Corrosion Protection.ppt [Read-Only] - University of Iowa

1 Corrosion ProtectionBasic Corrosion theory and protection methodsAuthor: Dr. Thomas J. Langill 2006 American Galvanizers AssociationCorrosion & Corrosion ControlWhat is CorrosionHow/Why Does Corrosion OccurCorrosion CostsForms of CorrosionCorrosion Control MethodsHot-dip Galvanizing (HDG) Process Coating Characteristics Performance in Corrosive EnvironmentsGalvanized Steel in ActionWhat is CorrosionCorrosion (n) The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its Galvanic SeriesZINC -AnodeSTEEL -CathodeThis arrangement of metals determines what metal will be the anode and cathode when the two are put in a electrolytic cell (arrangement dependent on salt water as electrolyte).Bimetallic CoupleCathodeAnodeElectrolyteExternal Circuit+-ConventionalCurrentElectronsEle ctronsReturn Current PathBare Steel CorrosionMicroscopic anodic and cathodic areas exist on a single piece of anodic areas corrode, new material of different composition is exposed and thus has a different electrical potentialForms of CorrosionGeneral Identified by uniform formation of Corrosion products that causes a even thinning of the substrate steelLocalized Caused by difference in chemical or physical conditions between adjoining sitesBacterial Caused by the formation of bacteria with an affinity for metals on the surface of the steelGalvanic/Dissimilar Metal Caused when dissimilar metals come in contact.

2 The difference inelectrical potential sets up a Corrosion cell or a bimetallic coupleCorrosion CostsDirect CostsNACE, CC Technologies, & FHWA jointly produced a report in 2001 detailing the costs of Corrosion $276 billion USD annually of US GDP (1998)Indirect CostsCatastrophe Public safety, property damage, environmental contaminationNatural Resources Waste production, increased energy consumptionPublic Outcry Traffic, inconvenienceMethods of Corrosion ControlBarrier protection Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrateCathodic protection Employs protecting one metal by connecting it to another metal that is more anodic, according to the galvanic seriesCorrosion Resistant Materials Materials inherently resistant to Corrosion in certain environmentsBarrier ProtectionPaintPowder CoatingsGalvanizingCathodic ProtectionImpressed CurrentGalvanic Sacrificial AnodeGalvanic Zinc Application Zinc Metallizing Zinc-rich Paints Hot-dip GalvanizingCathodic ProtectionImpressed Current External source of direct current power is connected (or impressed) between the structure to be protected and the ground bed (anode)

3 Ideal impressed current systems use ground bed material that can discharge large amounts of current and yet still have a long life ProtectionGalvanic Sacrificial Anode Pieces of an active metal such as magnesium or zinc are placed in contact with the corrosive environment and are electrically connected to the structure to be protected Example: Docked Naval Ships cathodic ProtectionGalvanic Zinc Application Zinc Metallizing (plating) Feeding zinc into a heated gun, where it is melted and sprayed on a structure or part using combustion gases and/or auxiliary compressed air Zinc-rich Paints Zinc-rich paints contain various amounts of metallic zinc dust and are applied by brush or spray to properly prepared steel Hot-dip Galvanizing Complete immersion of steel into a kettle/vessel of molten zincGalvanic Zinc ApplicationsZinc MetallizingZinc-rich PaintsHot-dip Galvanizing ProcessSurface PreparationGalvanizingInspectionSurface PreparationDegreasing Removes dirt, oils, organic residuePickling Removes mill scale and oxidesFluxing Mild cleaning.

4 Provides protective layerDegreasing/Caustic cleaningZinc-iron metallurgical bond only occurs on clean steelGalvanizingSteel articles are immersed in a bath of molten zinc ( 830 F)> 98% pure zinc, minor elements added for coating properties (Al, Bi, Ni)Zinc reacts with iron in the steel to form galvanized bath removalInspectionSteel articles are inspected after galvanizing to verify conformance to appropriate defects easily identified through visual thickness verified through magnetic thickness gauge BondEdge ProtectionMicrograph of galvanized edgeSame thickness at cornerInfluencers of Coating DevelopmentSteel Surface ConditionsSteel Chemistry Silicon PhosphorousThe Sandelin CurveCoating AppearanceNewly GalvanizedNewly GalvanizedNewly GalvanizedNewly InstalledNo SpangleHighly SpangleDull CoatingShiny & Dull CoatingThe Zinc PatinaForms as zinc reacts with the environmentConsists of zinc oxide, zinc hydroxide.

5 And zinc carbonateProtects the galvanized coating by providing an additional layer of Corrosion resistancePassivation Cycle123 Time0 48 hrs. 6 mo. 2 PerformanceAtmosphericLiquid (Chemicals, Fresh H2O, Salt H2O)SoilHigh TemperatureLow TemperatureConcreteAtmospheric: Service Life of HDGL iquid: Effect of pH on HDG steelPerformance in Soil> 200 different soil typesComplex Corrosion kinetics in soilVariables include: Porosity Resistivity Organic material Moisture content pH TemperaturePerformance in Various TempsHigh Temperature < 392 F (200 C)Low Temperature > -75 F (-60 C)Concrete: Rebar CorrosionStainingCrackingSpallingComplet e FailureConcrete: Galvanized RebarUnprotected RebarGalvanized RebarZinc is NaturalAirSoilWaterFeatures of HDG CoatingsZinc-iron intermetallic layersHarder than the substrate steelZinc patina Barrier protectionCathodic protectionMetallurgical bond to the substrate steelPaintableEdge and corner protectionZinc is a natural and healthy metalBenefits of HDG CoatingsMaintenance-free for 50 100 years in most atmospheric environmentsLong term performance in soils, water, and chemical environmentsNo touch-up required High & Low temperature performanceApplication independent of weather100% recyclableDry Bridge Road BridgeDate Galvanized1999 SectorBridge & HighwayEnvironmentRuralLocationAlexander .

6 NYHarrisburg Airport Transportation FacilityDate Galvanized2004 SectorBuilding & ArchitectureEnvironmentUrbanLocationHarr isburg, PAAES-PR Total Energy Power PlantDate Galvanized2002 SectorElectrical, Utility & CommunicationEnvironmentIndustrialLocati onSan Juan, Puerto RicoLeprino FoodsDate Galvanized2002 SectorFood & AgricultureEnvironmentRuralLocationWaver ly , NYAspinwall Water Treatment PlantDate Galvanized2001 SectorWater & MarineEnvironmentIndustrialLocationPitts burgh, PA