The challenges facing zinc silicates at high temperatures – part 1

It is an established truth that a system utilizing a zinc silicate primer will have the longest lifetime and offer the best corrosion protection in the most challenging atmospheric conditions. However, the issue of corrosion under insulation has emerged as a significant problem.

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

It is an established truth within the coating industry that a system utilizing a zinc silicate primer will have the longest lifetime and offer the best corrosion protecting in the most challenging atmospheric conditions, such as offshore and in chemical plants and refineries.

However, as the coating industry has matured, the issue of corrosion under insulation – CUI – has emerged as a significant problem. Not only does it occur in elevated temperature areas, it is also hidden under insulation and cladding making it difficult to detect and remedy.

Zinc silicates' limitation in temperature performance

Historically, the coating system specified for temperatures above 120°C was for one coat of an Inorganic zinc silicate (IOZ) followed by a silicone or silicone aluminum top coat. This seemed the natural choice, as the silicate binder essentially becomes glass once the coating has cured, giving it a very high temperature resistance.

However, the addition of the zinc dust for corrosion protection limits the maximum temperature performance of the coating. Metallic zinc has a melting point of 420°C, and once the zinc reaches this it rapidly reacts with oxygen in the atmosphere, degrading the metallic zinc into zinc oxide.

Figure 1 shows a zinc silicate before heating (top) and after heating (bottom). The white, “fuzzy” particles are the metallic zinc reacting with oxygen and being converted to zinc oxide.

image_showing_a_zinc_silicate — Figure 1: SEM image showing a zinc silicate before heating (top) and after heating (bottom) to a temperature above 420°C.

This process creates two problems. Firstly, zinc oxide does not offer galvanic corrosion protection, so the coating starts to lose its corrosion protection properties. Secondly, zinc oxide has a larger volume than metallic zinc meaning it occupies more space within the coating film. This increases the risk of microcracking in the coating, which increases the need for additional corrosion protection.

The formation of zinc oxide is a natural by-product of the galvanic corrosion protection zinc silicates offer, whereby metallic zinc “rusts” in place of the substrate forming zinc oxide or “white rust” as it is sometimes called. This formation of zinc oxide in corrosive environments adds a barrier element to the coating, by filling in the porous structure that naturally occurs in zinc silicate coatings. This formation of zinc oxide also occurs when zinc is heated above its melting point, albeit through a slightly different chemical reaction. The challenge for IOZ coatings is that above the melting point of zinc the zinc oxidation occurs as such a rapid rate that it compromises the coating’s galvanic corrosion protection properties.

Furthermore, the galvanic corrosion protection of zinc is an electrochemical reaction, and these occur more rapidly at elevated temperatures. In cases where moisture is present – such as in CUI conditions – the zinc in an IOZ coating will be consumed rapidly, reducing the coating’s lifetime compared to the same conditions at lower temperatures.

It is for this reason that the National Association of Corrosion Engineers (NACE) has stated that IOZ is not a preferred solution for use in the temperature range of 4°C - 175°C, which is exactly where CUI is most likely to occur. It is generally accepted that at temperatures above 175°C there is insufficient moisture present for CUI. This is true for steady state conditions, however field conditions are rarely steady state, and especially during shutdown and maintenance periods temperatures will drop into the CUI range. This causes corrosion inducing condensation to occur in the insulated system and CUI is now a potential risk. The insulation has a tendency to trap moisture, meaning that even when the temperature returns to above 175°C CUI is still a factor to consider.

Reversed potential of IOZs

The third issue regarding IOZs is reversed potential. This is where the the coated steel structure becomes anodic to the zinc and sacrifices itself in place of the zinc between 60°C and 80°C. This can lead to premature and accelerated breakdown of the structure the IOZ coating is supposed to protect. In addition to only occurring in a very narrow temperature gap, this polarity reversal requires the presence of dissolved oxygen, bicarbonates and nitrites, while chlorides and sulfates inhibit the process. This means that areas susceptible to CUI tend to be less susceptible to the reversed potential of a zinc silicate[1], and it is less likely to be something one would experience in a processing plant.

Summary

Therefore, there are three main reasons why the performance of IOZs is challenged at elevated temperatures;

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 reduced coating lifetime compared to similar conditions at ambient temperatures.
Zinc oxide is created at a faster than normal rate at temperatures above 420°C. The 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 – this can be seen in Figure 1 – which can create microcracking in relatively brittle IOZ coatings. This will increase the need for metallic zinc for galvanic protection, further reducing the coating’s lifetime.

The challenges faced by IOZs at elevated temperatures have led to engineers and specifiers to look for alternative solutions to protect against CUI. Such alternatives could include other types of high temperature coatings using different coating chemistries, TSA, or removing the insulation altogether where possible.

If an IOZ is being used, high zinc loadings tend to be specified to ensure that there is sufficient metallic zinc available for corrosion protection for as long as possible, even in imperfect conditions. For those of us who hold a particular fondness for IOZ coatings it begs the question, can something be done to improve their performance in high temperature conditions?

We discuss this topic further in our article ’The challenges facing zinc silicates at high temperatures – part 2’.

References
[1] Zhang, X. G. [1996] ‘7.2.4 Polarity Reversal’ Corrosion and Electrochemistry of Zinc. Plenum Press, pp. 203 - 208.

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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.