Understanding the corrosion environment Teach-in Understanding the corrosion environment
Any method be made more effective… Different methods for corrosion control Coupons Online Monitors Inhibition programs Any method be made more effective…
…When you understand the effect of the corrosion environment Corrosion rates vary with process conditions
5.5% NaCl
5.5% NaCl, 5 atm
5.5% NaCl, 85 °C
5.5% NaCl, 10 °C, 15 atm
It helps to know the effect of variations in the field To interpret coupon and monitor data… It helps to know the effect of variations in the field
Wait for a failure…? Rely on past experience? To locate where to place sensors & coupons… Wait for a failure…? Rely on past experience?
Tell you what has already happened, not what will happen Coupons Online Monitors Tell you what has already happened, not what will happen
OLI tools can help OLI gets the chemistry right
? Dew point pH Phase splits
Active Corrosion (dissolution) Understand what’s happening in your system Active Corrosion (dissolution) pH Protective Scale Passive Film
Activation controlled Determine the rate limiting redox processes Passive region Activation controlled Rate-limiting cathodic process
Determine pitting potential and max growth rate No Pitting Pitting
Pro-active Analysis Test Corrective Actions Determine optimum pH Screen alloys and inhibitors Assess process changes Focus Lab work Eliminate potential problems before they occur
The Corrosion Analyzer Tool for understanding the corrosion environment Mechanistically-based software tool Speciation Kinetics of uniform corrosion Partial anodic and cathodic processes Transport properties Repassivation
The Corrosion Analyzer Based on the OLI Engine Complete speciation model for complex mixtures Phase and chemical reaction equilibria Accurate pH prediction Redox chemistry Comprehensive coverage of industrial chemical and petroleum systems
The Corrosion Analyzer Based on the OLI Engine Thermophysical properties prediction Phenomenological and unique aqueous process models including kinetics and transport “Out-of-the-box” solution and technical support
The Corrosion Analyzer What It Does… Predict metal dissolution regime, passive films, and surface deposits Predict uniform corrosion rates and the potential for pitting corrosion Generate real solution stability (Pourbaix) Diagrams Produce theoretical polarization curves
The Corrosion Analyzer So you can gain insight on … Corrosion mechanisms Rate-limiting partial processes for your operating conditions Effects of process and materials changes Therefore Focusing lab time Reducing risky plant/field testing Managing design, operation, and maintenance
Today’s seminar “Hands-on” and “How-To” Using example problems Examining plots and diagrams Understanding the basis of the predictions
Today’s Seminar Perform “Single point” calculations Construct / interpret real solution Pourbaix Diagrams Calculate corrosion rates Evaluate the effects of pH, T, comp / flow Evaluate polarization curves Gain insight to corrosion mechanisms See rate limiting steps Can I read them? Can I trust them? Determine the likelihood of pitting to occur For your actual field or lab conditions
Simulating Real World Corrosion Problems Welcome to the CORROSION TEACH-IN Simulating Real World Corrosion Problems
Gas Condensate Corrosion Scope Gas condensates from alkanolamine gas sweetening plants can be highly corrosive. Purpose Diethanolamine 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
Gas Condensate Corrosion Objectives Determine the dew point of the acid gas Remove the condensed phase and perform corrosion rate calculations Mitigate the corrosion
Gas Sweetening Sour Gas Absorber Acid Gas Absorber liquor regenerator
Acid Gas Concentrations Species Concentration (mole %) H2O 5.42 CO2 77.4 N2 0.02 H2S 16.6 Methane 0.50 Ethane 0.03 Propane Temperature 38 oC Pressure 1.2 Atm. Amount 100 moles
Application Time
Dew Point Dew Point = 37.6 oC pH = 3.93 ORP = 0.576 V
Corrosion Rates: Flow Conditions Flow conditions have a direct effect on mass-transfer Static Pipe flow Rotating disk Rotating cylinder Complete agitation
Application Time
Carbon Steel Corrosion @ Dew Point Corrosion Rate = 0.7 mm/yr Corrosion Potential = -0.43 V Repassivation Potential = > 2 V Current Density = 60.5 A/cm2 H2CO3(aq)= ½ H+ + HCO3- - e H2S(aq)= ½ H2 + HS- - e HS-= ½ H2 + S2- - e H+= ½ H2 - e
Mitigation Adjusting solution chemistry Temperature profiling Alloy screening Cathodic protection
Adjusting the Solution Chemistry Changing operating pH Add acid or base
Application Time
Adjusting solution pH = 8.0
Screening Alloys Select an alloy that has a preferential corrosion rate 13% chromium 304 Stainless
Application Time
13 % Cr Steel Corrosion @ Dew Point Corrosion Rate = 0.06 mm/yr Corrosion Potential = -0.32 V Repassivation Potential = > 2 V Current Density = 5.7 A/cm2 H2CO3(aq)= ½ H+ + HCO3- - e HS-= ½ H2 + S2- - e
304 Stainless Steel Corrosion @ Dew Point Corrosion Rate = 0.0036 mm/yr Corrosion Potential = -0.15 V Repassivation Potential = > 2 V Current Density = 0.3 A/cm2
304 Stainless Steel Stability @ Dew Point Passivation is possible due to Cr2O3
Explaining common observations using Stability Diagrams Why Iron Rusts Explaining common observations using Stability Diagrams
Basics Iron is inherently unstable in water & oxidizes via the following reactions to form rust Its severity depends on (among others) Conditions (T/P), Composition, pH, and oxidation potential These four can be plotted on a single chart called a stability diagram
Start example
White 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 potentials H2O 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 region H2O is stable and deaerated H2O is stable and aerated Fe2O3 reduces and dissolves in water Fe(II) oxidizes and precipitates as Fe2O3 Elemental iron, Fe(0) oxidizes to Fe(II) in the presence of water FeO(OH), rust is stable in water at moderate to high pH’s White area is region of iron corrosion Water Oxidation Line Water Reduction Line Fe3O4 coats the iron surface, protecting it from corrosion Fe(III)3+ is the dominant ion Fe(II)2+ is the Elemental 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 corrosion At higher pH (10-11), Fe0 forms Fe3O4, a stable solid that precipitates on the iron surface, protecting it from further attack.
