V.S. Raja

AC impedance study on the activation mechanism of aluminium by indium and zinc in 3.5% NaCl medium

Abstract Activation of aluminium by In 3+ and Zn 2+ ions in chloride media has been investigated by using potentiodynamic polarisation, potential vs. time measurements, electrochemical impedance spectroscopy and scanning electron microscopy studies. It is suggested that indium exhibits In + and In 2+ intermediates during the dissolution and redeposition processes. Incorporation of any of these lower valent ions changes the defect structure of the passive film, leading to an increase in anionic vacancies and a decrease in the number of electrons, promoting active dissolution of aluminium. The role of zinc is to moderate localised attack by increasing the electron concentration and cation mobility, thereby lowering the corrosion rate at activated sites.

Influence of nitrogen on the pitting corrosion behavior of 904L weld clad

Abstract The effect of nitrogen on the weld cladding of 904L filler metal towards microstructure and electrochemical corrosion was investigated. The initial addition of nitrogen to the shielding gas was found to decrease the pitting resistance of the clad as compared to that obtained without nitrogen addition. Subsequent additions of nitrogen, however, gradually enhanced the pitting resistance. Microstructural examination showed that secondary austenitic phase formed along the interdendritic cell boundaries of the primary austenite, in the absence of nitrogen addition to the shielding gas, offered good resistance to the pitting corrosion of the interdendritic cell boundaries. On initial introduction of N, reduction in the formation of secondary austenite occurred making the clad inferior to pitting corrosion resistance. Subsequent additions of N decreased the formation of secondary austenite, enhanced pitting resistance of the weld clad by stabilizing the passivity of the interdendritic cell boundaries.

Effect of carbon on corrosion behaviour of Fe3Al intermetallics in 0.5 N sulphuric acid

Electrochemical corrosion behaviour of iron aluminides, produced by electroslag remelting technique, having the compositions (1) Fe–15.6Al–0.05C, (2) Fe–15.6Al–0.14C, (3) Fe–15.6Al–0.5C and (4) Fe–15.6Al–1C were investigated in 0.5 N H2SO4 media. Corrosion rates of these alloys were found to increase with carbon content. This was attributed to the preferential attack of the carbide phases. These alloys exhibited typical active–passive–transpassive behaviour. In addition, they displayed a secondary anodic current maxima during polarization. The resistance of aluminides to breakdown of passivity was assessed by varying addition of chloride ions in the same media.

The self regulating nature of In on the potential of Al in 3.5% NaCl solution

Abstract The role of indium on the corrosion potential of high purity Al (99.999%) and commercial purity Al (99.33%) has been investigated, employing potential vs time, polarization and wavelength dispersive X-ray analysis (WDX) techniques. Indium has been found to shift the corrosion potential of Al towards either anodic or cathodic direction depending on the purity of Al providing an optimum value for cathodic protection of steel. WDX analysis shows that in Al containing Fe, indium reduces the cathodic activity of phases such as Al 3 Fe. In high purity Al, the deposited indium provides additional cathodic sites. These processes are responsible for lowering E corr of commercial purity Al on the one hand and increasing E corr of high purity Al on the other hand through simply alloying aluminum of various purity with indium.

Corrosion behaviour of solar reflector coatings on AA 2024T3 - an electrochemical impedance spectroscopy study

Abstract Electrochemical impedance spectroscopy in the 100 kHz–5 mHz frequency range was applied to the study of corrosion behaviour of solar reflector coatings on AA 2024 exposed to 3.5% NaCl solution. Solar reflector coatings were obtained by sulphuric acid anodization of the alloy in presence of oxo-anions of molybdenum or vanadium. Corrosion behaviour of the oxide films was evaluated by determining the film resistance and capacitance values with exposure time. Comparison of the results reveals that, vanadium addition confers better barrier properties and corrosion resistance than molybdenum additions. Analysis using damage function based on the impedance at 100 mHz shows that solar reflector coatings have improved corrosion behaviour than normal sulfuric acid anodized oxide films on AA 2024.

