Typically, the Jotun Paint School training will cover a range of topics including:
Corrosion is a reaction between a metal and the surrounding environment. An environment's corrosiveness depends on its composition and its temperature.
To be able to reduce the corrosion rate the electrical circle forming the "corrosion cell" must be broken.
Corrosion can be reduced or prevented by:
- Applying a suitable paint system
- Installing a cathodic protection (CP) system
- Combine the two protective measures: Paint backed up by a CP system.
The importance of pre-treatment
To be able to meet the specified design life of a paint system it is essential to carry out the pre-treatment properly. The most critical factors are:
Mechanical cleaning methods
- Grind all sharp edges, weld seams etc.
- Clean the surface thoroughly
- Make the pre-treatment in accordance with the specified standard
Use of mechanical cleaning tools such as steel brushes, grinding equipment or machining, does not achieve the same degree of cleanliness and roughness as blast-cleaning. The adhesion between the substrate and the paint system will therefore be reduced. Needle guns, for example, often cause excessive roughness or break-up of the substrate.
To ensure maximum paint adhesion, a rough surface is required. In view of this, blast cleaning is the best preparation method. It removes old paint, rust and scales and gives a clean, rough surface. Possible methods are dry blast cleaning, slurry blast cleaning (addition of water), and wet blast cleaning (water with addition of abrasives). Dry blast cleaning gives a clean, dry surface and the required roughness, but causes considerable dust which contaminates the immediate environment. Slurry- and wet blast cleaning give a rough, clean surface without dust, but create flash rust. It has been found that much of the abrasives remain on the substrate after blast cleaning and may on some alloys cause a risk of corrosion at these points. For preparation of stainless steel, aluminium and galvanised steel, it is important to use non-metallic abrasives.
Ultra high pressure water jetting
This preparation method is becoming increasingly common. It removes contamination, corrosion products and old paint by applying water to the substrate under extremely high water pressure (up to 2500 bar). The method has two essential advantages: no blasting dust is created to contaminate the immediate environment as with traditional blast cleaning, and water-soluble salts are removed from the substrate. It is important to use clean water so that the substrate is not contaminated by the water used. The method gives a clean surface, but will not give any extra roughness to the steel. The original roughness of the steel is retained where intact paint is removed, but the corrosion pattern on corroded areas will be considered as roughness where corrosion has occurred. One disadvantage with water-cleaning is a tendency to form flash rust as moisture is added to the substrate. The degree of flash rust depends on the relative humidity, the temperature of the steel and atmosphere, and the cleanliness of the surface.
Key factors for correct application
- The correct paint system to be used for a given application will be determined by the surface treatment, application condition and exposure/ service conditions
- Two-pack paints will only work if they are mixed thoroughly by a mechanical agitator.
- Stripe coating of edges, welding seams, etc. will extend the lifetime of the paint system significantly.
- Application and curing of paints will be greatly affected by the environmental conditions. Check that the steel temperature is minimum 3oC above the dew point.
Generic types of paint
Paints can be divided into three groups according to their drying or curing mechanism. These main categories can be divided into different subgroups based on the chemical composition. Paints within one generic group have many identical properties. Different chemical modifications give the paints specific properties.
- Physically drying paints. The drying process consists exclusively of the evaporation of solvents.
- Oxidative drying paints. The drying process combines evaporation of solvents with a chemical reaction between the oil in the paint and the oxygen in the air.
- Chemically curing paints. The drying process involves a chemical reaction between the base and a hardener. Before use the two components must be mixed properly.
Safety and health
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- Surface treatment may involve several safety and health hazards
- Seek necessary information from the material safety data sheet
- Use adequate personal protection equipment to protect yourself during the work
- Stay fully informed about activities in the vicinity of the work place