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DOI: 10.1016/j.jmrt.2018.10.007
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Available online 7 December 2018
Analysis on surface film formed on high-strength carbon steels in acidic phosphate solution and its relationship with localized corrosion in a 3.5% NaCl solution
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Eun Hye Hwanga, Jin Sung Parka, Hwan Goo Seongb, Sung Jin Kima,
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sjkim56@sunchon.ac.kr

Corresponding author.
a Department of Advanced Materials Engineering, Sunchon National University, Jungang-ro, Suncheon, Jeonnam 540-742, Republic of Korea
b POSCO Technical Research Laboratories, Kumho-dong, Gwangyang, Jeonnam 545-090, Republic of Korea
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Received 06 July 2018, Accepted 15 October 2018
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Abstract

The nature of a passive film formed on high-strength carbon steel and degradation of its passivity in acidic phosphate solutions were characterized by X-ray photoelectron spectroscopy and electrochemical polarization measurements. This study reveals that the steels show typical passivation and localized corrosion behaviors. The passivating film is composed mainly of FePO4·2H2O at a lower potential of 0.4VSCE, but the major component of the film is changed to γ-Fe2O3 at a higher potential of 1.1VSCE. At more than 1.5VSCE, a number of shallow pores and/or pits were observed primarily at coarse-sized second phase particles. The four-point bending test suggested that the pre-existed pits/pores act as stress intensifiers under the subsequently applied tensile loading condition, resulting in higher anodic dissolution rates and lower resistance to stress corrosion cracking (SCC).

Keywords:
Carbon steels
Corrosion
Surface film
Phosphate solution
Full Text
1Introduction

Several engineering metallic materials, such as stainless steels, Al alloys, and Ti alloys, can be used in aggressive environments due mainly to the passive films, which are thin (nanometer scale) and protective oxide layers formed on the metal surface. These alloys, however, can often be susceptible to pitting corrosion caused by the localized breakdown of passive films [1]. This suggests that surface passivation is a prerequisite condition for the localized corrosion phenomenon. In this regard, carbon steel is not generally considered to be attacked by localized corrosion, and few investigations have been performed on the passive film on carbon steels [2]. Typical local corrosion behaviors are also observed when carbon steels are exposed to a specific environment [3–7]. In particular, the localized corrosion behaviors of carbon steels have been reported in a CO2/H2S containing solution [3–5] or oxidizing acid such as concentrated sulfuric acid [6,7]. According to the sandwich model [8,9], the passive film formed on carbon steels exposed to such environments is composed mainly of an inner layer of Fe3O4 and an outer layer of γ-Fe2O3. The stabilized oxide film can act as a protection barrier against the migration of aggressive species into the underlying steel [10]. In general, passive film is not expected to be formed on carbon steels in aqueous corrosion environments containing Cl. During the descaling and pickling process using phosphate solutions, however, passive film can be formed on carbon steels. In some cases, a pre-formed passive film with local defects can remain on the steel surface, which can exhibit localized corrosion when subsequently exposed to an aqueous environment. In addition, under applied tensile stress conditions, the risk of stress corrosion cracking (SCC) induced by the transition of pit-to-crack mechanism can also be increased [11]. In this regard, the current study revealing the nature of a passive film formed on high-strength carbon steel and degradation of its passivity in phosphate solution can provide a significant insight into future perspectives on localized corrosion and/or SCC of the steel. Several researchers have examined the anodic dissolution and passivation behavior of iron in phosphate solutions [10,12,13]. In particular, Flis [10] has reported that the passivation behavior and mode of SCC in phosphate solutions can depend strongly on the C content in the steel as well as the type of surface layers. Considering that the susceptibility to SCC increases with increasing strength of steel [11,14–16], the increasing demand for high-strength steels used in a variety of industrial applications suggests that the passivation behavior and subsequent localized corrosion and/or SCC can be one of the major technical issues that need to be addressed. Nevertheless, the nature of the passive film, its local breakdown properties, and subsequent corrosion behavior in aqueous environment containing Cl is not been completely understood.

