Feature

Methods for Samplings andAnalyzing Soluble Salts onSteel Surfaces: AComparative Study

by S. Flores, Pontificia Universidad Católica del Perú Laboratorio deCorrosión; and J. Simancas and M. Morcillo, Centro Nacional deInvestigaciones Metalúrgicas, CSIC

Soluble salts, particularly chlorides and sulfates, on rusty steel frequentlyproduce premature failure of coatings.1-4 Because of this problem,laboratory and field methods for sampling and analyzing soluble salts havebeen developed.4-12

This article presents a comparative study on the sensitivity and reliability ofdifferent methods for extracting and analyzing soluble contaminants onsteel surfaces (Table 1 ). Two field methods, swabbing4,5 and the Bresle5,6

method, and one laboratory reference test, the Mayne method, werecompared for their efficiency in the extraction of saline contaminants.Propriety indicator test strips4-7 and a chloride ion-selective electrode wereused as a field test and laboratory method, respectively, to assess thereliability and sensitivity for detecting soluble chlorides. Finally, solublesulfate was determined indirectly from the concentration of ferrous ion(Fe2+) using proprietary indicator test strips in the field.5,8 The turbidimetricmethod was used as a laboratory reference method for soluble sulfates.

Experiments were carried out on clean, unalloyed steel specimenscontaminated with known amounts of chlorides and sulfates (Part A) andrusted, unalloyed steel specimens weathered for 1 year in 7 atmospheres ofdifferent aggressiveness (Part B).

Experimental

Part A

Specimens of fresh cold rolled steel (5 x 5 cm2 [0.78 x 0.78 in.2]) weredegreased with acetone, and their surfaces were contaminated with knownamounts of NaCl (0, 50, 150, 500, and 1,000 mg Cl-/m2 [0, 0.000175,0.00525, 0.0175, and 0.035 oz Cl-/m2]) and FeSO4.7H20 (0, 1,000, 1,500,and 2,000 mg SO4

2-/m2 [0, 0.035, 0.0525, and 0.07 oz SO42-/m2]). The

contamination process was as follows: contaminating solutions wereprepared by dissolving the appropriate amount of salt in the minimum

volume of distilled water, which was diluted with ethanol to a knownvolume. The required volume of contaminating solution was applied byvolumetric pipette over the steel surface and then spread evenly with aglass rod (Fig. 1). Immediately afterwards, contaminated specimens wereplaced for a while in an oven at 100 C (212 F). The ethanol and the dryingprocess enable fast solvent evaporation and hence minimize rusting.

Part B

Samples of steel that had been rusted for 1 year at 7 test sites representingdifferent atmospheric conditions were used.13,14 The outermost non-adherent rust was removed by wire brush cleaning the surface to an SSPC-SP 2, Hand Tool Cleaning (St2 in ISO 8501-19), rust grade.15,16 Solublesalts in the remaining adherent rust were extracted by the swabbing and theMayne methods. The extracts were analyzed for chlorides (indicator teststrips and chloride ion-selective electrode) and for sulfates (indicator teststrips and turbidimetric analysis).

Sampling

Soluble salts were extracted from steel surfaces using 3 different methods:the swabbing, Bresle, and Mayne methods.

