ISO 8502-6 Bresle Patch: Correct Application Technique for Chloride Testing on Blast-Cleaned Steel
Soluble ionic contamination — particularly chlorides — is one of the leading causes of premature coating failure. Chloride ions trapped beneath a protective coating create osmotic cells that drive blister formation, underfilm corrosion, and adhesion loss. The Bresle patch method (ISO 8502-6), combined with conductimetric measurement (ISO 8502-9), is the most widely specified field test method for measuring soluble contamination on blast-cleaned steel surfaces.
Despite its widespread use, the Bresle patch method is frequently performed incorrectly. Errors in patch application, extraction volume, contact time, and measurement procedure can produce results that are up to 40% lower than the actual contamination level — leading to acceptance of surfaces that should have been rejected. This guide covers the correct technique in full detail.
The Standard: ISO 8502-6 and ISO 8502-9
The Bresle patch method involves two ISO standards used in combination:
- ISO 8502-6 — Extraction of soluble contaminants for analysis using the Bresle sampler. This standard defines the patch type, extraction liquid, contact time, volume, and extraction procedure.
- ISO 8502-9 — Field conductimetric determination of water-soluble salts. This standard defines the measurement procedure, calibration, and calculation method to convert conductivity (µS/cm) to soluble salt content (mg/m² NaCl equivalent).
Equipment Required
Before performing the test, verify the following equipment is present and functional:
- Bresle patch samplers — ISO 8502-6 compliant, 1250 mm² exposed area (standard size). Must be certified compliant; non-standard patches give different results.
- Distilled or deionized water — ASTM Type II or equivalent (conductivity ≤2 µS/cm). Do not use tap water or bottled drinking water.
- Syringe with needle — 5 mL syringe minimum. The syringe must provide a controlled injection volume.
- Calibrated conductivity meter — Resolution ≤0.1 µS/cm, calibrated against traceable standards. Range: 0–2000 µS/cm.
- Calibration solution — Traceable conductivity standard appropriate for the expected range.
- Timer — For accurate 10-minute contact time.
- Thermometer — Surface and ambient temperature must be within 5°C for valid results.
- Recording sheet — Document location, time, ambient conditions, and results.
Step-by-Step Procedure
Pre-Test Environmental Check
Before applying any patches, measure and record: ambient air temperature, relative humidity, steel surface temperature, and dew point. The surface temperature must be ≥3°C above the dew point temperature. If the surface is too cold, condensation will dilute the extract and give falsely low results. Record all environmental data on the inspection report.
Calibrate Conductivity Meter
Calibrate the conductivity meter against a certified calibration solution before each test session and whenever the meter has been transported or stored. The calibration solution should be within the expected measurement range (typically 84 µS/cm or 1413 µS/cm standard solutions). Record calibration date, solution batch, and reading versus nominal. Do not proceed if calibration error exceeds ±2%.
Select Test Locations
Select representative test locations across the prepared surface. Per most project specifications, at least 1 reading per 50–100 m² of prepared surface is required, with additional readings in areas of suspected higher contamination (welds, crevices, areas with previous rust staining, water drainage paths). Identify and mark each location clearly for documentation.
Apply the Bresle Patch
Remove the Bresle patch from its protective packaging without touching the adhesive or the interior of the membrane. Peel the backing carefully, center the patch over the selected test location, and press firmly from the center outward to eliminate air pockets. The entire perimeter must be firmly adhered to prevent leakage. On rough blast-cleaned surfaces, apply firm, sustained finger pressure for 30 seconds around the full perimeter. Critical: Any leak will cause extract to escape, giving falsely low results.
Inject Extraction Liquid
Fill the syringe with exactly 2.5 mL of distilled/deionized water (ASTM Type II). Insert the syringe needle through the patch valve (self-sealing septum) at an angle. Inject the water slowly and evenly into the patch cavity. The water should distribute over the entire surface within the patch. If bubbles or resistance are felt, withdraw and re-inject. Start the 10-minute timer immediately after injection is complete.
