Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials (2021)

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7 1.1 Introduction Electrochemical properties of earthen materials such as electrical resistivity, pH, salt con- centrations, and organic content are commonly used to characterize their corrosion potential. AASHTO test standards, adopted in the early 1990s, are among the most common practices in the United States to determine the electrochemical properties of earthen materials. However, the AASHTO test methods do not apply to all earthen materials, which encompass a broad range of physical and electrochemical characteristics, nor do they distinguish issues inherent to particular types of infrastructure construction. For example, AASHTO T 288 is used to determine the minimum resistivity (rmin) of earthen materials at a saturated or slurry state. However, a slurry state does not represent a condition that occurs during the effective service lives of earth retaining structures. The minimum resistivity obtained from such a test is not representative of the resistivity of earthen materials experienced at any time during the service lives of metal elements placed within them. Also, AASHTO T 288 specifies how test specimens are prepared by separating the sample into fractions according to particle size and only including the fraction passing the No. 10 sieve in the test specimen. Resistivity measurements for earthen materials are affected by soil texture; thus, including only the finer portion of the sample within the test specimen renders resistivity measurements that are different from what would be measured if all of the particle sizes inherent to the sample were included. The AASHTO tests may not be appropriate for determining the corrosivity of coarser types of earthen materials and do not consider prac- tical limits on moisture contents that may be experienced in the field. Results from the current AASHTO practices cannot be interpreted beyond establishing a common reference point for comparing the corrosivity of different soils under laboratory conditions. This research eval- uated alternative test methods that may be more appropriate for particular applications—for example, mechanically stabilized earth (MSE) walls—and considered a wider range of fill types incorporating larger particle sizes. 1.2 Research Objectives The main goal of this research was to develop a test protocol for characterizing the potential for in-service corrosion of steel in earthen materials. The objectives needed to achieve this goal were as follows: 1. Identify, sample, and characterize representative earthen materials; 2. Determine the effects of different techniques for electrochemical measurement and different procedures for specimen preparation (e.g., aggregate size) on the measured electrochemical properties of compacted soil or leachates extracted from the solid samples; 3. Establish links between laboratory and field measurements for proper interpretation of laboratory test results; and C H A P T E R 1 Background

8 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials 4. Develop a test protocol and corresponding characterization of corrosion potential that more accurately reflects the corrosivity of earthen materials as compared with the conven- tional methods. In pursuit of these objectives, the following questions and technical challenges were addressed: • Quantifying sampling and testing errors associated with measurements of electrochemical properties, considering – Diversity in the site and environmental conditions, earthen materials used in construction, and construction practices and – Variations in test procedures for measuring electrochemical properties. • Evaluating how the composition of earthen materials and water chemistry affects measure- ments of electrochemical properties, considering – The porosity, mineralogy, and texture (tortuosity) of earthen materials and – The relevance of laboratory tests, test parameters, and sample preparation techniques to actual field conditions and applications. • Relating laboratory measurements of electrochemical properties to performance observed in field measurements, considering – How well sampling strategies and the end points for laboratory measurements (e.g., the final moisture content) apply to the specific field conditions; – Material characterizations that are consistent with the available field performance data; – The reliability of laboratory and field tests; – Service life design and asset management practices for transportation infrastructure; and – Barriers to implementation. 1.3 Review of Current Practices for the Characterization of the Corrosion Potential of Earthen Materials This section summarizes the current test methods used to evaluate the corrosivity of earthen materials and presents the suggested interpretation of results. Laboratory tests for measurements of (a) electrical resistivity, (b) chloride content, (c) sulfate content, and (d) pH were included in the investigation. Sample preparations and limitations associated with each test method are also presented in this section. 1.3.1 Factors Affecting the Corrosion Potential of Geomaterials Several electrochemical parameters influence the corrosivity of an earthen material, including electrical resistivity, degree of saturation, pH, dissolved salts (ions), and redox potential (Elias 1990). These properties can also be affected by contamination from constituents not typically components of soil, including less-common minerals from mining and contamination from natural petroleum or man-made fertilizers. The effects from contaminants were not directly included in this study. Most salts are active participants in the corrosion reaction, with the exception of carbonate, which forms an adherent protective scale on the surface of most metals and inhibits corrosion rate. According to the literature, chloride, sulfate, and sulfide are the major components promoting corrosion in steel reinforcements embedded in earthen materials/ concretes (Ahmad 2003; Romanoff 1957). Given the relationship of resistivity to salt content, Romanoff (1957) and King (1977) established it as the most significant indicator of corrosion potential in earthen materials. Sagues et al. (2009) identified the following factors affecting corrosion of metals in soils and water: • Key factors are temperature, oxygen concentration, resistivity, pH, carbonate scaling tendency, acids, alkalis, salts, distribution of soil particle size, porosity, water content, and microbial activity.

