Zinc Rich Paints

The corrosion control behaviour of carbon steel coated with a zinc‐rich paint containing two metallic compounds, Al2(SO4)3 and CaO, as anticorrosive additives was examined recently, under wet and dry cyclic corrosion test conditions. The zinc‐rich paint coating without the two metallic compounds formed a white corrosion product (often referred to as zinc salts) and red iron rust on the surface after the corrosion test, whereas the coating with the metallic compounds showed reduced surface corrosion products. 

Interestingly, the corrosion current density of the painted steel substrate decreased drastically due to the incorporation of metallic compounds in the paint. The zinc‐rich paint coating modified with the metallic compounds contained dispersed (Zn5(OH)8Cl2·H2O) phase and possibly very fine CaSO4 particles, which remarkably improved the protectiveness of the zinc‐rich paint coating, improving cathodic protection.

So what does all this mean? Well, steel is often employed as a structural material in infrastructures. However, the steel used in construction corrodes through reactions with corrosive agents in the environment in which it is placed, such as oxygen and water, under wet and dry cycles. Therefore, effective counter measures are required to prevent atmospheric corrosion for maintaining the steel infrastructures, learn how to correctly calculate surface areas of steel for steel by reading our blog post on coating calculations-the math here. 

Several approaches have been proposed and applied to prevent the corrosion of steel, including the application of protective coatings, the inclusion of corrosion inhibitors, and galvanic protection. Among them, a paint coating is the most frequently used protective measure because of its versatility. In particular, zinc-rich paints are widely used to protect steel infrastructures because of their high anticorrosive properties. 

The main component of a zinc‐rich paint is metallic zinc powder, which is considered to have anticorrosive properties owing to its sacrificial effect as well as the protective effect of its corrosion products. In general, the anticorrosive properties of conventional paint coatings consisting mainly of a resin are attributed to the barrier effect of the continuous resin‐rich film, which provides a semi-permeable layer or layers to the penetration of atmospheric corrosives to the underlying steel substrate. 

On the other hand, the zinc‐rich paint coating exhibits a lower barrier effect because of the lower resin fraction in the film. The main role of the resin in the zinc‐rich paint coating is to bind the metallic zinc powder particles in the coating, and the coated film inevitably contains many defects owing to the lower fraction of the resin. Consequently, corrosives from the surrounding environment can more easily penetrate the zinc‐rich paint coating, and then the corrosion of metallic zinc powder particles results in the generation of alternative paths for corrosives.

Moreover, further marine corrosion of the metallic zinc particles in corrosion leads to a decrease in their sacrificial anti-corrosive effect after the initial stage of corrosion. Therefore, to improve the anticorrosive effect of the zinc‐rich paint coating, it is indispensable to increase the protective effect of the corrosion products of Zn. Effects of alloying elements to metallic zinc coating for steel protection have been intensively investigated over a number of years. For example, the addition of metallic Al to hot‐dipped galvanised coatings improved the corrosion protection performance of the coating in general terms. It has been reported in a Japanese study that alloying Al to zinc‐based metallic coating stabilised the corrosion products of Zn. As for the zinc‐rich paint coating, it was also pointed out that metallic Al added to the coating improved the barrier effect accompanied by the formation of Al oxide.

These imply that Al3+ eluted from the coatings modified the corrosion products of Zn, resulting in the improved protection of the coatings against corrosion. On the other hand, ion species other than Al3+ can also modify the corrosion resistance of Zn. The same study examined passivating products of Zn in a Ca(OH)2 solution and found that Ca2+ progressed the growth of Ca[Zn(OH)3]2·2H2O (calcium hydroxyzincate) that passivated Zn surface. In addition, it was reported that SO42− in marine droplets decreased the corrosion rate of Zn due to the formation of NaZn4Cl(OH)6-SO4·6H2O.

For rust grown on steels, it has been reported that the supply of nonferrous metallic ions to the rusts resulted in the modification of their structure, leading to improvements in the protectiveness of the rusts. Furthermore, it has been reported that the supply of metallic cations to a steel surface in the early stage of the corrosion process resulted in the accelerated formation of the protective rust layer. These reports imply that the presence of ion species such as Al3+, Ca2+, and SO42− in zinc‐rich paint coating from the early stage of corrosion can quickly improve the protective performance of the coating. To achieve a favourable situation in a zinc‐rich paint coating, water‐soluble metallic compounds are focused. 

Recently, it was demonstrated that water‐soluble metallic compounds added to an epoxy‐based paint provided metallic cations in the paint during the corrosion process, improving the corrosion resistance of the paint-coated steel substrate by modifying the rust structure itself. Based on the aforementioned, water‐soluble metallic compounds added to a zinc‐rich paint could modify the corrosion products of Zn, and thus improve the results of a protective effect of the zinc‐rich paint coating. In the study, the structure and protectiveness of the corrosion products formed in zinc‐rich paint coatings containing metallic compounds on carbon steel were examined. 

About Dangle Rope Access

Here at Dangle, we provide a variety of comprehensive inspection, access, coatings, and composite (IACC) industrial services. Our services are available to both the private and public sectors.

We offer high-quality proven solutions that will help reduce maintenance costs in both the long and short-term. We are based in Dundee, Scotland and also have offices based in Edinburgh, along with our newly established training centre in Northern Ireland Dangle Academy. Due to our company size and structure, we are able to offer a flexible and versatile approach to the way we run our business and the services that we offer our clients. And, as a leading painting company, we’ve worked on several renewable energy projects in the UK, Europe, and the US. 

Dangle is an award winning rope access company specialising in industrial coatings. We can help you to optimise your assets, operational and maintenance costs, with the latest developments in corrosion protection.

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