Adhesion of the film on the surface is the key to coating effectiveness. Proper surface preparation enhances adhesion of the film to the surface. When coating a surface prepared by blasting, the real issue is not time: the key criterion is that the steel temperature shall always be at least 3 °C (5 °F) higher than the calculated dew point. This safety margin is sufficient for all types of coatings. Consideration of factors such as time, temperature, and humidity must be the responsibility of the inspector.
The following factors are caused by moisture:
• High atmospheric humidity enhances condensation of moisture on the surface.
• Condensation can result in flash rust that causes the coating to fail.
• Surface condensation, if painted over, may result in blistering and delamination.
• It has been estimated that 60 to 80% of all premature coating failures are caused either completely or partially by inadequate or improper surface preparation.
Common surface preparation defects include flash rust, which occurs if the surface is subjected to environmental factors before the first coat is applied, and prevents the coating from adhering properly to the substrate surface after blasting. The presence of high humidity conditions often causes the formation of rust bloom on the metal surface, resulting in blistering and delamination.
The common term “hold the blast” (also referred to as maintaining the “white metallic luster”) refers to the prevention of rust bloom formation between the blasting and coating cycles. Proper environmental controls with dehumidifiers can help in holding the blast between the blast and coat cycles.
The life of the coating cannot be guaranteed on large, cool metal surfaces, such as ship interiors or petroleum and chemical storage tanks, unless the surface is clean and dry when the coating is applied. Blasting and coating operations on metal surfaces encounter several problems when relative humidity (RH) is high or surface temperatures are low.
High humidity in the environment condenses on the colder surface in the form of droplets, causing the metal surfaces to rust immediately after being blasted. Consequently, the coating does not adhere properly, and the coating process might need to be repeated. In addition, condensation also causes blistering, blooming, and peeling problems in paint jobs.
Because the rate of condensation and corrosion in a specific environment increases significantly when the RH is 45% or above, controlling the humidity below 45% is an important factor in preventing corrosion.
Several environmental controls are recommended for ensuring a quality coating:
• Maintain the temperature of the steel surface to be coated at a minimum of 3 °C higher than the air dew point.
• Maintain a 10 °C dew point differential between the air inside and the air outside of a tank to be coated.
• Maintain the RH of the surrounding air below 40%.
Dehumidification
Dehumidification is the simplest and most cost-effective solution to ensure proper surface preparation and coating application because it removes the moisture from the coating system. Dehumidifying the air inside a tank while blasting and coating helps to achieve and maintain the level of moisture in a range that will prevent condensation and thus rust bloom. An adequately designed and applied dehumidification system during blasting and coating operations will ensure that the RH of air inside the tank is maintained at the level specified by the coating manufacturer for optimum performance of the coating to enable timely completion. Benefits of this process include the following:
• The air dew point is maintained at least 3 °C below the surface temperature to prevent condensation and flash rust, a common reason for premature coating failures.
• Condensation is prevented between coats, thereby reducing the possibility of intercoat delamination and improving curing properties.
• Build-up of hazardous and inflammable vapors is avoided inside enclosed areas being blasted.
• Ventilation air is provided for on-site personnel.
• A quality coating is ensured when adhering to the paint manufacturer’s specification.
• The life of the coating is increased by a factor of 1.5 to 2 times.
• Intercoat adhesion is appreciably improved.
• A coating can be applied at any time of the year.
• Downtime can be accurately scheduled.
• Project time and cost can be reduced by as much as 35%.
A dehumidification system offers several advantages for the contractor:
• The entire surface can be blasted before coating application begins.
• Duplication of surface cleaning is not necessary.
• Downtime is not required for bad weather.
• Adhesion problems are unlikely to arise.
• Increased productivity will reduce time and costs for the project.
Dehumidification also offers advantages to the coating manufacturer. The coating will provide better performance and longer service life, and disagreements with the contractor will be avoided. For the coating inspector, a dehumidification system helps to meet the manufacturer’s specifications for application and achieve customer satisfaction. Figures 1 and 2 show an example of an extremely complex structure in a high RH area. It is very important to control the factors previously described so that the project can be completed efficiently.