• Swabbing Method—The swabbing method relies on surface rinsing.4,5,7,10

A pure cotton wool swab was soaked in a beaker containing 10 ml (0.35oz) of distilled water. The whole metal surface was swabbed with a cottonball held by tweezers (Fig. 2). After swabbing, the cotton ball was swirledin the water and squeezed. This process was repeated at least 4 times. Thecotton ball was subsequently placed in the beaker, and the test area wasdried with a fresh cotton ball, which was also then immersed in the beaker.The water and the cotton balls were then swirled and stirred for at least 2minutes to assure thorough mixing. The total volume of extract thusobtained was 10 ml (0.35 oz). The test was performed in triplicate at eachsite of contamination.• Bresle Method—The Bresle method, also called the plastic patch method,was developed in 1989 by Bresle.5,6 It uses a flexible cell consisting of aself-adhesive plastic patch with a 40-millimeter (1.6-inch) hole punched inthe center and covered with a thin latex film to provide a sample area of12.5 cm2 (1.94 in.2).11 After the plastic test patch was attached to the testsurface, 2 ml (0.07 oz) of distilled water was injected by syringe into thesampling compartment (Fig. 3).12 Special care was exercised to preventtrapping air between the latex film and the specimen surface. After 20seconds, the extracting liquid was removed, and the process was repeated 3times using the same washing liquid. A final washing was performed with 2ml (0.07 oz) of fresh water, so the total volume of the final extract was 4ml (0.14 oz). A single test was carried out at each of 4 contamination sites,150 and 500 mg/m2 for chloride and 1,000 and 1,500 mg/m2 for sulfate.(Editor’s Note: Divide by 10 to compute concentration inmicrograms/cm2.)• Mayne Method—This method, also known as the boiling and rinsingmethod,4 was first used by Mayne.1 A volume of 200 ml (7 oz) of distilledwater was placed in a 250-milliliter (8.8-ounce) beaker; the water was

boiled for 10 minutes and cooled in a nitrogen stream. Each specimen wasthen immersed in the water for 30 minutes (Fig. 4), after which it wasremoved and rinsed with distilled water that was also collected in thebeaker. The extract thus obtained was filtered and diluted to 200 ml (7 oz)for subsequent analysis. The test was performed in triplicate at eachcontamination site.

Chloride Analysis

Chloride ion in the extracts was analyzed using proprietary test stripindicators4,7,11 and a chloride ion-selective electrode.

• Chloride Indicator Test Strips—The chloride indicator test strip methoduses strips impregnated with silver dichromate, which reacts with chlorideion to produce a white column of silver chloride in the strip. The columnheight is proportional to the chloride content in the solution; a proprietaryunit scale running along the strips is converted into concentration units by acalibration table (Fig. 5). The detection limit is 30 ppm Cl-.• Chloride Ion-Selective Electrode—When immersed in a chloride solution,an electrochemical sensing electrode develops a potential proportional tothe concentration of chloride ion. Potential measurements are convertedinto concentration units with the aid of a calibration graph previously runfrom standard solutions. The detection limit is approximately 1 ppm.

Sulfate Analysis

Sulfate ions in the extracts were determined by a second proprietaryindicator test strip method and turbidimetric analysis.

• Indicator Test Strip for Sulfate—The sulfate content in the extracts wasobtained indirectly through the ferrous ion content, which was determinedby using proprietary indicator test strips.5,8,11 These test strips are soakedwith 2,2'-bipyridine, which reacts with Fe2+ to form a dark red complex.The color is compared with a color scale to obtain a concentration value(Fig. 6). The color scale allows researchers to assign the strip referenceconcentrations (3, 10, 25, 50, 100, 250, and 500 ppm) but no intermediatevalues.• Turbidimetric Analysis—The sulfate ion content in the extracts was alsodetermined by a turbidimetric analysis according to ASTM D 516-68.9 Theoperational principle involves making a suspension of barium sulfate byadding BaCl2 to the solution under controlled conditions. The degree ofturbidity, which is proportional to the amount of sulfate in the solution, isdetermined by measuring the suspension absorbance at 400 nm.Absorbance values thus obtained are converted into concentration unitswith the aid of a calibration curve previously run from standard solutions.

Results

Fresh Steel Contaminated with Chloride and Sulfate

For fresh steel contaminated with known amounts of chloride and sulfate,Table 2 and Table 5 show the soluble contaminants extracted using theboiling, swabbing, and plastic patch methods and compared to the levels ofthe applied contamination.

Steel Rusted in Contaminated Atmospheres

Table 3 , Table 4 , and Table 6 list the soluble chloride and sulfatecontamination levels provided by the field and laboratory referencemethods.

Discussion

Sampling: Soluble Salts Extraction Efficiency

It should be noted first that this study was conducted with non-rusted steel.The extraction efficiency of the boiling, swabbing, and plastic patchmethods was assessed on the basis of the contamination applied andcomparing the results obtained with the field methods against thoseobtained with the laboratory tests performed with a chloride ion-selectiveelectrode (Cl-) and by turbidimetric analysis (SO4

2-).

