The Importance of Associations and Standards for Powder Coaters
April 22, 2024Can You Powder Coat Over an Existing Coating?
May 14, 2024Custom coaters do not have the luxury of choosing the kind of material that we apply our coatings to – we have to make the best of what we are given. It is our job to properly apply an engineered coating system to our customer's product. This is especially an issue in the market segments we focus on at Aegis because hot rolled steel is the most common material that we encounter.
This post will cover the source of mill scale, why it is an issue, and what you need to do to ensure that the coating you apply will achieve the life for which it was designed. The advice given in this article is applied to product which will be used in exterior or harsh environments – however I would advise that even in internal / controlled environments, it is still best to remove the mill scale.
The Root of Mill Scale
Mill scale is also known as iron oxide. It is a hardened surface on hot rolled steel that is formed during the milling process. In his book “Corrosion Prevention by Protective Coatings”, Munger explains that the oxide layer coaters typically encounter is formed by the latent heat once the metal has been shaped and is cooling. If the mill scale were to provide a uniform and impenetrable barrier to the steel underneath then it would not be a concern. Unfortunately mill scale and steel have different thermal coefficients. During this cooling period, the scale hardens and cracks (211). It is these cracks that are of concern for the long term success of the engineered coating system applied by the custom coater.
Back to basics
At the heart of the issue with mill scale is corrosion. Without getting into the detailed particulars of the corrosion process here it is in a nut shell:
There are four components that are generally required for a corrosion cell to exist – they are easily remembered through the acronym A.C.M.E.
- Anode (-): An area on the steel plate (negative) that is more reactive resulting in the steel deteriorating and being released as positive ions (cations) which flow to the Cathode. This is where oxidation occurs.
- Cathode (+): An area of the steel plate (positive) that is less reactive which receives positive ions (cations) and excess electrons from the anode. It releases negative ions (anions) to the anode.
- Metallic Pathway: The metallic pathway that connects the anode to the cathode, allowing electrons to flow from the anode to the cathode. In the case of steel, it acts as its own metallic pathway.
- Electrolyte: Generally a water based pathway that allows ions to flow between the anode and the cathode. (The presence of salts greatly increases the transfer of ions)
In the presence of these four components a corrosion cell is setup and steel will rust. Other variables such as humidity, temperature, exposure, etc, will determine the rate of corrosion.
In a test I conducted in the environmentally controlled confines of my office I took two steel plates which were blasted to a SSPC-SP10 / NACE No.2 Near White finish. As seen in the photos, a piece of road salt was added to a bare steel plate (Plate C). With the addition of water, an electrolyte was created resulting in a corrosion cell. The progression of corrosion can be seen in the following pictures. Because the composition of the steel is not entirely uniform, anodes and cathodes exist on the surface of the steel.
In this photo you can see Plate C is compared to Plate A – which was “monitored” on the same corner of my desk as Plate C – the effects of an electrolyte are clear. (Plate B is at my home staining my patio table for the sake of coating and corrosion science – much to my wife’s disapproval).
Get rid of it!
If you have ever taken a grinder to mill scale you likely have found that it polishes quite nicely but requires far too much effort to remove with hand tools. There are two effective methods for removal: pickling in acid and abrasive blasting. I only have experience with the latter and will explain it here.
The abrasive blasting process uses media (there are a variety of types – garnet, steel shot, mineral) to clean a substrates surface of rust, scale, and other contaminants.
BEFORE AND AFTER BLASTING - This picture shows two identical pieces – one with tightly adhering mill scale, and one freshly blasted. Apart from effectively removing the mill scale from the steel, the blasting also provides a totally clean surface that is ready to coat. This introduces a new variable that the coater must be aware of.
Blasting creates a profile in the steel that is determined by the type of media used and the air pressure behind it – at Aegis we typically see measurements between peaks and valleys around 1 – 2 thou with the use of a popular mineral media. This profile improves the adhesion of the coating to the steel.
Caution is advised however, as traditional powder coating is applied at 2 - 3 mils over chemical pretreatment. With the blast profile at 2 thou, a coating thickness of 2 - 3 mils could leave some areas dangerously thin. To overcome this risk, any steel we blast, we always recommend a primer at 2.5 - 3.5 mils, followed by a top coat.
The reason for this is demonstrated as follows:
If my blast profile is 1.5 thou, and my single coat is 2.5 thou, in some areas I am only left with 1 thou of coating. This is too thin to provide an effective barrier, and pinpoint corrosion will occur. When applying a primer at 2.5 – 3.5 thou, and our top coat at 2.5 – 3.5 thou, we have an effective coating system that bonds well, provides excellent moisture / chemical resistance (through the epoxy), and UV stability, color, and gloss retention through the exterior grade top coat.
Solvent wiping is not an effective means of preparing hot rolled steel for long term use, especially not exterior use.
Case in point
We had a new customer bring us a bus shelter that had to be stripped and recoated. It had been in service for two years and the coating had started to peel off. Upon closer inspection, the coating was doing its job – the problem was that the mill scale was “failing”.
You can see the coated scale being lifted from the framework.
Where the coating is still intact is contrasted to areas where the mill scale has completely rusted off and where it was still intact.
As we did not coat this bus shelter originally I can only assume that the steel was degreased and possibly chemically pretreated prior to coating. When the bus shelter arrived on site it would have looked no different from a properly prepared steel structure. However, over the course of two years it was subjected to 8 seasonal changes (think of the expansion and contraction caused by the temperature changes – this is constantly disrupting the bond of the scale to the steel combined with the moisture from rain and snow), was installed near a road way where pollution would have settled on it and gradually penetrated the coating. Furthermore, it was likely never touched up if and when breaches occurred. In a matter of only two years it was in our facility to be stripped and recoated.
For the sake of argument, assume that the cost to degrease and put a single coat on the bus shelter was $125 – System A. To sandblast, prime, and top coat may have been $225 – System B. The cost to strip, prime, and top coat was $300+ (System C). In the short term, the customer may think they saved money by going with the cheaper alternative (System A) – after all, powder coating is tough isn’t it? However, as evidenced by this example, they paid $425 (System A+C) for something that would have cost $225 if done right the first time. Assuming a 15+ year coating life when properly applied, System B would have cost them $15/yr whereas System A+C cost them $28/yr. Furthermore, this does not take into account the cost to remove and reinstall the bus shelter.
It is likely that the customer simply asked for the bus shelter to be painted blue, and the original coater gave them just that – a blue bus shelter. Far too often we receive RFQ’s that state the color the customer would like the product to be without any indication of where the product will be used, what the coating will come in contact with, or how long the coating is expected to last. While a full exploration into understanding customer’s expectations of coating performance is beyond the scope of this article, it is still worth noting. As custom powder coaters we have a responsibility to understand what material we are expected to work with and how best to prepare it for the natural life of the coating.
When applied over a properly prepared surface with suitable primer and exterior grade top coat, according to a major powder manufacturer’s data, a two coat system will last 15+ years in a marine or industrial environment (C5-I/M as per ISO 12944-6). I make this statement with the condition that the coating has not been breached, and that it has been properly maintained.
Conclusion
As a custom powder coater, we cannot choose the condition of the steel that we get to coat. It is not enough to respond to a complaint of a failed coating on mill scale with “My coating is fine, your mill scale sucks!” I believe that we have a responsibility to provide our customers with a coating that will last and perform according to the manufacturers’ design and provides them with "peace of mind". In order to do this we must remove any barrier that we are aware of prior to coating. It is likely that the coaters who originally coated the bus shelter thought nothing of the mill scale – but now that you are aware or the problems it can cause I hope you reconsider.
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