CORROSION CONTROL BY COATINGS AND PAINTING

INTRODUCTION.

This chapter presents information on the effective use of organic coatings to protect metal, particularly steel, shore facilities from corrosion. This chapter provides a general understanding of the principles involved in coating operations, so that guide specifications for actual coating of facilities can be used most effectively.

Scope.

This chapter covers coating of metal surfaces encountered at Naval shore activities, including their selection, application (and surface preparation), and maintenance in a state of acceptable condition. It does not cover painting of other substrate surfaces or painting of ships or aircraft. The coatings covered are limited to organic paint materials, with the exception of inorganic zinc coatings which are included, because they are most commonly used in conjunction with organic coatings and are applied by spray, like organic coatings. Powder coatings are also described, although their application is somewhat different from typical brush, roller, or spray application.

Coatings as Part of Corrosion Control Program.

While coating metal surfaces is the most commonly used method of corrosion control, it cannot be used effectively without interacting with other methods of corrosion control. These include, but are not limited to, designing proper components; selecting proper materials, components, and cathodic protection; and using corrosion inhibitors.

METHODS BY WHICH COATINGS PROTECT METALS FROM CORROSION.

Coatings have three basic mechanisms for protecting metals from corrosion, although more than one of these mechanisms may be used by a coating. These mechanisms are:

• barrier protection
• cathodic protection
• corrosion inhibitors.

Barrier Protection. Most coatings provide corrosion protection by forming a barrier relatively impermeable to moisture and electrolytes necessary for corrosion. Obviously, for optimum protection, the barrier should be as impermeable, thick, and continuous as practical.

Cathodic Protection of Steel. Some protective coatings have a high loading of fine zinc particles, so the particles in the cured film are in electrical contact with each other and with the underlying steel. This permits a type of cathodic protection. Presently, two basic types of zinc-rich coatings are used on steel: organic and inorganic products.

Inhibitive Pigments. Some pigments are added to primers to inhibit corrosion at the coating/metal interface. Red lead is the most common example of an inhibitive pigment.

COMPOSITIONS AND PROPERTIES OF COATINGS.

Components of Coatings and Their Functions. All ingredients used to formulate a coating can be placed in one of three basic categories: solvent, resin, and pigment. Each of these categories has a special function in the coating’s formulation.

Solvent. The solvent is used to dissolve the resin material that actually forms the coating film. It also reduces the viscosity of the product to permit easier application, as well as affecting its leveling, drying, durability, and adhesion. Because the different organic polymers in different resins greatly differ in their solubilities, some resins require much stronger than others to dissolve them. In water-based coatings, the water is a dispersing rather than a dissolving agent.

The emission of virtually all organic solvents in coatings gives rise to photochemical smog. Thus, there is a great pressure to reduce the amount of solvent in coatings or to use water-based coatings.

Resin. The resin is the binder or film-forming part of the coating that is responsible for most of the properties of the coating. Thus, coatings are identified by the generic types of their resins. The resin and the solvent portions of coatings are sometimes called the nonvolatile and the volatile vehicle, respectively, and are sometimes referred to collectively as the vehicle.

Pigment. The pigment constitutes the solid portion of a coating. It is generally heavier than the liquid vehicle portion and may settle out on prolonged standing. Pigments are usually modified or unmodified natural earth materials, although less stable organic pigments are occasionally used. The chief function of the pigment is to provide opacity (hiding) to protect the organic vehicle from degradation by sunlight. Titanium is the pigment most frequently used to give opacity to white paints and light tints. Pigments also provide color, improve adhesion and weather resistance, decrease moisture permeability, and control gloss. Leafing pigments, such as aluminum, tend to form parallel plates in the film to effectively increase its thickness by increasing the path moisture must penetrate. Other things being equal, the finer the pigment particle size and the less the pigment/resin ratio, the glossier will be the coating.

The pigment and the resin portions are sometimes called the solids portion, since they remain after all the solvent has evaporated. Obviously, the greater the coating solids, the greater will be the dry film thickness received from a given wet film thickness. There are also many additives or extenders added to coatings to modify gloss or consistency, emulsify components, improve weathering, or obtain some other desirable  property. They are considered to be in the resin or pigment part depending whether they exist in the coating as a liquid or solid.