(c) Before we decide that coatings are the ultimate answer to corrosion control on underground
structures and that we need look no further, must be understood that there is a very important catch to any
statement on the use of coatings. A coating if it is to perform as intended, must be "perfect", and must "remain"
perfect.
1 Adhesion -- The coating must be tightly bonded to withstand handling, and backfilling
and prevent moisture and corrosion accumulation at the coated steel surface.
2 Electrical Resistance -- The coating must provide electrical insulation (inasmuch as
corrosion is electro-chemical) and must be suitable for use with cathodic protection for long term protection.
3 Water Resistance -- The coating must ensure that the material's initial high electrical
resistivity does not diminish. Properties of water and electrical resistivity are related in that, as a coating absorbs
water, electrical resistivity deteriorates.
4 Chemical Resistance -- The coating must withstand corrosion effects of soil chemicals
and must have resistance to oils on pipelines and underground storage tanks.
5 Soil Stress Resistance -- The coating must resist the deformation forces of alternate
wetting and drying cycles on the soil.
6 Economical Asset -- The coating must provide long-term protection.
c. On underground structures such as pipelines, it is not economically or practically feasible to apply an
insulating coating that will meet these criteria over the full length of the pipeline.
(1) Many types of coatings have been developed over the years. Vinyls and epoxies are often used
for submersion service. Coal tar enamel, extruded polyethylene, and epoxies are all used underground.
(2) The best coating material if not applied and handled properly provides a poor overall job.
(a) Properly cleaned surface -- Surface preparation is more than likely the single most important
factor in securing a good coating job.
(b) Proper priming -- Not too thick or thin (some coatings do not require a primer).
(c) Proper application of coating -- Temperature, thickness, and cure.
(d) Proper handling of coated surface -- After application, do not mishandle (drop, bang, or scar)
coated pipes.
16-7
QM5200
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