Characterization of Electrodeposited Mn-Cu-Zn Coating for Corrosion Protection

Abstract

Preparation of new metal alloy coatings capable of protecting steel from corrosion is a major objective of electrochemical deposition and functional galvanotechnics. A growing interest of researchers in alloys electrodeposition is mainly caused by the cost-effectiveness of metal coatings due to inexpensive and eco-friendly electrolytes used. In that regard, of particular interest was the possibility of obtaining manganese alloys characterized by high anti-corrosion performance with suitable mechanical properties. Manganese, a metal with high negative standard potential (-1.18V, SHE) is chemically active and easily reacts with air oxygen and moisture. Despite their high effect, electrocoatings of pure manganese are fragile and in case of impact are easily removed from a steel surface to get protected from corrosion. To decrease chemical activity and brittleness of manganese, it is alloyed with metals of lower negative standard potential, e.g. with Cu, Zn and others. The goal of present research is obtaining of high quality coating of Mn-Cu-Zn alloy, determination of parameters of electrolysis, electrolyte composition, effect of cathodic current density on chemical composition, morphology and structure of coatings. Also, corrosion tests of electrodeposited alloys are studied. Mn-Cu-Zn alloys were electrodeposited from sulphate bath, containing citrate or EDTA and their mixtures as complexing ligands. The impact of bath composition and deposition parameters on alloy composition, cathodic current efficiency, structural and electrochemical properties was investigated. At a higher current density structural and electrochemical properties was investigated. At a higher current density ( \(\geq 37.5 \mathrm{~A} \mathrm{dm}^{-2}\) ) a uniform surface deposit of \(\mathrm{Mn}-\mathrm{Cu}-\mathrm{Zn}\) was obtained. Optimal \(\mathrm{pH}\) of electrolyte \(\left(0.3 \mathrm{~mol} / \mathrm{dm}^3 \mathrm{Mn}^{2+}+0.6 \mathrm{~mol} / \mathrm{dm}^3\left(\mathrm{NH}_4\right)_2 \mathrm{SO}_4+0.1\right.\)\(\mathrm{mol} / \mathrm{dm}^3 \mathrm{Zn}^{2+}+0.005 \mathrm{~mol} / \mathrm{dm}^3 \mathrm{Cu}^{2+}+0.05 \mathrm{~mol} / \mathrm{dm}^3 \mathrm{Na}_3 \mathrm{Cit}+0.15 \mathrm{~mol} / \mathrm{dm}^3 \mathrm{EDTA} ; \mathrm{t}=30^{\circ} \mathrm{C} ;\)\(\mathrm{t}=30^{\circ} \mathrm{C} ; \mathrm{T}=20 \mathrm{~min}\) ) for silvery, nonporous coating of \(\mathrm{Mn}-\mathrm{Cu}-\mathrm{Zn}\) alloy was within 6.5-7.5; coating composition: \(71-83 \% \mathrm{Mn}, 6-7.8 \% \mathrm{Cu}, 11.5-20 \% \mathrm{Zn}\), current efficiency up to \(40 \%\). XRD patterns revealed \(\mathrm{BCT}\) (body centred tetragonal) y-Mn solid phase solution (lattice constants \(a=2.68 Å ; c=3.59 Å\) ). Corrosion tests on deposited alloys were carried out in an aerated \(3.5 \% \mathrm{NaCl}\) solution. The corrosion current density (icor) of the electrodeposited alloys on carbon steel was 10 times lower than corrosion rate of pure zinc and manganese coatings. Triple alloy coatings corrosion potential (Ecorr \(=-1140 \mathrm{mV}\) vs. \(\mathrm{Ag} / \mathrm{AgCl}\) ) preserved negative potential value longer (more than three months) compared to carbon steel substrate \(\left(E_{\text {corr }}=-670 \mathrm{mV}\right.\) vs. \(\left.\mathrm{Ag} / \mathrm{AgCl}\right)\). Tafel polarization curves of \(\mathrm{Mn}-\mathrm{Cu}-\) \(\mathrm{Zn}\) alloy coatings in aerated \(3.5 \% \mathrm{NaCl}\) solution did not exhibit typical passivation behavior, which can be described by the formation of low solubility of adherent corrosion products on the alloy surface. Corrosion testing on Mn-Cu-Zn electrocoating in a chlonine environment demonstrates that it is the best cathodic protective coating for a steel product.