The effects of thioureido imidazoline and NaNO2 on passivation and pitting corrosion of X70 steel in acidic NaCl solution
Introduction
Because of the good properties such as high strength, high impact toughness and low ductile-brittle transition temperature, X70 steel is widely used in various industries, particularly in oil and gas industry as a pipeline material. However, in acidic chloride containing environments which are commonly present in oil fields, the steel experiences severe corrosion and different methods have to be applied to extend the life of the steel facilities [1], [2], [3]. Inhibitors are frequently applied to control corrosion. In acidic solutions, imidazoline and thiourea show high inhibition efficiency and have been used widely in sour CO2 [4], [5], [6] and hydrochloric acid [7], [8] containing environments. Usually, in acidic solutions the oxide films on steel surface are not stable, therefore inhibitor molecules could adsorb directly on metal, creating an inhibiting film on the substrate surface. As the result the interactions between the metal and the inhibitor components would affect the inhibition mechanism and efficiency. In a previous study, Tan et al. reported [9] that when imidazoline is used alone, some small anodic zones may form on steel surface, which increases the pitting susceptibility of the steel. This may explain observations made by Martin et al. [10] that generally the organic inhibitors containing nitrogen cannot effectively inhibit corrosion of steels in CO2 solution or other acidic environments, and the inhibitors containing S and P show better inhibitive effects. Different substituents can be introduced to improve the inhibition efficiency of imidazole. Recently Milosev et al. [11], [12] studied the inhibition of imidazole, benzimidazole and its methyl and mercapto derivatives for copper in chloride solution, and the results showed that the differences in inhibition efficiency are related to differences in composition and structure of the protective layer formed. Liu et al.’s study [13] showed that in a CO2-containing oilfield produced water an imidazoline derivative can partially inhibit the growth of planktonic iron-oxidizing bacteria. Zhang et al. [14] reported that for mild steel in hydrochloric acid solution, the chloride-substituted compound shows better inhibitive performance than the fluoride substituted one. Mohammad et al. [15] reported that a series of imidazolines showed good inhibition for mild steel in CO2–NaCl solution by forming an imidazoline film on the metal surface.
On the other hand, although thiourea as inhibitor is extensively used, its concentration in solution has a great influence on the inhibition efficiency. If the thiourea concentration is not controlled in a suitable range, corrosion may be accelerated [16]. For example, in an acetic-sodium acetate buffer system with pH 3.7, the highest inhibition is obtained for X70 steel when the thiourea concentration is 50 mg/L. The corrosion current density increases when the thiourea concentration further increases [17]. Zhao et al. [18] studied the inhibition effect of oleic-based imidazoline quaternary ammonium salt (OIMQ) and thiourea (TU) for carbon steel in CO2 saturated brine solution, and an excellent synergistic inhibition effect between OIMQ and TU was observed. Du et al. [19] synthesized an imidazoline derivative (1-(2-thioureidoethyl)-2-alkyl imidazoline) inhibitor and reported that the inhibitor showed good inhibition properties for Q235 steel in CO2 saturated saltwater at room temperature. The inhibition efficiency increased with increasing in concentration of inhibitor and temperature. Du et al. [20], [21] also reported the inhibition performance of a thioureido imidazoline inhibitor (TAI) in CO2 corrosion conditions. The results indicated that the values of inhibition efficiency show a peak-value-phenomenon at a concentration of 0.15 mmol/dm3 owing to the changed adsorption mode. The chemical adsorption of the inhibitors occurred by the formation of coordination bonds between the heteroatom of the inhibitors and Fe on the metallic surface. Recently Zeng et al. [22] studied the inhibition effect of TAI for the flow accelerated corrosion at different locations of X65 carbon steel elbow in a simulated formation water of oil field. It was reported that TAI is an anodic type inhibitor by remarkably inhibiting the anodic process.
For carbon steels in acidic environments, general corrosion is the main corrosion type, hence most previous studies paid attentions to the effects of inhibitors on general corrosion. However, in practical industrial environments pitting corrosion of carbon steels happens extensively, including in acidic systems, which may be due to the effects of the complicated conditions such as flowing, varying pH and temperature, the enrichment of aggressive ions and the presence of corrosion products or other deposits. Therefore, it is interesting to examine the inhibiting effects of organic inhibitors for both general and pitting corrosion in acidic chloride solutions. NaNO2 may effectively promote passivation of steels in acidic solutions, as the result steels would show passivation-pitting behavior instead of active corrosion. However, there were few studies on the compounded effect of NaNO2 and organic inhibitors. It was reported [23] that some sulfur-containing organic compounds including TU may inhibit corrosion of aluminium in NaNO2 solution, but the effect between NaNO2 and TU was not mentioned. In this paper, the effects of a thioureido imidazoline inhibitor on passivation and pitting behavior of X70 steel in acidic chloride solution containing NaNO2 are studied. The attention was mainly paid to both the inhibition of TAI on pitting corrosion of steel and the compounded effect of the organic inhibitor TAI and the inorganic inhibitor NaNO2.
Section snippets
Experimental methods
The studied material was X70 steel with following chemical composition(wt%): 0.0623% C, 1.29% Mn, 0.010% P, 0.0015% S, 0.19% Si, 0.0026% O, 0.003% N, and Fe. The X70 steel rod with a cross section of 1 cm2 was cut into samples 0.5 cm in thickness. The sample was sealed with epoxy resin after a copper wire was soldered on, leaving an area of 0.06 cm2 as the working area. Before test, the working area was abraded with silicon carbon abrasive papers step by step from 240# up to 1000#, then was
Immersion 1 h in the test solutions containing different concentrations of TAI
The X70 steel samples were immersed in the test solutions with different concentrations of TAI for 1 h, then polarization curves were measured. Fig. 3 shows the results, and Fig. 4 shows the changes of the corrosion potential Ec and the pitting potential Eb with TAI concentration. In the test solution without TAI, X70 steel showed active-passive-pitting behavior which is due to the effect of NO2− [28]. NO2− is an oxidizing agent and the following cathodic reaction in acidic solution may occur
Conclusions
- (1)
In acidic 0.2 mol/L NaCl + 0.05 mol/L NaNO2 + TAI solution, when the amount of TAI inhibitor was less than 110 ppm, in the early stage the anodic dissolution of X70 steel was facilitated and the pitting corrosion potential decreases. When TAI amount reached 160 ppm, self-passivation happened for X70 steel in the solution and the corrosion current density decreased by two orders of magnitude, but the pitting corrosion potential was still decreased. The possible reason is that the residual thiourea
Acknowledgement
The authors would like to thank the National Natural Science Foundation of China (Contract 51210001) for supports to this work.
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