Selecting the correct paint system for protection against corrosion assures that the most economical and best technical solution is achieved.
A weak paint system might be cheaper at first but additional maintenance cost in the lifetime of the building might boost the overall cost. On the other hand an excess of paint for a very low corrosion environment will be a mismanagement of the project budget.
Identifying the corrosion category of the environment is the most important factor to consider before selecting a protective coating.
When selecting a paint system it is vitally important to define the conditions in which the structure, facility or installation is to operate. To establish the effect of environmental corrosivity, the following factors must be taken into account:
• humidity and temperature (service temperature and temperature gradients)
• the presence of UV radiation
• chemical exposure (e.g. specific exposure in industrial plants)
• mechanical damage (impact, abrasion etc)
ISO 12944 distinguishes 6 basic atmospheric corrosion categories:
C1 | Very Low |
C2 | Low |
C3 | Medium |
C4 | High |
C5 | Very High |
C6 | Extreme |
The characteristics of the above categories are as follows;
For every steel structure project, we set the relevant corrosion category together with our clients and determine the paint system according this category.
Alkyd paints are the modern descendant of oil paints. Instead of pigment suspended in oil, alkyd paints are typically formed by an alkyd resin dissolved in a thinner. These one-component primers are easy to apply and economical but offer limited
protection against rust in the long run. For low corrosive environments, these paints can be used for budget purposes.
Epoxy coatings are typically comprised of an epoxy base and a curing agent. A wide variety of coating properties can be achieved by manipulating either of these components:
Epoxy polyamide coatings offer great moisture resistance, epoxy mastic coatings offer exceptional film thickness and phenolic epoxy coatings offer good chemical resistance.
And due to this versatility, you’ll find epoxies used as a primer, intermediate coat or even a topcoat depending on the needs of the application.
Advantages
• Abrasion resistant
• Chemical resistant
• Good performance when submerged
• Can easily build film thickness for enhanced abrasion resistance
• Can be formulated to exhibit a wide variety of favorable coating properties
Disadvantages
• Chalks when exposed to UV light
• Low flexibility
Polyurethane coatings are widely used —often as a topcoat— in applications where durability and abrasion resistance are key considerations.
Most often, polyurethanes are chosen as the topcoat of a total protective coating system. For example, polyurethane might be applied as a topcoat above a zinc-rich primer and epoxy intermediate coat.
Polyurethanes are versatile enough to be formulated to handle a wide variety of service environments.
As a generic coating type, zinc-rich coatings refer to organic or inorganic coatings with high loadings of zinc dust.
The zinc provides galvanic protection of the steel surface, meaning that it will corrode instead of the steel beneath it.
As the zinc-rich coating corrodes it forms a barrier between the steel and its environment
Zinc-rich primers will often be applied as part of a two-coat (zinc-rich primer, polysiloxane topcoat) or three-coat (zinc-rich primer, epoxy intermediate coat, polyurethane topcoat) system.
The prevailing service environment —along with factors such as cost, accessibility of the asset, expected labour costs and desired service life of the coating system— will define which coating system is best-suited for your application.
Coating selection happens on a case by case basis, and we can evaluate the specific needs of your application, offering several systems that could work based on our decades of experience in the industry.
Contact our team to get more information about the coating options of your metal building project.
