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Basic corrosion theory and protection methods

Basic corrosion theory and protection methods
Corrosion Protection Basic corrosion theory and protection methods Author: Dr. Thomas J. Langill © 2006 American Galvanizers Associationƒƒƒ Corrosion Corrosion Control What is Corrosion How/Why Does Corrosion Occur Corrosion Costs Forms of Corrosion Corrosion Control Methods Hotdip Galvanizing (HDG) Process Coating Characteristics Performance in Corrosive Environments Galvanized Steel in Actionƒ What is Corrosion Corrosion (n) The chemical or electrochemical reaction between a material and its environments that produces a deterioration of the material and its properties.The Galvanic Series ZINC Anode STEEL Cathode This 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 Couple Conventional Current Electrons Electrons External Circuit Return Current + Path Cathode Anode ElectrolyteBare 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 potentialƒƒƒƒ Forms of Corrosion General Identified by uniform formation of corrosion products that causes a even thinning of the substrate steel Localized Caused by difference in chemical or physical conditions between adjoining sites Bacterial Caused by the formation of bacteria with an affinity for metals on the surface of the steel Galvanic/Dissimilar Metal Caused when dissimilar metals come in contact, the difference in electrical potential sets up a corrosion cell or a bimetallic coupleƒƒƒ ƒƒ Corrosion Costs Direct Costs Indirect Costs NACE, CC Catastrophe Technologies, Public safety, property damage, environmental FHWA jointly contamination produced a report in 2001 detailing the Natural Resources costs of corrosion Waste production, increased energy 276 billion USD consumption annually Public Outcry 3.1 of US GDP (1998) Traffic, inconvenienceƒƒƒ Methods of Corrosion Control Barrier Protection Provided by a protective coating that acts as a barrier between corrosive elements and the metal substrate Cathodic Protection Employs protecting one metal by connecting it to another metal that is more anodic, according to the galvanic series Corrosion Resistant Materials Materials inherently resistant to corrosion in certain environmentsBarrier Protection Paint Powder Coatings Galvanizingƒƒƒ Cathodic Protection Impressed Current Galvanic Sacrificial Anode Galvanic Zinc Application Zinc Metallizing Zincrich Paints Hotdip Galvanizingƒƒ Cathodic Protection Impressed 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.ƒƒ Cathodic Protection Galvanic 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 ƒƒƒ ƒƒƒ Cathodic Protection Galvanic 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 Zincrich Paints Zincrich paints contain various amounts of metallic zinc dust and are applied by brush or spray to properly prepared steel Hotdip Galvanizing Complete immersion of steel into a kettle/vessel of molten zincGalvanic Zinc Applications Zincrich Paints Zinc MetallizingHotdip Galvanizing Process Surface Preparation Galvanizing Inspectionƒƒƒ Surface Preparation Zinciron metallurgical bond only occurs on clean steel Degreasing Removes dirt, oils, organic residue Pickling Removes mill scale and oxides Fluxing Mild cleaning, provides Degreasing/Caustic cleaning protective layerGalvanizing Steel 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 Zinc bath removal galvanized coating.Inspection Steel 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.Metallurgical BondEdge Protection Same thickness at corner Micrograph of galvanized edgeƒƒ Influencers of Coating Development Steel Surface Conditions Steel Chemistry Silicon PhosphorousThe Sandelin CurveCoating Appearance Newly Newly Galvanized Galvanized Dull Coating No Spangle Newly Newly Installed Galvanized Shiny Dull Highly Spangle CoatingThe Zinc Patina Forms as zinc reacts with the environment Consists of zinc oxide, zinc hydroxide, and zinc carbonate Protects the galvanized coating by providing an additional layer of corrosion resistancePassivation Cycle Time 0 – 48 hrs. 1 48 hrs. – 6 mo. 2 6 mo. – 2 yrs. 3Environmental Performance Atmospheric Liquid (Chemicals, Fresh H O, Salt H O) 2 2 Soil High Temperature Low Temperature ConcreteAtmospheric: Service Life of HDGLiquid: Effect of pH on HDG steelƒƒƒƒƒƒ Performance in Soil 200 different soil types Complex corrosion kinetics in soil Variables include: Porosity Resistivity Organic material Moisture content pH Temperatureƒƒ Performance in Various Temps High Temperature 392 F (200 C) Low Temperature 75 F (60 C)Concrete: Rebar Corrosion Staining Cracking Spalling Complete FailureConcrete: Galvanized Rebar Unprotected Rebar Galvanized RebarZinc is Natural Air Soil WaterFeatures of HDG Coatings Zinciron intermetallic layers Harder than the substrate steel Zinc patina Barrier protection Cathodic protection Metallurgical bond to the substrate steel Paintable Edge and corner protection Zinc is a natural and healthy metalBenefits of HDG Coatings Maintenancefree for 50 – 100 years in most atmospheric environments Long term performance in soils, water, and chemical environments No touchup required High Low temperature performance Application independent of weather 100 recyclableDry Bridge Road Bridge Date Galvanized 1999 Sector Bridge Highway Environment Rural Location Alexander, NYHarrisburg Airport Transportation Facility Date Galvanized 2004 Sector Building Architecture Environment Urban Location Harrisburg, PAAESPR Total Energy Power Plant Date Galvanized 2002 Sector Electrical, Utility Communication Environment Industrial Location San Juan, Puerto RicoLeprino Foods Date Galvanized 2002 Sector Food Agriculture Environment Rural Location Waverly , NYAspinwall Water Treatment Plant Date Galvanized 2001 Sector Water Marine Environment Industrial Location Pittsburgh, PA