Measuring salt, dust and surface contamination is a critical check in the preparation of steel structures before anti-corrosion coatings are applied. The presence of soluble salts (chlorides, sulfates, nitrates), of dust left by blasting, or of oils, greases and other contaminants on the metal surface seriously compromises the adhesion of the coating and accelerates under-film corrosion (osmosis, blistering, detachment). The technologies available include the Bresle method for soluble salts, the dust tape test for dustiness, and detection kits for organic contamination (oils, greases).
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The Bresle method for soluble salts
The Bresle method (ISO 8502-6, ISO 8502-9) is the most widespread procedure for the quantitative measurement of soluble salts. A transparent adhesive cell (Bresle patch) is stuck to the prepared metal surface; a known volume of deionised water is injected into the cell with a syringe, left in contact for about one minute with agitation, then drawn off. The electrical conductivity of the solution, measured with a calibrated conductivity meter, is proportional to the concentration of salts on the surface. Converted into mg/m² by established formulas, it gives the saline contamination value. Complete kits are available with cells, syringes, a conductivity meter and calibration solutions.
The dust tape test
The dust tape test (ISO 8502-3) measures the dust remaining after blasting: transparent adhesive tape is applied to the surface and the quantity and size of the adhering particles are then compared visually with standardised reference scales (quantity from 0 to 5, size from 1 to 5). The result is expressed as a dustiness category (for example “dust quantity 2 / dust class 3”). For highly critical surfaces, kits with cameras and automatic image analysis software are used to obtain quantitative results.
Organic contamination
Checking for organic contamination (oils, greases, hydrocarbons) is done by indirect methods: the water-break test (water spreads evenly only on a clean surface), the coloured-water test, detection kits with UV fluorescent dyes, and solvent sampling with laboratory analysis. For critical applications (aerospace, food packaging, medical) there are portable UV fluorescence instruments for detecting hydrocarbon residues on metal.
Applications
Surface contamination checks are required by the more severe anti-corrosion specifications (marine environments, offshore, the chemical industry): typically a maximum of 50 mg/m² of chlorides for standard marine systems, and 20 mg/m² for very severe environments. Dustiness must generally be classified as “class 2” or lower before the primer is applied. Documenting the measurements (with a photograph of the Bresle patch, the conductivity value, the contamination calculation and the comparison against the specification) constitutes objective evidence that the preparation procedures have been followed.
Reference standards
The reference standards are: ISO 8502-1 (extraction and analysis of surface corrosion products), ISO 8502-2 (chlorides on cleaned surfaces), ISO 8502-3 (dust tape test), ISO 8502-6 (Bresle sampling) and ISO 8502-9 (Bresle sampling with a field conductivity meter). Calibration of the conductivity meter is carried out with ISO 17025 certified KCl solutions of known conductivity. Dust test kits must carry standardized visual reference documents.
Integrating the measuring kits with digital reporting systems (mobile apps, dedicated software) allows every inspection to be fully documented: sampling data, a photograph of the Bresle patch, georeferencing of the measuring point, automatic calculations, and generation of campaign reports that comply with the standards. This level of documentation is fundamental in marine and offshore anti-corrosion projects, where conformity with the surface preparation specifications is verified by FROSIO- or NACE-certified inspectors.
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