b'Corrosion ConsiderationCorrosionisdefinedasthedeteriorationofa dissolved chemical elements (ions) and serve asmetallic structure due to its interaction with the the electrical connection between different partssurrounding environment. of the structural element.The water containingSteel Underground - How Long Does It Last? dissolvedchemicalelementsiscalledanSteelfoundationsupportsaresubjectedtoa electrolyte.The presence (or absence) of theserange of corrosive forces that are quite different ions,aswellas their natureandconcentration,fromsteelexposedtoatmosphericconditions. determinesthe electricalconductivity,orTheperformanceofsteelandgalvanized resistivity, of the electrolyte.structural steel elements underground are not asAeration: The availability of oxygen (aeration)well understood as is the life expectancy of steel in the soil surrounding the metal is also essentialproducts in above ground applications. to the corrosion process.The cyclic process ofFor corrosion to initiate, steel requires not only wetting and drying of the soil causes oxygen tooxygen but also the presence of dissolved salts in be present in the soil.It is also the reason thatwater.Ifeitheroftheseitemsisabsent, mostcorrosionusuallyoccursnearthesurfacecorrosion will not occur. where the wet-dry cycle is more severe.Thecausesofcorrosiononburiedmetallic Under these conditions, metal ions will migratestructuresaregenerallyunderstood,butthis from the anodic (+) locations on a metallic objectknowledgebasedoesnotalwayspermitan and transfer to the cathodic (-) locations.It isaccurate prediction of a design life when placed thislossinmetalattheanodiclocationsthatin a corrosive environment.This chapter is not resultsinthedegradationoftheundergroundintendedasarigoroustechnical text;ratherit metallic structure.provides general knowledge to help the reader to Controlling Factors for Corrosionestablishing whether corrosion could be a critical Soil Type:Some soil types are more corrosivefactorinaspecificfoundationsupport thanothers.Thephysicalandmineralogicalapplication. composition of soils, which are a result of:A qualified engineer, knowledgeable inTheir origin, decomposition and depositiondesign for corrosion environments, shouldThe plant life and its decompositionbe consulted when foundation supportTopography of the landproducts are to be used in a known corrosiveenvironment. AlloftheseinfluencethesoilscorrosivityCorrosion occurs by an electrochemical process. potential.The soils having greatest concern areIn order for corrosion of an underground metallic those which produce water soluble acid formingstructure to occur, the following must be present: chemicalelements.Examplearecarbonates,1.)Electrical potential, 2.)Dissolvedsaltsin bicarbonates, chlorides, nitrates and sulfates, orwater (electrolyte) and 3.) Aeration base (alkaline) forming chemical elements suchElectrical Potential: Corrosion is initiated by as sodium, potassium, calcium or magnesium.a difference in electric potential (electric charge) Thesoilsthathavethehighestcorrosivebetweentwopointsonaburiedmetallic potential,aresoilsdescribedorclassifiedbystructure.This electrical potential can be caused geotechnicalengineersassilty,loamy,clay,bystrainsinthemetalorbetweencomponent organic(peats,cindersandashes),andsoilsparts, or contact with different soil types along which are poorly aerated.Granular soils (sandsthe shaft, or non-homogeneities in metal, etc.A and gravels) which are highly aerated can draindifferenceinelectricalpotentialcausesthe waterawayrapidly.Inwelldrainedsoilthedevelopment of anodes and cathodes along electrolyte is not constantly in contact with thethesurfaceofthemetal.Theremustbean steel and the corrosion process is reduced.electricalconnectionbetweentheanodesand Soil Resistivity:The resistivity of the soil is onecathodes for corrosion to occur. ofthesimplesttestsforsoilcorrosivity.ToElectrolyte:Water or moisture in the soil that obtainthesoilresistivity,onepassesacurrentsurroundsthepileorpiershaftmaycontain through the soil and measures the resistivity ofECP Technical Design Manual Corrosion Life2021 Earth Contact Products, L.L.C.2021-09 Chapter 8 - Page 147 All rights reserved'