Therefore, in the present study, the nature of the passive film formed in an acidic phosphate solution was characterized by means of electrochemical polarization measurement and X-ray photoelectron spectroscopy (XPS). Based on the results, the relationship between the local breakdown of a passive film and the subsequent aqueous corrosion behavior in 3.5% NaCl solution was clarified.

2Experimental2.1Specimen preparation

The steels tested in this study were two types of carbon steel with a carbon content of 0.1–0.2wt.% and, 0.2–0.3wt.%, respectively. The steels also have Mn<1.5wt.%, Si<0.2wt.%, P<0.01wt.%, and S<0.01wt.%. To differentiate these two steel samples with lower and higher C-contents, the samples are referred to as LC (i.e. low carbon steel) and MC (i.e. medium carbon steel). The LC and MC specimens were austenitized by heating to 930°C and subsequently cooled in air and water, respectively. To toughen the quenched MC specimens, tempering heat treatment was also performed at 200°C for 30min. The LC specimen consisted of ferrite/scattered pearlite, as reported elsewhere [13], whereas the MC specimen was composed of tempered martensite with fine carbide precipitated along the martensite lath. The size distributions of the 2nd phase particles in the two steels tested were measured quantitatively using an image analyzer with 60 image frames (1000× magnification).

2.2Potentiodynamic polarization measurements

To evaluate the electrochemical corrosion behavior of the steels in a phosphate solution, potentiodynamic polarization tests were carried out in 1M NaH2PO4+0.1M H3PO4 solution. The specimens were polarized dynamically from around −0.2V to 2V with respect to the open circuit potential (OCP) at a scan rate of 0.2mV/s. The exposed area of specimen is 1cm2. For the sealing process during the electrochemical measurements, the rubber O-ring was used in the typical flat-type corrosion cell. For the electrochemical tests, a platinum grid and saturated calomel electrode (SCE) were used as the counter and reference electrode, respectively. Before the tests, the specimens were ground to 2000 grit paper and cleaned ultrasonically in ethanol.

2.3XPS analysis

To characterize the nature and composition of the surface film formed on the steel, the steel specimens were polarized at three different potentials, 0.4, 1.1, and 1.5VSCE by potentiostatic polarization, and the surfaces were analyzed by XPS. After potentiostatic polarization in a 1M NaH2PO4+0.1M H3PO4 solution, the specimens were rinsed immediately with ethanol and kept in a vacuum desiccator. XPS spectra were recorded using a VG Scientific Escalab 250 with Al Kα radiation and a spot size of 500μm. The spectra were analyzed using spectral data processor (SDP) v 3.0 software, which allows smoothing and deconvolution of the curve. As the standard procedure in XPS analysis, the C 1s peak from contaminant carbon at 285.1eV was used as a reference for charge correction [17].

2.4Surface morphology observation by FE-SEM

After the potentio-dynamic/static polarization and immersion test in the 1M NaH2PO4+0.1M H3PO4 solution, the specimens were rinsed immediately with ethanol, and kept in a vacuum desiccator until the surface morphology was observed by field emission scanning electron microscopy (FE-SEM). The SEM images were obtained with a probe current of 0.5nA and accelerating voltage of 10kV. For effective identification of the 2nd phase particles distributed throughout the matrix, back-scattered electron (BSE) mode and energy dispersive spectroscopy (EDS) analysis were employed.

2.5Four-point bent beam test

To evaluate the SCC susceptibility, a four-point bent beam test was performed according to ASTM G39 [18]. Fig. 1 presents schematic diagrams showing the configuration of the test jig and the dimensions of the specimens. To clarify the influence of the pre-formed pits on the SCC resistance, one of the two MC specimens was pre-exposed to a 1M NaH2PO4+0.1M H3PO4 solution for 6h prior to the SCC test. The SCC test was conducted in a 3.5% NaCl solution at room temperature, and the applied stress levels were kept at 70%, 80%, and 90% of the yield strength (YS) of the steel. The deflection distance of the four-point loaded specimen was calculated using the following relationship:

where y is the deflection of the specimen, σ is the yield strength of the specimen, H is the distance between the outer supports, A is the distance between inner and outer supports, E is the modulus of elasticity, and t is the specimen thickness.

