Guidelines for Detection and Remediation of Soluble Salt Contamination Prior to Coating Steel Highway Structures (2019) / Chapter Skim
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3 Key Issues for Salt Detection and Remediation This section provides guidance related to nine soluble salt detection and remediation questions. Following each question is a summary of the key points and a more detailed discussion of the issue.
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4 bridge through two primary mechanisms. First, salts under a coating may osmotically draw moisture through the coating to the steel-coating interface, forming osmotic blisters.
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5 Conductivity, sulfate ion density, and nitrate ion density are also useful methods under certain circumstances. When requiring or reporting soluble salt surface concentrations, it should be clearly stated what is being measured.
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6 How Much Salt Should Be Allowed on Surfaces Prior to Painting? Key Points • When abrasive blasting and re-painting a bridge structure, maximum chloride ion level of 10 µg Cl–/cm2 is suitable for most environments.
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7 • The methods are sensitive to low levels of soluble salt contamination. Since typical industry requirements are near the detection limits for the technique, significant measurement variability may be observed in the field.
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8 • All methods are sensitive to very low levels of soluble salt contamination and should be performed with care. As a result, a significant relative standard deviation (on the order of 50%)
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9 steel prior to abrasive blasting or the environmental conditions since surface preparation)
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10 • Dry abrasive blasting, wet abrasive blasting, and ultrahigh pressure water jetting to a high degree of cleanliness (e.g., SSPC-SP 10/NACE No. 2, Near-White Metal Blast Cleaning; SSPC-SP 10 (WAB)
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11 The contractor should develop surface preparation procedures and processes that will reduce soluble salt levels in addition to removing the paint, rust, and mill scale. The plan should employ processes that recognize soluble salts are more likely to be present on corroding steel with pitting and/or pack rust than on surfaces from which an intact coating has been removed.
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12 Discussion To ensure effective management of soluble salts prior to painting, the specifier should address them in the contract requirements. Specification statements should clearly state what level of contamination is allowable (see Table 3)
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13 ions extracted from representative rusted steel specimens. ISO 8502-9 notes that this constant may vary by ±12% in "extreme" conditions.
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14 purposes of specifying surface cleanliness prior to coating." The standard does not suggest what level of salt is acceptable in any given application. The standard provides an equation to convert a conductivity measurement to the equivalent NaCl density.
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