Painting and hot-dip galvanizing are widely used to protect engineering components and structures from corrosion. They can also provide colour, identification, improved appearance and other surface properties.
The performance of any protective coating depends on more than the coating material itself. Surface preparation, component design, application, environment, inspection and maintenance can all influence the resulting service life.
Paint is a liquid or otherwise applied coating material which forms a protective film on a surface. In engineering it is commonly used for corrosion protection, decoration, identification and functional surface requirements.
Industrial paints and coating materials vary considerably in composition, but important constituents can include:
Engineering protection is normally achieved using a complete coating system rather than considering the final colour coat in isolation. Depending on the coating specification, a system can include:
Not every modern coating system requires separate primer, undercoat and topcoat layers. The number and type of coats depend on the coating technology, substrate, environment and required performance.
Surface preparation is one of the most important stages in achieving a durable protective coating.
Depending on the substrate and coating system, preparation may involve:
The required preparation should be established from the coating specification and the condition of the substrate rather than assuming that one preparation method is suitable for every coating.
A primer is the first coat in many protective paint systems and is applied directly to the prepared substrate.
Depending on the particular coating system, a primer can provide:
The appropriate primer depends on the substrate, environmental exposure and subsequent coating layers.
Intermediate coats may be used between the primer and finishing coat to build the required coating thickness and provide additional barrier protection.
The term undercoat is commonly encountered in traditional paint systems, but modern industrial protective-coating specifications more often define the individual coats by their coating type, function and required film thickness.
The topcoat forms the outer surface of the paint system and is directly exposed to the service environment.
Depending on the application, the topcoat may be selected to provide:
Paints can use different carrier systems. Two broad categories are water-based and solvent-based coatings.
In a solvent-based coating, organic solvents may be used to provide the viscosity and application characteristics required. Solvent subsequently evaporates as the coating dries or cures.
Water-based coatings use water as an important part of the liquid carrier. These include polymer dispersions and other coating chemistries designed for aqueous application.
The fact that a coating is described as water-based or solvent-based does not by itself define its complete curing mechanism or engineering performance. The binder chemistry and complete coating formulation must also be considered.
Paint coatings do not all form their final film by the same mechanism. Depending on the coating, film formation can involve:
Some production coating systems are heated or stoved to accelerate or complete curing. The required temperature and time depend on the particular coating specification.
The term lacquer has traditionally been used for coatings which form a film principally through evaporation of volatile solvent, leaving the film-forming material behind.
Modern coating terminology and chemistry are more varied, so the actual product specification should be used when determining the drying or curing mechanism of a particular coating.
Protective coating specifications commonly define the required film thickness. Insufficient thickness may reduce protective performance, while excessive coating thickness can also create problems depending on the coating system.
Coating thickness should therefore be controlled and inspected in accordance with the applicable coating specification and measurement procedure.
Component geometry can have a significant effect on coating quality. Designers should consider whether surfaces can be adequately prepared, coated, inspected and maintained.
Potential problem areas include:
Where corrosion protection is important, coating access should be considered during component design rather than after fabrication is complete.
Hot-dip galvanizing protects iron and steel by immersing the prepared component in molten zinc. The zinc reacts metallurgically with the iron in the steel to produce a series of zinc-iron intermetallic layers, normally with a zinc-rich outer layer.
The resulting coating provides both a physical barrier between the steel and its environment and sacrificial protection. Zinc is anodic relative to steel and can therefore provide protection to small areas of exposed steel adjacent to the zinc coating.
The precise plant process varies, but batch hot-dip galvanizing generally includes the following principal stages:
A hot-dip galvanized coating protects steel in two important ways.
The zinc coating physically separates the underlying steel from the corrosive environment.
Zinc is anodic to steel. Where a small area of steel becomes exposed adjacent to the coating, the surrounding zinc can corrode preferentially and provide cathodic protection to the exposed steel.
A component intended for hot-dip galvanizing should be designed with the galvanizing process in mind. The complete fabricated component must be handled, cleaned, immersed in molten zinc, withdrawn and drained.
Important considerations include:
Cleaning solutions, flux and molten zinc must be able to enter and leave the appropriate parts of a fabricated component. Air and gases must also be able to escape as the component is immersed.
Poor venting or drainage can result in uncoated areas, trapped process fluids, excessive zinc accumulation and serious process hazards.
Vent and drain arrangements should therefore be agreed with the galvanizer when designing hollow or complex fabrications.
Fabricated steelwork can contain residual stresses from rolling, forming, welding and other manufacturing operations. Heating the complete assembly during hot-dip galvanizing can allow distortion to occur in susceptible fabrications.
The risk depends on component geometry, material thickness, fabrication sequence, weld arrangement and structural stiffness. Large thin plates, asymmetric assemblies and heavily welded structures can require particular attention.
The zinc coating adds material to the surface and can therefore affect threads, holes, fits and other close-clearance features.
Components requiring galvanizing should be detailed with the intended coating process in mind. Thread allowances and mating components should follow the relevant fastener and galvanizing requirements rather than assuming that an uncoated fit will remain unchanged after coating.
Galvanized steel can also be painted or powder coated where additional corrosion protection, colour or appearance is required. A combined galvanized and organic coating system is often referred to as a duplex system.
Painting galvanized steel requires appropriate surface preparation and a coating system compatible with the zinc surface. The condition and age of the galvanized surface can influence the preparation required.
The most appropriate corrosion-protection system depends on the component and its operating environment.
| Consideration | Paint coating | Hot-dip galvanizing |
|---|---|---|
| Colour | Very wide range of colours and finishes available. | Natural zinc appearance unless subsequently painted or otherwise finished. |
| Application | Can be applied to components and structures using several different application methods. | Fabricated component must be suitable for immersion in the galvanizing process. |
| Protection mechanism | Principally depends on the particular coating system providing protection to the substrate. | Provides barrier protection together with sacrificial protection from zinc. |
| Surface preparation | Preparation depends on substrate and specified coating system. | Cleaning, chemical preparation and fluxing form part of the hot-dip galvanizing process. |
| Component geometry | Access for preparation and coating is important. | Immersion, venting, drainage, handling and bath capacity must be considered. |
| Maintenance | Inspection and maintenance requirements depend on coating and environment. | Can provide long-term protection, but coating condition should still be considered throughout service. |
The required protection cannot be selected solely from the name of the coating. Corrosion severity depends on the actual service environment.
Factors can include:
A coating system should therefore be selected against a defined service environment and required performance rather than simply specifying "painted" or "galvanized" on an engineering drawing.
Protective coatings should be inspected where their condition is important to the integrity or service life of the component.
Inspection can consider:
The appropriate inspection method and acceptance criteria should come from the applicable coating specification rather than from a general visual assessment alone.
Corrosion protection is most effective when considered during the design of the component rather than added after the geometry and fabrication method have already been fixed.
Good engineering design should consider:
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