Contact Time: 10 Minutes
Allow the extraction liquid to remain in contact with the surface for exactly 10 minutes. During this time, soluble salts dissolve into the liquid. Agitating the patch periodically (pressing gently to move the water across the surface) can improve extraction efficiency — some procedures specify 3–5 agitation events during the 10-minute period. Do not remove the patch or insert the syringe during this time.
Extract the Sample
After exactly 10 minutes, insert the syringe needle through the valve again and slowly withdraw the liquid. Extract as much liquid as possible — typically 2.0–2.3 mL will be recovered (some absorption into patch material is normal). Withdraw the syringe carefully. Immediately dispense the extracted liquid into a clean sample vial or directly into the conductivity meter cell. Do not delay — evaporation and further absorption will affect results.
Measure Conductivity
Immediately after extraction, measure the conductivity of the extracted liquid using the calibrated conductivity meter. Ensure the conductivity probe is fully immersed and the cell is free of air bubbles. Allow the reading to stabilize (typically 10–30 seconds) and record the value in µS/cm. Also measure and record the conductivity of the blank (a 2.5 mL sample of the same distilled water, uncontacted) as a quality check.
Calculate Contamination Level
The contamination level in mg/m² NaCl equivalent is calculated from the net conductivity (sample conductivity minus blank conductivity) per ISO 8502-9. For a standard Bresle patch (1250 mm² area) with 2.5 mL extraction volume, the conversion factor is published in ISO 8502-9 Annex A. Most calibrated field kits include a reference chart or calculator. Report the result in mg/m² NaCl equivalent.
Acceptance Criteria by Service Environment
Acceptance limits for soluble contamination are set by the project specification and coating manufacturer, not by ISO 8502-6 itself. The following limits are commonly specified in industry:
| Service Environment | Typical Limit (mg/m² NaCl equiv.) | Reference |
|---|---|---|
| Offshore / marine immersion | ≤20 mg/m² | ISO 12944, Shell DEP, ExxonMobil GP |
| Buried pipeline | ≤20 mg/m² | Pipeline owner specifications |
| Marine atmospheric | ≤30–50 mg/m² | ISO 12944-4 |
| Industrial atmospheric | ≤50–80 mg/m² | Various owner specs |
| General maintenance | ≤100 mg/m² | Coating manufacturer data sheets |
Common Errors and How to Avoid Them
- Using tap water instead of distilled water: Tap water conductivity can range from 200–800 µS/cm, completely obscuring the surface contamination signal. Always use ASTM Type II water (≤2 µS/cm) or verify blank conductivity is ≤2 µS/cm before proceeding.
- Patch leaking due to rough surface: Apply extra pressure along patch perimeter for 60 seconds on surfaces with profile >Rz 50 µm. If leaking persists, the test location should be changed to a flatter area nearby.
- Incorrect extraction volume: Using more or less than 2.5 mL changes the dilution factor. The ISO 8502-9 calculation is specifically calibrated for 2.5 mL with the standard 1250 mm² patch.
- Delayed measurement after extraction: Measure conductivity within 2 minutes of extraction. Evaporation concentrates the extract and falsely increases readings; absorption into the syringe walls decreases them.
- Not measuring blank conductivity: Always measure the conductivity of a blank (uncontacted 2.5 mL of the same distilled water batch) and subtract from the sample reading to get net conductivity.
- Testing on a wet surface: Surface moisture dilutes the extract. Ensure surface is dry before patch application. If the surface temperature is below the dew point, condensation will form and the test is invalid.
Documentation Requirements
A compliant inspection record for Bresle patch testing must include: date and time of test; ambient temperature and relative humidity; surface temperature and dew point; steel structure/zone identification; test location coordinates or sketch; blank conductivity reading; sample conductivity reading; calculated contamination level (mg/m² NaCl equivalent); acceptance/rejection decision; inspector name and certification number; conductivity meter make, model, and last calibration date.