Portugaliae
Electrochimica Acta

Long term wet corrosion resistance of metals depends on the stability of their corrosion product layer. With immersion corrosion tests, such stability can be predicted. EIS and potentiodynamic polarization were complemented with XPS to investigate the characteristics of Ni corrosion product layer formed after 1 hr. and 72 hr. immersion in 3.5% NaCl solution. Two time constants with decreasing Nyquist semi-circle size and phase angle maxima, based on EIS characterization during the immersion times, indicated the formation of an increasingly porous and less adherent corrosion product layer. The product formation shifted the Ni corrosion potential more negatively and increased cathodic and anodic current densities, during potentiodynamic polarization. XPS characterization suggested that a rapid nucleation of NiO could increase H2O adsorption, subsequently triggering the formation of different forms of Ni(OH)2 in the corrosion product layer. Consequently, the corrosion resistance of the Ni coating decreased after 72 hr. immersion in 3.5% NaCl solution.

An algorithm is presented for the calculation of corrosion parameters with mixed charge-transfer and diffusion control, based on the polynomial method, and having good accuracy and precision.

The feasibility of using extract of Grewa Venusta (wild jute tree) root as corrosion inhibitor with mild steel was investigated using gravimetric and electrochemical techniques in 1.0 M hydrochloric acid. The inhibitor’s concentration, temperature and time were varied in the range of 0-10% v/v at 2% v/v interval, 30 - 75 oC at 15 oC interval and 1-6 hours at 1 hour interval. Characterizations of the extract were done by quantitative method (AAS) and Gas Chromatography-Mass Spectrometry (GC-MS). Scanning electron microscope (SEM) was used to analyze the surface morphology of the samples. The synergetic effect of the inhibitor was evaluated by addition of halide ions (KBr-, KCl and KI-). The results showed that corrosion rate increased with an increase in temperature, and decreased with an increase in inhibitor’s concentration and time; maximum inhibition efficiency was 97.60% and 99.88% in the presence of KI- addition, and was assumed to occur via adsorption of the inhibitor molecules on the metal surface. The extract contains metallic elements such as calcium, magnesium, mono acetate (C5H10O4) and 4H-Pyrazole (C9H10F6N2S) that suppressed the anodic dissolution. The adsorption of the extract molecules on the mild steel surface obeys Langmuir adsorption isotherm. The results showed that the inhibitor acted as a mixed-type inhibitor.

The inhibition performance and mechanism of N1,N1,N3,N3-tetrakis((3,5-dimethyl-1H-pyrazol-1-yl)methyl)propane-1,3-diamine (BF2) and N1,N1,N2,N2-tetrakis((3,5-dime-thyl-1H-pyrazol-1-yl)methyl) benzene-1,2-diamine (BF4) for the corrosion of mild steel in 1.0 M HCl were investigated using weight loss method and electrochemical measurements. The results show that both tetrakis pyrazole derivatives act as good inhibitors, and inhibition efficiency follows the order: BF4 > BF2. Two tetrakis pyrazole derivatives are mixed type inhibitors exhibiting predominantly cathodic behavior. The Nyquist plots showed that, after increasing inhibitors’ concentrations, charge-transfer resistance increased and double-layer capacitance decreased, involving increased inhibition efficiency. The adsorption of both inhibitors on a steel surface obeyed Langmuir model, thus, the thermodynamic and kinetic parameters were calculated and discussed. Quantum chemical parameters are calculated using the Density Functional Theory method (DFT). Correlation between theoretical and experimental results is discussed.

A Ni-Al composite was electrodeposited on a Ni substrate, and its corrosion behavior was observed in 2 M NaCl solution, compared with a pure Ni coating. The Al particles increased the porosity of the composite and encouraged charge percolation, both at the corrosion product layer-solution interface and at the substrate–solution interface, based on EIS characterization. This phenomenon greatly decreased the corrosion potential, and increased both cathodic and anodic current densities in the active region, as well as the passive current density in the passive potential range, during polarization of the composite. Although a continuous Al3+ ions supply to the passivation front was suspected, based on the longer passivation potential of the Ni-Al composite, the simultaneous consumption of the Al products by the chloride ions is the reason for serious cracking and localized collapse of the composite corrosion layer, as confirmed by SEM. This conferred lower corrosion resistance on the Ni–Al composite, compared to the pure Ni coating, in the 2 M NaCl solution.