1、Item No. 24216NACE International Publication 05101This Technical Committee Report has been preparedby NACE International Task Group 018* onCorrosion Control of Structural Steel Pilings inNonmarine ApplicationsState-of-the-Art Survey on Corrosion ofSteel Piling in Soils December 2001, NACE Internatio
2、nalThis NACE International technical committee report represents a consensus of those individual memberswho have reviewed this document, its scope, and provisions. Its acceptance does not in any respect precludeanyone from manufacturing, marketing, purchasing, or using products, processes, or proced
3、ures not included inthis report. Nothing contained in this NACE International report is to be construed as granting any right, byimplication or otherwise, to manufacture, sell, or use in connection with any method, apparatus, or productcovered by Letters Patent, or as indemnifying or protecting anyo
4、ne against liability for infringement of LettersPatent. This report should in no way be interpreted as a restriction on the use of better procedures or materialsnot discussed herein. Neither is this report intended to apply in all cases relating to the subject. Unpredictablecircumstances may negate
5、the usefulness of this report in specific instances. NACE International assumes noresponsibility for the interpretation or use of this report by other parties.Users of this NACE International report are responsible for reviewing appropriate health, safety,environmental, and regulatory documents and
6、for determining their applicability in relation to this report prior toits use. This NACE International report may not necessarily address all potential health and safety problems orenvironmental hazards associated with the use of materials, equipment, and/or operations detailed or referredto within
7、 this report. Users of this NACE International report are also responsible for establishing appropriatehealth, safety, and environmental protection practices, in consultation with appropriate regulatory authorities ifnecessary, to achieve compliance with any existing applicable regulatory requiremen
8、ts prior to the use of thisreport.CAUTIONARY NOTICE: The user is cautioned to obtain the latest edition of this report. NACEInternational reports are subject to periodic review, and may be revised or withdrawn at any time without priornotice. NACE reports are automatically withdrawn if more than 10
9、years old. Purchasers of NACE Internationalreports may receive current information on all NACE International publications by contacting the NACEInternational Membership Services Department, 1440 South Creek Dr., Houston, Texas 77084-4906 (telephone+1281228-6200).ForewordThe purpose of this technical
10、 committee report is to provideuseful corrosion information for engineers, designers,consultants, and others involved in the design,maintenance, and rehabilitation of structures with steel pilefoundations. The report contains information obtained froma survey of the open literature on corrosion of s
11、teel pilings.The intended audience includes structural, geotechnical,and bridge engineers who may not be familiar withcorrosion science and engineering. Accordingly, the reportincludes a short overview of the mechanism of pilingcorrosion in soils. See the references and bibliography foradditional in
12、formation on corrosion of steel piling in soils.This NACE technical committee report was written by TaskGroup 018 on Corrosion Control of Structural Steel Pilingsin Nonmarine Applications. This task group is administeredby Specific Technology Group (STG) 05 onCathodic/Anodic Protection and is sponso
13、red by STG 01 onConcrete and Rebar and STG 03 on Protective Coatingsand Linings. This report is published by NACE Internationalunder the auspices of STG 05._*Chairman John Beavers, CC Technologies, Inc., Dublin, Ohio.NACE International2IntroductionField investigations by Romanoff and other researche
14、rs atthe National Institute of Standards and Technology(NIST)(formerly the National Bureau of Standards NBS)(1)in the 1960s and earlier demonstrated that steel pilings arenot significantly affected by corrosion in undisturbed soil,regardless of the soil type and properties.1On the otherhand, recent
15、examinations of steel piles exposed duringbridge-pier construction in several states have revealedsevere corrosion damage, including complete severing ofthe piles in corrosive soil strata. The problem appears to beassociated primarily with the use of man-made materialssuch as slag and cinders for fi
16、ll around the piling. Extensivecorrosion damage has also been observed in relatedstructures such as reinforced soil structures in similarenvironments.Mechanism of Piling Corrosion in SoilsAll commonly used engineering metals, including carbonsteels, corrode because they are thermodynamicallyunstable
17、. One principle of thermodynamics is that amaterial always seeks the lowest energy state. A significantamount of energy is put into a metal when it is extractedfrom its ores, placing it in a high-energy state. In thecorrosion process, the energy of the metal is reduced as itreverts to a corrosion pr
