1、Standard Practice for Assessment of Corrosion of Steel Piling for Non-Marine Applications AASHTO Designation: R 27-01 (2015) American Association of State Highway and Transportation Officials 444 North Capitol Street N.W., Suite 249 Washington, D.C. 20001 TS-1a R 27-1 AASHTO Standard Practice for As
2、sessment of Corrosion of Steel Piling for Non-Marine Applications AASHTO Designation: R 27-01 (2015) 1. SCOPE 1.1. This standard practice is focused on corrosion of steel piling for non-marine soil applications. 1.2. This standard practice is divided into two parts: Part IEnvironmental Conditions Ca
3、using Corrosion of Steel Piling, and Part IICorrosion Considerations for New and Existing Piling. 1.2.1. Part I of the standard practice describes the current knowledge of the mechanism of underground corrosion to aid the reader in better understanding the controlling factors and identifies the know
4、n factors that cause corrosion of piling in non-marine applications. 1.2.2. Part II of the standard practice describes procedures that should be followed to assess the soil corrosivity at a specific site and offers guidance in the selection of corrosion mitigation procedures for new piling installat
5、ions. Methods are described to evaluate the present condition of existing steel piling. Guidance is provided in the continued use of existing steel piling or reuse of steel piling in new or rehabilitated structures. 1.3. This standard practice does not preclude testing and test methods used to asses
6、s design parameters for the placement or continued use of piling. 1.4. Test methods not currently available as AASHTO or ASTM Methods are included in the Appendices of the NCHRP Report 408 (Beavers and Durr 1997). 1.5. This standard practice may involve hazardous materials, operations, and equipment
7、. This standard practice 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 limitations prior to use. 2. REFERENCE
8、D DOCUMENTS 2.1. AASHTO Standards: T 206, Penetration Test and Split-Barrel Sampling of Soils T 207, Thin-Walled Tube Sampling of Soils T 255, Total Evaporable Moisture Content of Aggregate by Drying T 291, Determining Water-Soluble Chloride Ion Content in Soil T 306, Progressing Auger Borings for G
9、eotechnical Explorations 2.2. ASTM Standards: D512, Standard Test Methods for Chloride Ion in Water D516, Standard Test Method for Sulfate Ion in Water 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-
10、1a R 27-2 AASHTO D1452, Standard Practice for Soil Exploration and Sampling by Auger Borings D2487, Standard Practice for Classification of Soils for Engineering Purposes (Unified Soil Classification System) D2488, Standard Practice for Description and Identification of Soils (Visual-Manual Procedur
11、e) D4220, Standard Practices for Preserving and Transporting Soil Samples D4972, Standard Test Method for pH of Soils G51, Standard Test Method for Measuring pH of Soil for Use in Corrosion Testing G57, Standard Test Method for Field Measurement of Soil Resistivity Using the Wenner Four-Electrode Me
12、thod 3. SIGNIFICANCE AND USE 3.1. This standard practice identifies the factors that cause corrosion of steel piles subjected to subsurface, non-marine environment in underground conditions. It provides procedures to assess the corrosion potential of the piles and offers recommendations on the ways
13、to mitigate the corrosion. By determining the existing condition, the remaining life of the piles can be predicted. PART IENVIRONMENTAL CONDITIONS CAUSING CORROSION OF STEEL PILING 4. MECHANISM OF UNDERGROUND CORROSION 4.1. Corrosion of structural steel in soils is electrochemical in nature. When st
14、eel corrodes, the iron atoms in the steel undergo oxidation and lose electrons (Equation 1). Other components in the soil are reduced and gain the lost electrons (some combination of Equations 2, 3, and 4). The electrochemical reaction associated with oxidation is the anodic reaction and the electro
15、chemical reaction associated with reduction is the cathodic reaction. The sites where the anodic and cathodic reactions take place are termed the anode and cathode, respectively. The combination of the anode and the cathode, coupled with current flow between the two, is called a corrosion cell. 4.2.
