About LTBP

This research was conducted 

as part of the Federal Highway 

Administration’s Long-Term 

Bridge Performance (LTBP) 

Program. The LTBP Program is 

a minimum 20-year research 

effort to collect scientific performance field data, from 

a representative sample of 

bridges nationwide, that will 

help the bridge community 

better understand bridge 

deterioration and performance. 

The products from this program will be a collection of 

data-driven tools including 

predictive and forecasting 

models that will enhance the 

abilities of bridge owners to 

optimize their management 

of bridges.

SUMMARY REPORT

Research, Development, and Technology | Turner-Fairbank Highway Research Center | 6300 Georgetown Pike, McLean, VA 22101-2296

http://www.fhwa.dot.gov/research/

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. 

FHWA Publication No.: FHWA-HRT-14-026

FHWA Contacts: Paul Y. Virmani, HRDI-60, (202) 493-3052, paul.virmani@dot.gov; and 

Justin Ocel, HRDI-40, (202) 493-3080, justin.ocel@dot.gov.

This document presents the result of a laboratory study of methodology for extraction and analysis 

of soluble salts from steel substrates.

Methodology for Analysis of 

Soluble Salts from Steel Substrates
2

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. Ionspecific 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 g/cm2 chloride concentration on the surface, the chloride recovery rate 

by 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 patch and sleeve 

methods were also evaluated. Acidic solutions 

were used as extraction liquid in patch cell or 

sleeve. The chloride recovery rates from freshly 

doped steel surfaces (no aging) with 30 g/cm2

were 101 percent for the patch test method and 

107 percent for the sleeve test method. When the 
3

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 g/cm2; it was 

about 1 g/cm2 for patch/titration, and about 

5 g/cm2 for sleeve/chloride ion detection tube.

Appleman classified chloride levels on blast 

cleaned steel.(9) Chloride levels above 50 g/cm2

were considered highly contaminated, levels 

below 30 g/cm2 were classified as low chloride, 

and levels between 30 and 50 g/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 g/cm2

or greater, the surface has to be re-cleaned; if the 

chloride concentration was less than 10 g/cm2, 

the surface was considered clean. Chloride levels 

between 10 and 50 g/cm2 indicated the surface 

cleanliness was marginal.

Research Objective

The objective of this study is to evaluate extraction and analysis methods for determining the 

amount of soluble salts on steel substrates.

Three extraction methods were used in this 

study. The first one was a paint test cell. A glass 

tube was clamped on to a steel plate, and an 

O-ring kept the steel plate/glass tube watertight. 

The second one was the latex sleeve method, 

which is part of a field test kit. The third extraction method was the boiling method, boiling steel 

plate in deionized water.

Extraction solutions were analyzed by ion chromatography. Unless specifically noted, all the 

ionic concentration measurements were carried 

out with ion chromatography method. Anion 

concentration was used to represent the amount 

of salts in a sample. The three anions that commonly exist on steel bridge surfaces are chloride, 

sulfate, and nitrate. Ion chromatography can 

determine the concentrations of all three anions 

simultaneously with accuracy below 1 ppm.

Ion Chromatography as an 

Analysis Method

Ion chromatography is based on the principle of 

separating ionic species when a liquid mobile 

phase moves past a stationary phase.(10) The 

liquid phase carries the sample to be tested, 

and the stationary phase is made of permeable 

polymer. The stationary phase is composed 

of ion-exchange polymer particles packed in a 

column. Anions or cations are separated by differences in the rate at which they pass through 

the column. A reversible exchange of ions 

between a liquid phase and a stationary phase 

takes place in the column. The time a specific ion 

spent in the column, or retention time, is based 

on the attraction between the ion and charged 

sites on the stationary phase. An anion exchange 

column contains positively charged groups on the 

stationary phase, attracting anions in the mobile 

phase. Different anions can be separated because 

they have different affinities towards the stationary phase. To separate two ions, one of the ions 

must be preferably adsorbed by the stationary 

phase so that there is a significant difference in 

the retention time between the two ions.

Ions are detected by electrical conductivity after 

they are separated by the ion-exchange column. 

The concentration of the ions to be tested is 

relatively low, and the high concentration of the 

eluent can obscure accurate detection of the ions. 

In suppressed ion chromatography, the background conductivity of the eluent is reduced prior 

to conductivity measurement. For anion analysis 

using KOH as eluent, the solution passes through 

a cation-exchange membrane suppressor, and 

cations K+ are replaced by hydrogen ions H+. 

