Surface Soluble Salt Measurement – Conductivity Verses Ion-specific Methods of AnalysisBill Corbett
Chemical contaminants on a surface can include chlorides, ferrous ions, sulfates and nitrates, among other types of soluble salts. Chloride may come from deicing materials or marine/coastal environments, ferrous ions are a by-product of corrosion, sulfates can be airborne, particularly in industrial environments (e.g., coal-fired power plants) and nitrates may come from the soil (e.g., fertilizers). These chemicals are deposited onto surfaces while the structure is in service, or during transportation of new steel to the fabrication shop, or from the shop to the field. They can typically be removed from surfaces by pressure washing or water jetting using clean water or water with the addition of a proprietary salt removal-enhancing solution. The effectiveness of the pressure 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 are configured with difficult-access areas, as contamination will tend to concentrate and become trapped 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. Provided there is a sufficient quantity of water in the service environment, and the concentration of the water-soluble contaminant trapped beneath the coating system is high enough, water can be drawn 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.
It is for these reasons that many specifications require inspection of surfaces for chemical contaminants after surface preparation operations are complete, but before application of the primer. 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. SSPC Guide 15, “Field Methods for Retrieval and Analysis of Soluble Salts on Steel and Other Nonporous Surfaces” describes common methods for sampling and analysis of soluble salt contamination, with the intent of assisting the user in selecting an extraction and analysis procedure. Guide 15 is contained in Volume 2 of the SSPC Steel Structures Painting Manual, “Systems and Specifications.” A copy of the Guide is available from SSPC (www.sspc.org).
Common methods of extracting soluble salts from surfaces for analysis include: surface swabbing; latex patches/cells (ISO 8502, Part 6) and latex sleeves. Common methods of analysis of the extracted soluble salts include ion-specific test strips/tubes for chloride, ferrous ion and nitrate salts; drop titration for chloride; and turbidity meters for sulfate ion detection. Each of these methods of analysis are considered “ion-specific.”
Except when chemical additives are employed, the methods of reducing the surface concentrations (i.e., pressure washing [low or high pressure], steam cleaning or other methods) are not ion-specific. So consideration may be given to performing the analysis of the extracted solution using a non-ion specific method of analysis known as conductivity (ISO 8502, Part 9), rather than conducting multiple ion-specific tests on the extracted sample(s), since the method of removal typically addresses all soluble salts. In this case, a sample is extracted from the surface using any of the methods listed above (swab, latex or latex patch) using distilled or deionized water. Once the extraction is complete, the solution is placed directly onto a conductivity meter (verified for accuracy first; see below) that will accommodate small samples and that automatically compensates for the temperature of the liquid (temperature compensation is very important for the proper use of conductivity meters).
The conductivity meter displays the concentration of the ionic contamination in millisiemens/cm (mS/cm) or microsiemens/cm (µS/cm). To convert from mS/cm to µS/cm, multiply mS/cm by 1000 (e.g., 0.35 mS/cm is 350 µS/cm). Note that for the values from the conductivity meter to have any meaning, the area of the surface being sampled and the volume of water used in the extraction must also be known, which will be the case when using the sampling methods listed above, particularly ISO 8502, Part 6 and Part 9. The conductivity meter will not reveal the type of ionic contamination; that is, it will remain unknown whether the conductivity reading is due to chloride, ferrous ion, nitrate, sulfate or other soluble salts. All that is known is that there is ionic contamination in the extracted test sample. Naturally the conductivity of the extraction solution (the distilled or deionized water) should be tested (known as a “blank”) and any conductivity reading of the water deducted from the reading of the surface extraction sample(s). For example, if the conductivity of the surface extraction is 354 µS/cm and the conductivity of the distilled/deionized water is 3 µS/cm, the reported conductivity is 351 µS/cm.
Many specifications have established thresholds for the maximum amounts of surface salt contamination based on the type of salt (e.g., 7 µg/cm2 chloride; 10 µg/cm2 nitrate and 17 µg/cm2 sulfate). If conductivity testing is substituted for ion-specific testing, then the specifier will need to establish thresholds based on conductivity values (in µS/cm). For example, the US Navy has established thresholds of 70 µS/cm for atmospheric (non-critical) service and 30 µS/cm for immersion (critical) service.
There can be considerable cost savings associated with changing from ion-specific testing to conductivity measurements, since each ionic contaminant of interest must be analyzed using different methods. And none of the kits contain re-usable supplies, so contractors must purchase many kits for each project. Naturally these costs are passed on to the owner, as part of the contractor’s bid. By performing conductivity instead of ion-specific analyses, the costs are reduced since the conductivity meter can be used for thousands of readings, as long as it remains accurate and within the manufacturer’s tolerance. Most of the portable conductivity meters come with a standard solution (known as a buffer solution) with a known conductivity for verifying the accuracy of the meter. Verification of accuracy before each use is recommended.
Finally, it is worth mentioning that there are a few devices on the market that perform both extraction and analysis of the surface, and display the surface salt concentrations in PPM, mS/cm, µS/cm or µg/cm2. Similar to the conductivity meter these instruments are not ion-specific, but are typically more costly than a portable conductivity meter. They do not use any expendable supplies (other than distilled water) and they too compensate for temperature.