16、 Figure 1 is a schematic of a corrosion cell. As shown in Figure 1, the electrons produced by the oxidation reaction flow from the anode to the cathode in the steel where they are consumed by the reduction reaction. Note that the direction of current flow is opposite to the direction of electron flo
17、w since, by definition, current is the flow of positive charge. In the soil, current must flow from the anode to the cathode to maintain charge neutrality. Current flow in the soil is carried by ions, moving through the water in pore spaces between the soil particles. 2015 by the American Associatio
18、n of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a R 27-3 AASHTO Figure 1Schematic of a Corrosion Cell 4.3. Oxygen reduction (Equation 2) usually controls the rate of corrosion of steel in soils. This reaction is, in turn, controll
19、ed by the rate of movement of oxygen through the soil and water to the steel surface. In the absence of oxygen, reduction of water (Equation 3) can occur. However, this is normally slow enough to cause no significant corrosion damage to steel. Hydrogen ion reduction (Equation 4) occurs when the soil
20、 is very acidic and can significantly contribute to the rate of corrosion of steel in such soils. The iron ions produced by oxidation of the steel can eventually react with components in the soil to form corrosion products. For example, Equation 5 shows the iron ions reacting with water to produce r
21、ust. Other corrosion-related products are formed on the surface of the metal by the reduction reactions. These products include hydroxide ions (Equations 2 and 3) and hydrogen gas (Equations 3 and 4). 4.3.1. Oxidation of Iron: Fe Fe+ 3e(1) 4.3.2. Oxygen Reduction: O2+ 2H2O + 4e 4OH(2) 4.3.3. Water R
22、eduction: 2H2O + 2e H2+ 2OH(3) Anodic ReactionFe Fe+ 3eSoilAnodeSteelCurrentFlowinSoilElectronFlowCurrentFlowin MetalCathodeCathodic ReactionO + H O + 4e4OH2 2 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable
23、 law.TS-1a R 27-4 AASHTO 4.3.4. Hydrogen Ion Reduction: 2H+ 2e H2(4) 4.3.5. Formation of Rust: 2Fe+ 3H2O Fe2O3+ 6H+(5) 4.4. If the anodes and cathodes are very close to each other and evenly distributed on the steel surface, uniform corrosion of the steel occurs. On most underground steel structures
24、, rates of uniform corrosion are low and rarely cause service failures. The most notable exception is where the soil pH is below 4. 4.5. Where the anode and cathode sites are well separated on a steel surface, a “macrocell” is formed. Severe corrosion and resulting service failures can occur when th
25、e anode in a macrocell is confined to a relatively small area of the steel. In this instance, the form of corrosion is frequently referred to as pitting. Once macrocells are started on a steel surface, the products of the electrochemical reactions generally cause the macrocell to continue. For examp
26、le, the reduction reactions cause an increase in the pH at the cathode. Steels form protective films in elevated pH environments, reducing the rate of corrosion. On the other hand, the reactions at the anode reduce the pH. The acidic environment created at the anode causes protective oxide films on
27、the steel to break down, increasing the corrosion rate. As the pH at the anode decreases, the reduction of hydrogen ions may occur locally, further increasing the rate of attack. 4.6. One of the common macrocells on piling in soils is caused by variation in oxygen concentration over the steel surfac
28、e. Oxygen macrocells frequently develop in stratified soils. Anodes are formed where the oxygen concentration is low, such as in clay soils, and cathodes are formed where the oxygen concentration is high, such as in sandy soils. The water table is another area where oxygen macrocells usually develop
29、. A macrocell also may develop where there is variation in the chloride content of the soil, such as along roads with saltwater runoff. In this macrocell, anodes form in regions of high chloride concentration. 4.7. Other factors that affect the rate of oxygen macrocell corrosion include the relative
30、 surface area of the anode and the cathode, soil resistivity, and microbiological activity. Where the cathode is large and the anode is small, a large current is concentrated at the anode, leading to high rates of corrosion at the anode. Soil corrosivity generally increases with decreasing soil resi
31、stivity. Where the soil resistance is high, a high current flow between the anode and cathode cannot occur due to the high voltage (IR) drop in the high resistance soil path. Microbiological activity, or the presence of welds or inclusions in the steel, can also aggravate macrocell corrosion by aidi
32、ng in the creation of the anodic sites. Further discussion of the effects of resistivity and other parameters on corrosion is given in the NCHRP Report 408. 4.8. Piling and other underground structures also can undergo accelerated corrosion as a result of stray current flow in the soil. Sources of s
33、tray current include cathodic protection systems for other structures, direct current (DC) electric transit systems, mining activity, and high voltage DC electric power lines. DC electric current, flowing parallel to a structure, will jump onto that structure if that structure has a lower resistance
