Methodology for Analysis of Soluble Salts From Steel Substrates

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This document presents the result of a laboratory study of methodology for extraction and analysis of soluble salts from steel substrates.

INTRODUCTION AND BACKGROUND

Coatings applied on surfaces contaminated with excessive amounts of adverse soluble salts do not provide expected service life. Steel meant to be used without protective coating, such as weathering steel and stainless steel, can also suffer from corrosion damage caused by high concentration of soluble salts under corrosive conditions. Soluble salts often contain chloride, nitrate, and sulfate as adverse anions. Although most of the salts are soluble in water, they cannot be easily removed from steel surface by washing or abrasive blasting. Salts may also exist in pits and crevices on corroded surfaces within or under rust.

Atmospheric contaminants are one major source of soluble salts on steel bridges while deicing salts are another significant source of salt deposition on bridges. Even abrasives for cleaning steel surfaces sometimes contain detrimental amount of soluble salts, which can provide additional salts to the substrate during blast cleaning instead of removing in-situ residual salt.

There are number of field and laboratory methods to determine the amount of soluble salts on steel surfaces. SSPC Guide 15 describes the most commonly used field methods for the extraction and analysis of soluble salts.(1) It also includes laboratory reference methods for extraction and ion concentration analysis. The guide not only provides sampling and testing procedures but also discusses advantages and limitations of each method.

In SSPC Guide 15, the field methods are categorized into two major groups: methods either measuring total conductivity or determining concentration of specific ions.⁽¹⁾ Conductivity methods are further divided into two subgroups: fully automated single-step methods or multistep conductivity measurement techniques. Ion-specific methods are all multistep methods since there is no automated method available.

A fully automated method integrates extraction and analysis procedures. A device is attached to a metal surface, and a certain amount of extraction water is dispensed to dissolve or extract soluble salts from the surface. The device then measures the conductivity of the solution. A multistep method has separate steps for extraction and analysis. Extraction of soluble salts can be done with swab, latex sleeve or patch cell, or special filter paper. The extraction solution is collected for measuring conductivity or concentration of specific ions.

Field methods measuring chloride ions generally use paper strip, test tube, or drop titration. For sulfate ions, a colorimetric method or optical comparative method can be used to measure the turbidity of the solution. There is nitrate test strip available for determining nitrate concentration. Paper strips are also used for ferrous ion test.

The laboratory reference method for extracting soluble salts is the boiling method, which uses reagent water to extract salts. Sonic enhancement can also be applied for salt extraction. Commonly used laboratory reference methods for detection of specific ions are titration, ion chromatography, and ion-selective electrode. Ion chromatography can simultaneously test multiple ions with great accuracy.

The extraction methods using swabbing, latex patch cell and sleeve have been evaluated in FHWA studies in the past.^(2,3) The extraction efficiency could be increased by using acidic fluid instead of deionized water; however, it affects the conductivity when estimating the amount of salts in the sample. Suitable equations were provided so that the actual chloride concentration on the substrate could be calculated based on the conductivity reading.⁽²⁾  Guidelines and recommendations to improve extraction efficiency and analysis accuracy were provided in those FHWA studies.

Extraction and analysis of chloride ion have been studied in the past. (See references 4–9.) De-ionized water was used in those studies to extract soluble chlorides, sometimes at elevated temperature. Titration or photometry methods were applied to determine chloride concentration. Flores found that on average the patch method overestimated chloride concentration by 50 percent, while the swabbing method underestimated sulfate concentration by 20 percent.⁽⁷⁾ Considerable decrease in the extraction efficiency on rusted steel surfaces was due to the difficulty of extracting contaminants at the steel/rust interface. Methods using indicator test strips for determining soluble chloride were fairly accurate with non-rusted steel but provided low values for rusted steel. Rust hinders chloride extraction because the steel/rust interface is the preferential location for chloride ions to accumulate. Removal of rust on steel surface breaks down the barrier, improving chloride extraction from the steel surface.

A study compared chloride recovery rates among different curing conditions and extraction methods.⁽²⁾ Chloride was put on steel panels and then retrieved with different extraction methods. For specimens with 30 μg/cm² chloride concentration on the surface, the chloride recovery rate by the swabbing method was 95 percent for freshly doped steel surface (no aging). The recovery rate decreased for the doped specimens that have aged for 4 h: 80.7 percent recovery rate at 98.6 °F (37 °C) and 57 percent relative humidity, and 43.6 percent recovery rate at 98.6 °F (37 °C) and 78 percent relative humidity. The chloride recovery rate of the patch and sleeve methods were also evaluated. Acidic solutions were used as extraction liquid in a patch cell or sleeve. The chloride recovery rates from freshly doped steel surfaces (no aging) with 30 μg/cm² were 101 percent for the patch test method and 107 percent for the sleeve test method. When the specimens were aged for 4 h at 98.6 °F (37 °C) and 57 percent relative humidity the chloride recovery rates were 97.7 percent for the patch test method and 99.3 percent for the sleeve test method. The chloride recovery rates further decreased to 79.9 percent for the patch test method and 60.4 percent for the sleeve test method when the doped specimens were aged at 98.6 °F (37 °C) and 78 percent relative humidity for 4 h. This study also demonstrated that analytical methods had detection limits for chloride. The detection limit (threshold) for swab/ion detection strip was 3 μg/cm2; it was about 1 μg/cm2 for patch/titration, and about 5 μg/cm² for sleeve/chloride ion detection tube.

Appleman classified chloride levels on blast cleaned steel.⁽⁹⁾ Chloride levels above 50 μg/cm² were considered highly contaminated, levels below 30 μg/cm² were classified as low chloride, and levels between 30 and 50 μg/cm² were considered marginal. These numbers were recommended for coating system selection. In addition, a different classification was introduced for protective coatings on corroded areas. If the level of chloride concentration was 50 μg/cm² or greater, the surface has to be re-cleaned; if the chloride concentration was less than 10 μg/cm², the surface was considered clean. Chloride levels between 10 and 50 μg/cm² indicated the surface cleanliness was marginal.