This section is a guide to anti corrosion coatings for long term protection of steel structures. Many aspects have been omitted in order to keep it brief.
Specifiers and Contractors should always refer to the relevant Standards for more detail or contact us for our assistance.
Recent surveys showed 80% of coating failures were due to incorrect or inappropriate specification with the balance due to incorrect application.
This section provides guidance on the use of different types of metallic coating systems for the protection of steel structures against atmospheric corrosion.
Firstly, determine the coatings durability or "lifetime to first major maintenance".
The following five ranges are commonly used:
Secondly, determine the environment at which the structure is to be located and the atmospheric corrosivity at its specific location. Atmospheric environment examples are:
Guidance on years of service to first major maintenance to be expected for metal sprayed coatings for different environmental conditions are provided in the table below:
S = Sealed.
* = Would have very high durability but would be uneconomical.
NR = Not Recommended.
Coating Material Selection.
TSZ = Thermal Sprayed Zinc. Recommended for pH 5 - 12. Weight of wire required 1.2kg/m2/100µm.
* Excellent protection against atmospheric corrosion.
* High so called “cathodic (long distance protective) reaction".
* Life cycle time proportional to coating thickness.
* Inadequate in the case of higher chloride / salt content in air as for coastal regions.
* Corrosion behavior similar to that of hot dip galvanizing.
* Suitable for dry heat up to 250 degrees C.
TSA = Thermal Sprayed Aluminium. High temperature, seawater immersion and marine splash zones. Recommended for pH 4 - 9. Weight of wire required 0.35kg/m2/100µm.
* Especially sufficient for SO2 containing industrial atmosphere.
* Sufficient for marine atmosphere and in seawater.
* High heat resistance.
* Highly resistant against acidic contamination.
* Low cathodic long distance protective reaction in the atmosphere.
* Cathodic long distance protective reaction high in a strong electrolyte only, e. g. sea water.
* Suitable up to 500 degrees C.
TSZA: 85/15 alloy (85% zinc, 15% aluminium) provides twice the protection of zinc. Weight required 1.0kg/m2/100µm.
* High corrosion resistance also in low acid medium.
* Higher corrosion resistance against atmospheric corrosion than zinc and aluminium.
* Higher corrosion resistance against chlorides and especially SO2 atmospheres than zinc.
* Inadequate for applying in salt water without organic coating.
* Cathodic long distance reaction, however less than for pure zinc.
For new steel, all surface imperfections such as sharp edges, weld splatter, weld folding and slag shall have been removed from surfaces. All edges shall have a radius of curvature of no less than 2.0mm.
Grit blasting is the only method of preparing surfaces for metal spraying. It removes rust, mill scale and produces a suitably roughened surface profile. Clean, dry, compressed air shall be used for blasting. Moisture separators, oil separators, traps or other equipment may be necessary to achieve this requirement.
A high standard of surface preparation is required to develope the optimum thermo-mechanical bond required for metal sprayed coatings. Steel surfaces to be arc sprayed with zinc or aluminium or to be flame sprayed with zinc should be abrasive blast cleaned to at least Class 2.5 to produce a sharp angular profile of at least 50µm. Steel surfaces to be flame sprayed with aluminium or aluminium alloys should be abrasive blast cleaned to Class 3 to produce a sharp angular profile of at least 75µm. Profile depth should be measured using a surface profile gauge or replica tape. Coating thicknesses in excess of 300µm may require an increase in surface profile.
Metal sprayed coatings should not be applied if any discolouration of the cleaned surface has occured, or when the temperature of the substrate is below the dew point. Any visible rust that forms on the surface of the steel after blast cleaning shall be removed by re-blasting the rusted areas to meet the requirements of the specification before metal spraying.
The first metal spray coat should be applied within 4 hours after the last cleaning and before any discolouration of the prepared surface occurs. Spray angle should be near to right angles to the substrate as possible, with a spray range or stand off between 100 - 200mm. The sprayed metal coating should overlap on each pass. Two or more coats of this layer should be sprayed alternatively in vertical and horizontal passes to ensure uniform coverage to the minimum specified coating thickness.
Due to the thermal spray process, the coating typically has around 10% porosity. The purpose of sealing is to fill this porosity and delay oxidisation of the coating . Sealing extends the life of the coating as well as providing an even finish appearance. Ensure the coating is free of moisture before application and all solvents have evaporated before subsequent coats to avoid solvent entrapment.
Metal sprayed anti corrosion coatings are normally put into service without any finishing, however paint systems can be applied to suit particular environmental conditions or if aesthetics require. This is a Duplex Coating.
When steel protected only by paint is damaged or weathered, it can corrode in the unprotected area and additionally proceed to corrode the steel under the paint, a phenomenon called “scribe creep”. The corrosion products of iron are many times higher than zinc, resulting in progressive failure or paint peeling away from the damaged or weathered site. By contrast, when the steel is protected by TSZ that is then painted, any damage to the paint, even if it goes through to the substrate, does not result in significant steel attack. The TSZ coating galvanically protects the steel, minimizing scribe creep failure of the paint and also greatly reducing the severity of steel corrosion.
150 - 200 micron TSZ alone can provide 25+ years to first maintenance in most environments. Should the steelwork require a paint system for aesthetic reasons, it is not necessary to retain the same 150-200 micron of the TSZ as it is there to galvanically protect the steel with the paint acting as a barrier. An example duplex coating specified: 100 micron TSZ, 50 micron epoxy sealer, 100 micron epoxy build coat, and 50 to 100 micron polyurethane.
TSZ applied by arc spray equipment achieving high production rates, poor machine settings, rough finishes or higher coating thicknesses may produce a coarse surface profile of the finished coating. This may be an issue in some cases when TSZ is to be part of a duplex system. A specification of a low build paint system or areas of low paint build during application may cause ‘peaks’ in the TSZ to activate, causing white zinc oxide and blistering the barrier coating. AS/NZS 2312 recommends a profile height of less than 50 microns for TSZ before paint system is applied.
TSZ coatings have a porosity of approx 10%, depending on the specification or desired finish a low viscosity sealer with good wetting properties such as vinyl, acrylic, or thinned epoxy are used. Higher volume solids are preferable to reduce the number of sealing coats required. Be sure all solvents have evaporated before subsequent coats to avoid solvent entrapment. Always discuss with your paint representative for the correct sealers/topcoats, application, drying times and coating thicknesses for paint application over TSZ coatings.
Sealing or painting of thermal sprayed Aluminium is not recommended for submerged or splash zone applications.
Application examples HERE.