18、oduct, which in many casesincludes compounds that are identical to ores, e.g.,hematite.Corrosion of metals in aqueous (water-containing)environments, including soils, is electrochemical in nature.The metal atoms are oxidized (lose electrons) and speciessuch as water or oxygen are reduced (gain elect
19、rons). Themetal ions generated by the oxidation of the metal normallythen react with water or other species in the environment tocreate oxides, hydroxides, and other corrosion products. Inthe case of steel, these products of corrosion are calledrust. Products are also created by the reduction reacti
20、ons.These products include hydroxide ions and hydrogen. Asummary of typical reactions for the corrosion of steel andassociated reduction reactions is given in Equations (1)through (5).Oxidation of Iron Fe Fe+2e(1)Oxygen Reduction O2+2H2O+4e 4OH(2)Water Reduction 2H2O+2e H2+2OH(3)Hydrogen Ion Reducti
21、on 2H+2e H2(4)Formation of Rust 2Fe+3H2O Fe2O3+6H+(5)There are many other possible steel corrosion products in asoil environment, including magnetite (Fe3O4), ironhydroxide (Fe (OH)3), and various carbonates and sulfates.The composition of the corrosion products is dependent onthe species present in
22、 the environment. For example, thecorrosion products are dominated by reduced (loweroxidation state) iron species such as magnetite whenoxygen is not present.Because the common engineering metals arethermodynamically unstable in natural environments, theuseful life of an engineering structure is det
23、ermined by therate of corrosion, referred to as the corrosion kinetics. Thecorrosion kinetics can be controlled by the rate of theoxidation reaction, the rate of the reduction reaction, orcurrent flow between the locations on the metal surfacewhere the two reactions are occurring. For example,tenaci
24、ous and protective oxide films may form on the metalsurfaces, limiting the rate of metal oxidation. Stainlesssteels and aluminum are corrosion-resistant in manyenvironments because they form thin protective oxide films.Carbon steels also form protective oxide films in elevatedsolution pH and in some
25、 carbonate environments.The rate of general corrosion of carbon steels is usuallylimited by the rate of the reduction reaction. In the case ofunderground corrosion of steels, oxygen reduction is thedominant reduction reaction controlling the corrosion rate.In this environment, pH values are not norm
26、ally low enoughfor hydrogen ion reduction to be significant, and the rates ofwater reduction are low. For the oxygen reduction reaction,the rate-controlling process is generally the diffusion ofoxygen through the soil or electrolyte to the metal surface.This rate is controlled by the concentration o
27、f oxygen in thesoil and the thickness of the water layer through which theoxygen must diffuse. The most severe conditions aregenerally those in which a thin water layer is present on themetal surface, providing a short diffusion path for theoxygen. These conditions are normally encountered inmoist,
28、but not saturated, porous soils, especially in zonesthat are alternately wet and dry due to fluctuations of thewater table.The electrochemical reactions can occur uniformly on ametal surface, leading to a general corrosion of the metal.At one instant in time, metal oxidation may be occurring at aloc
29、ation, while one of the reduction reactions may occur at_(1)National Institute of Standards and Technology (NIST) (formerly National Bureau of Standards NBS), Gaithersburg, MD 20899.NACE International3the same instant on an adjacent atom, consuming theelectrons liberated by the metal oxidation react
30、ion. Aninstant later, the location of the reactions may switch. Formost underground steel structures, rates of generalcorrosion are usually low and can be predicted.Therefore, general corrosion rarely causes servicefailures.It is also possible for the oxidation and reduction reactionsto be separated
31、 on a metal surface, where the metaloxidation occurs predominantly at one area while thereduction reaction occurs predominantly at another area.This is referred to as a macrocell. One type of macrocell isa differential aeration cell, shown schematically in Figure 1.The differential aeration cell is
32、probably the most commoncorrosion cell that is experienced on pilings, pipelines, andother types of underground structures. The area at whichnet oxidation occurs is called the anode and the area atwhich net reduction occurs is called the cathode. In themetal, the electrons liberated by the oxidation
33、 reaction flowfrom the anode to the cathode where they are consumed bythe reduction reaction. Note that the current flow in themetal is shown in the opposite direction because current, bydefinition, is the flow of positive charge. In the soil, electricalcurrent, in the form of migrating ions, must f
