Painting of hot-dip galvanized steel - Duplex systems
Many analyses have shown that a duplex system, i.e. hot-dip galvanizing plus painting (fig.1), is a most cost effective way to protect steel, particularly with regard to its service life up to the first re-painting. Although hot-dip galvanizing alone usually gives fully satisfactory protection against corrosion, additional protection is sometimes desirable in aggressive environments. It can, for example, be advantageous if future maintenance is likely to be difficult to carry out, or if the zinc coating is thin, as on continuous galvanized thin sheet steel.
Aesthetic reasons — giving the matt grey zinc surface a more pleasing appearance — can also play a role. Similarly, paint can be used to give prominence to or warn of the existence of a structure, or even the opposite — to camouflage the structure. Insulation against galvanic corrosion if the galvanized steel is to be connected to another metal, such as copper, can be another reason for painting.
Service life of the duplex system
A duplex system generally has a much longer service life than each individual coating. Dutch experiments show that the service life of the system can be calculated according to the following formula:
LT = K(LZn+Lp)
= the service life of the duplex system in years
Lp = the estimated service life of the paint coating in years (in the actual environment), if it is applied directly to steel
K= environment-dependent factor between1,5 and 2,3:
1,5 when the system is exposed in environmental class C5 (see fig 2-2) or in sea-water.
1,6 – 2,0 when the system is exposed in environmental class C3 - C4 or when the wet period for the system is lower then 60 %.
2,1- 2,3 when exposed in environmental class C2.
The synergistic effect is explained in picture 2. Of course, the galvanized surface must have been correctly pretreated to ensure lasting paint adhesion. To obtain this, the zinc surface must be cleaned thoroughly and the correct type of paint must be used. The painting of zinc-coated surfaces requires somewhat more care than some other materials. Small quantities of impurities on the zinc surface, or the incorrect paint type, can lead to premature blistering and/or flaking.
It was often recommended in the past that the zinc surface should be exposed outdoors for 1-2 years before painting. The recommendation was correct before 1950. At that time the atmosphere, at least in Scandinavian countries, was relatively clean, which meant that the corrosion products that formed on a zinc surface consisted almost entirely of basic zinc carbonates.
Therefore, after a period of exposure, paint was not applied to a reactive zinc surface but to an inert layer of carbonates. The result was generally good, even with paints which would today be regarded as unsuitable. Today this method is suspect since the atmosphere contains sulphur oxides pollution and the corrosion products of zinc often contain water-soluble zinc salts. Paint applied to such a layer, which is to some extent water-soluble, will probably blister or flake, regardless of the paint system.
Fig. 2. To the left: Schematic drawing showing how micro-cracks in paint coatings on steel give rise to underfilm rusting and flaking. To the right: Micro-cracks in paint coatings on galvanized materials are filled with corrosion products from the zinc, but these have a smaller volume than rust from steel and do not cause flaking.
A shiny zinc surface is often considered to be clean enough to paint. In many cases this is not true, and this leads to poor results.
A freshly galvanized surface is a good substrate for painting only if:
• The article has not been water quenched. Quench water is seldom clean. Different salts that may be deposited on the zinc surface can later decrease or totally destroy the adherence of the paint coating.
• The article has not been stored inside the galvanizing plant. The air in the plant contains flux dusts (particles of zinc and ammonium chloride). These particles adhere to the surface and create a water soluble film which reduces the adherence.
• The article has not been stored or transported outdoors in a damp atmosphere. The risk of condensation, which leads to wet storage strain, is significant. It is not always visible to the naked eye, but will cause problems after painting.
• The article has not been stored more than about six hours between galvanizing and painting. The precise critical time is, of course, dependent of the cleanliness and dryness of the surrounding air.
A “fresh” zinc surface is seldom as clean as one can be led to believe by the lustre. Thin films of oil and grease from gloves, shoes and hoisting slings can increase the impression of a shiny and clean surface. Contamination exists in thin films which are transparent and very difficult to spot with the naked eye.
Hot-dip galvanized thin sheet is often chromated or oiled for protection against wet-storage stain. The chromate layer is somewhat water soluble, and painting over a film of oil seldom meets with success. Layers such as these must therefore be removed before painting. Chromating is, for environmental reasons, nowadays replaced by other methods of passivation.
