Introduction
Chemical contaminants on a surface can include chloride, ferrous ions, sulfates and nitrates, among others. These chemicals or “salts” are deposited onto surfaces while the structure is in service, or during transportation of new steel to the fabrication shop. They are soluble in water, so they can typically be removed from surfaces by pressure washing or water jetting using clean water (hot water works better than cold) or water with the addition of a proprietary salt removal-enhancement solution. The effectiveness of the washing step is dependent on the condition of the surface. That is, contamination is relatively easy to remove from smooth surfaces, but may be more challenging if the surfaces are pitted or crevices are present, as contamination will tend to concentrate in these areas. If the salts are not detected or are not adequately dissolved and rinsed from the surfaces, they can become trapped beneath a newly-installed coating system. If there is a sufficient quantity of water in the service environment, (e.g., immersion) the water-soluble contaminant trapped beneath the coating system will draw the water through the coating film by a process known as “osmosis.” This drawing force can be quite powerful and will continue until the concentration of salt in water is the same on both sides of the coating film (the concentration reaches equilibrium). This process creates a build-up of water and pressure beneath the coating film, oftentimes enough to cause blistering of the coating (known as osmotic blistering), underfilm corrosion and premature coating failure.
Additionally, if soluble salts on the surface are not sufficiently removed prior to abrasive blast cleaning, recycled abrasive media can become contaminated, which can lead to contamination of surfaces that were not originally contaminated. Note that SSPC-AB 2[1]requires that recycled abrasive be routinely tested for water soluble contaminants (maximum concentration of 1,000 µS/cm) to verify that this “transfer of contamination” is not occurring.
It is for these reasons that some specifications require inspection of surfaces for chemical contaminants after surface preparation operations are complete, but before primer application. Because this type of contamination cannot be detected visually, the surface must be sampled and the “surface extraction” tested for the contaminant(s) of concern.
Industry Standards
Industry standards are primarily focused on surface extraction and analysis procedures, as well as frequency and locations of testing. Common standards include:
- SSPC Guide 15, Field Methods for Retrieval and Analysis of Soluble Salts on Steel and Other Nonporous Surfaces
- SSPC Guide 24, Soluble Salt Testing Frequency and Locations on New Steel Surfaces
- NACE SP0508, Methods of Validating Equivalence to ISO 8502-9 on Measurement of the Levels of Soluble Salts
- ISO 8502 Preparation of steel substrates before application of paints and related products – Tests for the assessment of surface cleanliness
Part 5: Measurement of chloride on steel surfaces prepared for painting–Ion detection tube method (ISO 8502-5:1998)
Part 6: Extraction of soluble contaminants for analysis – The Bresle method (ISO 8502-6:1995)
Part 9: Field method for conductometric determination of water-soluble salts (ISO 8502-9:1998)
Part 10: Field method for the titrimetric determination of water-soluble chloride (ISO 8502-10:1999)
Since there is no “industry standard” for tolerable levels of chemical contaminants on a surface, the project specification must indicate the maximum quantity of soluble salts that can remain on the surface and be safely coated over. For example, the specification of the coating of the interior of a water storage tank may specify a relatively low surface concentration of soluble salt, since the service environment is “immersion.”
[1] Cleanliness of Recycled Ferrous Metallic Abrasives
Latest Extraction and Analysis Procedures
Soluble Salt Detection Meters
The Concentric Ring meter (shown) uses saturated filter paper from the surface to determine soluble salt levels. To use the device, a syringe is filled with 1.6 ml of high purity water, then ejected onto a clean unused sample paper, taking care to retain all the water on the paper. The wetted paper is then placed onto the area under test and pressed firmly into any contours and irregularities to remove any entrapped air, then the timer is started on the meter. After 2 minutes, the saturated paper is removed from the surface, placed onto the gold-plated electrodes, and the lid is closed, ensuring that the magnetic catch is fully engaged. The reading will automatically be displayed and stored into memory together with paper size, temperature, date and time. Half size or quarter size papers can be used for testing on small surfaces. The paper size is automatically detected, and the reading is adjusted.
