Page 698 - Corrosion Engineering Principles and Practice

652 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 653

Environmental test rooms: Environmental test rooms which permit
various combinations of temperature, fogging, humidity levels,
and shower effects to simulate rain can reproduce in an accelerated
mode many of the factors present in an actual exposure. For the
most part, these tests use small cut or machined specimens, but
assemblies of simulated components and complete systems can
also be tested as shown in Fig. 11.35.

14.9.5 Holiday Detection
Holidays are pinholes and voids in normally non-conductive organic
coatings that allow current to pass through the protective coating to
the metal base material. These discontinuities are usually not detect-
able with normal or even enhanced visual inspection techniques.
A holiday detector will help to find coating defects by impressing an
electrical voltage across the coating (Fig. 14.31). An electrode passes
over the entire coating surface and as it crosses a defect, it gives off an
electrical discharge, or spark, which signals the operator that a holi-
day has been detected. The operator can then mark the holiday for
subsequent repair prior to acceptance.
Holiday detectors must be selected for the thickness and type of
coating being tested. Excessive voltage can stress or damage thin coat-
ings if the detector has been set too high. Adherence to the manufac-
turer’s instructions is recommended, including both the manufacturer

(a)
FIGURE 14.31 High voltage holiday detector kit (a); low-voltage holiday
detector (b); holiday testing with high voltage detector (c). (Courtesy of
Tinker & Rasor)