Scanning Auger electron spectroscopy study of the oxide film formed on dendritic and interdendritic regions of C containing Fe3Al intermetallic

The oxide films formed during early stage of oxidation at 800 °C on dendritic and interdendritic regions of the cast Fe–16Al–1C (wt.%) alloy were studied using scanning Auger electron spectroscopy. Microhardness measurement and elemental depth profiles by Auger spectroscopy reveal that the carbide, Fe3AlC0.69, is the major constituent of the interdendritic region, while dendrites are predominantly Fe3Al phase. Between the two, the interdendritic region is found to be more prone to oxidation than the dendritic region, which was attributed to presence of carbides with low-Al content. In spite of the difference in oxide film thickness exhibited by both the phases, they consist of an inner aluminium oxide layer and an outer iron oxide layer.

Encapsulating 8-hydroxyquinoline in graphene oxide-stabilized polystyrene containers and its anticorrosion performance

We report the formation of encapsulated graphene oxide/polystyrene (GO/PS) containers via Pickering emulsion polymerization of oil-in-water emulsions stabilized by GO sheets. Without further functionalization, the as-synthesized GO sheets were used for emulsion stabilization to minimize the process step. A one-step process was developed to encapsulate 8-Hydroxyquinoline (8-HQ) in GO/PS containers. FT-IR study confirmed the encapsulation. The inhibiting effect of 8-HQ was found to affect the kinetics of the emulsion polymerization and shift the polymerization temperature to a higher value. A detailed study of stability, size, and morphology of encapsulated containers (PS/GO) was carried out as a function of GO-to-oil ratio and oil volume fraction in the formulation, to optimize the process parameters. A possible mechanism for the formation of GO/PS containers at various GO-to-oil ratios was discussed. The corrosion inhibition performance of 8-HQ-encapsulated containers was studied in an epoxy coating. These encapsulated containers induced long-term anticorrosive property to the coating. The use of cost-effective materials and a scalable method used in this work can be utilized to prepare environmentally friendly coatings for large-area applications.

Effect of Thermomechanical Treatment on the Environmentally Induced Cracking Behavior of AA7075 Alloy

The influence of thermomechanical treatment on the stress corrosion cracking behavior of AA7075 aluminum alloy forgings was examined in 3.5% NaCl solution by varying the extent of thermomechanical working imparted to each of the conditions. The results show that inadequate working during billet processing resulted in inferior corrosion and mechanical properties. However, more working with intermediate pre-heating stages also led to precipitation of coarse particles resulting in lowering of mechanical properties marginally and a significant reduction in the general/pitting corrosion resistance. The results obtained in the present study indicate that optimum working with controlled pre-heating levels is needed during forging to achieve the desired properties. It is also demonstrated that AA7075 in the over aged condition does not show any environmental cracking susceptibility in spite of the microstructural variations in terms of size and volume fraction of the precipitates. However, the above microstructural variations definitely affected the pitting corrosion and mechanical properties significantly and hence a strict control over the working and pre-heating stages during billet processing is suggested.

Effect of BTA on electrochemical corrosion and stress corrosion cracking behaviour of type 304 stainless steel in 1 M HCl

Effect of benzotriazole (BTA) on polarization and stress corrosion cracking (SCC) behaviour of type 304 stainless steel in 1 M HCl was investigated. The anodic polarization curves showed that with BTA additions the anodic polarization kinetics in the active region was not affected, though a reduction in critical current density, icrit, and passive current density, ip, was observed. However, BTA was found to influence significantly the cathodic reaction kinetics. SCC results using smooth tensile test specimens showed an increase in time-to-failure, tf, with BTA additions. Crack growth rate studies using single-edge notched (SEN) specimens showed an increase in threshold stress intensity for SCC, KISCC, and a decrease in crack growth rate, da/dt, with BTA additions. While the adsorption isotherms derived from weight loss data followed a Langmuir adsorption isotherm signifying a monolayer adsorption, the adsorption isotherms derived from SCC test data deviated from this at higher BTA concentrations. The paper discusses the possible reason for this deviation.

Graphene-based anticorrosive coatings for copper

The present study was focused on the development of environmentally friendly graphene-based anti-corrosive coatings and understanding the effect of these coatings on the electrochemical corrosion behavior of copper. Through effective functionalization of graphene (<=5 layers) with 3-aminopropyltriethoxysilane (APTES), the corrosion current density was reduced by ∼20 times in magnitude as compared to that of the uncoated copper. This enhanced corrosion protection is attributed to the high surface area of graphene, electrochemically produced highly conductive few layer graphene, its barrier properties, reduced water uptake/oxygen/salt permeation, and homogeneous dispersion of graphene throughout the coating.