Steelwork: Paint to Protect
The biggest concern with steelwork is corrosion. Besides enhancing comesmetics , which is of secondary importance , paint add an extra level of protection with the steel from condensation . It is the priming coat that protects the steel while the exterior coat, alternatively , shields the priming coat against wear and tear .
Protecting steel from corrosion would be considered at the design stage itself . The designer must ensure that all junctions and steel members exposed are easy to access for maintenance reasons and that the structure does not allow water to stagnate or dust to accumulate at any point.
When to re-paint:
Paint failure usually starts at the sharp angles and crowns of a structure as the paint tends to recede , like in the case of corrugated surfaces. The surfaces should be repainted as soon as the paint shows signs of failure like chalking, cracking, blistering or rusting above 0.5 percent of the surface area . Brand new structures must be painted within a week of its erection , as rust is known to work very quickly and can take in a matter of days in certain tropical climates and it is most likely that the paint film will encapsulate these pockets of rust leading to early deterioartioin .
For the best results, paint would be added when the weather is dry , the temperature reasonably warm minimum of 10 degrees Celsius and the humidity is below 85%.
It should be certain that the exterior isprepared correctly , the right primer and paint obtained and the work executed in suitable conditions.
To be certain of paint’s effectiveness , wipe and clean the surface extensively to remove water, dust, and rsut grease ; in water-rich environments, wash steel to remove all air borne salts. Make time to allow the surface to dry (completely dry) before adding any paint. Allow only for a minimum intervening period between preparing the surface and applying the paint , as even a week’s interval could reduce paint life by one fourth . Welds must be continuous, without crevices or gaps , and should be ground smooth or blast cleaned . No air-pockets should be allowed to remain.
Paint may be added either by spray or with a brush. Sprayed paint film is always a little thicker and its adiabatic expansion sometimes entraps moisture. This must be avoided. Paint like depends on the thickness of paint film. Ideally, it should be about 125 microns, with an additional 25 microns where the conditions are severe with the design, or location of the structure.
Typically , two coats of primer, one undercoat and one final coat, are essential for adequate protection against corrosion.
Choosing the primer isof utmost importance , for it is the priming coat which protects the structure . This selection will depend on the method of surface preparation possible on site. To add effective protection, the primer should be a high quality insulator against electro-chemical corrosion; be durable; should dry at a reasonable rate, that is neither too fast nor too slow; must adhere to steel and have an affinity for the finishing coat.
Typically, the most used primers are either lead based or zinc based. Red lead is an excellent primer for poorly prepared surfaces. Metallic lead, a powdered form of lead, is tolerant of poor surface finish and has good corrosion resistance. The primer should contain a minimum of 25 percent metallic lead pigment, and at least 40 percent if very good resistance is needed. Where the use of lead would be dangerous, zinc chromate primers are used. It should have not less than 15 percent chromate, and at least 40 percent for better protective qualities.
As zinc film is very thin, zinc chromate does not adhere to rough surfaces. These primers are less resistant to corrosion than lead-based primers and require a perfectly smooth surface for application.
Clean the surface thoroughly by shot blasting or phosphate treatment. Apply two coats of red lead primer followed by 2 coats of aluminium paint. Spray aluminium metal of 76 to 127 micron thickness. Apply zinc chromate primer and follow with durable top coat. All thin sections must be heavily galvanized or painted with a red lead primer and finished with a top coat suitable to the environment.
The paint system adopted for structures made of steel will depend on whether the environment is mild, moderate or severe . Inland areas with low rainfall; interiors of domestic buildings, offices and schools and factory interiors which are free from the problems of condensation or chemical pollution can be considered as areas with a more mild climate .
Moderate environment areas are those with a lot of rainfall or continuously high humidity; urban or industrial belt ; sites near fresh water or calm sea water; interiors where condensation is heavy and areas affected by pollution from chemicals like sulphur dioxide, sulphur, ammonia or weak acids. Severe environment areas are those affected by salt laden air; sites located close to chimneys and industrial processes and areas where the general level of industrial pollution is very high.
Measuring the thickness of paint:
Paint film thickness can be measured with an electrometer. The easiest method is to place a magnet in contact with the film coated surface of steel and gradually draw it away till detachment takes place . This can then be measured by a calibrated spring. The force of attraction with the magnet and steel serves as measures of the intermediate layer of paint film. This instrument is less than precise means to check the thickness of paint on the horizontal surface. This thickness can also be estimated by the consumption of paint per square meter.
A good paint job, one in which the aspects of surface preparation, paint selection and painting procedure are carefully attended to, ensures a longer life of the paint and much more protection to the steelwork.