Fig. 1.

Schematic diagram of (a) four point bent beam test jig and (b) specimen dimension.

(0.09MB).

After the test, the surfaces and cross sections of the specimens were inspected thoroughly to identify the cracks by optical microscopy (OM).

3Results and discussion3.1Potentiodynamic polarization measurements

The potentiodynamic polarization curves on tested steels in weakly acidic phosphate solution are presented in Fig. 2. In contrast to the typical uniform corrosion properties commonly observed in carbon steel, the tested steels exhibited passivation and localized corrosion behaviors similar to the case of high-alloyed stainless steels. A thin and protective surface film was expected to be formed on the steel surface. Mayne et al. [19] has reported that the passivating film on iron is composed mainly of γ-Fe2O3 with FePO4·2H2O. The formation of a monolayer of FeHPO4 under applied potentials below oxide passivation has also been proposed by Floryanovich et al. [20]. Whatever the passivating film is, the underlying steel can frequently be susceptible to localized corrosion attack by the local breakdown of the surface film. It is well known that the local pits almost always nucleate at some chemical or physical heterogeneity at the surface [1,21,22]. In this regard, pitting initiation sites were observed after the potentiodynamic polarization test, which are presented in Fig. 3. It is evident that a number of pits were initiated at coarse-sized 2nd phase particles distributed throughout the LC specimen. On the other hand, typical pits and 2nd phase particles contained in the pits were rarely observed in the MC specimen. This may be caused by the greater cleanliness of the MC specimen, as shown in Fig. 4 which presents the size distribution of 2nd phase particles in the steels. The mean size of the 2nd phase particles observed frequently in the MC specimens was much smaller than that in the LC specimen. However, the fact that MC specimen has less population of micro-pits than LC specimen is based on SEM observation under a magnification of 1300 times, and finer-pits which may present in MC specimen may not have been detected. Actually, finer pits initiated at around 2nd phase particles with fine size could also contribute to the increase in susceptibility to overall localized corrosion. As presented in Fig. 2, MC specimen showed a higher current density than LC specimen, suggesting that there were other factors that were much more dominant than the size of particles contained in the specimens. This will be discussed in the following paragraph. Regardless of the steel type, the particles were characterized mostly as Ti,Nb(C,N) and Al–Ca–Mg–O–S, which can be identified in Fig. 5. This suggests that the susceptibility of the tested steels to pitting corrosion depends primarily on the size of the 2nd phase particles.

Fig. 2.

Potentiodynamic polarization measurements of two tested steels in 1M NaH2PO4+0.1M H3PO4 solution.

(0.12MB).
Fig. 3.

Surface morphology observation of LC specimen after potentiodynamic polarization test; (a) 2500× magnification and (b) 1000× magnification.

(0.34MB).
Fig. 4.

Size distribution of the inclusions and precipitates in the two tested steels.

(0.07MB).
Fig. 5.

Nature of the 2nd phase particles in tested steels analyzed by (a) FE-SEM and (b) EDS.

(0.4MB).

Another metallurgical factor influencing the stability of the passivating film is the C-content in the steel. As shown in Fig. 2, the MC specimen with a higher C-content exhibited a higher current density in the passive region, suggesting that the addition of C to steel can deteriorate the passivation phenomenon. This is consistent with a previous study conducted by Flis [10]. He attributed the phenomenon to the porous characteristics of surface film with incorporated C. Although the microstructures are different for the two tested specimens, it was found from our preliminary polarization test that there was no significant difference in the electrochemical corrosion behaviors between the two MC specimens with different microstructures. In this regard, it is assumed that the effect of microstructure on the corrosion behavior in acidic phosphate solution is insignificant. Considering these facts, it is reasonable to assume that the effect of the C-content in the specimen on the corrosion resistance in acidic phosphate solution is much more dominant than the size of particles contained in the specimen.

