The protection of steel using different surface coatings has been a practice ever since steel was more commonly used as construction material in the world. Analysts believe that protective coats today represent sales of over billions US dollars per year and that this figure will have increased to more within less than 5 years. Where there is steel, there is corrosion and also the corrosion related spending has risen to billions. Indeed, developed and industrialized Countries have an individual corrosion related expenditures. This means that the cost of corrosion is significantly higher than the sums spent on industrial coatings worldwide.
A common method to protect steel from attack by corrosion is to coat the structure with different types of paints. One part contains anticorrosive pigments and additives, others react with surface humidity leading to changes in the paint film by which a protective barrier is formed to block the contact between the air / oxygen and the metal surface. Many of these systems are very effective, but if they show bubbles, cracks or other defects in the coating, for example, due to damage during labor and / or transport and construction, the barrier will be broken and the effect will be lost. In coastal climates, a completely intact coating that has been applied too thin (for example, at the edges not rounded correctly) may allow chloride ions on the surface to penetrate the paint film to the steel surface underneath. It is therefore very important that the paint specifications are followed to the letter, otherwise errors may occur. It usually costs four to five times more to correct the errors than the original cost was, because of all the extra work afterwards.
The above is one of the driving forces of the global use of hot dip galvanizing to protect steel since many decades. Hot dip galvanizing eliminates many troubles that may occur with surface treatment in the form of paint application. Having said this, it is understood that also hot dip galvanizing has its own parameters that must be followed to achieve a good result. Tubular closed details must for example be provided with a vent hole per meter with 25 mm diameter, to avoid that the details explode when heated. Among architects, it was of course unpopular to drill holes in carefully designed structures, and to fill them was a laborious and expensive process.
Another concern is the risk of twisting of thin steel parts. Even a very small rotation means, for example for a long I-balk that the bolt holes no longer match the holes on the second beams. It also occurs that some steel alloys have a different surface structure than ordinary steel and this can be a problem for hot dip galvanizing. Thus it can be difficult to achieve a uniform design of a welded construction composed of several different steel types. Some metals, such as cast iron are difficult to hot dip, and cannot be dipped in a liquid zinc bath with temperature at 450 degrees, because this would lead to bursts.
During the early 1930’s, a system for film galvanizing called Zinc Rich Coating (ZRC) (also called cold galvanizing compound) was developed and represents as same or sometimes superior properties than hot dip galvanizing. Zinc-rich coating (ZRC) technology has been used for years, finding application in marine industry, civil infrastructure or all type of ferrous materials. Zinc-rich layer is used as a single coat or as a component of the paint system. Coating systems consisting of zinc-rich paints offer decorative and barrier as well as anti-corrosive protection to the steel substrate. Sacrificial pigments such as zinc pigments require zinc in large quantities in order to enable flow of electric current. At the initial stage the zinc dust film being in electrical contact with the steel surface plays the role of a sacrificial anode providing cathodic protection. At a later period, zinc corrosion products such as zinc oxide or/and zinc hydroxide are formed, thus barrier protection begins to dominate. The greater the amount of zinc corrosion products which form on the zinc particles surface, the higher the probability of losing electrical contact between zinc particles themselves or zinc particles and the steel substrate. Thus it may lead to reduction and in the end complete loss of protection based on the cathodic mechanism. High electrical conductivity of coating is required for providing the first period of protection to the steel substrate.
Protective performance of these coatings can be markedly affected by the shape and size of zinc dust, the pigment volume concentration (PVC) and dry film thick-ness (DFT) of the applied coating. At the first step, ZRCs act via sacrificing mechanism and electrical contact between zinc particles and the steel substrate is essential to the coating protective performance. As organic and inorganic binders which are commonly used in ZRCs are naturally non-conductive, these coatings are formulated with high percentage of metallic zinc dust to ensure electrical conductivity. Therefore, one of the key features of a ZRC is its high content of powdered metallic zinc. Sufficient electrical contact between the film and the steel substrate is established because of high zinc content. High content of zinc powder results in higher porosity of the resulting dry film. Dissolution of active metallic zinc dust takes place soon after immersion in the electrolyte forming a non-conductive zinc oxide corrosion product. As the zinc oxide film forms, the electrical con-tact with the steel substrate is gradually reduced, and the potential of the steel exceeds the protection potential. It has been observed that cathodic protection mechanism is short lived due to the loss of electrical contact between the zinc dust and the steel substrate. It seems that only at the beginning of immersion, zinc particles provide the cathodic protection of the steel substrate, and subsequent formation of zinc corrosion products reinforces the barrier effect of the primer.
Why need of ZRCs?
Every year corrosion is costing billions to the industry. This corrosion doesn’t merely make the assets appear neglected, more importantly, it causes structural instability and consequently poses a serious safety hazard. It also causes severe disruption of the production process and thus profitability.
ZRCs differentiates from other anti-corrosion methods in combining both Passive and Active protection in an easily applied film galvanizing system that not only delivers active cathodic protection but also provides a passive physical shield.