The plastic patch method provided excess values for chloride (150 percentextraction efficiency average), whereas the swabbing method resulted inextraction efficiencies close to 100 percent (Table 2 , top). In his study onrusty steel specimens contaminated with known amounts of chloride, Tatorfound the swabbing method (analysis for Cl- by the indicator test stripmethod) to provide extraction efficiencies well below 100 percent (50-70percent).17 This result is not surprising when one considers the difficulty ofleaching chloride that accumulates in the inner layers of rust. This difficultyalso applies to the plastic patch method. (See Research News, “SSPCResearch on Performance Testing of Abrasives and Salt RetrievalTechniques,” p. 28.)

With respect to sulfate (Table 5 , top), the plastic patch and swabbing fieldextraction methods provided slightly lower concentrations than the actualvalues (first column in Table 5 ) and the values obtained by applying thelaboratory reference extraction method (boiling method).

Analysis of Soluble Salts

This section discusses the reliability and sensitivity of field analyticaldeterminations of chloride and sulfate compared to laboratorydeterminations using arbitrary methods.

• Chlorides—On non-rusted steel, both the swabbing/test strip and theplastic patch/test strip combinations provided excess extraction efficiencies

(Table 2 , bottom): 131 percent for the former and 167-189 percent for thelatter. Again, the plastic patch method gave overestimated values.

It should be noted that the extraction of soluble salts in the boiling methodwas performed using 200 ml (7 oz) of distilled water, so the chlorideconcentration in the final extract is usually below the detection limit of thetest strips (30 ppm). Because of the low chloride concentration, theboiling/test strip method failed to detect the chloride levels applied on freshsteel (Table 2 , first column, bottom). Given that the volume of the finalextract for swabbing and plastic patch methods was less than thecorresponding volume in the boiling method (10 ml [0.35 oz] for swabbingand 4 ml [0.14 oz] for plastic patch), the chloride concentration could beabove the detection limit of the test strips.

For rusted steel, as can be seen in Table 3 , the swabbing/test strip methodfailed to detect low concentrations of chloride, and it provided values wellbelow those yielded by the arbitrary laboratory method (boiling/chlorideion-selective electrode). This result may have occurred because of thepreferential accumulation of chloride ions at the steel/rust interface,1,18-20

which hindered their removal. This result confirms the observations ofAppleman, who claimed that surface rinsing techniques such as theswabbing method extract soluble contaminants preferentially from the outersurface of the rust.4 Like the patch test, the swabbing method will fail toextract most of the soluble salts because neither the underlying rust nor thesteel/rust interface is sampled.

Even though the boiling method does not ensure complete extraction ofsoluble contaminants from the steel/rust interface, it is more efficientbecause dissolved ions are more readily soluble and mobile at hightemperatures (100 C; 212 F).4

Table 4 compares the sensitivity of the chloride analysis method (selectiveelectrode and indicator test strip) applied after using the swabbing methodto sample rust contaminated with chloride. The low sensitivity of theindicator test strip precluded detection (30 ppm detection limit) of the lowcontaminant concentrations extracted by the swabbing method. The moresensitive chloride ion-selective electrode (detection limit 1-2 ppm),however, did allow such low concentrations to be detected.

• Sulfates—For non-rusted steel, as can be seen in Table 5 (bottom), boththe swabbing/indicator test strip and the plastic patch/indicator test stripmethods provided lower values, averaging 86 percent and 74 percent,respectively, particularly at the lowest sulfate concentration (1,000 mg/m2).

For steel rusted at 2 test sites (Table 6 ), the sulfate concentration was notdetected by the swabbing-indicator test strip technique. However, thelaboratory method (boiling/turbidimetric) supplied consistent results. Aswith chloride, preferential accumulation of sulfate at the steel/rust interfacemight hinder its extraction.

Conclusions

Field sampling techniques for soluble salt contaminations (swabbing andplastic patch methods) are appropriate for use on non- rusted steelsurfaces: e.g., to determine whether the abrasive left any contaminants onthe surface or to detect residual contaminants on the steel surface afterblasting. Nevertheless, extractions by the plastic patch method lead tooverestimated chloride concentrations (50 percent average in excess), andthe swabbing method results in underestimated sulfate concentrations (byan average of about 20 percent).