Another type of soluble salt meter (primarily used by the US Navy) combines the extraction and analysis processes. The meter (shown left) is a handheld, automated method for extracting and detecting soluble salts on flat surfaces; specially designed instrument heads allow for measurement on curved surfaces. Since the meter is attached to the surface using magnets, as indicated by the number 6 in the right image, its use is limited to carbon steel surfaces. The measurement area is circular and is fixed at 1250 square millimeters (mm2).
The pre-packaged kit consists of the soluble salt meter with a rechargeable battery, a 500 milliliter (mL) bottle of deionized water that accommodates a fixed volume fluid dispenser, and a 250 mL bottle of standard solution and syringe for verifying the accuracy of the meter.
Prior to conducting an extraction and analysis of the surface, the accuracy of the conductivity meter is verified by injecting the meter with the recommended quantity of calibration solution, verifying that the instrument display matches the conductivity of the standard solution, and flushing the meter using deionized water.
To perform an extraction, a flexible hose is attached from the deionized water fluid dispenser to the meter, then the meter is attached to the surface. Three (3) mL of deionized water is injected into the measurement chamber with one press of the dispenser attached to the de-ionized water bottle. The meter will agitate the solution and automatically obtain a reading, which is displayed on the LCD screen and simultaneously stored in memory. The meter is flushed with deionized water so that it is ready for subsequent measurements.
Latex Sleeves for Ion-specific Analysis – Chloride-Sulfate-Nitrate (CSN) Test Kit
If ion-specific testing for chloride, sulfate and nitrate surface contamination is required, then a special kit is used to perform both the extraction and analysis. The CSN kit contains latex sleeves, pre-measured extraction liquid, chloride ion detection tubes (Kitagawa), nitrate test strips, and pre-measured chemicals and a meter for sulfate analysis.
A premeasured amount of extraction solution is emptied into the latex sleeve (Figure 1) and the sleeve is attached to the prepared, uncoated surface (Figure 2). An extraction is performed (Figure 3), the latex sleeve is removed, and the solution analyzed for chloride (Figure 4), nitrate (Figure 5) and/or sulfate (Figure 6). For this method, the reading from the tube, strip and meter (in PPM) is the same as µg/cm2. This only works because the opening of the latex sleeve is 10 cm2 and the premeasured amount of solution is 10 mL, so the values cancel one another out.
Non-ion specific Analysis (Latex Patch/Conductivity per ISO 8502-6/8502-9)
For this procedure, a latex patch or magnetic cell (Bresle Patch, Figure 7; DeFelsko Patch or PosiPatch, Figure 8) is attached to surface. The Bresle Patch and DeFelsko patch contain compressible foam with adhesive backing to create the seal. The PosiPatch is attached magnetically. A prescribed amount of distilled water is injected into the patch/cell (Figures 9, 10) and the water inside the patch/cell is agitated for a few minutes. Once the extraction is complete, the solution is removed from the patch/cell and placed onto a conductivity meter (Figure 11) or Soluble Salts Tester (SST, Figure 12). Conductivity results are displayed in microsiemens/cm (µS/cm) or millisiemen (mS/cm); the SST instrument can also display surface concentrations in µg/cm2 or mg/m2. Either can be compared to the maximum allowable contamination level referenced in the project specification.
This analysis will not reveal the type of surface contamination; only that some type of water-soluble ionic contamination was extracted from the surface, causing an increase in the conductivity of the distilled water used for the extraction. If ion-specific analysis is required, chloride ion detection strips (Figure 13) or tubes (Figure 14), nitrate strips, and/or sulfate detection meters (shown previously) may be used.
The methods highlighted in this article all have advantages and limitations, vary considerably in cost, and have varying extraction efficiencies and limits of detection. As a Quality Control or Quality Assurance Inspector, it is important to fully understand the proper procedures to follow for the specified method. As a specifier, it is important to select a method so that all parties use the same one on a given project. It is also important to specify the threshold for each type of salt (or one threshold if conductivity is used, since it is non ion-specific) and verify the correct units are referenced:
Surface Concentrations: mg/m2 or µg/cm2
Conductivity: µS/cm or mS/cm
