Page 700 - Corrosion Engineering Principles and Practice

654 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 655

of the coating and the manufacturer of the holiday detector. ASTM
G62-07 Standard describes two test methods for holiday detection in
pipeline coatings.
ASTM G62-0 Method A is designed to detect holidays such as pin-
holes and voids in thin-film coatings from 0.0254 to 0.254 mm in thick-
ness using ordinary tap water and an applied voltage of less than
100 V DC. This method is effective on films up to 0.508 mm thickness
if a wetting agent is used with the water and may be considered to be
a non-destructive test because of the relatively low voltage. But Meth-
od A will not detect thin spots in the coating, even those as thin as
0.635 mm. However, this can be achieved with ASTM G62-07 Method
B because of the higher applied voltages, between 900 and 20000 V
DC. Method B is considered destructive because the high voltages
involved generally destroy the coating at thin spots.

14.10 Surface Preparation

Surface preparation is a critical part of coating projects and must provide
a surface that is compatible with the coating material to be applied. The
main concerns in this aspect of protective coatings are the cleanliness of
the surface required and the surface roughness or profile that essentially
provide anchorage to the coating. It is believed that, of the cost of
a coating job, as much as one-half to two thirds goes for surface
preparation and labor.
In some cases it is impossible to provide the best prepared surface
because there is insufficient money or time to do the job and/or it is
not permitted because of possible product contamination, fire hazard,
or some other reason. It must however be recognized that when there
is a lesser surface preparation, the longevity of a coating system will
likely be compromised.
The surface properties of the substrate to which a coating is
applied will influence its performance. Surfaces in compression often
increase coating performance because they tend to be attacked less
than surfaces that are less stressed; on the other hand, surfaces in
tension may cause coatings to fail earlier than they would otherwise.
Differences in expansion rates between a coating and a substrate
impose stresses on the coating which may lead to cracking and failure.
Additionally, both inadequate and excessive coating thickness may
lead to failure.

14.10.1 Principles of Coating Adhesion
Of the cleaning methods available to provide the cleanest and largest
effective surface per unit area, the use of abrasive blasting produces
a surface closest to the ideal surface profile. However, the best of
blasted surfaces can still be badly contaminated with residual metal
oxides, extraneous dirt, particles of the abrasive materials, and
adsorbed gases on the metal. As a consequence, it is necessary to