Influence of pitting and iron oxide formation during corrosion of carbon steel in unbuffered NaCl solutions
Introduction
Iron as carbon steel has been one of the most extensively studied metals in the environment. Under neutral conditions, oxide or hydroxide layers from its corrosion remain on the surface forming distinctive layers. These layers have a significant structure that tends to be determined by the anions present in the solution [1]. Moreover, the corrosion kinetics becomes independent of pH, and hydrogen ion reduction is no longer an important reaction [2], [3], [4]. The major reaction governing corrosion in most practical applications is the reduction of oxygen present in solution [1], [2].
The rate of corrosion of steels in neutral NaCl solutions is initiated through two main mechanisms: formation and build-up of a passivating iron oxide layer and partial destruction of this layer by pitting [5], [6], [7]. It is generally accepted that pitting corrosion is preceded by the appearance of tiny corrosion seeds on the metal surface, which is naturally protected by an oxide layer [8]. In pure dry air at normal temperatures a thin protective oxide film forms on the surface of mild steel. Unlike that formed on stainless steels, it is not protective in the presence of electrolytes and it usually breaks down [5].
Several researchers examined early pit development in mild steel immersed in chloride solutions in the presence of various passivating agents [6], [7], [9], [10], [11], [12]. An increasing tendency toward stable pitting of carbon steel with increasing Cl concentration in neutral NaCl solution was demonstrated by Chen et al., 2000 [7]. It was also shown that pits in carbon steel may tend to occur around the damaged area, leading to accumulated local dissolution [9], [10], [12], [13].
Iron oxide scales deposited on a corroding iron surface are usually a mixture of different oxides [14]. Layer-type rust arises as a result of potential and chemical gradients across the film which varies with film thickness [15], [16]. Oxygen diffusion across the oxide scales and subsequent reduction have been studied by Stratmann and Muller, 1994 [17]; their main finding was that oxygen is predominantly reduced within the rust scale and not at the metal/electrolyte phase boundary.
Recently, the analysis of current–potential curves obtained from linear potential sweep technique with a superposition model has been successfully applied to study corrosion of carbon steel in aerated unbuffered NaCl solutions [18]. This methodology made it possible to separately evaluate the influence of the cathodic subprocess, oxygen reduction, and the anodic sub process, iron dissolution, and was successfully applied to characterize the corrosion behaviour of a rust-free carbon steel surface. The present work extends the application of this method to characterize the time evolution of carbon steel corrosion when a homogeneous oxide layer builds up on the metal surface. It will be shown that this approach allows the determination of how the oxide layer formation and pitting process affect corrosion rate evolution.
Section snippets
Experimental
Experiments were conducted in a conventional three-electrode cell in which the working rotating electrode was made of carbon steel, the counter electrode was a platinum wire, and the reference electrode was Ag/AgCl (sat. KCl). The working electrode was made of a 4-mm diameter × 5-mm length SAE 1010 carbon steel rod. The composition (in wt%) of the studied carbon steel was Fe–98.5, Mn–0.6, C–0.2, and traces of P, S, Si, Sn, Cu, Ni, Cr, and Mo. It was prepared according to a previously described
Polarization curves
Polarization curves for the carbon steel electrodes obtained at different immersion time are shown in Fig. 1, Fig. 2, Fig. 3, Fig. 4. Reproducible polarization curves with well-defined segments were obtained for immersion time not exceeding 12 h in 1, 0.5, and 0.1 M NaCl electrolytes. For longer time the curves were distorted due to random noise of varying intensity. In 0.02 M NaCl electrolytes the curves showed noise after 6 h of immersion.
Iron oxide particle detachment from the corroding carbon
Conclusions
The analysis of the evolution in time of carbon steel corrosion in NaCl solutions using a superposition model for periodic polarization curves allows a kinetics characterization of the system influenced by iron oxide build-up.
The results of this analysis showed that the anodic subprocess, dissolution of iron, is well described in terms of a pure charge-transfer controlled kinetics. On the other hand, the cathodic subprocess, oxygen reduction on iron, is well described in terms of a mixed
Acknowledgment
Financial support under FONDECYT project 1070930 carried out at the Facultad de Ingeniería of the Universidad de Antofagasta, Antofagasta, Chile, is greatly appreciated.
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