The cations K+ diffuse out of the membrane. 

OH- and H+ react to form H2O, which has low 

conductivity. With a low conductivity solvent 

as background, anions in the solution can be 

detected accurately.
4

Experiment Details

Specimens

Two types of steel plates were used in the 

study, A588 and A710. The A588 steel was 1⁄4 inch 

(6.35 mm) thick, virgin material fresh out of 

the mill. The A710 steel was previously stored 

outdoors under natural exposure, and its surface 

was rusted and pitted. It was 3/8 inch (9.525 mm) 

thick.

Rectangular steel test panels, 4 by 6 by 1⁄2 inch 

(101.6 by 152.4 by 12.7 mm) each, were prepared 

in the lab. The panels were cut to size with a band 

saw, and then blast cleaned with glass/aluminum 

oxide abrasive to near-white cleanliness.(11)

Glass panels of 4-by-6-inch (10.16-by-15.24-cm) 

dimension were used as reference surfaces. The 

glass panels were cleaned and dried after each 

test cycle. The test panels, curing condition, and 

doping levels are presented in table 1.

Steel Panel Surface

Scanning electron microscope (SEM) images of 

the steel surface were taken after blast cleaning 

to examine surface features and cleanliness. 

Figures 1, 3, and 5 are images of virgin A588 steel 

at low, intermediate, and high magnification, 

respectively. Figures 2, 4, and 6 are images of 

A710 steel at low, intermediate, and high magnification, respectively.

Methods

Chloride Concentration

(Cl- in g/cm2)

Cl-, SO42-, and NO3-

(Concentration of each 

individual ion, in g/cm2)*

Total 

Number 

of 

Panels

1 3 5 10 30 1 5 30

No Aging

Glass Panels 3 3 3 3 3 3 3 3 24

Virgin A588 

Steel Panels 3 3 3 3 3 3 3 3 24

Rusted and 

Pitted A710 

Steel Panels

3 3 3 3 3 3 3 3 24

Aging at 

98.6 °F 

(37 °C) and 

78 percent 

Relative 

Humidity

Glass Panels 3 3 3 3 3 3 3 3 24

Virgin A588 

Steel Panels 3 3 3 3 3 3 3 3 24

Rusted and 

Pitted A710 

Steel Panels

3 3 3 3 3 3 3 3 24

Subtotal 18 18 18 18 18 18 18 18 Total: 

144

Table 1. Test panels and extraction samples.
5

Figure 1. Low resolution SEM image of virgin A588 steel surface after blast cleaning.

Figure 2. Low resolution SEM image of A710 steel surface after blast cleaning.
6

Figure 3. Intermediate resolution SEM image of virgin A588 steel surface 

after blast cleaning.

Figure 4. Intermediate resolution SEM image of A710 steel surface after blast cleaning.
7

Figure 5. High resolution SEM image of virgin A588 steel surface after blast cleaning.

Figure 6. High resolution SEM image of A710 steel surface after blast cleaning.
8

Doping and Aging

Doping solutions contained either chloride as 

anion, or a combination of chloride, nitrate, and 

sulfate as anions. The latter was denoted as a 

3-anion solution in this study. The concentration 

of these three anions was equal in the solution. 

For example, a 1 ppm, 3-anion solution had chloride, nitrate, and sulfate concentration all at 1 ppm. 

Steel panels and glass panels were doped with 

chloride solution or a solution containing chloride, 

nitrate, and sulfate. The purpose of using glass 

panels is primarily for evaluating the accuracy of 

extraction methods. A small amount of solution 

was sprayed on the surface within a 1.38-inch 

(3.5-cm)-diameter circle. A fine graduated syringe 

was used to regulate the volume to be dropped 

on the surface. The panels were doped with salt 

solutions to achieve surface concentrations of 

1, 3, 5, 10 and 30 g/cm2. For each concentration 

level, three doped specimens were prepared. An 

example of calculating ion concentration on the 

surface is given below.

The salt concentration on steel surface is controlled by the dosage of the doping solution and 

the surface covered by the doping solution. Let:

D = concentration of doping solution, ppm (10-6).

V = volume of doping solution, mL.

A = doping area, cm2.

 = density of doping solution, g/mL (use water 

density at room temperature, 106 g/mL).

C = concentration of salt on the surface, g/cm2.

