Page 304 - Challenges in Corrosion Costs Causes Consequences and Control(2015)

282 CORROSION CONTROL AND PREVENTION

of the ship should be such that all surfaces of the tank interior can be accessible and
that coating and surface inspections can be performed. Crevices that can collect dirt
and form corrosion cells should be avoided during the design phase.
Another aspect of design that can influence corrosion prevention is the design of
welds. Proper sizing of the welds and planning the sequence of the welds can reduce
stress concentrations and distortions of the hull. Past experience has shown that lap
joints have been prone to failure on older ships; therefore, butt-welded joints should
be used whenever possible. Designs should also avoid intermittent spot welding as
this form of welding is more prone to corrosion.
The most important element of corrosion protection is the proper coating selection.
The coating should be selected during the design phase on the basis of: (i) the function
of the ship; (ii) type of tanks used, and (iii) the expected life of the ship. Because of the
high cost of coating application, care should be taken in choosing the proper coating.
Possible coating choices are:

Epoxies
∘ Coal tar epoxy; silicone-modified epoxy; electrodeposition epoxy
∘ High solids epoxy over a waterborne epoxy zinc primer
∘ Pure amine epoxy; epoxy amides; epoxy amino/amides
∘ Hydrocarbon (wax)-modified epoxy amides and epoxy amines
∘ High solids (low molecular weight epoxy resins) epoxy
Thermoplastics
∘ Thermal-spray thermoplastics
∘ 100% solids rust-preventive wax
Others
∘ Coal tar polyurethane
∘ Polyurethane (aliphatic polyol) top coats
∘ Zinc silicates
∘ Alkyd paints
∘ Calcium sulfate alkyd

Solvent-free epoxies are much more expensive than coal-tar epoxies or the
solvent-borne epoxies previously used in ship construction. Most of the cost in
coating a ship is in the cost of grit blasting the steel followed by applying coating.
Coal-tar epoxies of solvent-borne epoxies cost $1.80 and $2.80 per square meter
while solvent-free epoxy cost $6.60 per square meter. For the amount of coating
needed to coat a ship, it will cost $150,000 more than coal-tar epoxy and $120,000
over solvent-borne epoxy. The cost of ship is around $70–80 million, and the use of
solvent-free epoxy is not significantly expensive.
The application of solvent-free epoxy has a longer life than coal-tar epoxy, and
coal-tar epoxy has to be applied two to three times compared to only one time in the
case of solvent-free epoxy coating. To perform recoating, the tanks have to be cleaned