The comparative study of the corrosion stability of Zn-Ni-Co alloy coatings deposited from chloride and sulphate baths

Abstract

The ternary Zn-Ni-Co alloy coatings were electrodeposited on steel from chloride and sulphate baths. The deposition was carried out galvanostatically at different current densities at ambient temperatures from additives free baths with different ratios of Ni and Co. Corrosion behaviour of these coatings was evaluated from polarization measurements in 3% NaCl. The deposition current density exhibited small influence on the corrosion potential and corrosion current density of alloys obtained from both chloride and sulfate baths. On the other hand, [Co2+]/Ni2+] ion ratio showed a significant influence on corrosion behaviour of ternary alloy coatings, and this influence was more pronounced with alloys deposited from chloride baths. It was shown that increased corrosion protection of steel can be achieved by electrochemical deposition of ternary alloys that contained lower amount off the alloying elements, while the composition of the ternary alloy can be optimized by proper choice of the type and composition of the deposition bath.

Trojne Zn-Ni-Co legure su elektrohemijski taložene na čeliku iz hloridnih i sulfatnih rastvora na 25°C. Legure su taložene galvanostatski, različitim gustinama struje iz rastvora bez dodataka i sa različitim odnosom jona legirajućih elemenata, Ni i Co. Koroziona stabilnost dobijenih prevlaka je ispitivana polarizacionim merenjima u 3 % NaCl. Gustina struje taloženja ima mali uticaj na korozioni potencijal i gustinu struje korozije prevlaka dobijenih i iz hloridih i slufatnih rastvora. Veći uticaj pokazuje odnos jona legirajućih elemenata ([Co2+]/Ni2+]), pri čemu je ovaj uticaj više izražen kod prevlaka dobijenih elektrohemijskim taloženjem iz hloridnih rastvora. Pokazano je da se koroziona stabilnost čelika može produžiti elektrohemijskim taloženjem prevlaka trojnih Zn-Ni-Co legura koje sadrže malu količinu legirajućih elemenata, i da se sastav trojnih legura može optimizovati pogodnim odabirom vrste i sastava rastvora za taloženje.