13Active Corrosion (dissolution) Understand what’s happening in your systemActive Corrosion (dissolution)pHProtective ScalePassive Film
14Activation controlled Determine the rate limiting redox processesPassive regionActivation controlledRate-limiting cathodic process
15Determine pitting potential and max growth rate No PittingPitting
16Pro-active Analysis Test Corrective Actions Determine optimum pH Screen alloys and inhibitorsAssess process changesFocus Lab workEliminate potential problems before they occur
17The Corrosion Analyzer Tool for understanding the corrosion environmentMechanistically-based software toolSpeciationKinetics of uniform corrosion Partial anodic and cathodic processesTransport propertiesRepassivation
18The Corrosion Analyzer Based on the OLI EngineComplete speciation model for complex mixturesPhase and chemical reaction equilibriaAccurate pH predictionRedox chemistryComprehensive coverage of industrial chemical and petroleum systems
19The Corrosion Analyzer Based on the OLI EngineThermophysical properties predictionPhenomenological and unique aqueous process models including kinetics and transport“Out-of-the-box” solution and technical support
20The Corrosion Analyzer What It Does…Predict metal dissolution regime, passive films, and surface depositsPredict uniform corrosion rates and the potential for pitting corrosionGenerate real solution stability (Pourbaix) DiagramsProduce theoretical polarization curves
21The Corrosion Analyzer So you can gain insight on …Corrosion mechanismsRate-limiting partial processes for your operating conditionsEffects of process and materials changesThereforeFocusing lab timeReducing risky plant/field testingManaging design, operation, and maintenance
22Today’s seminar “Hands-on” and “How-To” Using example problems Examining plots and diagramsUnderstanding the basis of the predictions
23Today’s Seminar Perform “Single point” calculations Construct / interpret real solution Pourbaix DiagramsCalculate corrosion ratesEvaluate the effects of pH, T, comp / flowEvaluate polarization curvesGain insight to corrosion mechanismsSee rate limiting stepsCan I read them? Can I trust them?Determine the likelihood of pitting to occurFor your actual field or lab conditions
24Simulating Real World Corrosion Problems Welcome to theCORROSION TEACH-INSimulating Real World Corrosion Problems
25Gas Condensate Corrosion ScopeGas condensates from alkanolamine gas sweetening plants can be highly corrosive.PurposeDiethanolamine is used to neutralize (sweeten) a natural gas stream. This removes carbon dioxide and hydrogen sulfide. The off gas from the regeneration is highly acidic and corrosive
26Gas Condensate Corrosion ObjectivesDetermine the dew point of the acid gasRemove the condensed phase and perform corrosion rate calculationsMitigate the corrosion
27Gas SweeteningSour Gas AbsorberAcid GasAbsorber liquor regenerator
28Acid Gas Concentrations SpeciesConcentration (mole %)H2O5.42CO277.4N20.02H2S16.6Methane0.50Ethane0.03PropaneTemperature38 oCPressure1.2 Atm.Amount100 moles
42304 Stainless Steel Stability @ Dew Point Passivation is possible due to Cr2O3
43Explaining common observations using Stability Diagrams Why Iron RustsExplaining common observations using Stability Diagrams
44BasicsIron is inherently unstable in water & oxidizes via the following reactions to form rustIts severity depends on (among others)Conditions (T/P),Composition,pH, andoxidation potentialThese four can be plotted on a single chart called a stability diagram
46White area is region of iron corrosion Explaining the EH-pH diagram using Fe, showing solid and dissolved species over range of pH’s and oxidation potentialsH2O is oxidized to O2 and H+H2O is reduced to H2 and OH-Elemental iron, Fe(0)o, is stable and will not corrode in this regionH2O is stable and deaeratedH2O is stable and aeratedFe2O3 reduces and dissolves in waterFe(II) oxidizes and precipitates as Fe2O3Elemental iron, Fe(0) oxidizes to Fe(II) in the presence of waterFeO(OH), rust is stable in water at moderate to high pH’sWhite area is region of iron corrosionWater Oxidation LineWater Reduction LineFe3O4 coats the iron surface, protecting it from corrosionFe(III)3+ is thedominant ionFe(II)2+ is theElemental iron (gray region) corrodes in water to form one of several phases, depending on pH. At ~9 pH and lower, water oxidizes Fe0 to Fe+2 which dissolves in water (white region of the plot). As the oxidation potential increases (high dissolved O2) Fe+2 precipitates as FeOOH, or rust (green region). The lower the pH, the thicker the white region and the greater driving force for corrosionAt higher pH (10-11), Fe0 forms Fe3O4, a stable solid that precipitates on the iron surface, protecting it from further attack.