34、 in the direction of the current flow than the soil. The structure is cathodically protected where the current jumps onto the structure and corrosion is accelerated where the current leaves the structure. Stray current corrosion is most commonly observed on structures that have large dimensions in o
35、ne horizontal direction, such as pipelines. Sheet piling and other piling that are electrically continuous also can experience stray current corrosion. 4.9. To summarize, corrosion of structural steel in soils is electrochemical in nature and is caused by the presence of oxygen and moisture in the s
36、oil. Corrosion is most likely to occur at or above the water table in disturbed stratified soils having low resistivity. For example, fill soils containing man-made materials such as cinders, slag, or ash are known to cause significant corrosion of steel 2015 by the American Association of State Hig
37、hway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a R 27-5 AASHTO piles. Stray electrical currents flowing in the ground, from sources such as transit systems, also can contribute to corrosion of structural steels in soils. PART IICORROSION CONSI
38、DERATIONS FOR NEW AND EXISTING PILING 5. SUMMARY OF APPROACH 5.1. A preliminary investigation (Phase I Site Assessment) is performed to obtain pertinent available information on the surface and subsurface conditions at the site. Information obtained in this investigation may include the position of
39、the piling or pile cap with respect to the groundwater table, the soil characteristics, and the presence of contaminants in the soil. This information is used to determine whether a further (Phase II) assessment is required. In general, a Phase II site investigation is required unless: 5.1.1. The pi
40、ling or pile cap is or will be below the water table at all times or, 5.1.2. The Phase I site assessment provides the necessary information outlined in the Phase II site investigation to establish the corrosivity of the site. 5.1.3. In cases where there are multiple sites that require investigation,
41、 priority should be given to those sites known to contain corrosive materials such as slag, cinders, ash, or other man-made products. 5.2. In the Phase II Site Investigation, continuous soil sampling is performed to a depth of 1 m below the minimum water table. The testing protocol outlined in Figur
42、e 2 is used for analyzing the soil samples. For homogeneous soils, testing is performed every 60 to 90 cm, while testing is performed on each distinct soil layer for inhomogeneous soils. This testing is limited to resistivity, pH, and particle size. 5.3. The flowchart shown in Figure 3 is used, in c
43、onjunction with the results of the soil analyses, to determine the likelihood of significant uniform or macrocell corrosion. As shown in the figure, neither form of corrosion is likely to occur at a significant rate if the saturated soil resistivity is greater than approximately 2000 -cm. Depending
44、on the homogeneity, particle size, and pH of the soil, one or both forms of corrosion may occur at a significant rate if the saturated soil resistivity is below 2000 -cm. 5.4. The need for corrosion monitoring, corrosion mitigation, or pile repair or replacement is determined using the flowchart sho
45、wn in Figure 4. No further testing or analysis is required if the results of the soil analyses indicate there is a low probability of significant uniform or macrocell corrosion. Corrosion monitoring is recommended where one or both forms of corrosion may occur at a significant rate. This corrosion m
46、onitoring will establish the rate of corrosion such that a remaining life assessment can be performed. Corrosion mitigation is recommended if the predicted remaining life is less than the design life. Corrosion mitigation is recommended in cases where corrosive soils are present and there is insuffi
47、cient time for proper corrosion monitoring. Pile repair or replacement is required if there is no predicted remaining life. If the predicted remaining life is much less than the design life, the pile should be excavated and examined to better define the existing condition and the need for repair, re
48、placement, or corrosion mitigation. 2015 by the American Association of State Highway and Transportation Officials.All rights reserved. Duplication is a violation of applicable law.TS-1a R 27-6 AASHTO Figure 2Phase II Site Sampling and Testing Protocol Phase I AssessmentBelow Water Tableat All Times
49、No TestingRequiredPile/Pile Cap LocationAt or above Water TableContinuous Soil Sampling:Surface to 1 m belowWater TableVisual Examination, Identification, andThickness of Soil LayersHomogeneityYesNoTest Each DistinctSoil LayerTest Soil Every60 to 90 cmTextureMedium- toFine-Grained Course-GrainedpH, ASTM Method G51As-Received ResistivityASTM Method G57Send to LaboratorypH, ASTM Method D4972As-Received ResistivityASTM Method G57Send to LaboratorySaturated ResistivityASTM Method G57Sieve Analysis/ParticleSize DistributionEv