Then the concentration of salt on the surface is:

To calculate the volume of solution needed for a 

certain surface concentration, we rearrange the 

equation to the following:

For example, to create a 5 g/cm2 chloride concentration in a 14.6 cm2

 area using a 100 ppm 

chloride solution, the volume needed for doping is:

As shown in the example in figure 9, 0.73 mL of 

the solution is needed.

Following the doping process, the plates were 

maintained for 4 h in two different conditions. 

This process was named as “aging.” One condition was laboratory exposure at room temperature and humidity, and the other condition was in 

an environmental chamber at 100.4 oF (38 oC) and 

78 percent relative humidity.

Extraction 

Soluble salts were extracted from the doped 

panel with a paint test cell, as shown in figure 10. 

The device included a glass tube, an O-ring seal, 

a nonstick base, and a stainless steel clamp. 

The panel was clamped between the glass tube 

and the nonstick base; the O-ring seal kept the 

apparatus watertight. 1.69 fl oz (50 mL) of deionized water was poured into the glass tube. A 

glass rod was used to stir the liquid in the glass 

tube. After 2 min of stirring, the liquid was poured 

into a beaker. 

The latex sleeve was also used for extraction in this 

study. The CHLOR*RID International field test 

kit has a latex sleeve and specialty extraction 

solution for analyzing the steel specimens. 

The proprietary extraction solution is claimed 

to “enhance retrieval rates.”(12) The latex sleeve 

had a self-adhesive pad with 1.55 inch2 (10 cm2) 

opening, therefore the extraction area was 

1.55 inch2 (10 cm2).

 Figure 8. V equals the product of C times A divided 

by the product of D times rho.

V = C  A

D  

 Figure 7. C equals the product of D times V times rho 

divided by A.

C = D  V  

A

 Figure 9. V equals the product of 5 g/cm2

 times

14.6 cm2 divided by the product of 100×10-6 times 

106 g/mL.

 V = 5 g/cm2

 14.6 cm2 

= 0.73mL

(100  10-6)  106 g/mL

1.0 g/cm2 = 2.3×10-7 oz/inch2.

Water density = 1.043 oz/fl oz.

1 fl oz = 29.57 mL.
9

The boiling extraction method was also used 

to determine the amount of salts on steel panels after blast cleaning. The steel panel was immersed in deionized water and boiled for 1 h. 

The surfaces of the plate were thoroughly 

rinsed and the liquid collected for testing 

concentration of salts. This method was not used 

to recover salts on doped panels.

Filtration 

The extracted solution was filtered using a syringe 

filter with a pore size of 0.2 m. The syringe, filter, 

and sample vial were washed with deionized 

water before the filtration process. The final liquid 

sample was clean and transparent.

Steel Surface after Extraction 

Figure 11 shows a virgin A588 steel surface, and 

figure 12 shows an A710 steel surface, before the 

doping and extraction processes. Figure 13 and 

figure 14 show the surfaces afterward. With abrasive blast cleaning, the virgin A588 steel had a 

smooth and clean surface, while the pitted and 

rusted A710 steel surface was relatively rough. 

Blast cleaning has removed all rust from the steel 

surface. After the doping and extraction process, 

the A710 steel surface had significantly more rust 

stains than the virgin A588 steel.

Figure 10. Extraction setup using a paint test cell.

Figure 11. Microscopic image of virgin A588 steel surface before doping/extraction process.
10

Figure 12. Microscopic image of A710 steel surface before doping/extraction process.

Figure 13. Microscopic images of virgin A588 steel surface after doping/extraction process.
11

Findings and Discussion

Extraction from Glass Panels

Glass panels were doped with salt solutions to 

serve as a reference for examining extraction 

and analysis methods. As shown in figure 15, the 

extraction rate for all three anions is consistent 

among the tested specimens with salt concentrations ranging from 1 to 30 g/cm2

.

As shown in figure 16, the extraction rate for 

chloride, either doped with single anion solution 

(contains only chloride as anion) or with 3-anion 

solution (contains chloride, sulfate, and nitrate as 

anions,) is consistent. Therefore, the extraction 

and analysis method are effective. Temperature 

and relative humidity did not affect the extraction 

rate, as shown in figure 17 and figure 18.

Figure 14. Microscopic image of A710 steel surface after doping/extraction process.
12

Figure 15. Extraction from glass panels doped with solutions containing 3 anions. Aging at 

room temperature.