Extraction efficiency on rusted steel surfaces is considerably decreased bythe difficulty of removing contaminants that usually accumulate at thesteel/rust interface. Under these circumstances, the swabbing method andpatch test provide clearly underestimated concentrations.

The extraction procedures that use a chloride indicator test strip and theferrous sulfate indicator test strip for determining soluble contaminants arefairly accurate with non-rusted steel but provide highly underestimatedvalues for rusted steel. Based on the results obtained in this work, neitherindicator test strip allows detection of concentrations below 700 mg/m2.

References

1. J.E.O. Mayne, “The Problem of Painting Rusty Steel,” Journal ofApplied Chemistry, Volume 9, December 1959, pp. 673-680.2. M. Morcillo, et al, “Some Observations on Painting Contaminated RustySteel,” JPCL, September 1987, pp. 38-43.3. L. Igetoft, “Surface Cleanliness and Durability of Anti-Corrosive Paint,”Proceedings of the 2nd World Congress: Coatings Systems for Bridges,October 26-27, 1982, University of Missouri-Rolla, Rolla, MO.4. B.R. Appleman, “Painting Over Soluble Salts: A Perspective,” JPCL,October 1987, pp. 68-82.5. D.A. Bayliss and K.A. Chandler, Steelwork Corrosion Control, ElsevierScience Publishers BU, 1991, pp. 230-239.6. A. Bresle, “New Method for Sampling and Analysis of Impurities fromSteel Surfaces,” Expertus Keniteknik private report, Stockholm, Sweden,1989. Compare reference 5.7. K.A. Trimber, “Detection and Removal of Chemical Contaminants inPulp and Paper Mills,” JPCL, November 1988, pp. 30-37.8. NACE Task Group T-6G-22, “Surface Preparation of ContaminatedSteel Surfaces,” Materials Performance, March 1987, pp. 49-54.9. ASTM D 516, Standard Test Methods for Sulfate Ion in Water andWaste Water, ASTM, Philadelphia, 1979.10. G.H. Brevoort, “Abrasive Blasting and Salt Contamination: A CaseHistory,” JPCL, May 1988, p. 24.11. Bresle method, Quantab strips and Merckoquant test, TechnicalLiterature, KTA-Tator Inc., Pittsburgh, PA.12. Internal Report, ISO TC35/SC12/WG2 N 143, “Sampling of SolubleImpurities on Surfaces to be Painted—The Bresle Method,” 1991.

13. L. Uller and M. Morcillo, “The Setting Up of an IberoAmerican Mapof Atmospheric Corrosion. MICAT Study—CYTED-D,” Proceedings ofthe 11th International Corrosion Congress, Florence, Italy, 1990, Volume2, p. 2.35.14. M. Morcillo, “Ibero-American Map of Atmospheric Corrosiveness(MICAT). Preliminary Results,” Proc. 1st Pan-American Congress onCorrosion and Protection, Mar del Plata (Argentina), October 1992, Ed.NACE, Houston, TX, Vol. 1, pp. 211-225.15. SSPC Vis-1, Pictorial Surface Preparation Standards for Painting SteelSurfaces; SSPC Vis-3, Visual Standards for Power- and Hand-ToolCleaned Steel; and SSPC-SP 2, Hand Tool Cleaning, SSPC, Pittsburgh,PA, 1982.16. SIS 055900. Pictorial Surface Preparation Standards for Painting SteelSurfaces, Swedish Standards Institute, Stockholm, 1967. ISO 8501performed as SIS 1967 version—no longer printed, but still can be used.17. Evaluation of the Bresle method for detecting soluble salts on steel,KTA-Tator Inc., personal communication, 1992.18. B.J. Harrison and T.C. Tickle, “New Aspects of the AtmosphericCorrosion of Steel and their Implications,” J. Oil and Colour Chem.Assoc., August, 1962, pp. 571-590.19. A. Bresle, “The Corrosion of Steel and the Dangerous Chlorides,”Metal Finishing, August 1976, pp. 23-30.20. U.R. Evans, “Electrochemical Mechanism of Atmospheric Rusting,”Nature, June 1965, pp. 980-982.

March 1994