Background 9 • The maximum corrosion occurs at a critical moisture content in a soil mass. Above the critical moisture content, the corrosion rate is controlled by the conductivity of the soil–water mixture (i.e., controlled by activation, or the rate by which electrons from the metal are trans- ferred to oxygen molecules through the electrolyte). Below the critical moisture content, the corrosion rate is controlled by diffusion (i.e., controlled by the rate of oxygen diffusion toward the metal surface). • In generally, higher annual rainfall and higher temperatures produce groundwater that is highly corrosive (as temperature increases, ion mobility and corrosivity increase). • Carbonate scaling can contribute to reduced corrosion rates, so the presence of carbonates should be considered in determining the corrosivity of soils and water. • Other properties of water, such as dissolved oxygen content, may need to be considered for precise determination of corrosion potential. • Besides chloride, sulfate can also break down protective passive film (or carbonate scaling) and cause pitting corrosion (Vilda 2009). Hence, observations of the performance of metals in contact with earthen materials with higher sulfate contents but low chloride contents, are needed to improve characterizations of corrosion potential. • The observed elevated corrosion rates at sites not initially characterized as aggressive because of their soil/water properties were mainly attributed to microbially induced corrosion (MIC). MIC is expected to be more significant in marine environments and warmer climates and for metal in contact with soils that have a high organic content. MIC can be mitigated via use of engineered, free-draining fill that is free of organics and placement of metals in soils above the water table where there is an abundance of oxygen. 1.3.2 Current Laboratory Test Methods Most transportation agencies evaluate electrochemical properties of earthen materials using current AASHTO laboratory test standards, which were adopted in the early 1990s. Specified test methods to measure the electrochemical properties of geomaterials are AASHTO T 288 (resistivity measurement), AASHTO T 289 (pH measurement), AASHTO T 290 (soluble sulfate content measurement), and AASHTO T 291 (soluble chloride content measurement). AASHTO describes the electrochemical requirements for fill material to be suitable for MSE wall con- struction as shown in Table 1-1. Alternatives to AASHTO tests for measuring soil resistivity include ASTM G187, Tex-129-E, Tex-129-M, ASTM WK24621, and SC-T-143. Resistivity test methods are of two general types: • Measurements of voltage drop in response to an applied current passing through specimens that are compacted in a soil box (galvanostatic test) or • Conductivity measurements on aqueous solutions extracted from solid samples (leachates). Other differences between the tests are in terms of sample treatments that may include sieving, air drying, heating, methods of mixing, time of settling/curing, and filtering. Parameter Acceptable Range AASHTO Standard Minimum resistivity (Ω·cm) >3,000 T 288 pH 5–10 T 289 Sulfate content (ppm) <200 T 290 Chloride content Note: Ω·cm = ohm·centimeter, ppm = parts per million. (ppm) <100 T 291 Table 1-1. AASHTO requirements for fill materials in MSE walls.