34、low from theanode to the cathode to maintain charge neutrality. Thecurrent flows through the aqueous phase in pore spacesbetween the soil particles and can be carried by eitherpositively or negatively charged ions.In general, macrocells are especially insidious in that, oncethe oxidation and reducti
35、on reactions become separated,the electrochemical reactions create local environments thatexacerbate the attack. For example, the reduction reactionscause an increase in the electrolyte pH at the cathode.Steels form tenacious protective films in elevated pHenvironments. Therefore, the rate of metalo
36、xidation at the cathode is reduced. On the other hand,hydrolysis of the iron atoms at the anode creates hydrogenions that reduce the electrolyte pH. The low-pH, acidicenvironment created at the anode destabilizes any oxidefilms that may have been present, increasing the rate ofattack. As the pH at t
37、he anode decreases, the directreduction of hydrogen ions may occur locally, furtherincreasing the rate of attack.Factors that affect the rate of differential aeration corrosioninclude the relative area ratio of the anode and the cathode,soil resistivity, and stratification of the soil. When thecatho
38、de is large and the anode is small, a larger current issupported by the cathode and that current is concentratedat the anode, leading to high rates of attack at the anode.When the soil resistance is high, a high current flowbetween the anode and cathode cannot be supported dueto the high voltage (IR
39、) drop in the high-resistance soil path.Stratification of the soil creates ideal conditions for thedevelopment of the differential aeration cells. As suggestedin Figure 1, steel areas in oxygen-deficient layers, such aswet clays or regions below the water table, become theanodes while oxygen-rich la
40、yers, such as porous sands,become the cathodes. Further discussion of the effects ofresistivity and other parameters on corrosion is given in thenext section.NACE International 4 FIGURE 1 Schematic showing differential aeration cell. Factors Controlling Piling Corrosion Position of Water Table The p
41、osition of the water table with respect to the pile is probably the most important factor affecting corrosion of steel piling. Little evidence of corrosion has been found when the entire piling is below the water table or when a concrete piling cap extends below the water table, even in corrosive so
42、ils. This was one of the major conclusions of the original NBS work by Romanoff, and that conclusion has stood the test of time.1 A recent example is described by Picozzi.2 An investigation was conducted on steel H-piles in an industrial waste environment in conjunction with rehabilitation of the Bu
43、ffalo Skyway. The water table was above the concrete pile cap. In spite of the presence of disturbed fill soils and corrosive soil characteristics, little corrosion loss of the piles was detected. Mechanistically, the effect of water table position on corrosion is readily explained. As described abo
44、ve, most instances of severe underground corrosion are the result of differential aeration cells. When the entire structure is below the water table, oxygen concentrations near the piling are uniformly low and the differential aeration cells do not develop. The maximum dissolved oxygen content in an
45、 aqueous phase is only 8.0 x 10-4% (8 ppm) at 77F 25C) compared with 20% (200,000 ppm) in the atmosphere. The position of the water table also may influence piling corrosion in instances in which the water table is below the top of the pile. In one Army Corp of Engineers report,3 it was observed tha
46、t corrosion attack of pilings was low when the majority of the piling was located below the water table, NACE International5even when the region above the water table was incorrosive soils. Again, this behavior can be explainedbased on a differential aeration cell mechanism. With acorrosion cell, th
47、e most severe attack occurs when thecathode (oxygen-rich area) is large and the anode (oxygen-deficient area) is small. This would represent conditions inwhich most of the piling is above the water table.Soil TypeSoil type is also an important factor affecting pilingcorrosion. This is a broad catego
48、ry that includes soilparticle-size distribution, soil stratification, man-made versusnatural soils, and cation-exchange capacity. Theclassification of soils is based on particle-size distribution.In the Unified Soil Classification System (USCS),4clays aredefined as having a grain size of less than 5
49、 m(1.97 x 10-4in.) while silt has a particle size between5 m (1.97 x 10-4in.) and 75 m (2.95 x 10-3in.), and sandhas a particle size between 75 m (2.95 x 10-3in.) and4.75 mm (0.187 in.). Because of their small particle sizeand chemical properties, clays hold moisture better than siltor sand and tend to be deficient in oxygen. When a pile isdriven through a stratified soil containing layers of clay andsilt or sand, the steel areas in the clay strata become theanodes in the differential aeration cells and the s
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