Figure 16. Extraction of chloride from doped glass panels. Aging at room temperature.

1.0 g/cm2 = 2.3×10-7 oz/inch2

1.0 g/cm2 = 2.3×10-7 oz/inch2
13

Figure 17. Extraction from doped glass panels. Aging at 100.4 oF (38 oC) and 78 percent 

relative humidity.

Figure 18. Extraction of chloride from doped glass panels. Aging at 100.4 oF (38 oC)

and 78 percent relative humidity.

1.0 g/cm2 = 2.3×10-7 oz/inch2

1.0 g/cm2 = 2.3×10-7 oz/inch2
14

Background Salt Concentrations on Steel Panels

There is no method available that can extract all 

salt contaminants from steel surfaces, especially 

for rusted steel substrate with pits. The effectiveness of each extraction method varies. For this 

study, three extraction methods were used to 

determine the background salt contaminants on 

steel panels, and the results are presented below.

Determining Background Salt Concentration 

Using Paint Test Cell Extraction Method

Background soluble salts on virgin A588 steel 

panels are presented in table 2. The results 

are based on measurements of 12 panels. On 

average, chloride concentration was found to 

be 0.15 g/cm2, nitrate 0.03 g/cm2, and sulfate 

0.20 g/cm2. Concentrations of the three anions 

vary substantially among the tested specimens, 

as indicated by the standard deviation of the ion 

concentration measurements. The amount of 

chloride ranges from none to 0.33 g/cm2, nitrate 

from none to 0.20 g/cm2, and sulfate from none 

to 0.65 g/cm2. Chloride and sulfate were found 

on most of the specimens. However, nitrate was 

not detected on the surface of most specimens. 

Among the 12 specimens tested, nitrate was 

detected only on 2.

The amount of soluble salts on pitted A710 steel 

panels varies significantly, as shown in table 3. 

A total of 21 panels were tested. Concentrations 

of the three anions vary substantially among 

the tested specimens. The amount of chloride 

ranges from 0.29 to 4.28 g/cm2. On average 

chloride is 1.36 g/cm2, but the standard deviation is 1.01g/cm2

. For nitrate, the average is 

0.23 g/cm2, yet with a standard deviation of 

0.27 g/cm2. Sulfate concentration is 2.09 g/cm2, 

and the standard deviation is 1.56 g/cm2.

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 0.15 0.03 0.20

Standard Deviation 0.11 0.08 0.17

Max 0.33 0.20 0.65

Min 0.00 0.00 0.00

Table 2. Background soluble sales on virgin A588 steel with paint test cell extraction method.

1.0 g/cm2 = 2.3×10-7 oz/inch2

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 1.36 0.23 2.09

Standard Deviation 1.01 0.27 1.56

Max 4.28 0.83 7.79

Min 0.29 0.00 0.62

Table 3. Background soluble salts on pitted A710 steel with paint test cell extraction method.

1.0 g/cm2 = 2.3×10-7 oz/inch2
15

Determining Background Salt Concentration 

Using Latex Sleeve Extraction Method

The latex sleeve extraction method uses a 10 mL 

proprietary extraction liquid. The background 

concentration of virgin A588 steel is based on 

measurements from five panels, and the results 

are shown in table 4. Chloride concentration on 

the virgin A588 steel surface varies from 1.75 to 

2.48 g/cm2 with an average value of 2.05 g/cm2

and the standard deviation is 0.30 g/cm2. The 

average nitrate concentration is 1.45 g/cm2, and 

varies from 0.96 to 2.13 g/cm2 with a standard 

deviation 0.48g/cm2. For sulfate, the average 

concentration is 5.40 g/cm2, and varies from 

5.04 to 5.98 g/cm2 with a standard deviation 

0.34 g/cm2.

The background concentration of rusted and pitted A710 steel panels is shown in table 5. The 

results are based on measurements of five panels. 

Chloride concentration on the rusted and pitted 

A710 steel surface varies from 2.02 to 2.38 g/cm2, 

with an average value of 2.22 g/cm2 and 

a standard deviation of 0.15g/cm2. The average 

nitrate concentration is 1.52 g/cm2, and varies 

from 1.35 to 2.03 g/cm2 with a standard 

deviation 0.29 g/cm2. For sulfate, the average 

concentration is 5.24 g/cm2, and varies from 

4.72 to 5.94 g/cm2 with a standard deviation 

0.47 g/cm2.

