Effects of Zn content on surface deformability and corrosion resistance of MgZnMnCa alloys

The effects of the Zn content on the mechanical and corrosion properties of MgZnMnCa alloys were investigated. The results revealed that with increasing Zn content up to 4 wt.%, the Mg-4Zn-0.2Mn-0.2Ca (wt.%) alloy after solution treatment at 400°C for 24 h (ZM40-S) exhibited a volume fraction of 1.56 ± 0.05 (vol.%) of secondary phases with an average grain size of 83.6 ± 1.6 µm. This alloy in the at-cast condition exhibited indentation hardness (Hit) of 657.1 ± 17.8 MPa, which increased to 723.5 ± 29.8 MPa after solution treatments. In addition, the alloy also exhibited a Vickers hardness value of 66.8 ± 2.7 HV and an elastic modulus of 33.8 ± 0.6 GPa, close to the values for human bone. Further, the corrosion properties of this alloy were analyzed: it exhibited a cathodic polarization curve attributed to a hydrogen evolution reaction and an anodic curve corresponding to a passivation tendency, indicating the formation of an oxide layer at the surface; further, its corrosion rate was 0.23 mm/year.

Effect of pH and NaCl concentration on the hydrogen evolution reaction of X60 steel

Purpose When hydrogen evolution reaction occurs on a metal surface, on the one hand, the generated hydrogen atom may penetrate into the metal that causes the hydrogen embrittlement failure of materials; on the other hand, the hydrogen generation may increase the local pressure in the coating and cause coating blistering. The purpose of this study is to study the effect of NaCl concentration and pH on hydrogen evolution reaction of X60 steel. Design/methodology/approach A cathodic polarization curve 257E-2V vs OCP and EIS was obtained by conventional three-electrode system in different NaCl concentrations, 257E3.5 and pH. Second, various parameters such as hydrogen evolution, over-potential current–density polarization resistance and capacitance of double electric layer were obtained based on fitting of the experimental data. Finally, the reaction mechanism was determined by Tafel curves. Findings It was concluded that in different NaCl concentrations, diffusion layer induced by concentration polarization affects the diffusion process of H+ ions, which makes over-potential increase. Under great effect of concentration polarization, the reaction is different in acid and alkaline environments, and the dielectric layer shows the characteristic of meta-alkaline adsorption, which makes difference in mechanism. Originality/value This research not only has theoretical significance but also gains utilization prospect. Ultimately, this research could be applied to clear hydrogen evolution process and protect long-distance pipeline against delamination.

Effect of deposition current density and annealing temperature on the microstructure and magnetic properties of nanostructured Ni-Fe-W-Cu alloys

Ni-Fe-W-Cu alloy powders were obtained by electrodeposition from an ammonium citrate bath at current densities ranging between 70 and 600 mA cm-2. As the deposition current density increased, the contents of Fe and W in the alloy increased, and those of Ni and Cu decreased. The total cathodic polarization curve was recorded, and partial polarization curves for Ni, Fe and W deposition and hydrogen evolution were determined. The current efficiency of alloy deposition was measured. The powders contained an amorphous matrix and FCC nanocrystals of the solid solution of Fe, W and Cu in Ni. At high current densities, small-sized nanocrystals exhibiting high internal microstrain values were formed. Powder particles were dendrite- and cauliflower-shaped. The dendrites had a large number of secondary branches and higher-order branches containing interconnected globules. The density of branches was higher in particles formed at high current densities. The powders formed at high current densities exhibited higher magnetization. Annealing at temperatures up to 460?C resulted in structural relaxation, accompanied by an increase in magnetization. At temperatures above 460?C, amorphous matrix crystallization and FCC crystal growth took place, accompanied by a decrease in magnetization.

CATHODIC DEPOSITION OF Mg(OH)2 COATINGS ON PURE Mg IN THREE Mg SALTS AQUEOUS SOLUTIONS

Film-forming effects of cathodic deposition on pure Mg substrate at constant DC in aqueous solutions of magnesium nitrate (Mg(NO3)[Formula: see text]6H2O), magnesium chloride (MgCl[Formula: see text]6H2O) and magnesium sulfate (MgSO[Formula: see text]7H2O) respectively were investigated systematically. Typical processes were studied by potentiodynamic cathodic polarization and galvanostatic polarization and typical samples were analyzed by SEM and XRD. The results indicate that the depositing efficiency is not only the highest but stablest, and deposited coatings show the best uniformity with Mg(NO3)[Formula: see text]6H2O solution employed as depositing medium and applied current density [Formula: see text][Formula: see text]mA cm[Formula: see text]. Cathodic deposition leads to regular mass loss of Mg substrate. The cathodic polarization curve of pure Mg in magnesium nitrate solution shows more obvious pseudo-passivation, several Tafel regions with different slopes appearing before diffusion-limited current density region, and oxygen consumption is the major cathodic reduction reaction under specified current density. However, hydrogen evolution reaction is dominant in both Mg chloride and Mg sulfate solutions. The deposition coatings are all composed of continuous and uniform mesh-like “basic layer” adjacent to substrate and discrete distributed snowball-like particles on the microscopic scale. The phase compositions are all crystal Mg(OH)2, and the coatings deposited in Mg chloride solution have (011) preferred orientation.

