The main objective of this review is to describe some of the important topics related to the use of marine and protective coatings for anticorrosive purposes. In this context, “protective” refers to coatings for containers, offshore constructions, wind turbines, storage tanks, bridges, rail cars, and petrochemical plants while “marine” refers to coatings for ballast tanks, cargo holds and cargo tanks, decks, and engine rooms on ships. The review aims at providing a thorough picture of state-of-the-art in anticorrosive coatings systems. International and national legislation aiming at reducing the emission of volatile organic compounds (VOCs) have caused significant changes in the anticorrosive coating industry. The requirement for new VOC-compliant coating technologies means that coating manufacturers can no longer rely on the extensive track record of their time-served products to convince consumers of their suitability for use. An important aspect in the development of new VOC-compliant, high-performance anticorrosive coating systems is a thorough knowledge of the components in anticorrosive coatings, their interactions, their advantages and limitations, as well as a detailed knowledge on the failure modes of anticorrosive coatings. This review, which mainly deals with European experience and practice, includes a description of the different environments an anticorrosive coating system may encounter during service. In addition, examples of test methods and standards for determination of the performance and durability of anticorrosive coatings have been included. The different types of anticorrosive coatings are presented, and the most widely applied generic types of binders and pigments in anticorrosive coatings are listed and described. Furthermore, the protective mechanisms of barrier, sacrificial, and inhibitive coatings are outlined. In the past decades, several alternatives to organic solvent-borne coatings have reached the commercial market. This review also presents some of these technologies and discusses some of their advantages and limitations. Finally, some of the mechanisms leading to degradation and failure of organic coating systems are described, and the reported types of adhesion loss are discussed.

Anticorrosive coatings: a review

The electrochemical behaviour of sacrificial zinc–nickel coatings, electrodeposited onto low alloy steel is studied, in a neutral, saline, aqueous environment, in accordance with different preparation conditions (nature of the bath, density of electrodeposition current, degassing and chromate passivation). The stability of the sacrificial properties is determined by open circuit potential measurements over time, while the corrosion rate is evaluated using polarization resistance measurements and volt–amperometric diagrams. The effect of any porosity, revealing the substrate, is determined by Evans diagrams for various levels of simulated porosity. The corrosion behaviour of these deposits is closely linked to the nickel content and, to a lesser extent, to their structure. It is improved by a chromate passivation, but the chromating layer may deteriorate over time (long, continuous immersion). Over time, the corrosive products, based on zinc hydroxide, form a barrier film which slows down the dissolution of the material. After prolonged immersion, the sample surface is generally crazed due to the internal stresses present in the deposit.

Corrosion behaviour of zinc–nickel coatings, electrodeposited on steel

Zn–Fe, Zn–Ni and Zn–Ni–Fe coatings were electrodeposited galvanostatically on mild steel from acidic baths (pH 3.5) consisted of ZnCl 2 , NiCl 2 , FeCl 2 , gelatin, sulfanilic ( p -aminobenzenesulfonic) acid and ascorbic acid. Cyclic voltammetry showed that the effect of gelatin was more pronounced than that of sulfanilic acid, and that the deposition of the ternary alloy behaved differently from the deposition of the binary alloys. In all three systems, the Faradaic efficiency was higher than 88%, the rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, and the deposition was of anomalous type. For each applied current density, the concentrations of Ni and Fe in the ternary alloy were higher than the corresponding concentrations in the binary alloys. The hardness of Zn–Ni coatings was the highest, while that of Zn–Fe coatings was the lowest. The Zn–Ni–Fe coatings were the smoothest, had distinguished surface morphology, and contained ZnO in the bulk, not just on the surface. The lowest corrosion rate in each alloy system (214, 325 and 26 μm year −1 for Zn–Ni, Zn–Fe and Zn–Ni–Fe, respectively) was characteristic of coatings deposited at 30, 30 and 40 mA cm − 2 , respectively. The higher corrosion resistance of the ternary alloy was also reflected by a higher corrosion potential, a higher impedance and a higher slope of the Mott-Schottky line. The enhanced corrosion behavior of the ternary alloy was thus attributed to its chemical composition, phase content, roughness and the synergistic effect of Ni and Fe on the n -type semiconductor surface film.

Electrodeposition of Zn–Ni, Zn–Fe and Zn–Ni–Fe alloys

Zn–Ni, Zn–Co and Zn–Ni–Co coatings were electrodeposited on mild steel from an acidic chloride bath containing p-aminobenzenesulphonic acid (SA) and gelatin. These additives changed the phase content in the coatings, most likely as a result of their adsorption at the surface of the cathode. The effect of gelatin was more pronounced than that of SA. The Faradaic efficiency was higher than 90%. As the current density was increased or the bath temperature was decreased, the concentration of the nobler metal in the coating increased. Both concentrations of Ni and Co in the ternary alloy increased as the applied current density was increased. Nickel and cobalt were found to have a synergistic catalytic effect. The thickness of all coatings increased as the applied current density was increased. The hardness increased with current density to a peak value, and then decreased. The rate of Zn deposition was heavily influenced by mass-transport limitation at high applied current densities, while the rates of Ni and Co deposition were not. The anomalous codeposition was explained by the great difference between the exchange current densities of Zn and the iron-group metal. Potentiodynamic polarization scans and electrochemical impedance spectroscopy showed that the corrosion resistance of the ternary Zn–Ni–Co alloy coatings was approximately 10 times higher than that of Zn–Ni and 7 times higher than that of Zn–Co. The improved corrosion resistance of the ternary alloy was attributed to its surface chemistry, phase content, texture, and surface morphology. The ternary Zn–Ni–Co coating may thus replace the conventional Zn–Ni and Zn–Co coatings in a variety of applications.

Electroplating and characterization of Zn–Ni, Zn–Co and Zn–Ni–Co alloys

The nature of electrodeposited zinc coating corrosion inhibition by alloying with Fe group metals is not fully understood, because it depends on several factors including the microstructure. The crystallographic texture and lattice cell parameters were determined for nonchromated Zn, Zn-Co, Zn-Fe and Zn-Ni coatings, deposited from alkaline baths, and the results have been used to explain the observed differences in their corrosion behaviour in aerated chloride solutions and in neutral salt spray tests. The presence of Ni or Co in the zinc matrix increases the Zn coating corrosion resistance, while Fe has no effect. It has been suggested that the lower Zn-Ni alloy corrosion rate is due to the predominant presence in this layer of crystallographic planes with a higher packing density. The lower Zn-Co alloy corrosion rate compared with that of Zn and Zn-Fe has been associated with the lower values of its lattice cell parameter c/a (axial ratio).

Corrosion resistance and microstructure of electrodeposited Zn and Zn alloy coatings

Operating conditions for zinc and nickel codeposition from chloride baths were studied in order to obtain alloys containing up to 20% nickel. Satisfactory deposits were produced at 50°C using current densities ranging from 5 to 20 mA cm−2 and nickel to zinc ratios ranging from 6.8 to 37.5%.

R. Fratesi

Papers

Electrodeposition of zinc-nickel alloys from chloride solution

The effect of the surface roughness and the load application mode on hydrogen embrittlement