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 2.05 1.45 5.40

Standard Deviation 0.30 0.48 0.34

Max 2.48 2.13 5.97

Min 1.75 0.96 5.04

Table 4. Extraction from virgin A588 steel panels with latex sleeve method.

1.0 g/cm2 = 2.3×10-7 oz/inch2

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 2.22 1.52 5.24

Standard Deviation 0.15 0.29 0.47

Max 2.38 2.03 5.94

Min 2.02 1.35 4.72

Table 5. Extraction from pitted A710 steel with latex sleeve extraction method.

1.0 g/cm2 = 2.3×10-7 oz/inch2
16

Determining Background Salt Concentration 

Using Boiling Method

Steel panels were boiled in deionized water for 

1 h to extract soluble salts on the surface. The 

background concentration of virgin A588 steel is 

based on measurements of three panels, and 

the results are shown in table 6. Chloride concentration varies from 0.41 to 0.78 g/cm2

, with an 

average value of 0.61 g/cm2 and a standard 

deviation of 0.15 g/cm2. Nitrate was not detected 

in the extraction solution. For sulfate the average 

concentration is 0.57 g/cm2, and varies from 

0.52 to 0.64 g/cm2 with a standard deviation of 

0.05 g/cm2.

The background concentration of rusted and 

pitted A710 steel panels is shown in table 7. 

The results are based on measurements of three 

panels. Chloride concentration on the rusted 

and pitted A710 steel surface varies from 3.36 to 

3.99 g/cm2, with an average value of 3.62 g/cm2

and a standard deviation of 0.24 g/cm2. Nitrate 

was not detected in the extraction solution. For 

sulfate, the average concentration is 6.45 g/cm2, 

and varies from 5.30 to 7.06 g/cm2 with a 

standard deviation of 0.85 g/cm2.

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 0.61 0.00 0.57

Standard Deviation 0.15 0.00 0.05

Max 2.48 0.00 0.64

Min 1.75 0.00 0.52

Table 6. Extraction from virgin A588 steel panels using boiling method.

1.0 g/cm2 = 2.3×10-7 oz/inch2

Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Mean 3.62 0.00 6.45

Standard Deviation 0.24 0.00 0.85

Max 3.99 0.00 7.06

Min 3.36 0.00 5.30

Table 7. Extraction from rusted and pitted A710 steel panels using boiling method.

1.0 g/cm2 = 2.3×10-7 oz/inch2
17

Background Contamination Comparison

The effectiveness of each extraction method 

varies significantly. Table 8 shows the measured 

average concentration of contaminants from the 

three extraction methods. All steel panels are 

virgin A588 steel. Using a latex sleeve and the 

specialty extraction solution, all three contaminants are extracted from the steel surface 

with higher concentration than the other two 

methods. For example, the average chloride 

concentration is 2.05 g/cm2 based on the latex 

sleeve method, while the boiling method measures chloride concentration as 0.61 g/cm2

, and 

the paint test cell detects 0.15 g/cm2. For sulfate, 

the average chloride concentration is 5.40 g/cm2

from the latex sleeve method, 0.61 g/cm2 from 

the boiling method, and 0.15 g/cm2 from the 

paint test cell method. The boiling method does 

not detect any nitrate, while the paint test cell 

method measures the nitrate concentration 

as 0.03 g/cm2. The latex sleeve method finds 

1.45 g/cm2 of nitrate on the steel surface.

The extraction of the specialty solution used in the 

latex sleeve method is still effective, but diminished nonetheless when it is used on the rusted 

and pitted A710 steel panels, as shown in table 9. 

The boiling method extracts chloride and sulfate 

more efficiently than the other methods. However, 

it is completely inadequate to extract nitrate. 

The latex sleeve method extracts 1.52 g/cm2

nitrate from the steel surface, while the paint test 

cell method gets 0.23 g/cm2. The chloride concentration measured with the boiling method is 

3.62 g/cm2, 2.22 g/cm2 from the latex sleeve 

method, and 1.36 g/cm2 from the paint test cell 

method. The sulfate concentration measured with 

the boiling method is 6.45 g/cm2, 5.24 g/cm2

from the latex sleeve method, and 2.09 g/cm2

from the paint test cell method.

Mean Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Paint Test Cell 0.15 0.03 0.20

Latex Sleeve 2.05 1.45 5.40

Boiling 0.61 0.00 0.57

Table 8. Extraction from virgin A588 steel panels using different methods.

