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Corrosion products of zinc-manganese coatings: part I—investigations using microprobe analysis and x-ray diffraction

The anti-corrosion performance of Zn and Zn15Al organic coating was investigated mainly by neutral salt spray test and electrochemical impedance spectroscopy. X-ray diffraction, scanning electron microscope and energy disperse spectroscopy were applied to analyze the corrosion behavior of the coating during the test. The facilitated formation of protective corrosion product simonkolleite on the Zn15Al coating surface was observed. Also, layered double hydroxides (LDH) consisting of Zn²⁺ and Al³⁺ were detected at the interface of coating/steel substrate of Zn15Al. This LDH was believed to function as ion exchanger that prevents Cl⁻ from contacting the steel substrate.

Zn–Mn alloy electrodeposition on steel electrode in chloride bath was investigated using cyclic voltammetric, chronopotentiometric and chronoamperometric techniques. Cyclic voltammetries (CV) reveal a deep understanding of electrochemical behaviors of each metal Zn, Mn, proton discharge and Zn–Mn co-deposition.

The electrochemical results show that with increasing Mn²⁺ ions concentration in the electrolytic bath, Mn²⁺ reduction occurs at lower over-potential leading to an enhancement of Mn content into the Zn–Mn deposits. A dimensionless graph model was used to analyze the effect of Mn²⁺ ions concentration on Zn–Mn nucleation process. It was found that the nucleation process is not extremely affected by Mn²⁺ concentration. Nevertheless, it significantly depends on the applied potential.

Several parameters such as Mn²⁺ ions concentration, current density and stirring were investigated with regard to the Mn content into the final Zn–Mn coatings. It was found that the Mn content increases with increasing the applied current density j_imp and Mn²⁺ ions concentration in the electrolytic bath. However, stirring of the solution decreases the Mn content in the Zn–Mn coatings. The phase structure and surface morphology of Zn–Mn deposits are characterized by means of X-ray diffraction analysis and Scanning Electron Microscopy (SEM), respectively. The Zn–Mn deposited at low current density is tri-phasic and consisting of η-Zn, ζ-MnZn₁₃ and hexagonal close packed ε-Zn–Mn. An increase in current density leads to a transition from crystalline to amorphous structure, arising from the hydroxide inclusions in the Zn–Mn coating at high current density.

ZnMn coatings were electrodeposited at fixed applied potential on steel substrate from an acidic chloride bath containing 4-Hydroxybenzaldehyde (4-HB) and Ammonium Thiocyanate (NH₄SCN) as additives. Voltammetric and Electrochemical Quartz Crystal Microbalance (EQCM) studies were carried out to study the related electrochemical system in order to determine the optimal deposition conditions. Several parameters such as deposition potential, plating bath composition and additives were investigated with regard to the Mn content in the final coatings. The composition, morphology and crystallographic structure of the coatings were studied using Atomic Absorption Spectroscopy (AAS), Scanning Electron Microscopy (SEM), Energy Dispersive Spectroscopy (EDS) and X-ray diffraction (XRD). In addition, the corrosion behavior of the ZnMn alloys was investigated by means of potentiodynamic polarization curves and polarization resistance (R_p) measurements. Manganese contents of 11 wt% were obtained with plating baths without additives and increased up to 17 wt% Mn in presence of both additives. The 17 μm-thick coatings obtained from the bath with additives consisted of a compact and smooth ε-ZnMn hexagonal close-packed (HCP) single phase with a fiber texture along the 〈110〉 direction. By contrast, coatings deposited from additive-free electrolytes were rough with a dendritic morphology and consisted of a single ε-ZnMn phase. ZnMn coatings deposited in absence of additives were found to provide lower protective ability than Zn coatings, presumably due to their dendritic morphology and rough surface. Oppositely, better corrosion protection was obtained for coatings deposited in presence of additives compared to pure Zn coatings.

Ternary Zn–Co–Mo alloy coatings were deposited from a citrate–sulphate bath. In a pH range of 5.5–5.9 the coatings contained 2.3–3.6 wt.% molybdenum and 3.4–3.7 wt.% cobalt. DC and EIS measurements revealed that in the course of 24 h exposure to NaCl solution the corrosion resistance of Zn–Co–Mo alloy coatings was higher than that of Zn and Zn–Co coatings. On the basis of XRD, ALSV and XPS studies it can be stated that the beneficial effect on the corrosion resistance of Zn–Co–Mo coatings has a passive layer composed of: Zn(OH)₂, ZnO, Mo(IV) oxide and hydroxide and a small amounts of Co₃O₄.

The effects of a deposition current density (c.d.) on the corrosion behaviour of Zn–Mn alloy coatings, deposited from alkaline pyrophosphate solution, were investigated by atomic absorption spectrophotometry (AAS), X-ray diffraction (XRD), atomic force microscopy (AFM), optical microscopy, electrochemical impedance spectroscopy (EIS) and measurement of corrosion potential (E_corr). XRD analysis disclosed that zinc hydroxide chloride was the main corrosion product on Zn–Mn coatings immersed in 0.5 mol dm⁻³ NaCl solution. EIS investigations revealed that less porous protective layer was produced on the alloy coating deposited at c.d. of 30 mA cm⁻² as compared to that deposited at 80 mA cm⁻².

Zn–Mn coatings were electrodeposited on steel substrate from an acidic chloride bath containing a commercial additive used in zinc plating. The effects of deposition parameters such as stirring, applied current density or potential and temperature on the coating composition, microstructure and crystallography were investigated. It was found that this additive permits to obtain dense, compact and thick Zn–Mn coatings. The morphology is quite particular, displaying hexagonal pyramids whatever the manganese content of the coating. This morphology is associated with the main phase found in the coatings that is the HCP ɛ-Zn–Mn. The manganese content reaches a maximum value of 20 wt.% corresponding to a monophasic ɛ coating. Stirring of the solution has no influence on the composition of the coatings but induces essentially an improvement of the surface appearance as brightness increases and roughness decreases. The increase of the temperature also leads to an enhancement of the surface appearance but induces a decrease of the manganese content.