Portugaliae
Electrochimica Acta

Electroless nickel coatings possess several advantages over electroplating such as ability to coat any material and with uniform thickness. Besides, these coatings are used mainly for their wear resistance and corrosion resistance applications. The present study addresses the corrosion behavior of the coating based on electrical impedance spectroscopy. The effect of the four parameters viz. bath temperature, reducing agent concentration, nickel source concentration and annealing temperature, on the electrochemical characteristics (charge transfer resistance and double layer capacitance) are studied with the help of Taguchi method and grey relational analysis. It is found that the bath temperature has the most significant influence on the corrosion behavior of the coating followed by nickel source concentration. The microstructural characterization of the coating is done with the help of scanning electron microscope, X-ray diffraction analysis and energy dispersive X-ray analysis.

The structure and properties of the passive film formed on the surface of AISI 316 stainless steel in aqueous ethanoic acid have been investigated using EIS. Experiments were carried out at 30 ºC in different concentrations of ethanoic acid. Effects on the film properties due to the change of electrode potential, exposure durations and addition of chloride ions to the electrolyte were also studied. Impedance parameters were determined using simple model and equivalent electrical circuit. Results suggest the formation of multilayered passive film on the steel surface. The film possessed dual structure. Inner layers were thin and compact, whereas the outer layer was porous and defective. The measured capacitive behavior was of non ideal nature and hence replaced by constant phase element or CPE. Formation of the passive film and the change in its structure has been explained using impedance parameters.

The inhibition of copper corrosion in 1 M HNO3 by (E)-(4-(4-methoxybenzylideneamino)-4H-1, 2, 4-triazole-3, 5-diyl) dimethanol (MBATD) was investigated by polarization, ac impedance, adsorption studies, quantum chemical calculations, molecular dynamics methods and surface morphological studies. Polarization studies showed that MBATD acts as a mixed type inhibitor. Adsorption followed the Langmuir isotherm with negative value of free energy. Various thermodynamic and kinetic parameters were calculated to understand the energy changes associated during the inhibition process. The correlation between inhibitive effect and molecular structure is ascertained by quantum chemical calculations using density functional theory and Fukui functions. The molecular dynamics method has also been used to simulate the adsorption of inhibitor molecule on the metal surface.

Expired ranitidine was tested as a corrosion inhibitor for mild steel in 1 M HCl using different techniques: weight loss, potentiodynamic polarization, open circuit potential and electrochemical impedance spectroscopy. The polarization resistance (Rp) value increased with increase in the concentration of the inhibitor. Results obtained revealed that ranitidine performed excellently as a corrosion inhibitor for mild steel in this medium at 303 K. The protection efficiency increased with increasing inhibitor concentration. The maximum protection efficiency of 90% has been obtained at 400 ppm. On the other hand, the efficiency decreases with increasing temperature. The adsorption of the inhibitor on the mild steel surface followed Langmuir adsorption isotherm model. The activation and thermodynamic parameters of dissolution and adsorption were calculated and discussed. The negative value of ∆Gads (-40 kJ mol-1) indicates spontaneous chemical adsorption. Results obtained from polarization, EIS and weight loss measurements are in good agreement with each other.

The effect of five compounds of ethoxylated fatty esters with different number of ethylene oxide units on the corrosion of copper in 1 M HNO3 solution has been studied using weight loss and galvanostatic polarization techniques. The inhibition efficiency was found to increase with increasing concentration and number of ethylene oxide units per molecule. Inhibition effect was explained on the basis of adsorption of ethoxylated fatty esters on the metal surface through their ethoxy groups. The adsorption process was found to follow Temkin adsorption isotherm. Polarization data suggested that the used additives act as mixed type inhibitors. It was found also that the ethoxylated fatty esters provide a good protection to copper against pitting corrosion in chloride containing solutions.

The corrosion resistance of three stainless steel materials, namely, stainless steel (SS), stainless steel electroplated with zinc (SS-Zn) and stainless steel electroplated with zinc followed by blue passivation (BP), has been evaluated in an aqueous solution containing 3.5% NaCl. A potentiodynamic polarization study and AC impedance spectra have been used to investigate the corrosion behaviour of these metals. The corrosion resistance of these materials in 3.5% NaCl increased in the following order: SS>SS+Zn+BP >SS+Zn.