Exposed, matt surfaces
Exposed matt zinc surfaces are always covered with corrosion products. Their composition is difficult to determine. As a rule-of-thumb, it should be assumed that they contain water-soluble or hygroscopic salts which make them unsuitable substrates for painting. It follows that some form of cleaning is always necessary before matt surfaces are painted. Paint coatings are always more or less permeable to water. This means that if the water-soluble products are not removed from the zinc surface, blisters full of solution will form beneath the paint coating. Access to protective film forming substances is poor in the salt solution and results in corrosion of the zinc layer. Corrosion spreads between the paint and zinc and the corrosion products dislodge the paint coating.
Cleaning and surface preparation
All experience shows that sweep-blasting gives the best foundation for good adhesion between paint and zinc. Its moderate mechanical effect removes all corrosion products and other impurities from the surface, even the water soluble products. Agents that protect against wet-storage stain are removed effectively, as are different types of oil. Many zinc surfaces are very flat and shiny. For these, blasting is beneficial since it creates a key on the surface which enables better mechanical anchorage of the paint coating.
Recommended data for sweep-blasting of galvanized surfaces is given in fig. 3. It is important that the instructions given in the table are followed. Inadequate cleaning may impair paint adhesion while excessive mechanical working can destroy the zinc layer or cause built-in stresses, which could subsequently cause the paint coating to flake.
Fig. 3. Blasting data for sweep-blasting of zinc coatings.
When sweep-blasting is performed correctly about 10 µm of the zinc layer is removed. If sweep-blasting cannot be used industrial lacquering according to the following procedure is recommended:
Chromated goods require alkaline degreasing and sometimes brushing, or treatment with nylon wool impregnated with a grinding agent of aluminium oxide. The phosphate layer should be as thin as possible, but should give continuous full cover of 2 - 4 g/m2. A thickness under 1 µm is usually considered to be optimal.
In the case of manual painting, degreasing with an emulsifying degreasing agent is recommended, preferably combined with brushing or lapping with nylon wool and thorough rinsing with water - high pressure if possible.
Some commercial degreasing agents contain oxide-dissolving additives. This can be an advantage when the composition of the corrosion products is not known.
Choice of paint
The types of paint recommended below can be applied directly to hot-dip galvanized steel after it has been cleaned. If other paints are to be used a suitable primer, preferably an epoxy primer, must first be applied to the zinc surface.
When choosing paint, it should be remembered that a paint can contain up to 10 - 15 different components. Each manufacturer has his own formula for a given type. Suppliers of raw materials also have different formulae for binding agents, which means that the number of variations can be very great. Paints of the same type, but from different manufacturers, can therefore have different properties. The guidance below should therefore be regarded as general guidelines only. If in doubt, discuss your selection of paint with the manufacturer’s representatives.
Epoxy, urethane or chlorinated rubber paints are preferred for high-pressure spraying. Epoxy paint should be of the 2-component, polyamide type and the content of non-volatile matter should not be too high. The recommended pigment volume concentration is 28 - 33%.
Epoxy primer is the norm when other types of paint, such as urethanes and epoxy tars are used for the final coating. However, the general rules are always to check with the paint manufacturer that the primer and the covering paint are compatible with each other. Also, never mix primers and covering paints from different manufacturers. A difficulty with epoxy paints may be their tendency to chalk.
Chlorinated rubber and two-component urethane are more sensitive than epoxy in respect of raw materials and paint compositions when applied to zinc. For chlorinated rubber, a pigment volume concentration of 26 - 30% is preferred, ideally with zinc phosphate as an inhibiting pigment and aluminium and/or micaceous iron oxide as a barrier-builder. Urethane paints should have a pigment volume concentration of 30 - 35%, and preferably be pigmented with aluminium/micaceous iron oxide.
For manual painting, two-component, acrylic-modified urethane and acrylic-modified alkyl resin are suitable. Even latex paints based on acrylate resin or polyvinylacetate will serve, although they do not develop sufficient hardness or adhesion properties for 10 - 14 days. Certain types of paint based on modified rubber, or on asphalt-modified rubber, appear promising and easy to handle.
Aluminium pigmented asphalt solutions can be used on structures in water, but they have poor mechanical strength.
In general, painting with these paints/systems should not be carried out at temperatures below 8 - 10 °C. The paints mentioned are all of the air drying type and forced drying should be used with great care. Try to obtain a general coat thickness of 75 µm - locally never below 50 µm.
For stove-drying paints, powders and laminate coatings the supplier should always be consulted.