1.0 g/cm2 = 2.3×10-7 oz/inch2

Mean Contaminant Concentration (g/cm2)

Contaminant Chloride Nitrate Sulfate

Paint Test Cell 1.36 0.23 2.09

Latex Sleeve 2.22 1.52 5.24

Boiling 3.62 0.00 6.45

Table 9. Extraction from rusted and pitted A710 steel panels using different methods.

1.0 g/cm2 = 2.3×10-7 oz/inch2
18

The difference in extraction rates among the test 

methods is not fully understood. For the paint 

test cell method, extraction takes place at room 

temperature rather than elevated temperature. 

The extraction media is water, which may not 

dissolve as much salt as the specialty solution. 

On the other hand, the specialty extraction solution seems to be less effective for extracting 

salts from rust. As a field deployable method, the 

latex sleeve with a specialty extraction solution 

appears to be a good choice, especially for testing freshly cleaned steel without pits. The boiling 

method is considered a reliable method in the 

laboratory, and it can effectively extract chloride 

and sulfate from rusted and pitted steel. However, 

special attention has to be paid when it is used to 

determine nitrate contamination on steel. 

Extraction from Doped Steel 

Panels

Doped Virgin A588 Steel

Virgin A588 steel plates were doped with salt 

solutions to achieve surface concentration of ions 

ranging from 1 to 30 g/cm2. The background 

contamination was subtracted when the recovery 

rate was calculated. The paint test cell method 

was used to extract salts from the steel surface. 

Since background contamination varies among 

extraction methods, for this purpose the background concentration obtained with the paint test 

cell was applied. This was considered appropriate, as we were interested in water-soluble salts 

on the steel surface.

Figure 19 shows the recovery rate of doped specimens that were aged for 4 h at room temperature. The recovery rate for chloride ranges from 

89 percent to 118 percent among the 5 doping 

levels, with average recovery rate at 100 percent. 

The standard deviation is larger among low doping concentration specimens. The recovery rate 

for nitrate is between 47 percent and 77 percent. 

For sulfate, the recovery rate varies from 

73 percent to 104 percent. Standard deviation of 

the recovery rate for nitrate and sulfate varies 

widely among the tested samples. 

Chloride ions were retrieved from chloride doped 

specimens as efficiently as from specimens 

doped with 3-anion combined solutions, as shown 

in figure 20. Chloride was extracted from the 

steel surface in all cases, irrespective of whether 

the steel plate was doped with chloride only, 

or 3-anion combination solutions. The average 

Figure 19. Recovery of salts from doped A588 steel panels. Aging for 4 h at room temperature.

1.0 g/cm2 = 2.3×10-7 oz/inch2
19

recovery rate for chloride-doped specimens is 

100 percent, and 98 percent for 3-anion doped 

specimens. The standard deviation is also very low.

When aging conditions changed from room 

temperature and humidity to 100.4 oF (38 oC) and 

78 percent relative humidity, the recovery rate 

for nitrate and sulfate changed significantly. 

Figure 21 presents the recovery rate of anions 

from virgin A588 steel panels that were doped 

with single anion solution. Chloride demonstrates 

a high recovery rate of 98 percent with very small 

standard deviation. The recovery rate for nitrate 

varies from zero to 55 percent. Sulfate has a 

recovery rate between 78 percent and 104 percent. 

At doping level 1 g/cm,2 nitrate was not detected 

in the extraction solution. At 5 g/cm,2 about 

10 percent was recovered from the surface. 

When doping levels increased to 10 and 

30 g/cm,2 the nitrate recovery rate was about 

40 percent and 55 percent, respectively. The sulfate recovery rate fluctuated among the tested 

specimens, but on average, about 90 percent 

sulfate was extracted from the surface.

For A588 virgin steel panels doped with 3-anion 

solutions, chloride maintains a stable recovery 

rate at all doping levels, as shown in figure 22. 

Among 3-anion doped specimens, the average 

recovery rate for chloride is 96 percent. The 

recovery rates for chloride have small standard 

deviation.

Figure 20. Chloride recovery rate from doped virgin A588 steel panels. Aging for 4 h at room 

temperature.

1.0 g/cm2 = 2.3×10-7 oz/inch2
20

Figure 21. Recovery of salts from doped virgin A588 steel panels. Aging for 4 h at 

100.4 oF (38 oC) and 78 percent relative humidity.

