The painting of hot dip galvanized steel is an orthodox and well-proven practice in outdoor environments, both in Australia (AS/NZS 2312.2) and internationally (ISO 12944-5). The practice is commonly called “duplex coating”. However, there are examples of early failures of duplex coatings due to incorrect specifications and poor practice. The information on this website aims to avoid such failures by directing specifiers and applicators to the paint systems, surface preparation and application practices that will provide a durable duplex coating in a broad range of service conditions.
While good painting practices and generic products for the various exposure conditions have been nominated, this does not preclude the possibility of other paints and methodologies also performing satisfactorily. However, in selecting alternative products, specifiers are urged to select products only from those with verified records of satisfactory long-term performance in equivalent or more severe service conditions.
The information on this website is for general guidance only and requires strict compliance with the individual paint manufacturer’s detailed application instructions for each proprietary product. A complete PDF version of the GAA’s Guide to Paint Systems for Hot Dip Galvanized Steel in Atmospheric Service can be downloaded at this link.
Reasons for painting hot dip galvanized steel are primarily:
Decorative – to create an aesthetic colour and gloss or provide an identifying colour.
Enhanced durability – to increase service life.
Wider chemical resistance – in a situation where hot dip galvanizing alone may be vulnerable, such as outside the pH range of 6 to 12.
In outdoor service, remote from the coastal fringe and isolated from areas of severe industrial pollution, hot dip galvanizing is inherently durable. This contrasts with paints and other organic materials, which are degraded by solar radiation. Therefore, in most conditions of atmospheric exposure, little is to be gained from painting galvanized steel with a coating thickness more than 300 g/m2 (42 µm) unless aesthetic or colour considerations are important.
In benign internal situations, and particularly conditions of extreme impact or hard wear, unless a change in colour or gloss is considered necessary, hot dip galvanizing is usually best left unpainted.
A great deal of hot dip galvanizing is painted on a casual basis, with conventional latex or suitably primed solvent-based alkyd paint. The choice of alkyd paint is crucial and requires a clear recommendation from the paint manufacturer. In particular, the use of an alkyd primer in direct contact with the hot dip galvanizing risks delamination of the paint due to its saponification.
It is important to note that because these paint systems are quite thin, typically 70 to 120 µm for a three coat system, the zinc profile and localised areas of increased zinc thickness, such as at edges, may be visible in the finish, much in the same way as the grain is visible in painted timber. While this would rarely be a problem, higher build paints can be used as part of the painting specification.
While acknowledging the lesser user requirements of conventional (DIY) decorative paints, strict adherence to the appropriate surface preparation and prime coat specification are a key to reliability in all situations when painting over hot dip galvanizing.
The paint systems detailed under Service Conditions 1 and 2, on the Painting Systems page, are essentially decorative paint systems.
PAINTING FOR ENHANCED DURABILITY
Hot dip galvanizing is inherently very durable, so there will rarely be a need to paint over it to achieve the intended service life. Indeed, in higher corrosivity zones (C3 to C5) painting hot dip galvanized steel can actually accelerate corrosion of the zinc substrate and reduce the overall service life of the article from the expected hot dip galvanized-only life unless a judiciously selected, uniformly applied, high build paint system is applied and the integrity of that paint system is maintained through its service life.
Nevertheless, in circumstances where the hot dip galvanized coating is slowly being attacked by a corrosive environment, the application of a suitable coating which insulates the zinc from that environment can prolong the life of the hot dip galvanized coating. Therefore, in severe coastal and industrial service environments, the painting of hot dip galvanizing can significantly extend service life. Even in quite benign outdoor service conditions where hot dip galvanizing might last for many decades, it can be prudent to paint areas which are sheltered from the cleansing influence of rain to extend the service life of the structure even further.
The paint systems detailed under Service Requirements 3 to 5 (see Painting Systems) are essentially paint systems intended to provide enhanced durability performance, resistance to wear and trafficking and more aggressive atmospheric service conditions. The paints range from two-pack epoxies and polyurethanes to powder coatings. Most paints have specific restrictions for how they are to be applied and cured and these restrictions need to be strictly enforced. For example, below about 8°C, latex paints will not usually dry and many two-pack coatings have a limited recoating time. Powder coatings, because they are hard cured as soon as they are stoved, can offer logistical benefits. In addition, because they can be applied electrostatically, more uniform coverage can be achieved with intricate shapes than is possible with conventional paints. However, judicious selection of pretreatment and application by competent operators is critical to performance.
For effective duplex coatings the thickness of the paint system must be increased as the environment becomes more corrosive. For painting in a high corrosivity locale where the coating thickness and integrity is difficult to guarantee, hot dip galvanizing may need to be avoided and an alternative corrosion protection system sought.
The selection of a suitable corrosion inhibitive primer is another prerequisite. Indeed, many failures of duplex coatings can be traced to either an inappropriate primer or an inadequate total paint thickness.
The paint systems detailed under Service Requirements 3 to 5 also provide options for situations where an aesthetic finish is also needed.
THE EFFECT OF MICROCLIMATES
Where the surface is shielded from the cleansing influence of rainwater, which would otherwise wash pollutants (particularly coastal and other salts) from the surface, uncoated steel and hot dip galvanizing are more susceptible to corrosion than other corrodible metals. The corrosion in these shielded locations, for example under verandahs and open roofed areas, can be typically 3 to 5 times greater than that for surfaces exposed to rain. In such cases, the appropriate paint systems detailed under Service Requirements 3 to 5 should be considered and the paint system would need to extend beyond the microclimate by at least 150 mm.
Even in quite benign locations, where maximised service life is required, the application of one coat of a two pack epoxy primer at 75µm DFT to non-rain-washed surfaces, after cleaning and degreasing the surface, can significantly extend the service life of the structure. This added coat insulates the hot dip galvanizing from the accumulated pollutants. As this coat will not be exposed directly to sunlight, a two-pack epoxy primer (refer to AS 3750.13 or APAS 2971) will be quite durable.
In a similar vein, while a hot dip galvanized structure might be essentially exposed to the atmosphere, at some points it may be in contact with the ground, buried in soil or may be exposed to intermittent or continuous ponding of rainwater. In such situations localised painting or the application of a protective tape or wrap may be needed to avoid premature corrosion in these areas. Again, the protective coating would need to extend well beyond the microclimate. AS/NZS 4680 provides some commentary on this issue and the GAA can provide further advice.
PAINTING FOR ENHANCED CHEMICAL RESISTANCE
Hip dip galvanizing is recommended to be used within the pH range of 6 to 12. Outside this range, its service life is likely to be unacceptable. This includes exposure to strong acids and alkalis as well as salts of strong acids and weak bases and vice versa. Hot dip galvanizing may also catalyse the deterioration of certain organic chemicals that are exposed to it. This phenomenon is rare and causes no damage to the hot dip galvanizing.
Just as coatings provide enhanced protection in corrosive atmospheric service environments, judiciously selected paint systems can also protect hot dip galvanizing from aggressive chemicals. Such approaches are usually only taken where the chemical exposure is low or moderate. Otherwise, the safe principle to adopt in extreme exposure situations is the use of a substrate material which is inherently inert. In such situations, stainless steels and plastic composites often find advantage.
In specific chemicals exposure, the recommendations of an expert or an established successful case history should always be sought.