Electrochemical Characterization of Different Variants of a Commercial Trivalent Chromium Process (TCP) Coating on Aluminum Alloy 7075-T6

The electrochemical properties (open-circuit potentials, anodic and cathodic polarization curve currents, and polarization resistances) were evaluated for AA7075-T6 alloys coated with three variants of a commercial trivalent chromium process (TCP) pretreatment coating. The coatings were formed on degreased and deoxidized aluminum alloy specimens. Measurements were made in oxygenated 0.5 M Na2SO4 and 3.5% NaCl. Comparison of coatings formed by immersion and spray was undertaken. The three coating variants were 650 chromitAL, versions E, V, and C. Similar concentrations of Cr were in all three coating baths but there were differences in Zr, Zn, S (likely as sulfate), and Fe among the three. TCP coatings formed by immersion exhibited electrochemical properties similar to those formed by spray. Overall, the greatest level of corrosion protection was provided by 650 E based on electrochemical data and results from a 14 d thin-layer mist (3.5% NaCl, 55°C) accelerated degradation test. The coating provides both anodic and cathodic protection in low-chloride electrolytes and functions as more of a cathodic inhibitor in high-chloride electrolytes. Rotating disk voltammetric data revealed the coating inhibits the reduction of dissolved oxygen by providing a diffusional barrier and possibly blocking sites for O2 chemisorption on the cathodically-active intermetallic phases.

Electrodepositon and Properties of Ni-Diamond Composite Coatings

Ni-diamond composite coatings are conducted by electrodepositon. The crystal structure and surface morphology of the composite coatings were examined with X-ray diffraction (XRD) and scanning electron microscopy (SEM) and the mechanism of Ni-diamond co-deposition is conducted by catholic polarization and cyclic voltammetry method. The result reveals that diamond particles are successfully embed in Ni matrix and the coatings have an amorphous structure. Cyclic voltammetry indicate that nickel deposition process is an irreversible electrode reaction and cathode polarization curve shift towards positive with the increase of scan rates. Cathodic polarization curve shows that reduction potential of nickel to shift to more negatives with the addition of complexing agent and the slope of the polarization curve is decrease; The addition of brightener and dispersant hinder the deposition of Ni2+ and promote the growth of crystal nucleus; The join of wetting agents and diamond particles have no big influence on the deposition of nickel.

Polarization of Strained AISI 4340 Steel in Oxygenated NaCl Solutions

Electrode reactions involved in the stress corrosion of AISI 4340 steel in sodium chloride solutions were studied by pulse galvanostatic and potentiostatic experiments. The redox cell system involves two principal cathodic reactions and one anodic reaction. These electrochemical reactions are affected by the application of stress, by change in pH, by oxygen content and the heat-treated condition of the materials. The effect of stress is to shift the cathodic polarization curve in an electropositive direction with increasing tensile load. It is equivalent to cathodic control for the conditions investigated and is described by the electrochemical interpretation, based on the coupling of the redox branches which produce the largest corrosion current.

Current and Potential Relations For the Cathodic Protection of Steel In a High Resistivity Environment

In order to evaluate potential and current criteria for the cathodic protection of bare low-carbon steel in a high-resistivity environment, specimens were exposed in the laboratory for a period of two months to a soil having a resistivity of about 20,000 ohm-centimeter. Previous work in low-resistivity environments has shown that corrosion can be reduced to a negligible degree by polarizing a steel structure to —0.85 volt (protective potential) with reference to a copper-copper sulfate electrode. In such studies, cathodic polarization curves have also been shown to be useful in indicating the current density required for cathodic protection. In the present study the above criteria were again evaluated. In addition to protecting the steel at the protective potential (free of IR drop), the effect on protection of including IR drop caused by the protective current was also noted. Also, cathodic polarization curves were obtained on a recorder in conjunction with a bridge circuit to eliminate the IR drop. The results show that the best degree of protection was achieved on the specimen controlled at —0.77 volt (without IR) with reference to a saturated calomel half-cell. This is approximately equivalent to the protective potential —0.85 volt with reference to the copper-copper sulfate electrode. Applied current indicated by the break (change-in-slope) in the cathodic polarization curve agreed reasonably well with the actual current necessary to maintain polarization at —0.77 volt (free of IR). The current required for protection was about three times the magnitude of the corrosion current; therefore, the corrosion reaction was either under anodic control (unlike previous studies) or an equivalent type of control which was caused by high resistance at anodic areas. 5.2.4