It can also be recognized from Fig. 2 that the stability of the passivating film is dependent on the environmental factors of temperature and the presence of Cl. The higher temperature and Cl concentration appear to degrade the passivating film. In particular, the addition of 0.1M NaCl to the phosphate solution results in a remarkable decrease in pitting potential, indicating that the passivity degradation caused by the presence of Cl is significant, as the case of the passive film formed on other metallic materials such as stainless steel or Ti alloys.

3.2Characterization of the surface film formed under different potential regions

To characterize the nature and composition of the surface film formed under different potential regions, three potential regions were selected from the potentiodynamic polarization curve. As indicated in the experimental section, the MC specimen was polarized at three different potentials of 0.4, 1.1, and 1.5VSCE for 2h. The specimen surfaces were then observed by FE-SEM, as shown in Fig. 6. Although the potentials of 0.4 and 1.1VSCE lie in the passivation region, the surface morphologies observed were quite different, suggesting that the compositions and/or thickness of the surface film could be different. A number of shallow pores were observed at a potential of 1.5VSCE outside the passive region. XPS analysis shown in Fig. 7 can surely support the fact that there are clear differences in the main components of the surface films formed under the two different potential regions. At the lower potential region of 0.4VSCE, the major component of the surface film was Fe(PO)4·2H2O, with γ-Fe2O3 and Fe3O4 comprising a smaller portion, whereas the surface film formed at the higher potential region of 1.1VSCE was composed mainly of γ-Fe2O3 with smaller portion of Fe(PO)4·2H2O and Fe3O4. Although the surface film formed at 1.5VSCE shows a similar pattern to the case of the film formed at 1.1VSCE, the overall signal was too weak to analyze reliably. FePO4·2H2O, which was expected to be formed mainly at lower potential region, can be dissolved and the surface was covered eventually with γ-Fe2O3/Fe3O4, which can be identified from the surface morphology (Fig. 6d) observed after the potentiostatic polarization at 1.5VSCE. The thickness of the surface films formed under 0.4 and 1.1VSCE was analyzed by AES, and Fig. 8 presents the results of depth profile analysis. Assuming that the surface roughness is so small that it does not need to be considered, the thickness estimated based on the sputtering rate were 2 and 3nm for the film formed at 0.4 and 1.1VSCE, respectively.

Fig. 6.

Surface morphology observation on the passive film of MC specimens; (a), (c) and (e) potentiostatic polarization of 1.5VSCE, 1.1VSCE and 0.43VSCE in 1M NaH2PO4+0.1M H3PO4 solution, respectively. (b), (d) and (e) magnified view of (a), (c) and (e), respectively.

(0.55MB).
Fig. 7.

Fe 2p3/2 photoelectron spectra of the MC specimens polarized at (a) 0.43VSCE, (b) 1.1VSCE, and (c) 1.5VSCE in 1M NaH2PO4+0.1M H3PO4 solution.

(0.22MB).
Fig. 8.

Depth profile analysis using AES for the MC specimens polarized at; (a) 0.43VSCE, and (b) 1.1VSCE.

(0.19MB).
3.3Stress corrosion cracking (SCC) behavior and its underlying mechanism

