1、Standard Method of Test for Comparative Qualitative Corrosion Characterization of Steel Bars Used for Concrete Reinforcement (Linear Polarization Resistance and Potentiodynamic Polarization Tests) AASHTO Designation: T 373M/T 373-171Technical Section: 4f, Metals Release: Group 2 (June 2017) American
2、 Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001TS 4f T 373M/T 373-1 AASHTO Standard Method of Test for Comparative Qualitative Corrosion Characterization of Steel Bars Used for Concrete Reinforcement (Linear Polarization Resi
3、stance and Potentiodynamic Polarization Tests) AASHTO Designation: T 373M/T 373-171Technical Section: 4f, Metals Release: Group 2 (June 2017) 1. SCOPE 1.1. This test method describes procedures to evaluate the comparative qualitative corrosion performance of uncoated Corrosion Resistant Reinforcing
4、(CRR) M 334M/M 334 alloy steel bars to those of M 31M/M 31 or other selected reference bars, utilizing electrochemical polarization resistance and potentiodynamic polarization measurements in various test solutions. The test methods evaluate the corrosion performance of the uncoated CRR bars as a fu
5、nction of (1) pH of the environment and (2) concentration of chloride ions, in particular the Cl/OH threshold ratio. 1.2. Bars tested in accordance with these test procedures shall be characterized as corrosion-resistant when both of the conditions below are met: 1.2.1. The ratio of the polarization
6、 resistance (Rp) measured at pH = 10 to the polarization resistance measured at pH = 13 is greater than or equal to () 0.5, i.e., Rpat (pH = 10)/Rpat (pH = 13) 0.5, and 1.2.2. The measured pitting potential is greater than or equal to () 250 mV vs. Ag/Ag Cl reference electrode at a Cl/OH ratio equal
7、 to 2. 1.3. The values stated in either SI or inch-pound units shall be regarded separately as standard. The inch-pound units are shown in brackets. The values stated might not be exact equivalents; therefore, each system must be used independently of the other. 1.4. This standard may involve hazard
8、ous materials, operations, and equipment. This standard does not purport to address all of the safety concerns associated with its use. It is the responsibility of the user of this procedure to establish appropriate safety and health practices and to determine the applicability of regulatory limitat
9、ions prior to use. 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS 4f T 373M/T 373-2 AASHTO 2. REFERENCED DOCUMENTS 2.1. AASHTO Standards: M 31M/M 31, Deformed and Plain Carbon and Low-Alloy Steel Ba
10、rs for Concrete Reinforcement M 334M/M 334, Uncoated, Corrosion-Resistant, Deformed and Plain Chromium Alloyed, Billet-Steel Bars for Concrete Reinforcement and Dowels 2.2. ASTM Standards: G3, Standard Practice for Conventions Applicable to Electrochemical Measurements in Corrosion Testing G5, Stand
11、ard Reference Test Method for Making Potentiodynamic Anodic Polarization Measurements G59, Standard Test Method for Conducting Potentiodynamic Polarization Resistance Measurements G102, Standard Practice for Calculation of Corrosion Rates and Related Information from Electrochemical Measurements 3.
12、TEST EQUIPMENT AND MATERIALS 3.1. Containers500-mL capacity polypropylene (PP) beaker, or equivalent. The solutions used are characterized by a high pH value, hence, glass containers shall not be used. 3.2. VoltmeterHigh impedance voltmeter (at least one Mohm), measuring to 0.001 mV. 3.3. Potentiost
13、atPotentiostat with 20 V compliance voltage (e.g., Gamry Instruments series G 750). 3.4. Reference Electrode (i.e., saturated Ag/AgCl reference electrode or Accumet #13-620-52 Calomel Reference Electrode)The reference electrode should be used with a salt bridge or luggin probe to prevent contaminati
14、on and to allow it to be placed closer to the working electrode to minimize IR drops at high current densities. 3.5. Conductive Threaded Rod316SS or equivalent threaded rod is used to make the electrical connections to the bars. 3.6. Test SolutionTest solution, compatible with material under test (s
15、ee Section 5.4 for solution preparation). 3.7. Counter ElectrodeShall be a noncorrosive mixed oxide coated titanium, platinum or similar material or at least two distinct noncorrosive electrodes placed symmetrically in the vessel. 4. TEST METHOD 4.1. Description of TestsTwo well-established types of
16、 corrosion tests are included Linear Polarization Resistance and Potentiodynamic Polarization curves. This section presents a brief overview of each test, similar to test procedures and measurements described in ASTM G3, G59, and G102. 4.1.1. Linear Polarization ResistanceThe linear polarization res
17、istance technique, or simply polarization resistance, involves measuring the change in the open-circuit potential of the electrolytic cell when an external current is applied to it. For a small perturbation about the open-circuit potential (OCP), there is a linear relationship between the change in