47Q: We all know O2 is bad…But how much is bad? H2O is oxidized to O2 and H+H2O is stable and aeratedWater Oxidation Line10 ppm O20.1ppm H2500 ppm O280 ppm H20.1 ppb H23 ppb O20.1 ppT H20.1 ppT O2Pure water is here…No air, no acid, no baseH2O is reduced to H2 and OH-H2O is stable and deaeratedWater Reduction Line
48Region of instability Elemental Iron (Feo) Iron and water react because they are not stable togetherRegion of instabilityThe reaction generates 2OH-, which increases the pHThe reaction generates H2, which puts the EH near the bottom lineElemental Iron (Feo)
50The reaction ends within the Fe3O4 region The reaction ends within the Fe3O4 region. Fe3O4 is a solid that passivates the iron surface protecting it from active corrosionInitial ConditionsDI water, no Feo7pH, 0.4VFinal Conditions1 ppm Feo added9.38pH, 0.5V0.9 ppb Feo7.07pH, -0.27V0.1 ppm Feo8.48pH, -0.42V
51Overlaying the Fe3O4 mass on the diagram – once the pH reached 9, Fe3O4 began to precipitate 1.4 g Fe3O4 ppts from 1 FeoFe3O4 precipitates when 0.3 mg/l Feo has reactedThe ppt point lines up with the stability curve
55Cr will oxidizes, but the reaction goes through a tough Cr2O3 protective layer.
56Ni3Fe2O4 is stable in the corrosion region, and will also protect the surface.
57Simulating Real World Corrosion Problems Welcome to theCORROSION TEACH-INSimulating Real World Corrosion Problems
58Corrosion in Seawater Scope Metals used for handling sea water face both general and localized corrosion.Various grades of stainless steels have been used to mitigate the problems.Stainless steels owe their corrosion resistance to a thin adherent film of oxides on their surface.Disruption of the films can lead to localized corrosion and premature failure.
59Corrosion in Seawater Purpose Chlorine and oxygen in sea water can attack the films used to passivate the steels.The CorrosionAnalyzer will be used to model the effects of chloride and oxygen on the rates of uniform corrosion and the possibility of pitting on the surface of the metals.
60Corrosion in Seawater Objectives Reconcile a sea water sample for electroneutralityReconcile a gas analysisCalculate uniform rates of corrosion for304 stainless steel316 stainless steelS31254 stainless steel
61Corrosion in Seawater Objectives (continued) Determine the probability of pitting using the localized corrosion feature.
62Kinetic Model of General Corrosion: Mass-Transfer MetalSurfaceSolutionAll reactions take place on the metal surface.Films are a diffusion barrier to corrosive speciesReduce mass-transfer-limited currents.Mass-transfer from solution is calculated from a concentration- dependent diffusion coefficient.film
63Chemistry The rates of corrosion use a subset of the OLI Chemistry Neutral SpeciesH2O, O2, CO2, H2S, N2 and all inert gases, Cl2, SO2, So and NH3, organic molecules that do not undergo electrochemical reactionsAnionsOH-, Cl-, Br-, I-, HCO3-, CO3-2, HS-, S2-, SO42-, HSO4-, SO32-, NO2-, NO3-, MoO42-, CN-, ClO4-, ClO3-, ClO-, acetate, formate, Cr(VI) anions, As(III) anions, P(V) anions, W(VI) anions, B(III) anions and Si(IV) anions.
65Corrosion of 304 Stainless Steel in Deaerated Sea Water LabAnalyzer used to reconcile electroneutralityNaOH/HCl Used to adjust pHSpeciesConcentration (mg/L)Cl-19000Na+10700Mg+21300Ca+2400SO4-22750HCO3-150pH8.0Temperature25 oCPressure1 atm.
