Understanding Corrosion Categories and the Painting Process

Introduction

In another article, “Sheet Metal Enclosures for Electrical Switchboards: A Cornerstone of Electrical Safety and Reliability” we discussed the importance of sheet metal enclosures for electrical switchgear assemblies. We have also discussed the type of metal enclosures. We all know that all metals are prone to environmental degradation, and painting is one of the widely used processes to protect them.

In industrial and commercial environments, metal structures, enclosures and components are constantly exposed to various environmental conditions. these different environmental is being defined as different corrosion catagories. These conditions vary based on the site’s geo-location, as it has different environmental conditions. These conditions depend on humidity, the salinity of the moisture content in the air, and air pollutants. Prolonged exposure of the metal enclosure surface to such conditions can lead to corrosion and surface damage. Corrosion is one of the most common and costly forms of material degradation. To mitigate this, understanding corrosion categories and applying the appropriate protective painting processes is essential.

What Is Corrosion?

Corrosion is the deterioration of a material, usually a metal, caused by chemical reactions with its environment. This typically involves oxidation with oxygen during exposure to the environment. If environmental air contains moisture, salts, and pollutants, then it aggravates the oxidation process. The worst part is that if oxidation starts and red oxide (rust) is formed, it continues to damage the metal until completely converted into rust. Left untreated, corrosion can compromise structural integrity, safety, and longevity.

Corrosion Categories: ISO 12944 Standard

ISO 12944 is a set of international standards for the corrosion protection of steel structures using protective paint systems. It provides guidance on classifying environments based on it corrosivity. These classified environments are referred to as corrosion categories. These categories help determine the appropriate protective coating system for different environments and ensure its durability.

Atmospheric Corrosivity Categories:

Various corrosion categories are defined in the table below:

Each category considers factors such as humidity, salinity, air pollution, and temperature to determine the potential rate of corrosion.

Protective Painting Process

_{^{Figure 1A Typical Paint Shop (Picture Courtesy: Tricolite Electrical Industries Ltd)}}

To combat corrosion effectively, a tailored painting system is applied based on the corrosivity category of the environment. Here’s a step-by-step overview of the industrial painting process:

Surface Preparation
Primer Application
Intermediate Coat (optional)
Topcoat Application
Curing
Inspection & quality control

Surface Preparation

For adhesion and durability, proper surface preparation is critical.

Abrasive Blasting (e.g., SA 2.5 or SA 3): Removes rust, mill scale, and old coatings. This process is not used for clean CRCA sheet steel. Surface preparation for these steel sheets is done through a chemical pretreatment process.
Degreasing: Eliminates oil and contaminants.
Cleaning and Dust Removal: Ensures a clean surface for primer application.
Priming

In a highly corrosive atmosphere, the primer serves as the first layer of defence against corrosion.

Zinc-rich primers are common for high-corrosion areas.
Epoxy-based primers offer strong adhesion and chemical resistance.
Intermediate Coating (Optional)

For environments C3 and above, an intermediate coat enhances protection.

For intermediate coating, typically a high-build epoxy that adds thickness and durability.
Topcoat

The final layer provides UV protection, colour, and aesthetic finish.

Polyurethane or acrylic topcoats are used depending on exposure to sunlight and chemicals.
Curing

In industrial painting, curing is a critical step that ensures the paint or coating achieves its full performance properties, like chemical resistance, hardness, and corrosion protection. Typical cure temperature: 160–200°C (320–392°F) witha curing 10–20 minutes, depending on powder type and film thickness. Curing causes the powder to melt, flow, and crosslink into a hard, durable finish. It is not just about the paint “drying”; it’s about the chemical reactions completing so the coating system performs reliably in demanding environments.

6. Inspection and Quality Control

Post-application inspections ensure:

Correct dry film thickness (DFT)
Adhesion strength
Uniform coverage
Compliance with project specifications

The total coat thickness is dependent on the corrosion category. Other stages like priming, intermediate coat and topcoat are also dependent on corrosion category. Understanding the complete process of coating for corrosion protection must involve referring to the paint manufacturer’s recommendations, especially for the specific corrosion category involved. One of the manufacturer’s recommendations can be found in the link below.

Choosing the Right System

Selecting the right coating system depends on:

Corrosion category
Expected service life
Environmental conditions (humidity, temperature, pollutants)
Maintenance schedule

ISO 12944 also provides guidance on expected durability:

Low: up to 7 years
Medium: 7–15 years
High: 15–25 years
Very High: 25+ years

Applicable Standards & Guidelines

ISO 12944 – Corrosion protection of steel structures by protective paint systems
ISO 8130 – Powder coatings — terminology and test methods
ASTM D3359 / ISO 2409 – Adhesion testing
ASTM B117 / ISO 9227 – Salt spray corrosion testing

Conclusion

Understanding corrosion categories and applying a well-planned painting process is crucial for the longevity and safety of metal structures. Whether you are working on a marine platform, an urban steel bridge or an electrical enclosure, choosing the right protective coating system ensures lower maintenance costs and extended life cycles.

Understanding Corrosion Categories and the Industrial Painting Process

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