10 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials ASTM WK24621 and Tex-129-M are new test methods (under development during the course of this research, 2016–2019) being considered for implementation by ASTM and the Texas DOT. Tex-129-E was the Texas DOT standard that will be superseded by Tex-129-M. Tex-129-M is an improvement as compared with Tex-129-E, as it applies to testing coarse- graded samples, including gravel. For Tex-129-E, larger particles were crushed to render specimens with all particles passing a No. 8 sieve. However, the geochemical behavior and electro chemical properties of crushed particles are not representative of the larger-sized par- ticles from which they were derived and the electrochemical and geochemical activity that occurs on the surfaces of the larger particles (Borrock et al. 2013). The test procedure described in Tex-129-M allows all particle sizes up to a maximum of 1¾ inches to be included in the test specimen, and, relative to Tex-129-E, larger-sized boxes are employed to accommodate testing coarse samples. ASTM WK24621 also applies to coarse-graded samples. In general, tests for pH and salt content are performed on extracts obtained after a small solid sample has been diluted with deionized (DI) water. Specific details of specimen preparation, such as the size of the solid sample, fraction of earthen material included in the test specimen (e.g., portion finer than the No. 10 sieve), dilution ratio, soaking period, method and time of mixing, and filtration of solids, vary across the different test procedures. These factors can significantly affect the electrochemical results obtained. Alternatives to AASHTO tests for measurement of pH include ASTM D4972; SC-T-143; Tex-128-E; Tex-620-M, a test procedure developed at CorrTest and described as part of NCHRP Project 21-06 (Vilda 2009); and a new test method for determining the pH of light- weight aggregates that was being considered by ASTM Committee D18 during the course of this research. The latter two test methods and Tex-620-M are applicable for measuring the pH of relatively coarse-grained materials, while the other tests are more applicable to finer materials. Alternatives to AASHTO tests for measuring soluble salt contents include Tex-620-J and Tex-620-M. In addition, ASTM D4327 provides a more robust technique that uses ion exchange chromatography (IEC) to determine the soluble salt content. This technique can be applied to samples that are prepared in accordance with AASHTO T 290 and AASHTO T 291. In addition, the sulfate and chloride contents can be determined from the same specimen. The research team reviewed state DOT standard specifications to identify their practices for measuring electrochemical properties of earthen materials. Twenty-two states use AASHTO test methods, three states reference multiple test methods, one state publishes modifications to the AASHTO methods, 12 states do not use the AASHTO methods, 15 states use different electrochemical requirements, and one state references FHWA guidance instead of AASHTO specifications for its practice. 1.3.3 Comparison of Different Resistivity Test Methods Tests for measurement of soil resistivity include those performed on extracts or on compacted specimens at moisture contents that include as-received and saturated. Resistivity measure- ments may be made in situ or in the laboratory. Laboratory measurements have the advantage that the moisture content in the soil box is controllable. To obtain a comparable resistivity that is independent of seasonal and other variations in soil moisture content, resistivity should be determined under the most adverse condition (e.g., at saturation). The resistivity measured in the water-saturated soil box does not necessarily represent the actual site conditions but pro- vides a baseline for comparing the corrosivity of different earthen materials and is considered as the minimum resistivity in this study. AASHTO T 288, ASTM G187, Tex-129-E, Tex-129-M, and ASTM WK24621 are test methods for measurements of resistivity on compacted specimens. Tests performed on compacted

Background 11 specimens are useful in investigating the influence of moisture content, level of compaction, and distribution of particle size (i.e., tortuosity of the current flow path) on specimen resistivity. Figure 1-1 shows the typical process for measuring the resistivity of earthen materials with a two-electrode soil box, as in AASHTO T 288. A sample size that includes about 1,500 grams (g) of air-dried materials finer than No. 10 sieve is required for testing. The process is as follows: • The soil sample is placed in an acrylic plastic soil box (Figure 1-1a) in layers and compacted by applying finger pressure (Figure 1-1b). The soil box has inner dimensions of 150 × 100 × 45 millimeters (mm) (length × width × height). • To provide a proper electrical contact between the resistivity meter and the soil, two stain- less steel plates with dimensions of 150 × 45 mm are affixed to the side walls of the soil box (distance between stainless steel electrodes = 100 mm). • A measured amount of distilled or DI water with a resistivity greater than 20,000 Ω • cm, is gradually added to the soil sample. • The resistivity meter is then connected to the stainless steel electrodes, as shown in Figure 1-1e, and an alternating current consisting of a square wave with a frequency of 97 hertz (Hz) is passed through the soil sample. • The electrical resistance is measured from the corresponding voltage drop between the two electrodes. The resistivity of the soil sample is computed by multiplying the resistance by the soil box factor, which is a function of the geometry of the box. The process is repeated by remixing the soil sample with increasing amounts of distilled or DI water to produce resistivity measurements at various moisture contents (up to saturation state, shown in Figure 1-1d. A plot of resistivity versus moisture content renders the minimum resistivity and corresponding moisture content (Figure 1-2). soil sample resistivity meter water saturation compaction weight measurement soil box (a) (b) (c) (d) (e) Figure 1-1. Resistivity measurement using a two-electrode soil box.