67Screening Considerations Some alloys do not perform well in seawaterWe will evaluate 3 stainless steelsUniform corrosion ratesPitting possibilityConsidering both deaerated and aerated conditions
68Corrosion of 304 Stainless Steel in Deaerated Sea Water 300 years to lose 1 mm of metaloC
69Corrosion of 304 Stainless Steel in Deaerated Sea Water Large difference means that pits are unlikely to formRepassivation PotentialDifference = 0.05 VCorrosion PotentialOr if a pit forms, then it will passivate
70What’s on a Polarization Curve? Standard Tafel BehaviorTransition to mass-transfer limited current density
71What’s on a Polarization Curve? The curve is only valid in aqueous systems and will be bounded by the decomposition of water.Intersection indicates location of the corrosion potentialCurrent density at corrosion potential also read at intersection
72What’s on a Polarization Curve? Basic polarization curve with water decomposition and corrosion reaction
73What’s on a Polarization Curve? Polarization curve with water decomposition, corrosion reaction and two mass-transfer-limited reactions.
74What’s on a Polarization Curve? This is what is measured experimentallyTranspassive regionPassive regionCorrosion Potential and Corrosion currentActive Corrosion
75What’s on a Polarization Curve? TranspassivePassiveActivePolarization curve demonstrating a galvonostatic sweep. The arrows indicate how the potential is changing as one moves along the line.
76There are many processes that make up the polarization curve. Fe = Fe+2 + 2e-2H2O=O2+4H++4e-H2O + e- = ½ H2+OH-H+ + e- = ½ H2
77The Polarization Curve for 304 SS in Deaerated Water Oxidation of water to O2Measurable polarization curveBreakdown of water to H2Peak Current density in the pit with the highest corrosion rateOpen circuit potential and current densityCorrosion of 304 ss
78Kinetic Model of General Corrosion: Phenomena Partial electrochemical processes in the active state:Cathodic reactions (e.g., reduction of protons, water molecules, oxygen, etc.)Anodic reactions (e.g., oxidation of metals)Adsorption of species on the metal surfaceActive-passive transition influenced byAcid/base properties of passive oxide filmsTemperatureAdditional species that influence the dissolution kinetics of oxide layersSynthesis of the partial processes according to the mixed potential theoryMay 20, 1997OLI Systems, Inc,
89304 SS in Aerated SolutionThe corrosion potential is greater than the passivation potential = .37 V at max O2Pitting will occur
90304 SS Polarization in Aerated Water 8 ppm O2Corrosion potential shifted anodically of the repassivation potential.0 ppm O2The surface will couple galvanically with the pits to increase their rate of corrosion.
97Mitigation Change Alloys Cathodic Protection S31254 seems the best at 25 oCS31254 increased potential for pitting at higher temperaturesCathodic ProtectionShifting of potential to less corrosive potentials via a sacrificial anode.Analyzers do not model CPPolarization curves can help determine the change in potential.
98Simulating Real World Corrosion Problems Welcome to theCORROSION TEACH-INSimulating Real World Corrosion Problems
99Dealloying of Copper Nickel Alloys ScopeA copper-nickel pipe made of Cupronickel 30 has been preferentially dealloyed while in contact with a 26 weight percent calcium chloride brine. It appears that the nickel in the alloy has been preferentially removed.
100Dealloying of Copper Nickel Alloys PurposeThe OLI/CorrosionAnalyzer will be used to show the relative stability of nickel and copper in the cupronickel alloy in an aqueous solution. It will show that protective films were not present as originally thought.
101Dealloying of Copper Nickel Alloys ObjectivesInput information into the software and perform calculationsUse stability diagrams to display information about the alloy and the protective filmsChange the diagrams to view different aspects of the stability of the alloy
102Application: Dealloying of Copper-Nickel Alloys A cupronickel 30 pipe (30 mass % copper) was used.26 wt % CaCl2 solution was in contact with the pipe.Nickel was preferentially removed.Dealloyed cupronickel pipe.
103Questions? Why did the nickel dealloy from the pipe? What could we do to prevent this from occurring?Which tools are available to understand this phenomenon?