18、applied voltage (E) 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS 4f T 373M/T 373-3 AASHTO and the change in the measured current per unit area of electrode (i). The ratio E/i gives the polarizatio
19、n resistance (Rp) term, which is inversely related to the corrosion performance of the alloy. 4.1.2. Potentiodynamic PolarizationIn this test, the potential is made to change at a constant rate over a wide range, typically from a very cathodic to a very anodic potential, while the corrosion current
20、density is measured. The test indicates the potential regions where there is electrode activity, and provides a variety of information about the corrosion behavior of the sample. Active, passive, and transpassive regions, for instance, can be readily identified. When chlorides are present, the test
21、can be used to identify the concentration in which pitting corrosion is initiated. 5. EXPERIMENTAL PROCEDURE 5.1. Steel Sample Preparation: 5.1.1. Test samples of approximately 1-in. length and 5/16-in. diameter must be machined, using coolant spray or copious coolant flow over the specimen, from th
22、e rebar to be tested, taking care not to overheat or unintentionally induce microstructural changes to the alloy. 5.1.2. A screw thread is drilled and tapped (e.g., 6-32 by 3/16in.) into one end of the specimen, to enable electrical contact to be made. 5.1.3. The surface of the test specimen shall b
23、e polished with SiC paper down to 1000 grit. 5.1.4. Electrical contact shall be made to the specimen with a 316SS or equivalent threaded rod long enough to extend outside of the electrochemical cell. 5.2. Electrochemical CellA standard ASTM G5 three-electrode electrochemical cell arrangement shall b
24、e used. The body of the 500-600 mL cell must be made of polypropylene plastic due to the elevated pH of the test solution. A typical setup is shown below in Figure 1. Figure 1Typical Three-Electrode Electrochemical Cell Set-Up 2017 by the American Association of State Highway and Transportation Offi
25、cials. All rights reserved. Duplication is a violation of applicable law.TS 4f T 373M/T 373-4 AASHTO 5.3. Electrochemical Cell and Sample Assembly: 5.3.1. The specimen shall be covered with an impermeable two-part epoxy coating in such a way that only approximately 1/2in. in length of the specimen i
26、s left exposed; the remaining length of the specimen as well as the contacting rod must be covered with an impermeable two-part epoxy coating. 5.3.2. The impermeable two-part epoxy coating shall be applied with care to provide a crevice-free seal. 5.3.3. The exposed surface area shall be measured an
27、d recorded. 5.3.4. Before testing, after the equipment is set up and the electrochemical cell is ready to receive the test specimen, the exposed area of the specimen shall be repolished with care not to damage the impermeable two-part epoxy coating, degreased with acetone, and cleaned with methanol
28、and deionized water. 5.4. Test Solution PreparationTest solution shall be prepared with deionized water and certified reagent chemicals. Ideally, the experiments should be controlled by an automated reliable potentiostatic instrument and corresponding software. 5.4.1. Polarization Resistance: 5.4.1.
29、1. Before performing the polarization resistance experiment, the test specimen as prepared according to the procedure in Sections 5.1 and 5.3 shall be placed in the electrochemical cell and exposed to the aqueous solution for 30 min. 5.4.1.2. The open circuit potential (OCP) shall be measured. 5.4.1
30、.3. A potential variation of 15 mV to +15 mV over OCP shall be applied at a scan rate of 0.125 mV/sec, and the corresponding changes in the current density (in A/cm2) shall be recorded. 5.4.1.4. The polarization resistance term Rpis then calculated based on this data. In order to properly evaluate t
31、he corrosion behavior of the corrosion resistant reinforcement (CRR), and particularly to demonstrate the fundamental difference between the CRR and carbon steels, the polarization resistance tests shall be performed for two different pH levels, pH = 13 and pH = 10, while maintaining the ionic stren
32、gth (IS) of the solutions constant. Naturally aerated aqueous solutions shall be prepared immediately before the test. Note 2Carbon steel typically loses its passive film at pH values between 11 and 10, while CRR is expected to maintain its corrosion resistance at lower pH levels. Note 3A lower pH s
33、olution (i.e., pH = 9) can be used; however, the buffering capacity for such a solution is low. During testing, the pH value shall be monitored to assure that no significant drop in pH occurs. Note 4Ionic strength (IS) is defined as one-half the summation of the product of the concentration of each
34、ion and its charge squared, as shown in the equation IS= 1/2 cizi2(Lehmann et al., 1996).2Table 1 illustrates the composition of aqueous test solutions. Table 1Polarization Resistance Aqueous Test Solutions pH NaOH (mol/l) IS NaOH (g/l) K2SO4 (mol/l) K2SO4 (g/l) 13 10 0.1 0.0001 0.1 0.1 4.0 0.004 0.
