Environmental Engineering Reference
In-Depth Information
directly on the metal surface. They contain inhibitive pigments or zinc, and per-
form the principal job of corrosion protection. Top coats are used on the primer
coating, mainly for the sake of appearance. Nevertheless, they also provide a
diffusion barrier and close the pores in the primary coat. Pores, which are called
''holidays'' in paint terminology, are the starting points of paint failures and they
are minimized with the application of multiple top coats. In industrial and marine
applications, three to five top coats is often recommended.
Paints, whether primers or top coats, are classified according to the type of
resin used as vehicle in the paint formulation. Among the most commonly used
synthetics are alkyds, phenolics, chlorinated rubber, vinyls, and epoxies. Alkyd
resins have found wide application in the protection of home appliances and
machinery due to their fast drying property and durability in atmospheric expo-
sures. Phenolic finishes have excellent resistance to acids, chemicals, moisture,
and cold alkalis. They are used frequently as linings for drums used to ship corro-
sive chemicals. Vinyls and chlorinated rubber have the widest range of resistance
to corrosives from strong acids to strong alkalis and have good resistance to
penetration by water. Epoxies are also resistant to alkalis and many other chemi-
cal media. Table 4.6 shows the resistance of various coatings exposed to several
environments.
Surface preparation constitutes a very important factor in the application of
all types of coatings, particularly paints. The surface should be free from mill
scales, dust, rust, grease, welding flux, and impurities. A clean and rough surface
is required for good adhesion of paint; otherwise the paint film fails prematurely.
Different methods of surface preparation include chemical cleaning, scraping and
wire brushing, abrasive cleaning, water blasting, flame cleaning, and cathodic
cleaning. Abrasive cleaning by sand blasting is considered to be the best. Often
phosphatizing is resorted to for prolonging the life of the paint coating. It is a
false economy to save on surface preparation because the cost of labor for repaint-
ing far exceeds the cost involved in a good surface preparation carried out in the
first place.
4.4 CATHODIC PROTECTION
Cathodic protection implies that the component to be protected from corrosion
is made cathode. This is accomplished by the flow of current from an external
source to the component. The rate of corrosion is brought to zero so long as the
sufficient amount of current is passed. It is a widely used corrosion prevention
practice employed to protect steel structures, which include pipelines buried in
soil or immersed in water, water tanks, ship hulls, chemical equipment, reinforc-
ing rods in concrete, and others. Steel protected by galvanizing (zinc coating) is
also an example of cathodic protection.
Cathodic protection can also be applied for the protection of other metals from
