Concept · Inspection and testing

Holiday (discontinuity) testing

What holidays are, how low-voltage wet-sponge and high-voltage DC detectors find them, and how film thickness decides which one to use.

A holiday is a pinhole, void or missed spot that goes through the coating to the substrate. Linings and immersion coatings must be free of them, because corrosion starts there. A holiday test finds these flaws while a repair is still cheap.

The why: two ways to find a hole

Both methods use one idea. Sound coating is an electrical insulator, and a holiday is a path to the steel. The detector looks for that path.

The low-voltage wet sponge method uses water. A damp sponge passes over the film. At a holiday, water runs in and touches the steel. The circuit through the ground lead closes, and the detector alarms when the resistance falls below its preset threshold. The threshold is typically 80 or 90 kΩ. NACE SP0188 allows 9 to 90 V DC, and 67.5 V is the usual setting on metal.

A thick film turns a pinhole into a long, narrow channel. Surface tension can keep the water at the mouth of the channel. A low-sudsing wetting agent lowers the surface tension and raises the sensitivity. NACE SP0188-2024 requires the agent on films above 250 µm (10 mils). Wash it off areas that you will repair or topcoat, because it harms adhesion.

The high-voltage DC method uses a spark. Air insulates until the electric field is strong enough to break it down. Over sound film, the coating blocks the spark. Over a holiday, the spark finds a path through the air to the steel, and the detector alarms.

The voltage has two limits. It must be high enough to break down air across a gap as thick as the film. It must also stay below the dielectric strength of the coating, or the test damages sound film. Thicker film needs more voltage, but not in a straight line. Paschen's law describes how the breakdown voltage of an air gap grows with the gap.

Which method for which film

ISO 29601:2026 is a separate ISO method. It detects porosity in a paint system of any thickness on metal.

In the field

1. Make sure the coating is fully cured. Retained solvent can form a conductive path and a fire hazard.
2. Measure the average DFT with a gauge to ASTM D7091. The thickness sets the method and the voltage (DFT measurement).
3. Connect the ground lead to bare steel of the structure.
4. Check the detector before use and after a drop or a sponge change. For a wet sponge detector, test the alarm with resistors 10 kΩ below and above the setting. For a spark tester, check the voltage at the electrode on holiday-free coating.
5. For a wet sponge, use clean tap water. Do not use salt water or deionized water. Add the wetting agent above 250 µm.
6. Move the sponge or the electrode at no more than 0.3 m/s. Cover 100 % of the area with overlapping passes.
7. Mark every alarm. Repair the holiday, then test the repair again.

A worked example

A tank lining has an average DFT of 635 µm (25 mils). This is above 500 µm, so use high voltage. NACE SP0188-2024 sets the voltage from Paschen's law. With the film thickness d in µm:

V = 1,500 + 1.5 × (170 + 2.48 × d + 58 × √d)

1. 2.48 × 635 = 1,575.
2. √635 = 25.2, and 58 × 25.2 = 1,462.
3. 170 + 1,575 + 1,462 = 3,207, and 1.5 × 3,207 = 4,811.
4. 4,811 + 1,500 = 6,311 V, which is about 6,300 V.

Check: with d in mils the formula is V = 1,500 + 1.5 × (170 + 63 × d + 293 × √d). For d = 25 it gives 1,500 + 1.5 × (170 + 1,575 + 1,465) = 6,315 V. The two forms agree.

The old rule of 100 V per mil would give only 2,500 V here. That is far too low. The SP0188-2024 rationale says the old rule had no scientific basis and let coatings pass that later failed.

What the inspector records

• The method, the detector make and its serial number.
• The voltage or the sensitivity setting, and the DFT range that set it.
• The water and the wetting agent, for a wet sponge test.
• The result of each detector check.
• The number and place of the holidays, their repair and the re-test.

Exam traps

• Trap: "Add a wetting agent only if you like." NACE SP0188-2024 requires it on films above 250 µm. Only the 1999 recommended practice left it optional.
• Trap: "Spark test at 100 V per mil." The 2024 revision replaced this rule with the Paschen-based equation.
• Trap: "Spark testing starts only above 20 mils." ASTM D5162-24 allows 10 to 20 mils when the voltage is set correctly.
• Trap: "Distilled water gives the cleanest test." The standard calls for clean tap water. Deionized and salt water must not be used.
• Trap: "Test a zinc-rich primer on its own." Zinc conducts, so the detector alarms everywhere. Test after a non-conductive coat is on it (zinc-rich primers).
• Trap: "Spark test a lining that has been in immersion." The spark can damage the sound film. Test new coatings only.
• Trap: "ASTM D5162 covers concrete." For concrete, use ASTM D4787.

How the exams treat it

The AMPP CIP 1 practical guide lists two holiday stations with 10 points each. One is for the wet sponge detector and one is for the high-voltage DC detector. A FROSIO training body lists low-voltage and high-voltage stations in its course. See NACE SP0188 and ASTM D5162 for the standards, and pinholes for the usual cause of holidays.

Use it: Standard: NACE SP0188

84 practice items in the members area mention this topic (CIP 1: 35 · CIP 2: 35 · FROSIO written: 14).

Related

• Standards: ASTM D5162 · NACE SP0188 · ISO 29601 · ASTM D4787 · NACE SP0892
• Defects: Pinholes · Holidays and misses · Cathodic disbondment · Outgassing · Bugholes (surface voids in concrete)
• Inspection and testing: Dry film thickness measurement · Adhesion testing