Zn–Ni and Zn–Fe alloy deposits modified by P incorporation: anticorrosion properties

In this study we have investigated the electrodeposition of amorphous iron–nickel–phosphorus alloys from a sulfate electrolyte. Fe-Ni alloys are known to exhibit an ’anomalous‘ type of plating behaviour in which deposition of the less noble metal is favoured. We have found that the codeposition of phosphorus from hypophosphite in the electrolyte led to a reversal to a ’normal‘ behaviour. This reversal was due both to the suppression of iron and enhancement of nickel partial currents. The overall deposition process is dominated by the hydrogen evolution reaction. This is exacerbated by the low pH needed to codeposit sufficient phosphorus to achieve an amorphous structure.

Electrochemical characterization of Fe-Ni-P alloy electrodeposition

Eletrodeposition of Fe-Ni-Cr-based alloys was investigated in a divided and undivided cell. Using formic acid (HCOOH) as a complexing agent, a chloride-hypophosphite solution was developed. Effects of composition on the morphology, structure, and corrosion behaviour of the alloys were studied. Effects of plating variables on composition of the alloys and current efficiency are also reported. Energy dispersive spectroscopy (EDS) and Auger electron spectroscopy (AES) were used to evaluate alloy composition. These alloys were determined to be amorphous by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Scanning electron microscopy (SEM) photomicrographs were obtained of the surface of selected deposits. The corrosion behaviour of the alloys was studied in 0.9 wt % NaCl solution. The alloys containing phosphorus and carbon were found to be less active than those containing only phosphorus or carbon. The corrosion potential of Fe-Ni-Cr-based deposited alloys is more noble the higher the chromium content. Generally, Fe-Ni-Cr-based alloys exhibit a wide passivation range. Knoop hardness of the deposited alloys was determined. It is found that both phosphorus and carbon improved the Knoop hardness value of the deposited alloys.

Electrodeposition and characterization of amorphous Fe-Ni-Cr-based alloys

Electrodeposition of iron-chromium alloys was investigated in a divided and undivided cell. The influence of the current density on the composition of the alloy was determined. Energy dispersive spectroscopy and Auger electron spectroscopy were used to evaluate alloy composition. These alloys were determined to be amorphous by X-ray diffraction and transmission electron microscopy. The corrosion behaviour of the alloys was studied in 0.5m HCl, and they were found to be more corrosion resistant than conventional chromium electrodeposits, with corrosion currents ranging between 40–60 μA cm−2, compared to 1850 μA cm−2 for conventional chromium. The alloy electrodeposition mechanism was studied by cyclic voltammetry.

Electrodeposition and characterization of amorphous Fe-Cr-P-C alloys

Ternary Zn–Ni–P alloy coatings were electrodeposited from electrolytes based on choline-chloride ionic liquid. Their phases, microstructures, chemical compositions and corrosion behaviors were characterized as a function of electrodeposition current density. Varying the deposition current density has an influence on the composition and corrosion resistance of the obtained amorphous ZnNi-P deposit. The chemical composition results show that during electrodeposition from ionic liquids, P is incorporated into the Zn-Ni alloy through an indirect mechanism. The amount of phosphorus and nickel of the alloy decreases with an increase in the deposition current density while that of zinc increases, which corresponds to normal co-deposition mechanism. From the two electrolytes considered, the morphology of the Zn-Ni-P alloy electrodeposited evolved from a superficial network of fine cracks to larger cracks with an increase of the deposition current density. From polarization curves of coatings in 3.5% NaCl, it is shown that the P incorporated Zn-Ni alloy coatings exhibit a remarkably enhanced corrosion resistance and barrier properties, and the protective ability increases with decrease in the deposition current density. © 2014 The Electrochemical Society. [DOI: 10.1149/2.002407jes] All rights reserved.

Structure, Composition and Corrosion Resistance of Zn-Ni-P Alloys Electrodeposited from an Ionic Liquid Based on Choline Chloride

Iron-chromium-phosphorus (Fe–Cr–P) alloys were prepared by electrodeposition from an acidic citrate electrolyte using sodium hypophosphite as the source of phosphorus. These alloys form a passive oxide layer when exposed to air and are useful as protective coatings on steel. The current efficiency of the plating process reaches a maximum of 20% at a current density of 100 mA/cm2 where the alloy has 10% Cr and 19% P. X-ray diffraction patterns and TEM analysis show that the alloy is amorphous. TEM results also indicate that small oxide particles (5–20 nm) are dispersed in the amorphous structure. Besides Fe, Cr, and P, the alloys contain a low level of oxygen (4–7%) in the form of mixed iron and chromium oxides, as confirmed by AES analysis. When heated, the amorphous structure transforms into a mixture of Fe3P and Cr3P, along with α–Fe–Cr grains. This phase transformation occurs in the temperature range of 450–460 °C for alloys with 19% P.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0884291400004337

T. E. Mitchell

Papers

Structure of amorphous Fe-Cr-P alloys prepared by electrodeposition