27 July 2008

Protective Coatings and Paints -03 Corrosion Control by Coatings

Most commonly used method for protecting facilities structure is coating of surfaces. It can be used most effectively in conjunction with other control methods. These include but are not limited to proper design of components, proper selection of materials and components,
cathodic protection, controlling the environment, and use of corrosion inhibitors.

The mechanisms of coating for protecting metals from corrosion is by interfering with one or more of the conditions necessary for corrosion to occur. It can be name as follow:

a) Barrier
Most coatings provide corrosion protection by forming a barrier that is relatively impermeable to moisture and electrolytes (usually salts) necessary for corrosion. No organic coating is completely impermeable, but some are much more so than others. Obviously, the coating film must be thick enough and free of discontinuities (holidays) to achieve maximum barrier protection.

b) Inhibitors
Some pigments are used in primers for metals to inhibit the corrosion reaction at the metal/primer interface. Red lead and zinc chromate pigments, used for this purpose for many years, are seldom used today because of health and environmental concerns. Acceptable alternate corrosion control pigments are available.

c) Cathodic protection
Some coatings have a high loading of fine zinc particles to provide cathodic protection to steel surfaces. They convert anodic areas on the steel to cathodic areas. The zinc particles must be in electrical contact with each other and with the steel to provide this protection. The two basic types is organic zinc rich coatings and in-organic zinc.

"A film of inorganic zinc silicate coating is unique because the film is relatively porous. Initially, the coating protects steel by cathodic protection, but its corrosion products gradually fill the porosity, so that it becomes a barrier coating. This means, once this barrier coat become scratched to expose the steel substrate, cathodic protection will begin again until the damage is healed. An inorganic zinc silicate coating is also unique in that it reacts with the steel to form a chemical bond."

Regardless of the mechanism(s) of protection imparted by coatings, a multiple coat system is normally recommended for maximum protection of metals.
The primer is formulated to bond well to the substrate and provide a good surface for adhesion of additional coats. Zinc or inhibitive primer pigments can provide corrosion control, as described above.
Intermediate coats provide additional barrier protection and unite the primer and finish coats. The finish coat provides additional barrier protection as well as resistance to weathering, and color, texture, and gloss.

By the way, coating has not only for protecting of structure, but there is other purposes, just name it :
- Cosmetic Appearance, somehow; it can rise moral and productivy of people who has activity somewhere around, if we could choose compatible color
- Marking Paint, other than for rapid identification of building, the main purpose of marking is to provide safety for personnel and equipment. You can imagine the disaster will be happened if there is no marking line on airfield runway or street.
- Safety Colors and Designs. Safety colors and designs are used on military installations for rapid identification of hazards or safety equipment. There is a rule/regulation for safety which is base in color used (
- Reflective finishes on concrete floors of hangars and shops may increase the lighting for
workmen, particularly in sheltered areas
- Nonskid Surfaces. Nonskid surfaces are frequently made by adding grit to coatings on floors, ramps, and other walking surfaces to provide slip resistance
- Antifouling Coatings. Antifouling coatings prevent the attachment and growth of marine fouling organisms by the controlled release of toxic materials. Regarding the content of toxic material (mainly lead-base material), which is arise health, safety and environment issue, many goverment and holder of authority for corrosion control has significant work-out for limitating and subtituting this material.[yirf]

next : 4 Coating Composition
before : 2 Design Consideration

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