35、0333 5.8042 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS 4f T 373M/T 373-5 AASHTO 5.4.2. Potentiodynamic Polarization: 5.4.2.1. After preparing the test sample as described in Sections 5.1 and 5.3
36、 the specimen shall be immediately placed in the electrochemical cell and conditioned at 1200 mV (vs. Ag/AgCl reference electrode) for 2 min in order to reduce any possible air-formed oxide film. 5.4.2.2. Then the potential shall be gradually increased from 1200 mV at a scan rate of 1 mV/s. 5.4.2.3.
37、 If a cyclic scan is desired, the potential shall be taken to the beginning of the transpassive region (about +550 mV vs. Ag/AgCl when no chlorides are present) or to the pitting potential if pitting initiates at a potential below the transpassive potential, and reversed back to the starting potenti
38、al. 5.4.2.4. In the presence of chlorides, the onset of pitting corrosion is detected by a shift of the transpassive region to lower potential values during the forward (anodic) scan. This new potential value is referred to as the pitting potential (for a particular chloride concentration). Pitting
39、corrosion is initiated when a chloride threshold value (as represented by the ratio between Cland OHions prepared in solution) is exceeded. 5.4.2.5. A simulated concrete pore solution with a constant OH (0.55M KOH + 0.16M NaOH, as described by Taylor, 1997)3shall be prepared. The concentration of Cl
40、ions shall be changed to a desired level by adding NaCl salt (see Table 2 for examples). In order to verify and demonstrate the enhanced corrosion performance of CRR in highly aggressive environments, the potentiodynamic polarization curves shall be measured in at least two environments: (1) simulat
41、ed concrete pore solution with no chlorides and (2) same aqueous solution with a high concentration of chlorides (e.g., ratio Cl/OH = 2 or higher). Note 5Carbon steel rebar typically p resents chloride threshold ratios Cl/OH between 0.2 and 0.85, with 0.6 being the most commonly referenced value. Ta
42、ble 2 illustrates the chemical composition of simulated concrete pore test solutions. Table 2Simulated Concrete Pore Solutions for Potentiodynamic Polariza tion Tests Cl/OH KOH (mol/l) KOH (g/l) NaOH (mol/l) NaOH (g/l) OH (mol/l) Cl (mol/l) NaCl (mol/l) NaCl (g/l) 0 0.55 30.86 0.16 6.40 0.71 0.00 0.
43、00 0.00 1 0.55 30.86 0.16 6.40 0.71 0.71 0.71 41.49 2 0.55 30.86 0.16 6.40 0.71 1.42 1.42 82.98 3 0.55 30.86 0.16 6.40 0.71 2.13 2.13 124.48 Note 6The two sets of tests described above can clearly demonstrate the corrosion behavior of a selected CRR and show how it compares to regular carbon steel r
44、ebar when test parameters, such as pH values and Cl levels (both considered excessively aggressive in the case of carbon steels), are controlled. 6. REPORT 6.1. Polarization ResistanceReport the polarization resistance RPat pH = 10 and RPat pH = 13 for test solutions prepared in accordance with Tabl
45、e 1 for the comparison bar material (M 31M/M 31) and the uncoated CRR test bar material. Evaluate the ratio of Rp(pH = 10)/Rp(pH = 13). 6.2. Potentiodynamic PolarizationReport the transpassive or pitting potential in mV vs. Saturat ed Ag/AgCl electrode used in the test at Cl/OH = 0 and Cl/OH = 2 for
46、 test solutions prepared in accordance with Table 2 for the comparison bar material (M 31M/M 31) and the uncoated CRR test bar material. 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.TS 4f T 373M/T 37
47、3-6 AASHTO 6.3. Chemical ConstituentsReport chemical analysis percentages for the following elements: carbon, chromium, copper, manganese, molybdenum, nickel, nitrogen, phosphorus, silicon, sulfur, and vanadium. 7. PRECISION AND BIAS STATEMENT 7.1 No precision and bias statement has been developed f
48、or this test method. 8. KEYWORDS 8.1. Corrosion performance; corrosion test; qualitative; uncoated corrosion resistant reinforcing (CRR) bars. 1Formerly AASHTO Provisional Standard MP 18M/MP 18, Annex A2. First published as a full standard in 2017. 2Lehmann, H. P., X. Fuentes-Arderiu, and L. F. Bert
49、ello. “Glossary of Terms in Quantities and Units in Clinical Chemistry.” Pure and Applied Chemistry, Vol. 68, No. 4, 1996, pp. 9571000. 3Taylor, H. F. W. Cement Chemistry, Second Edition. T. Telford, London, 1997, xviii, p. 459. 2017 by the American Association of State Highway and Transportation Officials. All rights reserved. Duplication is a violation of applicable law.