Figure 22. Recovery rate of chloride from doped virgin A588 steel panels. Aging for 4 h at 

100.4 oF (38 oC) and 78 percent relative humidity.

1.0 g/cm2 = 2.3×10-7 oz/inch2

1.0 g/cm2 = 2.3×10-7 oz/inch2
21

Doped A710 Steel

After doping and aging in a laboratory environment, the average recovery rate for chloride 

from the rusted and pitted A710 steel panels was 

comparable to that of virgin A588 steel panels, 

as shown in figure 23. Chloride recovery rate varies 

between 73 percent and 100 percent, with an 

average of 91 percent among the 5 doping levels. 

The recovery rate for sulfate fluctuates between 

0 and 99 percent, with an average of 60 percent. 

The recovery rate for nitrate is from 0 to 

22 percent, with an average of 7 percent. The 

fluctuation of chloride and sulfate recovery rate 

may be attributed to the background contamination of individual specimens. Nitrate demonstrated a rather different scenario, involving 

factors beyond the background difference of 

specimens. The amount of nitrate in most extraction solutions was not detectable. Of those 

detectable, it is possible that background nitrate 

existing on steel prior to doping was released to 

the extraction solution and contributed to the total 

amount of nitrate in the solution.

The average chloride recovery rate was not 

significantly affected by the existence of sulfate 

and nitrate on doped A710 steel panels, as shown 

in figure 24. When doped with chloride only, the 

chloride recovery rate is in the range of 73 percent 

and 100 percent, with an average of 91 percent. 

When doped with 3-anion solutions, the recovery 

rate is between 76 percent and 132 percent, with 

an average of 104 percent.

The recovery rates for doped A710 steel panels 

after being aged at 100.4 oF (38 oC) and 78 percent 

relative humidity for 4 h were also examined. 

The rusted and pitted A710 steel panels did not 

release any nitrate into the extraction solution, 

as shown in figure 25. Recovery rate for sulfate 

varied from 37 percent to 118 percent, with an 

average of 66 percent. Only chloride maintained 

a relatively consistent recovery rate, ranging 

from 96 percent to 106 percent, with an average 

of 101 percent.

Recovery rate of chloride was not affected by 

the presence of sulfate and nitrate, as shown in 

figure 26. Recovery rate for A710 steel panels 

doped with chloride varies from 92 percent to 

98 percent, with an average of 95 percent. The 

chloride recovery rate for panels doped with 

3-anion solution varies between 96 percent and 

103 percent, with an average of 99 percent.

Figure 23. Recovery of salts from pitted steel panels. Aging for 4 h at room temperature.

1.0 g/cm2 = 2.3×10-7 oz/inch2
22

Figure 24. Recovery rate of chloride from pitted A710 steel panels. Aging for 4 h at room 

temperature.

Figure 25. Recovery of salts from pitted A710 steel panels. Aging for 4 h at 100.4 oF (38 oC) 

and 78 percent relative humidity.

1.0 g/cm2 = 2.3×10-7 oz/inch2

1.0 g/cm2 = 2.3×10-7 oz/inch2
23

Figure 26. Recovery rate of chloride from pitted A710 steel panels. Aging for 4 h at 100.4 oF 

(38 oC) and 78 percent relative humidity.

1.0 g/cm2 = 2.3×10-7 oz/inch2

Summary

• The paint test cell was used in the laboratory 

for extracting water-soluble salts from steel 

plate surfaces. Its effectiveness in retrieving 

soluble salts was tested with glass panels. The 

extraction of chloride, nitrate, and sulfate from 

glass panels was seasonable and consistent.

• Recovery rate of chloride ion was above 

90 percent for both virgin A588 steel and 

pitted A710 steel panels. Aging condition and 

existence of other anion species did not affect 

chloride recovery rate.

• Fluctuation of sulfate ion recovery rate could 

be attributed to a wide range of background 

sulfate concentration of specimens.

• Nitrate recovery rate was low for specimens 

with doping concentration below 30 g/cm,2 

The average recovery rate was about 

50 percent for virgin A588 steel panels. No 

substantial amount of nitrate ion was retrieved 

from pitted A710 steel specimens.

• The boiling method has better efficiency than 

paint test cell method. Both methods used 

deionized water as extracting media.

