Holiday Detection on a Buried Pipeline Coating
A contractor has applied a fusion-bonded epoxy (FBE) coating to a 12-inch (305 mm) buried steel pipeline. The specification requires a holiday-free coating verified by high-voltage holiday detection at a test voltage appropriate to the coating thickness. The coating is nominally 14 mils (350 µm) thick. During inspection, the technician uses a DC holiday detector set to 1,000 volts and reports the coating 'holiday-free.' The owner's inspector questions the voltage. Separately, the technician notes several small pinholes near a weld that the detector did not flag. The contractor wants to accept the coating and backfill.
1. Assess whether 1,000 V is an appropriate test voltage for this coating and explain the risk of both too-low and too-high voltage.
Read the worked answer
For a 14 mil (350 µm) FBE coating, 1,000 V is generally too low to reliably detect thin spots and pinholes. A common field rule of thumb for FBE is roughly 1,000–1,500 V per mil (or ~40–60 V/µm), but the governing value must come from the product data sheet and the project specification - the inspector should confirm against those, not rely on a single rule.
- Too low: thin spots and pinholes remain undetected; the coating passes inspection but fails in service (corrosion under disbonded coating).
- Too high: can cause false indications and, more seriously, damage or puncture otherwise-sound coating, creating holidays that then must be repaired.
The technician's 1,000 V reading is not credible as a 'holiday-free' result for this thickness. Re-test at the specified voltage, calibrated and verified on a known-good/known-bad reference.
2. The technician observed pinholes near the weld that the detector did not flag. What does this tell you, and what should happen before backfill?
Read the worked answer
Pinholes not flagged by the detector strongly indicate the test voltage was inadequate (or the electrode/ground return was faulty). A functioning detector at correct voltage should flag through-coating defects.
Before backfill:
- Verify detector calibration and ground continuity.
- Re-scan at the specified voltage.
- Mark and repair all indicated holidays per the coating manufacturer's repair procedure (typically abrade and re-coat, or use a compatible repair patch).
- Re-test repairs.
- Do not backfill until the coating passes at the correct voltage and repairs are documented.
The judgment call: a 'pass' result is only as good as the test method; an inspector must question a result that conflicts with visual evidence.