Effect of Phosphate-Based Inhibitor on Corrosion Kinetics and Mechanism for Formation of Passive Film onto the Steel Rebar in Chloride-Containing Pore Solution

In the present study, different contents, i.e., 1–3% of 0.5 M ammonium phosphate mono basic (APMB), were used as corrosion inhibitor to reduce the corrosion of steel rebar. Electrochemical impedance spectroscopy (EIS) results showed that up to 24 h of exposure, polarization resistance (Rp) and passive/oxide film resistance (Ro) gradually decreased in simulated concrete pore (SCP) + 3.5 wt.% NaCl solution owing to the reduction in pH of the solution. The steel rebar exposed in 2% inhibitor containing SCP + 3.5 wt.% NaCl solution exhibited 90% inhibition efficiency after 1 h of exposure. X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy confirmed the formation of thermodynamically very stable and sparingly soluble goethite (α-FeOOH), maghemite (γ-Fe2O3), and iron phosphate (FePO4) as passive/oxide film onto the steel rebar surface exposed to 2% inhibitor containing SCP + 3.5 wt.% NaCl solution.

The Effect of Surface Defects on Chloride-Induced Depassivation of Iron—A Density Functional Theory Study

Chloride-induced depassivation is a large contributor to the degradation of metals, but defects are likely to play a key role in that process. Here density functional theory calculations are used to investigate the mechanism of the initial stages of chloride-induced depassivation of iron by studying the Cl interactions with stepped α-Fe2O3 (0001) surfaces and how that can lead to degradation of the passive oxide film. The low coordinated Fe sites near the step edge and O vacancies facilitate high local coverages of adsorbed Cl, which enhance surface Fe vacancy formation significantly. The step edge also lowers the Cl insertion energy, relative to the flat surface, but insertion by exchange with O is still endothermic. This study illustrates the importance of surface defects, step edges, and O vacancies in the depassivation mechanism, but the findings generally support the point defect model as a description of the depassivation mechanism.

Electrochemical studies on pitting corrosion of tin in sodium borate solutions containing nitrate ions

Purpose The purpose of this paper is to study the electrochemical behavior of Sn electrode in Na2B2O7 solutions in the absence and presence of NaNO3 as a pitting corrosion agent. Design/methodology/approach The electrochemical behavior of Sn electrode was studied by using cyclic voltammetry and potentiodynamic polarization measurements and complemented with scanning electron microscopy examinations. Findings This paper shows that in the absence of NO3 − ions, the anodic polarization of Sn electrode exhibits active/passive transition. Addition of various concentrations of NO3 − anions to the borate solution enhances active anodic dissolution and tends to break down the passive oxide film at a certain pitting potential. The pitting potential, and hence the pitting corrosion resistance, decreases with increasing NO3-ion concentration and temperature but increases with scan rate and repetitive cycling. Addition of CrO42−, WO42− or MoO42− oxyanions to the borate nitrate solution inhibits the pitting corrosion of Sn. Originality/value This is the first study that shows the effect of NO3 − ion as a pitting corrosion agent.

Corrosion Behavior of Fe-Mn-Si-Cr-Ni-Co Shape Memory Stainless Steel in Highly Oxidizing Medium

A study was conducted on the corrosion behavior and characteristics of the passive oxide film of Fe-Mn-Si-Cr-Ni-(Co) shape memory stainless steels (SMSS) in a concentrated nitric acid (HNO3) solution, based on potentiodynamic polarization, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) analyses. The results indicated that Fe-Mn-Si-Cr-Ni-(Co) SMSSs exhibit a passive behavior similar to that of 304L austenitic stainless steel (304L SS). However, unlike 304L SS, their high silicon (Si) content renders them insensitive to intergranular attack in highly oxidizing environments. The XPS analysis also indicated that Si appears to be the main element responsible for the high protectiveness afforded by the passive film formed on Fe–Mn–Si–Cr–Ni–Co SMSS.