• The proprietary chloride extraction solution 

retrieved more soluble salts from steel surfaces 

than deionized water.

• Microscopic images revealed that the surface 

of steel specimens was porous, therefore it 

could provide much larger surface area than a 

smooth surface. This might lead to adsorption 

or absorption of some ions by steel.

References

1. SSPC (2013). SSPC Guide 15—Field Methods 

for Extraction and Analysis of Soluble Salts on 

Steel and Other Nonporous Substrates, The 

Society for Protective Coatings, Pittsburgh, 

PA.

2. Chong, S., Yao, Y., and Rosario, M. (2003). 

“Intra-Laboratory Assessment of Commercial 

Test Kits for Quantifying Chloride on Steel 

Surfaces,” Journal of Protective Coatings and 

Linings, pp. 42–60.

3. Chong, S., Yao, Y. and Lee, S. (2007). “Use 

of Conductivity Measurements to Estimate 

Chloride Concentrations on Steel or Painted 

Surfaces, Journal of Protective Coatings and 

Linings, pp. 23–27.
24

Researchers—This study was performed by Dr. Rongtang Liu, a senior chemist and corrosion 

engineer of SES Group and Associates, LLC. For more information about this research, contact 

Dr. Paul Y. Virmani, HRDI-60, 202-493-3052, paul.virmani@dot.gov or Dr. Justin Ocel, HRDI-40, 

202-493-3080.

Distribution—This summary report is being distributed according to a standard distribution. 

Direct distribution is being made to the Divisions and Resource Center. 

Availability—This summary report may be obtained from the FHWA Product Distribution Center 

by email to report.center@dot.gov, fax to 814-239-2156, phone to 814-239-1160, or online at http://

www.fhwa.dot.gov/research.

Key Words—Soluble salts, Corrosion, Protective coating, Extraction, Ion chromatography, Ion, 

Anion, Chloride, Sulfate, Nitrate.

Notice—This document is disseminated under the sponsorship of the U.S. Department of 

Transportation in the interest of information exchange. The U.S. Government assumes no liability 

for the use of the information contained in this document. The U.S. Government does not endorse 

products or manufacturers. Trademarks or manufacturers’ names appear in this summary report 

only because they are considered essential to the objective of the document.

Quality Assurance Statement—The Federal Highway Administration provides high-quality 

information to serve Government, industry, and the public in a manner that promotes public 

understanding. Standards and policies are used to ensure and maximize the quality, objectivity, 

utility, and integrity of its information. FHWA periodically reviews quality issues and adjusts its 

programs and processes to ensure continuous quality improvement. 

JANUARY 2014 FHWA-HRT-14-026

HRDI-60/01-14(WEB)E

4. Neal, D. and Whitehurst, T. (1995) “Chloride 

Contamination of Line Pipe: Its Effect on FBE 

Coating Performance,” Materials Performance,

pp. 47–52.

5. Appleman, B. (1987). “Painting Over Soluble 

Salts: A Perspective,” Journal of Protective 

Coatings and Linings, pp. 68–82.

6. Alblas, B. and van Londen, A. (1997). “The 

Effect of Chloride Contamination on the 

Corrosion of Steel Surfaces: A Literature 

Review,” PCE.

7. Flores, S., Simancas, J., and Morcillo, 

M. (1994). “Methods for Sampling and 

Analyzing Soluble Salts on Steel Surfaces: 

A Comparative Study,” Journal of Protective 

Coatings and Linings, pp. 76–83.

8. Appleman, B., and Weaver, R. (1992). 

Maintenance Coating of Weathering Steel: 

Interim Report, Report No. FHWA-RD-91-087, 

Federal Highway Administration, McLean, VA.

9. Appleman, B., Weaver, R., and Bruno, J. 

(1995). Maintenance Coating of Weathering 

Steel: Field Evaluation and Guidelines, Report 

No. FHWA-RD-92-055, Federal Highway 

Administration, McLean, VA.

10. Weiss, J. (2004). “Handbook of Ion Chromatography,” Wiley-VCH Verlag GmbH & Co. 

KGaA, Weinheim, Germany.

11. SSPC (2007). Joint Surface Preparation 

Standard SSPC-SP 10/NACE No. 2—NearWhite Metal Blast Cleaning, The Society for 

Protective Coatings, Pittsburgh, PA.

12. Chlor*Rid International, Inc., Product 

Information, accessed at http://www.chlor-rid.

com.