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Corrosion ProtectionBasic corrosion theory and
protection methods
Author: Dr. Thomas J. Langill© 2006 American Galvanizers Association
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Corrosion & Corrosion ControlWhat is CorrosionHow/Why Does Corrosion OccurCorrosion CostsForms of CorrosionCorrosion Control MethodsHot-dip Galvanizing (HDG)
ProcessCoating CharacteristicsPerformance in Corrosive Environments
Galvanized Steel in Action
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What is CorrosionCorrosion (n)
The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties.
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The Galvanic SeriesZINC - Anode
STEEL - 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).
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Bimetallic Couple
Cathode Anode
Electrolyte
External Circuit
+ -
ConventionalCurrent
Electrons Electrons
Return Current Path
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Bare Steel Corrosion
Microscopic anodic and cathodic areas exist on a single piece of steel.As anodic areas corrode, new material of different composition is exposed and thus has a different electrical potential
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Forms of CorrosionGeneral
Identified by uniform formation of corrosion products that causes a even thinning of the substrate steel
LocalizedCaused by difference in chemical or physical conditions between adjoining sites
BacterialCaused by the formation of bacteria with an affinity for metals on the surface of the steel
Galvanic/Dissimilar MetalCaused when dissimilar metals come in contact, the difference inelectrical potential sets up a corrosion cell or a bimetallic couple
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Corrosion CostsDirect Costs
NACE, CC Technologies, & FHWA jointly produced a report in 2001 detailing the costs of corrosion
$276 billion USD annually3.1% of US GDP (1998)
Indirect CostsCatastrophe
Public safety, property damage, environmental contamination
Natural ResourcesWaste production, increased energy consumption
Public OutcryTraffic, inconvenience
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Methods of Corrosion ControlBarrier Protection
Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate
Cathodic ProtectionEmploys protecting one metal by connecting it to another metal that is more anodic, according to the galvanic series
Corrosion Resistant MaterialsMaterials inherently resistant to corrosion in certain environments
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Barrier Protection
PaintPowder CoatingsGalvanizing
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Cathodic ProtectionImpressed CurrentGalvanic Sacrificial AnodeGalvanic Zinc Application
Zinc MetallizingZinc-rich PaintsHot-dip Galvanizing
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Cathodic ProtectionImpressed Current
External source of direct current power is connected (or impressed) between the structure to be protected and the ground bed (anode)Ideal impressed current systems use ground bed material that can discharge large amounts of current and yet still have a long life expectancy.
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Cathodic 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 protectedExample: Docked Naval Ships
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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 PaintsZinc-rich paints contain various amounts of metallic zinc dust and are applied by brush or spray to properly prepared steel
Hot-dip GalvanizingComplete immersion of steel into a kettle/vessel of molten zinc
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Galvanic Zinc Applications
Zinc Metallizing
Zinc-rich Paints
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Hot-dip Galvanizing Process
Surface PreparationGalvanizingInspection
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Surface Preparation
DegreasingRemoves dirt, oils, organic residue
PicklingRemoves mill scale and oxides
FluxingMild cleaning, provides protective layer
Degreasing/Caustic cleaning
Zinc-iron metallurgical bond only occurs on clean steel
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GalvanizingSteel 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 coating. Zinc bath removal
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InspectionSteel articles are inspected after galvanizing to verify conformance to appropriate specs.Surface defects easily identified through visual inspection.Coating thickness verified through magnetic thickness gauge readings.
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Metallurgical Bond
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Edge Protection
Micrograph of galvanized edge
Same thickness at corner
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Influencers of Coating DevelopmentSteel Surface ConditionsSteel Chemistry
SiliconPhosphorous
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The Sandelin Curve
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Coating Appearance
Newly Galvanized
Newly Galvanized
Newly Galvanized
Newly Installed
No Spangle
Highly Spangle
Dull Coating
Shiny & Dull Coating
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The Zinc PatinaForms as zinc reacts with the environmentConsists of zinc oxide, zinc hydroxide, and zinc carbonateProtects the galvanized coating by providing an additional layer of corrosion resistance
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Passivation Cycle
1
2
3
Time0 – 48 hrs.
48 hrs. – 6 mo.
6 mo. – 2 yrs.
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Environmental Performance
AtmosphericLiquid (Chemicals, Fresh H2O, Salt H2O)SoilHigh TemperatureLow TemperatureConcrete
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Atmospheric: Service Life of HDG
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Liquid: Effect of pH on HDG steel
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Performance in Soil> 200 different soil typesComplex corrosion kinetics in soilVariables include:
PorosityResistivityOrganic material Moisture contentpHTemperature
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Performance in Various Temps
High Temperature< 392 F (200 C)
Low Temperature> -75 F (-60 C)
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Concrete: Rebar Corrosion
Staining Cracking
Spalling Complete Failure
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Concrete: Galvanized Rebar
Unprotected RebarGalvanized Rebar
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Zinc is NaturalAir
Soil
Water
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Features 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 metal
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Benefits 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% recyclable
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Dry Bridge Road BridgeDate Galvanized1999
SectorBridge & Highway
EnvironmentRural
LocationAlexander, NY
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Harrisburg Airport Transportation Facility
Date Galvanized2004
SectorBuilding & Architecture
EnvironmentUrban
LocationHarrisburg, PA
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AES-PR Total Energy Power Plant
Date Galvanized2002
SectorElectrical, Utility & Communication
EnvironmentIndustrial
LocationSan Juan, Puerto Rico
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Leprino Foods
Date Galvanized2002
SectorFood & Agriculture
EnvironmentRural
LocationWaverly , NY
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Aspinwall Water Treatment Plant
Date Galvanized2001
SectorWater & Marine
EnvironmentIndustrial
LocationPittsburgh, PA