dissolution of chromium [25]. However, in this case, it is expected that the decelerated dissolution of the Cu added steels can be governed by either the blocking effect or the screening effect depending on the Cu content. Platinum group metals (Ru, Pd, Ir and Pt) are immune or well passivated in ambient solutions [26], and surface diffusion of these noble adatoms is responsible for the accumulation on the alloy surface. However, the Cu accumulation occurs
Fig. 10. EPMA mapping data before the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
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Fig. 11. EPMA mapping data after the 6 h immersion test: (a) 0 wt%, (b) 0.2 wt% and (c) 0.35 wt% Cu containing steels.
by simultaneous dissolution and re-deposition, with solid diffusion also present but not rate controlling. This is considered why the charge transfer resistance of the Cu containing steels levelled off and became independent of the Cu content after 6 h immersion. 3.2. Surface analyses Fig. 7 shows the SEM images after the removal of the corrosion product from the weight loss test. The specimen without Cu suffered much more damage than those with Cu. The greatest surface heterogeneity was observed on the surface of the 0 wt% Cu specimen, with the surface morphology of the Cu containing steels being different from that of the Cu-free steel. The corrosion was more severe on the Cu-free steel when compared with the Cu containing steels. The grain boundaries of the 0.2 wt% Cu specimen were corroded slightly more than those of the 0.35 wt% Cu specimen and a considerable amount of debris (corrosion products) remained on the surface of the 0.35 wt% Cu specimen. This result indicates that the corrosion products on the 0.35 wt% Cu specimen are more adhesive and protective than those found on the 0.2 wt% Cu specimen. Figs. 8 and 9 show the XPS spectra of the specimens after immersion testing for 6 h. The XPS analysis further explains the improvement in the corrosion resistance gained through Cu
addition. Table 4 lists the surface products which were obtained by an analysis of the XPS peaks. In Fig. 8, chemical compounds such as FeOOH and Fe2(SO4)3 were detected in all of the specimens. In Fig. 9, Cu, Cu2O and CuO were detected in the two specimens with added Cu. These results indicate that the precipitation of Cu and Cu oxides and a higher amount of FeOOH on the surface of the specimens improve the corrosion resistance of the Cu containing steel. FeOOH is formed by galvanic corrosion of the Fe particles to Fe2+, oxidation of the Fe2+ to Fe3+ by the dissolved oxygen, hydrolysis to Fe(OH)3, which is positively charged and in a colloidal form, and then electrophoretically deposited as FeOOH onto the Cu matrix [27]. Although Fe2(SO4)3 was detected in all of the specimens, its effect on the corrosion resistance of the low alloy steels was not corroborated. To determine changes in the distribution and concentration of the alloying elements on the specimen surface before and after the 6 h immersion test, an EPMA mapping was conducted. Figs. 10 and 11 present the EPMA mapping data before and after the test respectively. Before the immersion test, Cu was uniformly distributed on the surface of the specimen and the MnS was partially observed. After the immersion test, the Cu concentration on the surface was increased, indicating that the presence of the Cu enrichment layer on the surface protected the steel by suppressing active dissolution of the substrate.
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A comparison of the EPMA results with the XPS results was made because each depth resolution of the two methods is different (XPS 5–50 Å, EPMA 100–5 um). The surface analysis (XPS) shows that Cu and Cu oxides are precipitated onto the Cu containing low alloy steels and that FeOOH increased with the addition of Cu, however the sub-surface analysis (EPMA) exhibited only Cu accumulation. This means that (1) the Cu enrichment layer resulted in a higher number of sites favourable for the adsorption reaction of protective corrosion products (FeOOH) and (2) this process occurred at the electrolyte/corrosion product interface. 4. Conclusions The Cu containing steel presents much lower corrosion rates than Cu-free steel in a sulphuric acid solution. Gradual enrichment of the Cu on the alloy surface by re-deposition of Cu was confirmed through the use of EIS and surface analyses. The accumulated Cu compound on the surface, which was composed mainly of Cu and Cu oxides associated with FeOOH, improved the corrosion resistance of the low alloy steel by reducing the active dissolution of the substrate. Additionally, the improved corrosion resistance of the Cu containing steel was attributed to the high hydrogen overpotential suppressing the cathodic hydrogen evolution reaction. Acknowledgement This work was supported by the Korea Ministry of Knowledge Economy through the Strategic Technology Development Program. References [1] R.H. Perry, D.W. Green and J.O. Maloney, Perry’s Chemical Engineer’s Handbook, seventh ed., McGraw-Hill, New York, 1997. [2] C. Kato, H.J. Grabke, B. Egert, G. Panzner, Electrochemical and surface analytical studies on hydrogen permeation with Fe-Cu alloys in sulfuric acid with and without H2S, Corros. Sci. 24 (1984) 591–611. [3] N.D. Greene, C.R. Bishop, M. Stern, Corrosion and electrochemical behavior of chromium-noble metal alloys, J. electrochem. Soc. 108 (1961) 836–841. [4] J. Guo, M. Seo, Y. Sato, G. Hultquist, C. Leygraf and N. Sato, Electrochemical behavior and surface composition of copper containing ferritic stainless steel in sulfuric acid solution, Boshoku Gijutsu (J. Corros. Engng of Japan) 35 (1986) 283-288. [5] A. Higginson, R.C. Newman, R.P.M. Procter, The passivation of Fe-Cr-Ru alloys in acidic solutions, Corros. Sci. 29 (1989) 1293–1318.
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