Corrosion & Associated DegradationObjectives Corrosion process Environmental factors Common forms of corrosion Methods of corrosion control and prevention

Metal Corrosionthe destruction of a material by chemical or electrochemical reaction to its environmenttypically a transfer of electrons from one metal to another through an Oxidation-Reduction Reaction.

Oxidation - Reduction ReductionAnodic metal gives up electrons (oxidation)

Cathodic metal accepts electrons (reduction)

Or gases accept electrons (reduction)

Corrosion MechanismCathodic celldiscussion of emfgalvanic seriesintergranular corrosionoxidation-reduction of ironsalt effects

Slides on Impact of CorrosionAluminum corrosionPitting Crevice corrosionContaminantsEnvironments

Basics of CorrosionEMF series is a numeric rating of potential under ideal conditionsGalvanic Series is a practical listingGalvanic Protection

Steel CorrosionInitial Oxidation ReactionSecondary Oxidation ReactionRust

Corrosion potential calculationReduction Reaction must have higher potential than the oxidation reaction or they will not form a cathodic cell-0.440 V-0.763 VRelative measure of corrosion

Acceleration of CorrosionPhysical Characteristicsexposed area (less, increases corrosion rate)time of exposure (more time, more corrosion)Environmental Characteristicsacidic environmentsulfur gas environmenttemperature (high temps, more corrosion)moisture (oxygenated moisture)

PassivationA protective film in oxidizing atmosphereschromium,nickel, titanium, aluminumMetal oxide layer adheres to parent metalbarrier against further damageself-healing if scratchedSensitive to environmental conditionspassivated metal may have high corrosion rates

Forms of CorrosionUniform corrosion of a single metalusually an electrochemical reaction at granular levelrelatively slow and predictablerusting of exposed steel, tarnished silvereasily corrected with coatings and regular maintenance

Forms of CorrosionGalvanic Corrosion2 dissimilar metals, electrolyte, electrical connection and oxygenPitting CorrosionLocalized corrosion forming holes or indentationsDifficult to initially detect

Forms of CorrosionCrevice Corrosionnarrow crevice filled with ionized solutionOxygen-rich on the outside, oxygen-poor on the insidemetals oxidize with salt anions FeCl2 and pH rises in cathodic zoneH+ may destroy passivity

Forms of CorrosionIntergranular Corrosioncorrosion along grain boundaries at microscopic levelstainless steels and heat treated high-strength steelscarbides precipitate along grain boundaries leaving these areas with no alloyed ChromiumWelds can have this same depletion effect

Forms of CorrosionCavitation and Erosion in Pipeparticulate matterturbulent flowabrades away the corrosion product abrasion of zinc coatings

Forms of CorrosionStress Corrosion Crackingtensile stress and corrosive environmentscracks are initiated at corrosion areastensile stresses propagate the crackcorrosion further deteriorate cracketc.....

Reinforcement Corrosion

Corrosion ProductsFe + 2OH = Fe(OH)2

Oxidation of Fe(OH)2Fe(OH)3 (rust)Passivity barrier breaks downPresence of OxygenMoisture

Corrosion of Metals in ConcreteReinforcing Steel & Prestressing SteelConcrete is Normally Highly AlkalineProtects Steel from Rusting if Properly EmbeddedIf Corrosion Occurs, the Reaction Products are Greater in Volume Than the Original SteelCorrosion Initiation and Rate Depends OnAmount of Concrete Cover, Quality of ConcreteDetails of Construction, & Exposure to Chlorides

Avoiding Corrosive SituationsChoose couple metals close on the galvanic seriesUse large anode, and small cathode areasElectrically insulate dissimilar metalsConnect a more anodic metal to the systemAvoid turbulent flow and impingements in pipe systems

Examples of Corrosion in CESteel strapping or iron nails with copper pipe is ok, but they may rust with time.Never use Copper strapping or attachments with steel pipe, steel pipe will corrodecondensation on the bottom of cold water pipe

Corrosion PreventionCoatingsBarrier filmsInhibitive PigmentsSacrificial treatmentsPaintActive Cathodic Protection

electrochemical corrosion requires2 dissimilar metals (anode and cathode)electrolyte (solution that can support the flow of electrons)conductor (metallic pathway for electrons to flow between metals)potential is measured in emf seriesgalvanic series is practical listing of emf potentials in less idealized conditionsintergranular corrosion at boundaries, different potential for different structures e.g. pearlite is part carbide and part alpha ironoxidation reduction of ironsalt draw moisture to the surface, chlorides react with rust making is flake off faster and exposing more iron for the oxidation reactionAluminum - Steel corrosion is a galvanic reactionPitting - isolated breakdown of coating system, break in film is anode and the rest is a cathode (oxidation concentration cell)example of pitting of a bridge flangeCrevice Corrosion is caused by the formation of an oxidation concentration cell.contaminants in the air accelerate corrosion, sulfur and acid rain participate in reactionsSteel Structure Painting Council (oil base life prediction for coatings)