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1 Introduction Scope This guideline has been developed to help highway agencies and their representatives understand the issues associated with detection and remediation of soluble salt contamination prior to coating steel highway structures. It is designed to consolidate practical guidance based on the work performed under NCHRP Project 10-97, âDetection and Remediation of Soluble Salt Contamination Prior to Coating Steel Highway Structures.â More extensive discussion of the technical issues in this guideline are presented in the contractorâs technical report, available for download on the TRB website (trb.org) by searching for âNCHRP Project 10-97.â After a brief background on soluble salts, the document presents information as responses to a series of practical questions that an inspector, contractor, or designer may pose. This guideline also contains four appendices: â¢ Appendix A, which is a copy of SSPC Technology Guide 15, Field Methods for Extraction and Analysis of Soluble Salts on Steel and Other Nonporous Substrates; â¢ Topical training materials for soluble salt detection and remediation in the form of annotated PowerPoint slides: â Appendix B: Introduction to Soluble Salts, â Appendix C: Detection of Soluble Salt Contamination, and â Appendix D: Remediation of Soluble Salt Contami- nation. Technical Background Soluble salts are those that dissociate in solution into anionic and cationic components. The soluble salts ref- erenced in this guideline are soluble in water at nominal room temperatures. Soluble salts may be transferred to a steel bridge structure as an airborne aerosol (generally from marine or industrial sources), wind-blown debris, and debris transferred from vehicles or rainwater. In many cold climates, the most common source of soluble salts on bridges is deicing materials. When present on a prepared surface, these salts are known to impact coating performance. Soluble salts may cause osmotic blistering or increase steel corrosion at coating defects. Surface preparation methods used during bridge coating remove (remediate) soluble salts with varying degrees of effectiveness. Following remediation of the surface, detection methods are available to confirm the surface has a soluble salt level below an acceptance limit that may be designated by the coating specifier. Since the deleterious effects of salts have been realized and practical field measurement techniques have been developed, coating specifiers have recognized the benefits of requiring that cleaned surfaces meet certain soluble salt levels, especially in marine and immersion service. Commer- cially available soluble salt detection methods use various techniques to dissolve the soluble salts into a known solution volume for testing. These methods are sensitive to low levels of soluble salt contamination; salt levels near the lower detection limit (LDL) of these methods have been found to inhibit coating performance. When testing for soluble salts, the three most common criteria are conductivity, chloride-ion concentration, and total dissolved salt concentration. Conductivity is a measure of all salts dissolved in the solution based on the specific conductance of each individual ion in the solution. It is important to recognize that both the dissolved anion and dissolved cation contribute to the solution conductivity and that not all ions have the same conductance. Two solutions of the same chloride-ion concentration, one containing only magnesium chloride and the other only sodium chlo- ride, would have different conductivities. The measured conductivity is often mathematically converted into the surface concentration using the formula described in Inter- national Organization for Standardization (ISO) 8502-9 C H A P T E R 1
2 and reported as âtotal salt.â The conversion does not represent any particular salt: it is simply a mathematical construct that has proven to be a pragmatically useful way to report conductivity data. To determine the specific ions in the solution, individual ion-specific tests are used. Examples of these include chloride- ion detection tubes, nitrate filter papers, and sulfate turbidity tests. It is often not practical to specifically identify all forms of soluble salts during an inspection. Chlorides are among the most common soluble ionic species, contribute to coat- ing deterioration, and therefore commonly have specified limits. Ion-specific techniques for sulfates and nitrates are also commercially available and may be useful in environments where these salts are common (e.g., industrial environments). A concerned user could send the extraction fluid to a water analysis laboratory to detect the concentration of a variety of specific ions; however, this is not useful as a surface prepara- tion quality check. Industry standards vary for the quantity and type of con- tamination considered acceptable. Coating performance data suggest that the level of tolerable soluble salt contamination is a function of coating type and service environment. It is generally not possible to remove all soluble salts, though it is possible to reduce soluble salts levels using common surface preparation methods. The following definitions are important for understanding the remainder of this document: Soluble Salt Contaminants: Water-soluble inorganic com- pounds (such as sodium chloride, ammonium sulfate, or magnesium chloride) that contaminate a surface or product. When soluble salts are present on a steel surface prepared for coating, they may cause prema- ture coating failure, particularly by osmotic blistering. Soluble salt contaminants are sometimes referred to as âionic contaminantsâ or ânon-visibleâ contaminants. Chloride Ions: Negatively charged ions (Cl-) derived from the element chlorine. Chloride ions are present in an aqueous solution in which soluble salts containing chloride (e.g., sodium chloride, magnesium chloride) are dissolved. Equivalent Surface Concentration of Total Salt from Conductivity: Based on measuring the average con- ductivity from twelve representative samples of soluble salts, an ISO committee developed a formula for con- verting conductivity to surface concentration of salts. The formula is provided in ISO 8502-9 and The Society for Protective Coatings (SSPC) Guide 15. Instrument manufacturers refer to the calculated value as total salt, salt density, and/or salt.