The Critical Role of Chloride Control
Soluble chloride contamination on blast-cleaned steel is the primary cause of osmotic blistering, filiform corrosion, and premature coating failure in immersion and marine environments. Even after thorough blast cleaning to Sa 2½ or Sa 3, chloride ions can remain in surface pits, corrosion products, and the steel crystalline structure itself.
When a chloride-contaminated surface is overcoated, the chloride attracts moisture by osmosis through the semi-permeable coating membrane, building up osmotic pressure at the steel surface. This pressure eventually blisters and disbonds the coating, exposing the steel to accelerated corrosion.
ISO 8502 provides the test methods for quantifying this risk. Parts 6 and 9 are the two most widely used field methods — both extracting soluble salts from the surface and measuring the extract for ionic contamination.
ISO 8502-6: The Bresle Patch Method
The Bresle patch method (ISO 8502-6) is the gold-standard field method for measuring water-soluble ionic contamination on blast-cleaned steel. It uses a self-adhesive latex patch with an enclosed cell cavity pressed against the substrate, into which a known volume of deionized water is injected to extract surface contaminants.
Bresle Patch Procedure
- Clean the syringe with deionized water three times. Fill with exactly 3.0 mL of deionized water (conductivity ≤ 5 μS/cm). Measure and record the initial conductivity of the water.
- Remove the backing and press the Bresle patch firmly against the blast-cleaned surface, ensuring the cell is airtight. Standard patch cell area: 1250 mm² (Elcometer) — confirm the patch manufacturer's cell area as this is used in calculation.
- Inject 3.0 mL of deionized water through the syringe port into the patch cell. Leave the syringe in the port to prevent leakage.
- Agitate the water within the cell by gentle movement for 10 minutes minimum (or as specified). Some specifications require 30–60 minutes for maximum extraction.
- Withdraw the extract back into the syringe. Measure conductivity with a calibrated field conductivity meter. Record temperature and apply 25°C correction.
- Calculate NaCl equivalent concentration using the meter's conversion (typically: 1 μS/cm ≈ 0.5 mg/L NaCl for dilute solutions). Then convert to surface density using the patch area and water volume.
Calculation Formula (ISO 8502-6)
The result is expressed as mg/m² NaCl equivalent:
C (mg/m²) = [(κ_extract − κ_blank) × V × f] / A
Where: κ = conductivity (μS/cm), V = water volume (mL), f = conversion factor (μS/cm → mg/L), A = patch cell area (m²)
Worked example: Extract conductivity 85 μS/cm, blank 3 μS/cm, net 82 μS/cm → 41 mg/L NaCl. Volume 3.0 mL = 0.003 L. Mass NaCl = 0.123 mg. Cell area 1250 mm² = 0.00125 m². Result = 0.123 ÷ 0.00125 = 98 mg/m² NaCl equivalent.
Bresle patches from different manufacturers have different cell areas — typically 1250 mm² but sometimes 2500 mm². Always verify the patch cell area from the manufacturer's data sheet and use the correct area in your calculation. Using the wrong area introduces a 2× error in the result.
ISO 8502-9: The Field Conductimetric Method
ISO 8502-9 is a simplified field method that estimates total soluble salt contamination using a conductivity measurement of an aqueous extract, converted directly to mg/m² NaCl equivalent. It is faster than the Bresle method and requires less specialized equipment, making it suitable for high-frequency routine monitoring.
The method uses a standardized extraction device (such as the SCM400 or similar) that applies a fixed volume of deionized water to a defined surface area, collects the extract, and measures its conductivity directly. The instrument typically displays results directly in mg/m² NaCl equivalent, removing the need for manual calculation.
Method-by-Method Comparison
- Standardized latex patch with defined cell area
- 3.0 mL deionized water, 10–60 min contact time
- Conductivity measured externally with separate meter
- Manual calculation of mg/m² NaCl equivalent
- High reproducibility — recognized reference method
- Suitable for vertical, overhead, and complex surfaces
- Accepted by most owner specifications as primary method
- Consumable cost per test: moderate (patch + water)
- Integrated extraction and measurement device
- Faster: typically 2–3 minutes per reading
- Direct readout in μS/cm or mg/m² NaCl equivalent
- No manual calculation required
- Slightly lower precision than Bresle method
- Best on flat, accessible horizontal surfaces
- Often used as rapid screening before coating
- Confirm specification acceptance of this method before use
Acceptance Limits by Application
ISO 8502-6 and ISO 8502-9 define the measurement method but not the acceptance limit — this is project- and application-specific. The following table presents widely used industry limits:
| Application / Environment | Typical Acceptance Limit (mg/m² NaCl eq.) | Source / Reference |
|---|---|---|
| General atmospheric exposure (C3) | ≤ 50 mg/m² | General industry practice |
| Severe atmospheric (C4/C5) | ≤ 20 mg/m² | ISO 12944-4; most industrial specs |
| Offshore / marine atmosphere (C5-M) | ≤ 20 mg/m² | NORSOK M-501; ISO 12944 |
| Immersion / splash zone (Im1/Im2) | ≤ 10 mg/m² | NORSOK M-501; paint manufacturer specs |
| Potable water / food contact | ≤ 10 mg/m² | Client-specific; AWWA standards |
| Pipeline FBE external coating | ≤ 20 mg/m² | API 5L2; DIN 30670; client specs |
| Zinc silicate primer | ≤ 20 mg/m² | Paint manufacturer application guide |
| Thermal spray (TSZA / TSZN) | ≤ 10 mg/m² | ISO 2063; thermal spray specs |
The acceptance limit is ALWAYS defined by the project specification, the coating manufacturer's technical data sheet, or the asset owner's engineering standard — never by the ISO 8502 test standard itself. Using a wrong or assumed limit is a frequent source of disputes. Before testing, obtain written confirmation of the applicable acceptance limit from the project specification.
Which Method to Use — Decision Guide
| Situation | Recommended Method | Reason |
|---|---|---|
| Third-party witness inspection, contractual compliance | ISO 8502-6 (Bresle) | Higher precision; internationally recognized primary method |
| Offshore / immersion service projects | ISO 8502-6 (Bresle) | High-stakes application requires most accurate method |
| Routine production monitoring (high volume) | ISO 8502-9 (Conductimetric) | Faster throughput; suitable as QC screening tool |
| Dispute resolution or appeal of failed test | ISO 8502-6 (Bresle) + ISO 8502-2 or -12 laboratory | Reference method; laboratory confirmation |
| Contractor QC on general industrial work | Either — confirm with specification | Both acceptable when specification permits |
| Pre-coating check in confined spaces | ISO 8502-9 (Conductimetric) | Faster and less equipment to manage in confined areas |
For critical offshore and immersion service projects, many asset owners specify that ISO 8502-9 (conductimetric) is used for rapid field screening before coating application, and that any reading approaching the acceptance limit triggers an ISO 8502-6 Bresle patch confirmation test. This two-tier approach provides speed without sacrificing accuracy on borderline results.
Correlation Between Methods
Both methods measure total soluble ionic contamination and convert to NaCl equivalent, but they differ in extraction efficiency and measurement principle. ISO 8502-9 typically shows approximately 80–95% correlation with ISO 8502-6 Bresle results under equivalent surface conditions. The Bresle method generally extracts slightly more contamination due to longer contact time and sealed cell pressure.
When converting between methods or comparing results, a correlation factor of approximately 0.85–0.95 is commonly applied (i.e., conductimetric result × 0.9 ≈ Bresle result for typical surfaces). However, formal correlation studies specific to the surface type and condition should be conducted if method equivalence must be demonstrated to a specification authority.