The corrosion behavior of carbon steel was investigated in deaerated, moderately alkaline conditions (pH 9.8) at room temperature. This study was mainly focused on the influences of carbonate and bicarbonate ions, exposure time, and scale formation on the corrosion rate. The corrosion rate was calculated from Linear Polarization Resistance (LPR) measurements, Electrochemical Impedance Spectroscopy (EIS), analyzing Potentiodynamic Polarization (PDP) curves, and based on time-resolved dissolved iron concentration measurements through Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). Surface characterization of corrosion products was performed by means of Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray analysis (EDX). The results showed that the presence of carbonate and bicarbonate significantly modified the corrosion behavior of the steel electrodes and gave rise to corrosion rates of several hundreds of μm y−1, over time decreasing to levels around 100–200 μm y−1. This decrease in corrosion rate was related to scale formation that introduced a certain degree of mass transport limitations. Nevertheless, the corrosion rates remain relatively high, which has implications for engineering. To explain these high corrosion rates, a mechanistic model was proposed, including two combined effects: a chemical and an electrochemical effect. The different methods to determine corrosion rates were found to yield comparable results.
The aim of this work was to revisit second-harmonic generation (SHG) as a technique for the in situ analysis of oxide film formation on steel samples immersed in aqueous solutions and during electrochemical experiments. The measurements were compared to active reflectance spectroscopy (ARS), which is a more established in situ spectroscopy technique. Carbon steel electrodes were subjected to polarization in pH neutral and mild-alkaline electrolytes to provoke the formation of passive films. In situ SHG measurements allowed monitoring of compositional changes in the formed film, as the SHG intensity increased in the domain of stability of Fe 3 O 4 and decreased in the domain of stability of Fe 2 O 3 during potentiodynamic polarization. ARS, on the other hand, is suitable to monitor film thickness. ARS absorption increased along with film growth when moving in the anodic direction during potentiodynamic polarization. During 30 min cathodic polarization at −0.9 V Ag/AgCl sat. KCl in initially pH neutral the combination of SHG and ARS revealed the presence of an oxide film that was primarily composed of Fe 3 O 4 and with a thickness up to 0.7 nm.
Electrical resistance (ER) probes are a technique used to monitor the corrosion rate (CR) of metals in a wide range of applications and industries. However, limited data comparing CR measured with ERprobes, weight loss and electrochemical methods is available. The aim of this work is to provide data regarding the reliability of CR measurements with ER-probes by comparison with LPR, EIS and weight loss measurements and to provide suggestions for designing ER-probes depending on the expected corrosion mechanism and CR. To evaluate the ER-probes reliability in CR measurements, both uniform and localised corrosion in four different environments were studied. CRs obtained with ER-probes showed good agreement with weight loss and electrochemical measurements. Numerical models were used to optimise ER-probe geometry regarding the expected corrosion type and CR, and to study the effect of Joule heating. Design charts were proposed to support ERprobes design concerning the expected CR in certain applications.
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