104Which Tools are Available? A Pourbaix diagram can help us determine where metals are stable.CorrosionAnalyzer
105Creating the First Stability Diagram We will use the CorrosionAnalyzer to create a stability diagram for this system.Features of CorrosionAnalyzer diagramsReal-solution activity coefficientsElevated temperaturesElevated pressuresInteractions between species and overlay of diagrams.
107Application TimeTime to start working with the OLI Corrosion Analyzer
108The Pourbaix DiagramThere are quite a few things to look at on this diagram.Stability field for waterStability fields for nickel metal and copper metalStability fields for nickel and copper oxidesStability fields for aqueous species.We will now break down the diagram in to more manageable parts.
109Stability Diagram Features SubsystemsA base species in its neutral state and all of its possible oxidation states.Cuo, Cu+1, Cu+2Nio, Ni+2All solids and aqueous species that can be formed from the bulk chemistry for each oxidation state.
110Stability Diagram Features For each subsystemContact SurfaceBase metalsAlloysFilmsSolidsSolid LinesAqueous Lines
111Stability Diagram Features Natural pHPrediction based on the bulk fluid concentrationsDisplayed as a vertical lineSolidsAll solids included by defaultThe chemistry can be modified to eliminate slow forming solids.
112Stability Diagram Features PassivityThin, oxidized protective films forming on metal or alloy surfaces.Transport barrier of corrosive species to metal surface.Blocks reaction sites
113Water Stability Water can act as an oxidizing agent Water is reduced to hydrogen, H2Water can act as a reducing agentWater is oxidized to oxygen, O2To be stable in aqueous solution, a species must not react with water through a redox process.
115Water Stability – Natural Waters Surface waterOcean waterBog waterOrganic rich lake waterOrganic rich waterlogged soilsOrganic rich saline water
116Copper Pourbaix Diagram Oxidized SpeciesE Independent acid and base chemistryPredominant speciespH dependent redoxpH independent redoxReduced Species
117Copper Pourbaix Diagram Aqueous speciesStability field for passivating filmEquilibrium between species in contact with a solidEquilibrium between speciesNatural pHStability field for base metal or alloy
118Copper Pourbaix Diagram Stable copper metal in alloy extending into water stability field.The solution pH is in a region where the copper metal will be stable.Copper pipes are used for potable water for this reason.
119Nickel Pourbaix Diagram No Nickel metal extends into the water stability fieldThe solution pH is in a region where nickel is expected to corrode
120Ni Overlaid on Cu We need to know the Oxidation/Reduction potential CuCl(s) may form to protect the alloy at the solution pH.Since the nickel is part of a copper-nickel alloy, it is possible that copper could provide a protective film
123CorrosionAnalyzer Calculation The oxidation reduction potential is V
124Ni Overlaid on CuThe potential of V lies above the passivating film. Dealloying can occur.
125Conclusions Why did dealloying occur? No protective film at the operating pH and oxidation/reduction potential of the process fluid.Copper lies within the region of water stabilityNickel does not lie within the region of water stabilityThe presence of Cu+ ions in equilibrium with copper metal promotes replating of copper metal driven by the oxidation of nickel.
126Chemistry Standard OLI Chemistry 7400 components9100 individual species82 Elements of the Periodic Table fully covered8 additional elements partially covered.Stability diagrams have access all of this chemistry
127Chemistry Alloys 6 predefined classes supported User defined alloys Cu-NiCarbon Steels – Fe, Mn, and CFerritic Stainless steels – Fe, Cr, Ni, Mo and CAustenitic stainless steels - Fe, Cr, Ni, Mo and CDuplex stainless steels FCC phase - Fe, Cr, Ni, Mo, C and NUser defined alloys
128Limits to the Standard OLI Chemistry Aqueous PhaseXH2O > 0.65-50oC < T < 300oC0 Atm < P < 1500 Atm0 < I < 30Non-aqueous LiquidCurrently no Activity Coefficient Model (i.e., no NRTL, Unifaq/Uniqac)Fugacity Coefficients are determined from the Enhanced SRK
129Limitations of Pourbaix Diagrams No information on corrosion kinetics is provided.Diagram is produced from only thermodynamics.Diagram is valid only for the calculated temperature and pressureOxide stability fields are calculated thermodynamically and may not provide an actual protective film.Dealloying cannot be predicted from the diagram alone.