12 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials AASHTO T 288, ASTM G187, ASTM WK24621, Tex-129-E, and Tex-129-M differ in terms of sample treatments (whether or not the sample is dried before distilled water is added in increments), the particle size distribution of the specimen, the manner in which the soil sample is mixed with water, and the moisture conditions during the test. Table 1-2 summarizes the differences between different test methods in terms of sample treatment and specimen prep- arations. In the present research, the results from different test methods were compared with those from the relevant AASHTO standards, which serve as the nominal values. In the case of soil resistivity, these comparisons were made with respect to data from the AASHTO T 288 test. Test methods performed on aqueous extracts for measurement of resistivity/conductivity include Tex-620-M, the U.S. Geological Survey (USGS) Field Leach Test (Hageman 2007) and SC-T-143. Leachate tests commonly include • Preparing measured amounts of material for testing, • Adding a measured volume of DI water to the sample, • Agitating the mixture (simultaneous heating in some of the test methods), and • Measuring the pH, temperature, and conductivity of the aqueous solution (see Figure 1-3). 0 500 1,000 1,500 2,000 2,500 0 5 10 15 20 25 30 35 R es is tiv ity ( Ω m ) · Moisture content (%) min Figure 1-2. Resistivity versus moisture contents (adapted from McCarter 1984). Test Method Air/Oven Dry Particle Size Mixing Method Moisture Condition AASHTO T 288 Air/oven dried at 60°C <2 mm; crushing not allowed Water added incrementally, mixed thoroughly, placed in box; first increment cures for 12 h Water added in increments until saturated or until a minimum resistivity is reached ASTM G187 No Debris and particles > ¼ in. removed Unless tested as received, water is added and mixed as soil is placed within box; no curing As received or saturated ASTM WK24621 No; soaked for 24 h prior to testing All sizes Similar to ASTM G187, but aggregates have been soaked As received/saturated then drained Tex-129-E Oven dried at 60°C <2.36 mm; crushing allowed Water added incrementally, mixed thoroughly, placed in box; no curing Water added in increments until saturated Tex-129-M Air/oven dried at 60°C All sizes Water added incrementally, mixed thoroughly, placed in box; no curing Water added in increments until saturated Table 1-2. Comparison of different resistivity test methods in terms of sample treatment.

Background 13 The samples are syphoned via syringes and filtered before the analytical tests are conducted by IEC to determine the sulfate and chloride contents. The USGS Field Leach Test applies to poorly graded sands, gravels, and aggregate mixtures. This test measures the conductivity of leachate and has the advantage that the same sample may be used for measurements of resistivity, pH, and chloride and sulfate contents. However, this test may not render meaningful results for well-graded materials, where the tortuosity of the path between the particles affects the current flow and the resistivity measurement obtained. Different test methods such as SC-T-143, Tex-620-J, and Tex-620-M differ with respect to sample preparations that may include sample size, dilution ratio (the weight ratio of DI water added to the soil sample), the manner in which samples are mixed with water, and whether or not the extract is filtered before the test. Table 1-3 summarizes the differences between these test methods in terms of sample preparation. 1.3.4 Limitations of Current Test Methods AASHTO T 288, T 289, T 290, and T 291 are performed on specimens that are separated from the sample on the No. 10 sieve. In particular, for coarse fills with little to no material passing the (a) (b) (c) (d) Figure 1-3. Procedure to perform a typical leach test: (a) prepare measured amounts of material, (b) add DI water, (c) agitate, and (d) make measurements.

14 Electrochemical Test Methods to Evaluate the Corrosion Potential of Earthen Materials No. 10 sieve, a sufficient amount of fines for testing might be obtained from sieving a large quantity of the material. This process is impractical and may lead to inaccurate results, especially for gravel fills that have very little fine materials. Unless some breakage is anticipated during placement and compaction, crushing of larger aggregates to obtain the finer fraction is neither appropriate nor allowed by the AASHTO test standards. This is because most of the soluble ions are concentrated on the surfaces of the particles (diffusion of ions through the particles is negligible). Testing the finer portion of the material (that which passes a No. 10 sieve, i.e., finer than 2.00 mm) presumes that the finer fractions are significant sources of soluble salts. This is not necessarily the case when coarse fills that have very little or no material finer than the No. 10 sieve are used. In that case, an alternative method of test should be considered. 1.4 Knowledge Gaps and Study Purpose Gaps in knowledge that need to be addressed before alternative test methods for measure- ments of electrochemical properties can be recommended are summarized with the following questions: • How fine does the material need to be before testing the fraction passing the No. 10 sieve is appropriate? • How coarse should the material be before testing an aqueous extract for resistivity is appropriate? • How do results obtained from different test methods compare? • What is the precision and bias for individual test methods? • How well does the proposed characterization of corrosion potential correlate with perfor- mance and observed corrosion rates? Test Method Sample Size (g) (W:S) Mixing Method Settling Time (h) Filtration SC-T-143 2,000 1:1 Mix then let stand for 30 min; agitate for 3 min at 0-, 2-, and 4-h intervals 20 Yes Tex-620-J 30 (separated on the No. 4 sieve and then pulverized to pass a No. 40 sieve) 10:1 Heat sample to 150°F and digest on hot plate for approximately 16 h, stirring periodically None Yes Tex-620-M 100 (dried) 10:1 Shake vigorously for 30 min 1 No; tip of electrode placed 5 cm deep into the mixture Note: W:S = water to solids ratio by weight; h = hour. Table 1-3. Differences in test methods performed on extracts (leachates).