Protective Coating of Crude Oil Storage Tanks​​​​​​​

2. PROPERTIES OF PAINTS
Generally, the most obvious properties that a paint must have are shown by the sketch of Figure (2):
Figure (2): Paint basic properties requirements Often, a single paint will not have all the required properties so a number of coats, or
paint system, will be applied rather than a single coat. The first coat or primer must protect and adhere to the tank surface (The paint teeth). The paint film is built up by intermediate coats and a top coat or finishing coat provides protection against sunlight, abrasion, etc and often provides decoration.
Primers and topcoats must, of course, be compatible with one another, and coating suppliers should be consulted for such information. Due to application costs being far higher than material costs, the move is to reduce to a minimum the number of coats in a paint system with a single high-build system. Paints basically consist of solid particles, called pigments, dispersed in a liquid, known as the binder, medium, film-former or vehicle. When the paint dries, the binder binds the pigment particles

together in a coherent film that adheres to the surface.
​​​​​​​The binder provides most of the properties required to protect the substrate from the environment, and is the most important constituent of the paint. The binder generally consists of an organic polymer known as a resin dissolved in a solvent. Paints are generally categorized according to the binder type, with names such as alkyd, epoxy, polyurethane, etc. The pigment provides corrosion protection properties along with color to the film (binder). In addition to binder and pigment, most paints contain additional solvent (thinner) to thin the paint out so it can be more easily applied. Solvents completely evaporate and play no part in the dry film. Other additives include extenders, which are similar to pigments and added as a pigment to build up solid constituent or to modify paint properties.
Other additives include driers, anti-skinning agents and thixotropic agents which reduce sagging of the paint when applied to a vertical surface. Paints protect basically by providing a barrier between the substrate and theenvironment. They are electrically insulating which impedes the movement of the ions which take part in the corrosion reactions. Water and oxygen can both diffuse into paint coatings, although some are more impermeable than others and therefore provide greaterprotection.
An inert or barrier pigment such as iron oxide assists in this barrier effect. Leafing pigments, such as micaceous iron oxide (MIO) and leafing aluminum, provide an overlapping, shingle effect, providing a further barrier to the corrosive elements and strengthening the coating. Such pigments also protect the binder from UV egradation. Aluminum is also used for high temperature paints, but should not be used where chemical resistance is required. Such pigments are best used in top coats but can be added to primers. Lamellar pigments will affect the color and appearance of the film.

In addition to the barrier effect, primers usually contain pigments which can provide additional protection. Inhibitive primers contain chemical compounds which dissolve in water to some degree and form a protective passive chemical compound at the anodic sites on the metal. This reduces or prevents corrosion, but such pigments are only used in primers for atmospheric conditions, not for immersed environments where they can give rise to osmotic blistering. Red lead and zinc chromate both provide an inhibitive effect, although primers containing such compounds are no longer used due to their toxicity.
​​​​​​​These pigments have now been replaced by zinc phosphate, although this pigment probably provides protection more by barrier than inhibitive effect. Zinc-rich primers provide a barrier effect but, more importantly, have sufficient metallic pigment (90 per cent or more in the dry film) for them to be able to provide cathodic protection. The zinc powder is in sufficient concentration for there to be a conducting path between the zinc particles and the steel. Clearly, the surface must be well prepared so there can be electrical contact between the zinc and the steel. At a scratch in the coating, the zinc corrodes in preference to the steel. Additionally, zinc corrodes at a significantly lower rate than steel and the zinc corrosion products which form tend to block up the pores in the coating providing further barrier protection [3].​​​​​​​​​​​​​​