Page 627 - Corrosion Engineering Principles and Practice

590 C h a p t e r 1 4 P r o t e c t i v e C o a t i n g s 591

5. A great variety of factors will determine the success or failure
of an expensive coating job. These include such things as skill
of the workmen, proper inspection of the job in progress and
at its end, and provision of equipment to apply the coatings
properly and promptly, among numerous others.
Continuous testing and evaluation of candidate systems and
examination of systems actually in service more than pays its way in
providing a basis for proper selection. The information thus obtained
during the analysis of coating failures, either provoked during
accelerated testing or more observed in service can be highly precious.
Figure 14.2 provides a distribution by failure type of the 84 coating
failures mentioned earlier [3]. The following list summarizes many of
the factors that may have caused these failures and Table 14.1 briefly
describes common coating failures with their causes and remedies:
1. Water absorption: All organic coatings absorb water and those
used in corrosion protection are usually in the range of 0.1 to
3 percent by weight.
2. Moisture vapor transfer: This is the action of water vapor passing
in molecular form through an organic substance. Generally,
the lower the moisture vapor transfer rate, the better the
protection that will be provided by that organic coating.
3. Osmosis: Osmosis refers to the passage of water through a
semipermeable membrane, an organic coating in the present
context, from a solution of lower concentration to one of
greater concentration.

Blisters Adhesion

Physical damage

Cohesive

Loss of gloss
Delamination/
disbondment
Solvent entrapment
Pinholes
FIGURE 14.2 Breakdown of the root causes behind 84 coating failures
thoroughly investigated.