Which non-destructive testing method is used to detect holidays in coatings through electrical current?

• A. Holiday testing (electrical)
• B. Ultrasonic thickness measurement
• C. Visual inspection
• D. Infrared thermography

Answer: (A)

Holidays in a coating are defects where the protective layer doesn’t fully cover the surface, leaving a path for electrical current to reach the metal substrate. Electrical holiday testing uses a holiday detector to expose these paths. The coated surface is wetted with an electrolyte and a voltage is applied. If the coating is intact, current flow is very small; if there’s a holiday, the electrolyte reaches the metal and current flows, which the instrument detects, pinpointing the location of the holiday. This method is nondestructive and specifically designed to reveal discontinuities in the coating by electrical means.

Ultrasonic thickness measurement looks at coating thickness and substrate, not electrical leaks. Visual inspection checks surface appearance but may miss subsurface flaws. Infrared thermography detects temperature differences and heat patterns, not the direct electrical path through a holiday.