Article pubs.acs.org/IECR
Anoxic Corrosion Behavior of Pipeline Steel in Acidic Soils Maocheng Yan,* Cheng Sun, Jin Xu, and Wei Ke State Key Lab for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, China ABSTRACT: Acidic red soils are highly corrosive toward carbon steel, but the corrosive nature of this particular soil has not been well documented. Electrochemical techniques including Tafel plots, electrochemical impedance spectroscopy, and weigh loss test were used to characterize the properties and nature of pipeline corrosion in a typical acidic red soil at different water content values. The results show that corrosion rate and type and intensity of corrosion attack are significantly dependent on the soil water content and hence the soil moisture droplet or film absorbed on the steel surface. The corrosion rate increases with the water content up to ca. 30% soil water content (100% water-holding capacity), and it decreases with further increases in water content in the long-term test. The evolution of a kinetic parameter and a “Fe oxide facilitating corrosion” mechanism is proposed and discussed for the anaerobic corrosion of steel in acidic soils.
1. INTRODUCTION Although a combination protection system of coatings and cathodic protection (CP) is applied, external corrosion by soil is still one of the leading causes of pipeline leak and rupture.1−6 Corrosion of carbon steel is generally moderate in well-drained soil with high resistivity; however, carbon steel, e.g. pipeline steel, usually suffers from severe corrosion in highly resistive red soils in South China. In a long-term test program, the corrosion rate as high as 0.1 mmpy (ca. 4 mpy) is found for carbon steel in the red soil in the South China.7 The soil corrosivity rating systems tend to underestimate corrosivity of this kind of soil. Moreover, because of the high resistivity, red soils tend to retard or even shield CP current from reaching the steel surface under coating defects. Severe corrosion attack on pipeline steel has been detected under coating defect where CP system fails in field inspection in the red soil region. Now, there is a growing safety concern regarding the risk of corrosion of pipelines in the red soil regions. It has been accepted that corrosion in soils is an electrochemical process. The electrolyte or even just a thin adsorbed moisture film is essential for corrosion to proceed from the point of view of its electrochemical mechanism. Previous research mainly takes soil corrosion as an aerobic corrosion, and its mechanism depends essentially on the presence of oxygen. In this context, the influence of the soil water content (SWC) is intensively investigated with regard to O2.8−12 It has been generally expected that the O2 reduction is the cathodic reaction supporting the corrosion process, and the corrosion rate of the steel is supposed to be proportional with the content of dissolved O2 in the soil. Herein, the O2 diffusion process is the rate-determining step at a high water content8,11,12 whereas the ohmic overpotential induced by soil resistivity dominates the corrosion process at a low water content. The description has been shown to be correct in many soil conditions. Gupta et al.11 studied the influence of the SWC on corrosion in soils with different size fractions and found that corrosion rate increases with the SWC and that it decreases after the SWC approaches the saturation condition. A critical SWC is observed for all three soils concerned, where corrosion © XXXX American Chemical Society
is the most severe. The corrosion rate declines at an even higher water content, which has been attributed to the limited access of O2 diffusing through the soil in water-saturated conditions.8 The appearance of critical SWC for soil corrosion has also been observed and has been confirmed by other authors.8−12 However, for the case of corrosion of carbon steel in acidic soils, the above description on the corrosive nature is quite inadequate. The corrosive nature of acidic soil is different from other soils. The environmental conditions in the heavy, sticky red soil with high water content is considered to be anoxic, and the role of O2 is considered to be relatively limited. The oxygen reduction is not a dominant cathodic reaction in corrosion processes of the steel in the weak aeration conditions.13,14 The high corrosivity is contrary to the expected proportionality with content of O2 in the soil. A systematic study is still lacking on the corrosive nature and influencing factors of red soils. Hence, further studies are still needed to provide a better understanding of the properties and nature of pipeline steel corrosion in acidic red soils. In this context, a research program has been being conducted on this subject to improve the existing knowledge of corrosion of carbon steel in acidic soils. This program also desires to understand the corrosive nature of the red soils toward pipeline steel. The reaction of steel under anoxic conditions depends primarily on the properties and composition of the clay mineral fraction of the soils. On the basis of the physicochemical properties of Fe oxides in red soils, the role of Fe oxides residing in acidic red soils has been highlighted in our investigations. It has been reported just recently that corrosion aggressiveness of red soils is determined by its mineralogy and that corrosion of pipeline steel is facilitated by Fe oxide minerals residing in the red soil.14 However, a systematic study on the process of “soil Fe oxides facilitating corrosion” and Received: July 9, 2014 Revised: October 18, 2014 Accepted: October 21, 2014
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dx.doi.org/10.1021/ie502728a | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Industrial & Engineering Chemistry Research
Article
comprehensive understanding of the corrosive nature of acidic soils are still lacking. In the present work, influence of the SWC on the electrochemical and corrosion behavior of API X80 pipeline steel was investigated in an acidic red soil. The evolution of kinetic parameters and a possible mechanism of soil Fe oxides facilitating corrosion for the corrosive nature of the acidic soil have also been discussed.
2. EXPERIMENTAL SECTION 2.1. Soil Sample. The sample of a red clay soil was collected in Yingtan (28°15′20″ N, 116°55′30″ E), China. The soil was taken at 1 m depth underground. The soil is an Ultisol according to the US soil taxonomy15 and is regarded as a representative of red soils in South China. The soil contains 40−50 wt % clay minerals dominated by kaolinite and hydrous mica and also contains a small amount of vermiculite. The cation exchange capacity of the soil is 0.07−0.11 mol kg−1 at pH 7. Selected physicochemical properties and the chemical compositions of the red soil are summarized in Table 1. The soil was air-dried and passed through a 16-mesh sieve.
Figure 1. XRD spectra of the 104 Hz) to the capacitive response of the bulk soil media and represent it by a RC set of CsRs in the equivalent electrical circuit shown in Figure 6. 3.3. Further Analysis of EIS. The equivalent electrical circuit (ECC) shown in Figure 6 has been used to characterize the system. The high-frequency loop (f > 104 Hz) associated with the response of the bulk soil media can be represented by a pair of CsRs, where Rs and Cs represent the soil resistance and the soil capacitance, respectively. The low-frequency semicircle (Nernst impedance) associated with the corrosion process in the soil−steel interface can be represented by a pair of QdlRct in
Figure 6. Equivalent electrical circuit for EIS of the steel, where Rs represents the soil resistance and Rct the charge-transfer resistance; a constant phase element Qdl is used as a substitute of the double-layer capacitance at the steel−soil interface because of the nonideal capacitive response of the interface.
parallel (Figure 6). Rct is the charge-transfer resistance and a constant phase element (CPE) Qdl is used as a substitute of the double-layer capacitance Cdl at the metal−soil interface because of the nonideal capacitive response of the interface. The impedance of Q is defined as28,29 ZQ = [Y0(jω)n ]−1
(1) −1
where ω is the angular frequency (s ), Y0 the CPE constant (the admittance magnitude of the CPE) which can be converted into a capacitance, n the Q-power (0
