Corrosion protection paints and coatings are corrosion-resistant coating are usually applied on metal parts for the purpose of preventing corrosion. This type of coating can be made from substances such as zinc or iron and can be applied as a base layer, primer or topcoat through various techniques. It is commonly used to give corrosion protection to structures like pipelines and concrete bars and any metallic surface that demands some form of protection from adverse environmental conditions such as sea-going vessels. This article is to describe the best application process for corrosion protection paints and coatings.
What gives credence to successful application and hence optimal performance of coatings are
Processes for paint application
• Application surface preparation
• Film thickness of the paint and coating system
• Methods of application
• Conditions during application
Application Surface preparation
Preparing a metallic surface for protection paints application depends on the surface itself, the first step is to access the surface condition and determine the best method to clean it, it could be by removing old coatings, dirts and oil, chlorides and other surface contaminants with solvents such as epochem 502 heavy duty degreasers and epochem solvent cleaners other methods though expensive includes abrasive blast cleaning, wet abrasive blast cleaning, hand and power tool cleaning, flame cleaning and acid prickling. Of all the surface cleaning methods abrasive blasting is the fastest and cost-effective methods of surface preparation used for large areas of steel is best applied for removing mill scale, rust, old paint and similar contaminants. For coatings requiring only minimal surface preparation, the surface needs to be prepared in accordance to SSPC-SP 1 and SSPC-SP 2 or SSPC-SP 3. Tightly adhered rust, mill scale or previous coating can remain provided it cannot be removed by lifting with a dull putty knife using moderate pressure. Use SSPC-SP 7 (NACE 4) to expedite the preparation of large areas.
Film thickness of the paint and coating system
An adequate film thickness is crucial for an anti-corrosion coating system to perform optimally and provide good anti-corrosion protection lifetime as designed by the producers of the paint, etc. A lower thickness will result in premature failure of the coating and over-application can cause problems like solvent entrapment, loss of adhesion, cracking of the coating (including mud-cracking) and splitting of primer coats that is applied after surface preparation.
It is essential that application procedure should follow the anti-corrosion coating thickness specified by the manufacturers, allowing for practical application variations. The wet film thickness or (WFT) of the anti-corrosion coating is measured and can be converted to a dry film thickness (DFT) following manufacturer’s guidelines. The wet film thickness measurement can serve as an aid in determining how much anti-corrosion coating should be applied to reach the specified DFT. The dry film/wet film ratio is based on percentage of solids by volume of the anti-corrosion coating being used. Coating manufacturer’s data sheets sometimes list solids by weight as well as solids by volume, Epocoat paints usually indicated the solid materials constituents of all their paints.
The basic formula using solids by volume
WFT = DFT (% solids by Volume)
The actual DFT specified will depend upon the type of paint being applied and the nature of the substrate surface. To determine whether an anti-corrosion coating thickness is acceptable, there are guidelines produced by the coatings manufacturers, such as the 80-20 and 90-10 rules. For example, the 90-10 rule means: no measurement may be below 90% of the DFT specified without repair being undertaken and not more than 10% of the measurements may be in the range 90-100% of the DFT specified without repair being undertaken. The figures will depend upon the type of anti-corrosion coating being applied and the area of the vessel being coated. Measurements of the dry film thickness are influenced by the profile of the substrate, particularly when abrasive blasting has been used. Thin films (of less than 25_m) cannot be measured accurately over blasted surfaces using commercially available paint thickness gauges. Measurements of DFT at edges and corners are not accurate due to the measurement techniques used by these gauges. Coating companies can advise on suitable methods for these circumstances.
Method of paint application
The normal methods of application of anti-corrosion are by using hand brush, roller brush, conventional air spray or airless spray machines.
Use of brushes is a very slow method and demands a lot of human efforts and man-hours , this method is used to coat very sensitive locations especially where roller brush and paint sprayers cannot get to. it is essential to not overcoat or undercoat surfaces. painting with brush is usually essential when painting the surface tolerant primers where there is possibilities of rusting. Roller brushes are faster than hand brushes. we will briefly discuss the use of conventional air spray and airless sprayers
The conventional sprayer methods involves using either air assited sprayers or airless sprayers, for air assited conventional sprayers, they are used for applying single component zinc silicates to large surfaces. The equipment is relatively simple and inexpensive and is usually confined to fairly low-viscosity coating. Coating under pressure and air are fed separately to the spray gun and mixed at the nozzle. The coating is atomized and air is mixed with these droplets forming a fine mist of paint which is carried by the air pressure to the work surface. It is essential to use the correct combination of air volume, air pressure and fluid flow to provide good atomization and a coating film which is free from defects. Poor control gives rise to overspray and rebound from the work surface defects in the coating.
Airless spraying method
Airless sprayers work by pumping high-pressure paints or coatings through a hose to the nozzle of the spray gun. As the paint is forced through the tip of the nozzle, it gets broken up (atomized) into fine droplets that form the spray pattern. Airless sprayers are so called because the paint is pumped through the system under high pressure as opposed to using compressed air.
Although airless sprayers offer many advantages over traditional compressed air methods, they are known for having low transfer efficiency, i.e., the ratio of the amount of paint that makes it to the surface versus the amount that gets lost or dispersed in the atmosphere. For example, a transfer efficiency of 80% means that 80% of the paint sprayed was successful in reaching the surface while the remaining 20% was wasted.
The transfer efficiency of airless spraying can be problematic when it comes to anti-corrosive coatings with a high solvent content. While solvents are useful for lowering the viscosity of the coating, they frequently contain high amounts of volatile organic compounds (VOCs) and hazardous air pollutants (HAPs). These compounds are toxic and can cause several health problems if inhaled or ingested. Airless spraying methods can significantly increase the rate of emission of VOCs and HAPs due to the dispersion of the wasted paint into the atmosphere.
In an attempt to reduce the number of airborne contaminants released by airless spraying, many industries have turned to coatings with less solvent content. These coatings are known as high solids coatings and naturally have a much lower VOC and HAP content.