Coating, Powder Coating and Surface Treatment — Why Small Defects Can Become Large Problems

What Does Surface Treatment Do?

On coated metal outdoors, the coating can act as a barrier that limits access by water and dissolved salts. Its continuity and adhesion, the substrate, preparation and complete specified system determine protection. Colour and gloss are additional functions.

A system may include a cleaned and prepared substrate, primer where required, and one or more topcoats. Powder type, thickness, coating build, edge coverage and application all affect performance; no single parameter determines the outcome.

How Does Powder Coating Work?

Powder coating is applied as dry polymer powder — for example polyester, epoxy or a hybrid — to a conductive, earthed workpiece. The powder is usually electrostatically charged, then heated so it flows and cures into a continuous film. Cure temperature, object temperature, time and thickness must follow the specified system rather than a universal range.

Epoxy can chalk or discolour under UV. Polyester may be formulated for outdoor use, but actual UV, corrosion, chemical and mechanical performance must be documented for the specified system. The description “powder coated” does not establish performance by itself.

Wet Paint and Powder Coating

Wet paint is applied in liquid form with a water- or solvent-borne carrier. Powder coating is normally applied dry and then cured. Thickness, durability, geometry coverage, UV performance and repairability depend on formulation, system build and workmanship. Either can form an organic barrier in a corrosion-protection system.

What Happens When the Coating Is Scratched?

A continuous, well-adhered system limits electrolyte access. Corrosion through this mechanism requires damage that reaches the metal or affects adhesion enough for electrolyte to reach the metal or interface. Steel can rust, while aluminium can pit in chloride environments.

Small defects can matter, but are not inherently worse than large ones. Outcome depends on depth and geometry, substrate, material combination, anode-to-cathode area ratio, coating system and exposure. See What Is Galvanic Corrosion?

Why Microscopic Scratches Can Matter

Capillary action can retain electrolyte in a narrow defect. Local geometry, area effects and corrosion products can concentrate attack and lift the coating. A hairline surface mark that neither reaches the substrate nor affects adhesion does not automatically start these processes.

UV Light and Ageing

UV radiation, temperature, moisture and mechanical loading can affect organic coatings. Depending on binder, pigments, thickness and exposure, the coating may chalk, fade, lose gloss, change mechanically or become more permeable. These changes are system-specific and no fixed progression applies to every organic coating.

Salt, Moisture and Chemicals

Moisture can provide the electrolyte needed for metallic corrosion. Salt deposits can prolong wetness and form a more conductive solution. Chlorides can destabilise passive oxide layers, but the effect depends on material, concentration, temperature, drainage, drying and exposure.

If damage reaches the metal or interface, salt solution can contribute to attack or underfilm corrosion. Industrial emissions, de-icing salt and cleaners can also affect coatings. Suitability must be verified for the exact system, substance and environment.

How Corrosion Can Begin Beneath a Coating

Underfilm corrosion can develop where electrolyte reaches the metal or interface through a penetrating defect or lost adhesion. Filiform corrosion on aluminium and undercutting on steel are possible under certain conditions, rather than automatic outcomes of every defect.

Blistering and Flaking

Blisters can result from osmotic processes, trapped moisture or gas, poor adhesion or corrosion products. A blister alone is not proof of corrosion; investigation may need to assess adhesion, substrate and coating build.

Edge Corrosion

Sharp edges may have weaker coverage because of geometry, coating flow and application. Rounded edges and a system suited to the geometry can improve coverage, but performance must be verified for the actual design and process.

Damage During Transport and Installation

Handling can cause scratches or impact marks, but corrosion risk depends on whether the damage reaches the metal or affects adhesion. Drilling or cutting after coating exposes metal only where machining penetrates the complete system. Fasteners and field-worked surfaces can also create interfaces or moisture traps.

How Corrosivity Categories Relate to Service Life

ISO 12944 covers protective paint systems for steel structures. The 1998 edition used C5-I and C5-M; ISO 12944-2:2017 uses C1–C5 and CX. ISO 12944-1:2017 defines durability ranges to first major maintenance painting: H is 15–25 years and VH is more than 25 years. These are not warranties, exact field lives or the service life of a complete product. (ISO 12944-1:2017) (ISO 12944-2:2017)

C5-M is therefore an older designation. Selection must be based on the current category, substrate, preparation, complete paint system, durability range, workmanship and documented test evidence. See IP66, IK10 and C5-M.

Protecting the Metal — or Choosing the Right Material

Metals may be protected by paint, anodising or galvanising. A non-metallic material such as SMC does not have the metallic structure required for metal-based electrochemical corrosion. Stainless steel is still a metal whose resistance depends on alloy, environment and design.

Metallic, galvanic and other metal-based electrochemical corrosion cannot occur in the non-metallic SMC material itself. Surface damage does not expose corrodible metal within the SMC, but depth, fibre damage or exposure, surface treatment and the effects of chemicals, moisture, UV and mechanical loading must still be assessed.

Maintaining Surface Treatments

Regular visual inspection of edges, holes, fasteners and areas where salt and dirt collect can identify visible scratches, blisters and damage, but cannot rule out hidden underfilm corrosion or lost adhesion. Cleaning can reduce loading when carried out according to the system’s instructions.

A compatible repair applied after correct cleaning and preparation can restore a local barrier and reduce further attack. It does not guarantee that hidden corrosion has stopped or that factory performance has been restored.

Summary

Powder coating can form part of a complete corrosion-protection system whose performance depends on substrate, preparation, coating build, adhesion, application and exposure. Damage reaching the metal or affecting adhesion can admit electrolyte and permit underfilm corrosion. Inspection and correct early action can reduce attack, but cannot guarantee a service life.

Frequently Asked Questions