Coating Inspection and Quality Control
Coating Inspection and Quality Control
Inspection and QC ensure that the coating system meets the specification and will perform as intended. Key concepts include measurement of dry film thickness (DFT) using magnetic gauges (for steel) or eddy current (for non-ferrous), holiday detection (low and high voltage), adhesion testing (pull-off, cross-cut), and environmental monitoring (temperature, humidity, dew point). The inspector must understand sampling plans (e.g., SSPC-PA 2) for DFT measurement: typically, one spot measurement consists of three gauge readings, and the average must meet the specified minimum and maximum.
Traps include the difference between spot measurements and area measurements, and the 80-20 rule in SSPC-PA 2: no more than 20% of readings can be below the specified minimum, and no reading can be less than 80% of the minimum. Another trap is the use of the correct gauge for the substrate; magnetic gauges are for steel only. Holiday detection must be done after the coating has cured and with the correct voltage to avoid damaging the coating.
Worked example: if the specified DFT is 8-12 mils, and the spot readings are 7, 9, and 11 mils, the average is 9 mils, which is within the range. However, the 7 mil reading is below the minimum (8 mils) and is 87.5% of the minimum (7/8 = 0.875), which is above 80%, so it is acceptable if only one of five spot readings is below. Always refer to the specification for acceptance criteria.
Key points:
- DFT measurement per SSPC-PA 2: spot = average of three gauge readings.
- 80-20 rule: no more than 20% of readings below minimum, and none below 80% of minimum.
- Magnetic gauges for steel; eddy current for non-ferrous.
- Holiday detection: low voltage for thin films, high voltage for thick films.
- Adhesion testing methods include pull-off (ASTM D4541) and cross-cut (ASTM D3359).
- Environmental conditions must be recorded during inspection.