Definición y características de la ISP

Painting of metals is the most common and the oldest technique used to prevent corrosion. Nowadays, there are a variety of paint coatings available in the market with different application methods to choose from. Some examples are the liquid applied coatings such as epoxies, urethanes and polyureas that are suitable for application by spray or brush [67].

Paints basically are made of solid particles, called the pigment, which are mixed with resins, solvents and plasticizers, called the vehicle. Thinners and solvents are added to control the viscosity and achieve appropriate application properties [35]. The resins are used as a binder for the pigments to make a homogeneous film. The plasticizers give flexibility, toughness, and extendibility to coatings [4]. In general, corrosion protection of steels by the application of paint coatings requires more than a single coat of paint.

Frequently, protective coatings consist of three or more coats of paint, each having different properties and functions [35]. Paint systems for atmospheric applications are normally composed of two or three coats of paint. These are categorized as priming coat, undercoat or intermediate coat and top or finishing coat [4].

The priming coat is the first element applied to the metal surface to be protected.

The primer is considered as the most important component of a coating system. It provides an effective bond to the metal surface and also to the subsequent coat. The additional function of primers is their corrosion property. Depending on their chemical composition, they may act as barriers to water and chemicals, as sacrificial anodes to protect the underlying steel when the coating is damaged or as corrosion inhibitors [4].

Some priming coats contain corrosion inhibiting pigments. These pigments form an alkaline solution at the substrate surface in the presence of water to suppress the anodic or the cathodic reaction [68]. Primers should have an adequate thickness to cover

the asperities of the surface entirely. Sometimes when equipment and structures are manufactured, the time to get to their final destination could be weeks or even months;

therefore, priming coats are usually applied at the point of manufacture to protect the equipment from corrosion [4].

The undercoats are applied essentially to increase the thickness of the coating.

They are formulated to enhance the resistance to chemicals and moisture vapor transfer of the entire system. Intermediate coats represent an important part of coating systems intended to be used in severe environments like on offshore facilities. Adhesion to the primer is an obligatory requirement to keep the integrity of the system [35].

Finishing coats represent the first line of defense against the electrolyte that may be formed under the insulation. They are fabricated with similar properties to the intermediate coat to withstand the corrosive environment that they may be subjected to.

Usually topcoats have a lower pigment to vehicle ratio and are of a different color than the undercoat in order to assess the extent of coverage [4].

Weather conditions influence the performance of coatings during their application. A temperature of 21 °C and a relative humidity within the range of 50 and 60

% are considered the proper application conditions for most coatings [9]. For site application, the working conditions can be controlled by the use of covers to reduce the flow of air and moisture into the working area. Most of the time the painting periods on offshore structures and other outdoor applications are scheduled to be during the less humid months of the year [35]. In the case of the East Coast of Canada, where there is at least one hour of fog for more than 100 days per year, the suitable conditions for painting are restricted to specific months of the year.

On offshore structures, salt droplets represent an additional difficulty for field application. They can cause adhesion problems of coatings to the metal surface and

between the coats of paint. Chloride salts promote the formation of moisture over the surface to be coated even at low relative humidities of about 40%. Because of the severe and uncontrollable marine conditions, the life and performance of coating systems may be lower than those applied on land and under better conditions [35].

There are three standard methods that are commonly used for field applications:

rolling, brushing and spraying. Additional methods are usually employed in the working area of the coating companies. The labor cost associated with any type of work is higher on offshore structures than doing the same work on land; special consideration must be taken when selecting the type of application method of coating systems. Table 5.1 shows the average covered area in a working day, based on field experiences, that an operator would be able to paint using different methods.

Table 5.1: Paint coatings application coverage rates [35]

Method Area covered/ day (m²)

Brush 100

Roller 200 -400

Air spray 400 -800

Airless spray 800 - 1200

Liquid epoxies are frequently used for coating pipes. They are composed of two different materials that are mixed just before use. They can be applied by brush or spray.

These paints polymerize by the chemical reaction within the material itself that result from the mixing of the two components. Special consideration must be given to the mixture. If epoxies are not mixed in the proper ratio, they do not cure well, resulting in a poor quality coating [46].

Phenolic epoxies are frequently used for high temperature applications up to 149°C. Urethanes and polyureas are excellent coatings for cold service temperature applications. Most of urethanes are recommended to be used at temperatures below 66°C.

Moisture can affect the quality of urethane coatings, particularly during their application.

Experiences have shown good results with moisture-cure urethanes in cases where moisture is a problem [46].

Coal tar or asphalt based materials do not require special treatment before their application. They can simply be applied by paint glove or brush. Surface preparation does not represent an important element for these materials as it is for many other coatings.

They adhere well to the metal surface and are an excellent choice for irregular shape applications. The principal drawback of this type of coatings is the recommended service temperature at which they can be used. They may become too soft and may lose their adhesion property to the metal at temperatures higher than 49°C. On the other hand, at temperatures below -18 °C, some of theses coatings may become less flexible [46].