Protective coatings are often perceived as a final finishing activity. In reality, coatings are complex engineered materials whose performance depends on their chemical constituents, curing mechanisms, application conditions, and the environment in which they operate.
A clear understanding of coating fundamentals is essential for professionals involved in inspection, maintenance, asset integrity, and corrosion management. Many coating failures originate not from product quality, but from a lack of understanding of how coatings actually work.
What Is Paint Made Of?
From a chemistry perspective, a coating is made up of four primary constituents:
- Binder (resin) – forms the continuous film and provides adhesion, strength, and durability
- Pigments – provide colour, opacity, corrosion protection, or functional properties
- Solvents – control viscosity and application characteristics
- Additives – modify specific properties such as flow, drying, wetting, or stability
Each component plays a specific role, and changing one element can significantly affect overall coating performance.
Drying vs Curing — A Critical Distinction
One of the most common misconceptions in coatings is confusing drying with curing.
Drying is the physical change from a liquid to a solid state. This can occur through:
- Solvent evaporation
- Water evaporation (water-borne systems)
- Coalescence of resin particles
Curing, however, is a chemical reaction that develops the coating’s final mechanical, chemical, and protective properties.
Examples include:
- Epoxy coatings, which cure by chemical reaction between the epoxy resin and a curing agent
- Alkyd coatings, which cure by oxidation through reaction with oxygen from the air
A coating may appear dry to the touch but still be chemically immature. Premature exposure to service conditions before full cure can result in reduced adhesion, softness, blistering, or early failure.
How Coatings Protect Steel from Corrosion
Coatings protect steel by interrupting corrosion processes through one or more fundamental mechanisms.
1. Barrier Protection
Barrier coatings restrict the movement of water, oxygen, and ions to the steel surface. Performance depends on:
- Film thickness
- Cross-link density
- Low permeability
- Multi-coat system design
Laminar pigments such as micaceous iron oxide or aluminium flake are often used to improve barrier performance.
2. Passivation
Inhibitive pigments reduce corrosion by interfering with electrochemical reactions at the steel surface. Pigments such as zinc phosphate provide protection when moisture penetrates the coating.
3. Cathodic (Sacrificial) Protection
Zinc-rich coatings provide galvanic protection by sacrificing the zinc pigment in preference to the steel substrate. For effective protection, high zinc loading and good electrical continuity are essential.
Most protective coating systems rely on a combination of these mechanisms rather than a single method.
Why Coating Systems Use Multiple Layers
Traditional corrosion protection systems are deliberately designed as multi-layer systems, with each coat performing a defined function.
- Primer
Promotes adhesion to the substrate, provides corrosion protection, and forms a base for subsequent coats. - Intermediate Coat(s)
Provide film build, durability, and enhanced barrier properties. These layers usually account for the majority of the total dry film thickness. - Topcoat
Provides colour, UV resistance, weatherability, and protects underlying layers from degradation.
Altering or omitting layers can significantly reduce system performance and service life.
Primer Mechanisms and Selection
Primers are not interchangeable, and their performance depends on the protection mechanism they are designed to provide:
- Sacrificial primers – high zinc content for galvanic protection
- Inhibitive primers – corrosion-inhibiting pigments for passivation
- Barrier primers – low permeability systems that slow moisture ingress
Incorrect primer selection, low zinc content, or incompatibility with subsequent coats can compromise corrosion protection from the outset.
Pigment Volume Concentration (PVC) and Film Properties
Pigment Volume Concentration (PVC) plays a critical role in coating performance.
- Low PVC coatings
Tend to be glossy, less permeable, and more flexible - High PVC coatings
Tend to be matte, more permeable, and less well bound
At the Critical Pigment Volume Concentration (CPVC), significant changes in film properties occur. Understanding PVC is essential when evaluating durability, permeability, and corrosion resistance.
Coatings for Harsh and Demanding Environments
Standard coating systems are not suitable for all conditions. Specialist coatings are required for environments such as:
- Offshore splash and tidal zones
- High-temperature piping and equipment
- Corrosion under insulation (CUI)
- Abrasion, erosion, and immersion service
Developments in resin technology have led to surface-tolerant coatings, solvent-free systems, high-temperature resistant coatings, and underwater-applied coatings. These systems only perform as intended when correctly specified, applied, and cured.
Antifouling Coatings and Marine Applications
Modern antifouling coatings are no longer tin-based. Current systems include:
- Controlled biocide-release coatings
- Biocide-free foul-release systems
- Advanced polymer-based technologies
These coatings are designed to control fouling, reduce drag, and extend docking intervals while meeting environmental regulations.
Why Fundamentals Still Matter
Many coating failures are not caused by product defects, but by:
- Incorrect coating selection
- Inadequate surface preparation
- Insufficient curing time
- Misunderstanding of coating chemistry
Protective coatings should be treated as critical engineered systems, not cosmetic finishes. Understanding the fundamentals leads to better inspection decisions, improved specifications, and longer asset life.
Final ThoughtEffective corrosion protection is achieved when coating chemistry, application, environment, and inspection are properly aligned. A strong understanding of coating constituents and basic chemistry remains one of the most powerful tools for improving coating performance and asset reliability.