The challenges facing zinc silicate at high temperatures – part 2

Before formulating an inorganic zinc silicate (IOZ) for elevated temperature use – temperatures above 120°C – we must determine what the main challenges are for an IOZ in these conditions.

Michelle Ystad Eriksen, MSc, Materials chemistry and energy technology
Regional Marketing Director Performance Coatings - NEA.

3 key challenges for zinc silicate at high temperatures

As we saw in the previous article on zinc at elevated temperatures, there are three main challenges:

IOZ coatings offer corrosion protection through galvanic electrochemical reaction. At elevated temperatures the rate of this process is accelerated leading to an overconsumption of zinc and a reduced coating lifetime compared to similar conditions at ambient temperatures.
At temperatures above 420°C, the melting point of zinc, zinc reacts with oxygen in the air to form zinc oxide. Zinc oxide is not a galvanically active material and cannot offer galvanic corrosion protection, further reducing the lifetime of the coating.
Zinc oxide occupies a larger volume than metallic zinc, which can create microcracking in relatively brittle IOZ coatings. Microcracking might also occur due to expansion and contraction in the coated substrate as temperatures vary or because of vibrations and movement in the process equipment on the coated facility. Any microcracking will increase the need for metallic zinc to offer galvanic protection, further reducing the coating’s lifetime.

Due to these three challenges, the market tends to specify IOZ coatings with the highest level of zinc ( 85% by weight) to maximize the galvanic corrosion protection and lifetime of the coating. To further extend the coating’s lifetime, a silicone or aluminum silicone topcoat is used to reduce the IOZ’s exposure to oxygen and moisture. In service conditions where the structures are mostly dry these systems work well, however in areas where there is cyclic service or significant humidity this type of coating system has a limited lifetime.

Testing: how to increase the coating’s lifetime?

When formulating an IOZ for high temperature service it is important to remove or reduce the susceptibility to the above challenges. Since the melting temperature of zinc cannot be changed without introducing an alloy, we have to examine other ways to increase the lifetime of the coating.

When evaluating the performance of an IOZ coating, a determination of the necessary zinc loading is an essential first step. For general anticorrosive use there are a wide variety of standards correlating zinc loadings to coating lifetime, however these standards are only valid for temperatures up to 120°C. For service conditions above 120°C the highest zinc loadings are typically recommended to achieve the longest coating lifetimes. However, this beg the question of whether an increase in zinc content does indeed extend coating lifetime at elevated temperatures.

In order to examine this, IOZ formulations with three different zinc levels were applied to steel panels and exposed to dry heat at 540°C for seven days. This was done so that the zinc would melt and react with the oxygen in the atmosphere. The panels were then placed in salt spray chambers for six weeks to force the zinc to provide galvanic protection. As can be seen in Figure 1, all zinc loadings showed the formation of white zinc salts – these are primarily made up of zinc oxide and zinc chloride[1] .However, the panels with 80 and 85 wt% zinc were displayed red iron oxide as well, meaning that the zinc was no longer able to offer galvanic corrosion protection to the steel panel.

zinc diffrent stages — Figure 1: Panels with IOZ at different zinc loadings after seven days of exposure to 540°C, followed by six weeks of exposure to salt spray.

The results seem to indicate that a higher zinc loading gives a worse performance under elevated temperatures. If we consider the composition of an IOZ coating this makes sense, as a lower zinc loading means there is more binder and filler relative to the zinc dust, and the zinc is possibly more protected against oxidation when the temperature exceeds zinc’s melting point.

Taking this one step further, we wanted to explore whether we could further reduce the oxidation of zinc when heated above its melting point, while still ensuring galvanic corrosion protection. In order to achieve this, two additional raw materials were introduced to the coating; ceramic spheres and glass flakes. Figure 2 shows a SEM image of the resulting coating film.

Figure 2: SEM image of an IOZ coating where glass flakes and ceramic spheres have been added to the coating.

The ceramic spheres were used to bulk up the coating and push the zinc particles closer together, ensuring metallic contact between the zinc even though the loadings were lower than normal. The glass flakes are used for multiple reasons. They reduce the oxidation of the zinc. They also add a barrier element to the coating, preventing water from reaching the zinc and reducing its overconsumption for galvanic corrosion protection. Finally, they add additional flexibility to the coating, thereby reducing microcracking when the zinc oxidizes and increases in volume.

Figure 3: Panel with 65 wt% zinc, glass flakes and ceramic spheres after 7 seven days of exposure to 540°C, followed by six weeks of exposure to salt spray.

In Figure 3 we can see a panel with these added raw materials after seven days of exposure to 540°C, followed by six weeks of exposure to salt spray. Compared to the panels in Figure 1 there were virtually no zinc salts, but even more importantly there was no sign of iron oxide. This means that the IOZ was protecting the steel substrate with a more moderate consumption of zinc, thereby increasing product lifetime – even after the zinc had been exposed to temperatures above its melting point.

"Compared to the panels in Figure 1 there were virtually no zinc salts, but even more importantly there was no sign of iron oxide."

This claim is further supported by hot water immersion testing (Figure 4) to compare the product with 65 wt% zinc, glass flakes and ceramic spheres (left) and standard IOZ with 80 wt% zinc dust after six weeks’ exposure to tap water at 80°C. The difference in zinc salt formation was very clear, with the standard formulation having a significant higher level of zinc salt formation. In addition the panel with 80 wt% zinc dust started to show the formation of red iron oxide corrosion, indicating that that the coating was starting to lose its galvanic corrosion protection. There was no evidence of this on the 65 wt% coating.

Conclusion

Based on the test results above, it is clear that the coating with the lowest zinc content gives better galvanic corrosion protection at CUI and high temperature conditions. As has been described in this article and illustrated in the videos this is due to glass flakes introducing a barrier element to the coating and preventing overconsumption of the zinc, and ceramic spheres pushing the zinc closer together to ensure maximum utilization of the zinc which has been added to the coating.

References
[1] Zhang, X. G. [1996] Corrosion and Electrochemistry of Zinc. Plenum Press, pp. 169.

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