Pulsed cobalt-rich Zn–Co alloy coatings produced from citrate baths

• Published in: Surface & coatings technology Vol. 306; pp. 462 - 472
• Main Authors: Garcia, Julyana R., do Lago, Dalva C.B., Cesar, Deborah V., Senna, Lilian F.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 25.11.2016
• Subjects: Anticorrosive coatings; Citrate baths; Electrodeposition; Simple pulsed current; Zn–Co alloys; Zn–Co alloys; Citrate baths; Electrodeposition; Anticorrosive coatings; Simple pulsed current
• ISSN: 0257-8972; 1879-3347
• DOI: 10.1016/j.surfcoat.2016.01.044

Zn–Co alloy coatings deposited by pulsed current (PC) typically exhibit higher adhesion, lower porosity and smaller grain size than those deposited by direct current (DC). All of these features contribute to the improved anticorrosive performance observed in PC Zn–Co coatings. Additionally, an increase in cobalt content in Zn–Co coatings tends to decrease the material's grain size, which may influence the anticorrosive performance of the coating. Therefore, the present study investigates the deposition of anticorrosive cobalt-rich Zn–Co coatings on carbon steel using PC deposition. The electrodeposition process was executed under stirring conditions using baths containing Zn (II) ions (0.05molL−1), different Co (II) concentrations ([Co2+]=0.05 and 0.10molL−1) and sodium citrate (0.10molL−1) as the complexing agent. Four current densities (jc) and three pulse frequencies (F) were applied to produce the alloys. It was observed that the deposition process was anomalous under all of the studied conditions. Higher contents of cobalt were verified at higher current densities for both Co (II) concentrations, particularly at smaller pulse frequencies. The anticorrosive performance of the Co-rich Zn–Co coatings appears to depend on the relationship among the parameters jc, [Co2+] and F, because their joint effect influenced the chemical composition, thickness and microstructure of the coatings. In the current study, the best anticorrosive performances were observed for coatings containing 7.4 to 13wt.% Co, presenting nanometric grain size and one predominant crystalline phase (γ-Zn21Co5). These coatings showed a high stability in an aggressive environment for 24h of exposure, thus indicating that they could be useful as long-term corrosion protection coatings. •Co-rich Zn–Co coatings were deposited by pulsed current on steel from citrate baths.•The increase in jc and [Co2+] favored the production of coatings with high wt.% Co.•Coatings containing 7.4–13wt.% Co exhibited the best anticorrosive performance.•Anticorrosive coatings presented small grain sizes and predominant γ-Zn21Co5 phase.•Co-rich Zn–Co protective coatings rest on the relationship among jc, [Co2+] and F.