Fig. 9a shows the SCC resistance of the two specimens, evaluated using a four-point bent beam test. The threshold stress levels show that pre-exposure to the phosphate solution can cause a significant decrease in SCC resistance. For a clearer understanding, the surface morphology after immersion in the phosphate solution for 6h was observed, as shown in Fig. 9b. As noticed, a lot of pits were formed on the surface. The morphology was similar with that observed after potentiostatic polarization at 1.5VSCE. This suggests that when the steel is polarized at a much higher potential exceeding the passive region or is immersed for a long time, the passivating film of the FePO4·2H2O/Fe-based oxide formed on the steel could be deteriorated, leading to local breakdown. The pits formed on the steel could act as stress intensifiers under additionally applied tensile stress condition. Furthermore, hydrogen generation is also expected in the pits by a hydrolysis reaction (Fe2++2H2OFe(OH)2+2H+) [23] and/or hydrogen reduction reaction (H++eH) [24], leading to hydrogen uptake in the steel. The concept of hydrogen-facilitated anodic dissolution reported previously [25–27] could also be adopted. A mechanistic study considering these situations was conducted using electrochemical methods with the stepwise sequence involving potentiodynamic polarization in phosphate solution – cathodic hydrogen charging – potentiodynamic polarization in a 3.5% NaCl solution. Fig. 10a shows the potential role of pre-existed pits formed under potentiodynamic polarization in a phosphate solution followed by cathodic hydrogen charging in subsequent corrosion behavior in a 3.5% NaCl solution. This indicates that the steel with pre-existed pits did not show typical uniform corrosion behavior, and the polarization behavior was more like the unstable localized corrosion. The proposed mechanism described here is presented in Fig. 10b.

Fig. 9.

(a) Threshold stress of general MC specimen and pre-exposed MC specimen in a phosphate solution; (b) surface morphology of the MC specimen after immersion in a 1M NaH2PO4+0.1M H3PO4 solution for 6h; (c) EDS analysis on the marked region in (b).

(0.29MB).
Fig. 10.

(a) Potentiodynamic polarization measurements of the MC specimen in 3.5% NaCl solution, followed by potentiodynamic polarization in phosphate solution and subsequent cathodic hydrogen charging; (b) schematic diagram of SCC mechanism of tested steels.

(0.24MB).
4Conclusion

The nature of a passive film formed on high-strength steel in an acidic phosphate solution was characterized. Based on surface analysis, the relationship between the local breakdown of the passive film and subsequent aqueous corrosion behavior in a 3.5% NaCl solution was discussed. The main conclusions can be summarized as follows:

  • (1)

    High-strength steel classified as typical carbon steel exhibited passivation and localized corrosion behavior in an acidic phosphate solution. A number of pits observed in low carbon steel were initiated at the coarse-sized 2nd phase particles of Ti–Nb(C,N) or Al–Ca–Mg-based oxysulfide. The stability of the passivating film was dependent upon the carbon content, temperature and concentration of Cl ion.

  • (2)

    To characterize the nature and composition of the surface film formed under different potential regions, three potential regions (0.4, 1.1, and 1.5VSCE) were selected from the potentiodynamic polarization curve. XPS analysis indicated that, at the lower potential region of 0.4VSCE, the major component of the surface film was Fe(PO)4·2H2O, with γ-Fe2O3 and Fe3O4 comprising a smaller portion, whereas the surface film formed at the higher potential region of 1.1VSCE was composed mainly of γ-Fe2O3 with a smaller portion of Fe(PO)4·2H2O and Fe3O4. Although the surface film formed at a potential of 1.5VSCE showed a similar pattern to the case of the film formed at a potential of 1.1VSCE, the overall signal was too weak to analyze reliably, and a number of shallow pores were observed.

  • (3)

    The SCC test showed that pre-exposure to the phosphate solution can cause a significant decrease in SCC resistance, due mainly to lots of pits that could act as local stress intensifiers under the stress condition. The pre-existing pits did not show typical uniform corrosion behavior, and the polarization behavior was more like unstable localized corrosion.

Conflicts of interest

The authors declare no conflicts of interest.

Acknowledgments

This research was supported in part by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (grant number: 2016R1D1A3B03930523). Also, this work was partly funded and conducted under the Competency Development Program for Industry Specialists of the Korean Ministry of Trade, Industry and Energy (MOTIE), operated by Korea Institute for Advancement of Technology (KIAT) (No. P0002019, HRD).

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Copyright © 2018. Brazilian Metallurgical, Materials and Mining Association
Journal of Materials Research and Technology

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