Volume 40, Issue 6

Over the last 30 years, scanning electrochemical microscopy (SECM) has become a fundamental technique in corrosion research. With its high spatial resolution and its ability to study local electrochemistry, it contributes essentially to the understanding of corrosion processes. By using selective micro- and nano-sensors, concentration profiles of different corrosion relevant species, from protons to metal ions, can be established. This review provides a comprehensive overview about SECM based techniques and discusses various types of microsensors, including materials selection and preparation techniques, and it provides extensive tables on redox-couples for specific corrosion research applications.

The unique features of aluminium alloy (AA6061) and its composite (AA6061-SiC composite) and their distinct advantages over aluminium metal have made them popular in many engineering applications such as automotive, aerospace, and structural components. However, aggressive media like acid, alkaline or chloride media reduces their corrosion and fatigue resistance, hence the workability of these materials. There is no evidence available in the literature about the systematic review of corrosion and inhibition of these materials in aggressive acid, alkaline and chloride media. Hence, the present review paper gains significant importance. One of the most feasible strategies for managing material corrosion is the addition of an inhibitor to an aggressive medium. A critical review of the inhibition performance of reported organic and inorganic compounds as inhibitors against the corrosion of AA6061 and AA6061-SiC composite in acid, alkaline and chloride media is summarised in this paper. A correlation between the chemical structure and inhibition of the activity of the inhibitors is discussed. The details of the types of inhibition process and the general mechanism of inhibition involved have been discussed.

The influence of iron sulfide on the rate of cathodic and anodic processes of steels with ferrite, perlite and ferrite perlite structure was shown. It was established that the hydrogenation and corrosion rate of steels with different structures depends not only its structure, but also the nature of sulfide-containing products of corrosion. The well-known scheme of the influence of hydrogen sulfide on the processes of corrosion and hydrogenation of steel was supplemented by reactions of formation iron sulfides. They can affect the rate of recombination of hydrogen atoms and, respectively, its molation and absorption by metals.

The effect of cysteine on the corrosion characteristics of Cu5Zn5Al1Sn alloy in 3.5 wt% NaCl solution has been studied by electrochemical and surface characterization techniques in various immersion times. The results of electrochemical impedance spectroscopy (EIS) revealed that the degradation of Cu5Zn5Al1Sn alloy occurred in 3.5 wt% NaCl and was aggravated with increasing immersion time. The results of inhibition efficiency calculated from EIS data showed that cysteine can act as an effective anti-corrosion substance, which was also proved by the less eroded morphology of the alloy surface observed on scanning electron microscopy (SEM). Furthermore, the elemental analysis of alloy surfaces was investigated by Raman, electron dispersion spectroscopy ( EDS), and X-ray photoelectron spectroscopy (XPS), which confirmed the presence of S and N species. An adequate adsorption isotherm and inhibition mechanism was also suggested based on EIS results.

The ammonia-hydrazine shutdown protection method is widely used in thermal power plants in China because of its good protective performance. However, the toxicity and safety of hydrazine have restricted its use in many power plants, and it is necessary to improve the shutdown protection techniques. In this paper, electrochemical impedance spectroscopy and potentiodynamic polarization curves were used to study the effect of hydrazine in water solutions of different pH values, adjusted with ammonia on the corrosion behavior of carbon steel. It was found that in the water solution without hydrazine, the corrosion resistance of a carbon steel electrode increased with raising the pH. When the pH value of water increased to 10.5, the impedance value of the carbon steel increased significantly, the corrosion current density decreased from 26.06 μA·cm −2 at pH 10.0 to 2.36 μA·cm −2 , and the steel surface was passivated. Hydrazine had different effects on the electrochemical corrosion behavior of carbon steel in water solutions with various pH. When the pH of water was not higher than 10.0, hydrazine had a good corrosion inhibition effect on carbon steel. When the pH of water was not lower than 10.5, the addition of hydrazine inhibited the passivation of carbon steel and promoted the corrosion. The adsorption and substitutional oxidation of hydrazine in the anode region of carbon steel surface should be the reasons for the above phenomenon.

Herein protective coatings against corrosion are compared with regard to their protective efficiency. The coatings had been prepared over the course of time from differently trialkoxysilyl-functionalized commercial or synthesized precursors but were tested using the same techniques and in similar conditions. The coatings are compared according to the existing data on contact angles for water, free surface energy, thickness and protective efficiency, expressed in term of corrosion current density. Moreover, the influence of differently functionalized trialkoxysilyl precursors on the crosslinking of coatings is regarded. It was also noted that too hydrophobic additives can introduce certain defects which can detrimentally influence the protective efficiency. Spectroelectrochemical approach can give important insights into the degradation of protective coatings under electrochemically induced loads. Ex situ infrared reflection-absorption (IR RA) spectroelectrochemical approach can identify hydration, breakage of some siloxane bands or changes in the C–H spectral region. Careful examination can confirm the interruption of some bands between alloy and coating that are responsible for its adhesion. Raman imaging is appropriate to follow the formation and growth of pits that form in the coatings.

In this study, Zn–Ni–Cu and Zn–Ni–Cu–TiB 2 coatings were deposited using high-velocity oxy-fuel (HVOF) thermal spray technique on a mild steel substrate. Corrosion tests like neutral salt spray (NSS) following (ASTM B-117) standard and immersion cycle test following ASTM G-31, ASTM G1-03, standards were carried out for Zn–Ni–Cu and Zn–Ni–Cu–TiB 2 coated mild steel along with uncoated mild steel acting as a control. Both Zn–Ni–Cu and Zn–Ni–Cu–TiB 2 coated mild steel were corrosion resistant as compared to uncoated mild steel. Raman analysis following the immersion cycle test inferred that uncoated mild steel had all forms of rust. While Zn–Ni–Cu and Zn–Ni–Cu–TiB 2 coated mild steel developed very little rust. The characterization helped to understand the changes in the surface before and after tests. It was observed that both Zn–Ni–Cu and Zn–Ni–Cu–TiB 2 coated mild steel had little corrosion degradation of surface as compared to uncoated mild steel. Suggesting that both coatings performed significantly better compared to uncoated mild steel in corrosive environments. Polarization and EIS tests of both coated and uncoated mild steel in a 3.5% NaCl medium helped to understand the behaviour of coatings over a range of frequencies. Both coated samples had high polarization potential E corr values and lower polarization current I corr values as compared to uncoated mild steel. Inferring better performance of coatings in corrosive environments as compared to uncoated mild steel.