Organic
Organic coatings are widely used for corrosion protection applications on exterior surfaces and also for interior coatings and linings. In fact, organic coatings are used more for corrosion protection than any other protection method available; they can also provide enhanced appearance of a previously dull or unattractive metal. There are several types of organic coatings, which include paints, varnishes, enamels, and lacquers, and numerous organic materials to choose from for corrosion protection applications. The types of coatings are defined below in Table 51.
Organic coatings have three basic methods of protecting a metal substrate from corrosion: 1) by preventing the attacking agents from penetrating through to the metal (impermeability), 2) by inhibiting attacking agents, and 3) by functioning as a cathodically protective material. An impermeable coating will protect the metallic substrate from having to face otherwise harmful environments that contain corrosive agents. Organic coatings containing inhibitors can neutralize the attacking corrosive agents by reacting with them and possibly forming a protective film on the metallic substrate. Cathodically protective organic coatings contain additives which decreases the corrosion potential between the metal and the surrounding corrosive environment.
An organic coating system will often have three components: 1) a primer, 2) an intermediate coat, and 3) a topcoat. The primer is very important to the integrity of the coating system. It is the fundamental layer of the system and thus provides the basic adhesion between the metal substrate and the intermediate or subsequent layer of the coating system, as well as corrosion protection. The intermediate coat provides corrosion resistance and thickness to the coating system. The top-coat is also very important since it provides the first level of protection against corrosion and acts as a seal over the intermediate coat and primer. Typically it is thinner than the priming coat, has good wear and abrasion resistance, and usually determines the appearance of the organic coating system.
Proper coating selection is clearly one of the most important aspects in protecting the metal from corrosion. There are, however, three other important factors that should be given proper consideration along with selecting the proper organic coating in order to provide the optimal service life. The first (1) is surface preparation, which is important for providing a strong bond between the coating and the substrate. The second (2) is proper selection and application of a priming coat, which should have good adherence to the substrate and should be compatible with the coating. Poor adhesion or incompatibility could lead to coating failure. The third (3) important factor is the proper selection of a topcoat; however, the topcoat is important if the surface is prepared poorly or an improper primer is selected since the coating will fail anyway.
The ingredients of organic coatings usually include volatile and non-volatile components. The volatile components serve as thinners, while the non-volatile components act as the film-forming ingredients (e.g. resin, oil, wax etc.) and sometimes include pigments and plasticizers. The pigments have several functions; they provide protection against moisture penetration, resist corrosion, protect against sunlight, and add consistency and color to the coating. Plasticizers are used to keep the coating from cracking.
Organic coatings have some advantages and disadvantages when compared to metallic coatings. For instance, they are usually more economical, can be applied on top of metallic and inorganic coatings, come in various colors and have a broad range of physical characteristics. They are, however, more susceptible to mechanical damage, and they don’t offer any anodic protection to areas of the substrate that are exposed.
Table 52 provides a summary of various organic materials used in coating systems. The table shows the advantages and disadvantages of the different resin materials, including properties and characteristics, compatibility with other materials, as well as their performance and compatibility in certain environments.
Corrosion Preventive Compounds (CPCs)
The organic coating system is typically expected to be a longer-term protection method for metals, but there are temporary protective organic materials that can provide short-term protection against corrosion. These are called corrosion preventive compounds (CPCs). CPCs are generally separated into two categories: water displacing and non-water displacing compounds. They are often used on places where the protective coating has been damaged and the metal substrate is exposed until the coating can be reapplied. CPCs can be used on both interior and exterior surfaces for corrosion protection. Although some CPCs may appear to be a permanent film, they can usually be removed with an appropriate solvent, and are not expected to be a long-term solution to corrosion.
The water displacing compounds are usually clear or translucent, soft, oily compounds, however some form hard, dry films. These can fill cracks and crevices and form a thin protective layer that is less than 1 mm thick. Non-water displacing compounds are typically thick, colored, and can be either hard or soft and are usually used for longer periods than the water displacing compounds. Generally, CPCs are applied as fluids by wiping, brushing, spraying or dipping.
Three of the most common CPCs are described in military specifications. These are MIL-C- 16173, MIL-C-81309, and MIL-C-85054. MIL-C-16173 is a soft, water-displacing CPC that is sprayed on as a brown film. MIL-C-81309 is a very thin compound which forms a soft film after drying. MIL-C-85054, which is also known as Amlguard, forms a hard, clear film after drying and is of the most commonly used CPCs due to its superior protection capability. Some of the more common CPCs are categorized in Table 53.