Q: We all know O2 is bad…But how much is bad? H2O is oxidized to O2 and H+ H2O is stable and aerated Water Oxidation Line 10 ppm O2 0.1ppm H2 500 ppm O2 80 ppm H2 0.1 ppb H2 3 ppb O2 0.1 ppT H2 0.1 ppT O2 Pure water is here… No air, no acid, no base H2O is reduced to H2 and OH- H2O is stable and deaerated Water Reduction Line
Region of instability Elemental Iron (Feo) Iron and water react because they are not stable together Region of instability The reaction generates 2OH-, which increases the pH The reaction generates H2, which puts the EH near the bottom line Elemental Iron (Feo)
Start example on Page 36-39
The 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 corrosion Initial Conditions DI water, no Feo 7pH, 0.4V Final Conditions 1 ppm Feo added 9.38pH, 0.5V 0.9 ppb Feo 7.07pH, -0.27V 0.1 ppm Feo 8.48pH, -0.42V
Overlaying the Fe3O4 mass on the diagram – once the pH reached 9, Fe3O4 began to precipitate 1.4 g Fe3O4 ppts from 1 Feo Fe3O4 precipitates when 0.3 mg/l Feo has reacted The ppt point lines up with the stability curve
Start example on Page 40-41
If a constant source of O2 is present, then the EH and pH do not change, and we are stuck in the rust region The EH and pH does not change as Feo reacts with aerated water
Why is Stainless Steel stainless?
Cr will oxidizes, but the reaction goes through a tough Cr2O3 protective layer.
Ni3Fe2O4 is stable in the corrosion region, and will also protect the surface.
Simulating Real World Corrosion Problems Welcome to the CORROSION TEACH-IN Simulating Real World Corrosion Problems
Corrosion 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.
Corrosion 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.
Corrosion in Seawater Objectives Reconcile a sea water sample for electroneutrality Reconcile a gas analysis Calculate uniform rates of corrosion for 304 stainless steel 316 stainless steel S31254 stainless steel
Corrosion in Seawater Objectives (continued) Determine the probability of pitting using the localized corrosion feature.
Kinetic Model of General Corrosion: Mass-Transfer Metal Surface Solution All reactions take place on the metal surface. Films are a diffusion barrier to corrosive species Reduce mass-transfer-limited currents. Mass-transfer from solution is calculated from a concentration- dependent diffusion coefficient. film
Chemistry The rates of corrosion use a subset of the OLI Chemistry Neutral Species H2O, O2, CO2, H2S, N2 and all inert gases, Cl2, SO2, So and NH3, organic molecules that do not undergo electrochemical reactions Anions OH-, 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.
Chemistry Cations H+, alkali metals, alkaline earth metals, Fe(II) cations, Fe(III) cations, Al(III) cations, Cd(II) cations, Sn(II) cations, Zn(II) cations, Cu(II) cations, Pb(II) cations and NH4+.
Corrosion of 304 Stainless Steel in Deaerated Sea Water LabAnalyzer used to reconcile electroneutrality NaOH/HCl Used to adjust pH Species Concentration (mg/L) Cl- 19000 Na+ 10700 Mg+2 1300 Ca+2 400 SO4-2 2750 HCO3- 150 pH 8.0 Temperature 25 oC Pressure 1 atm.
Application Time
Screening Considerations Some alloys do not perform well in seawater We will evaluate 3 stainless steels Uniform corrosion rates Pitting possibility Considering both deaerated and aerated conditions
Corrosion of 304 Stainless Steel in Deaerated Sea Water 300 years to lose 1 mm of metal .0033 mm/yr @ 25 oC
Corrosion of 304 Stainless Steel in Deaerated Sea Water Large difference means that pits are unlikely to form Repassivation Potential Difference = 0.05 V Corrosion Potential Or if a pit forms, then it will passivate
What’s on a Polarization Curve? Standard Tafel Behavior Transition to mass-transfer limited current density
What’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 potential Current density at corrosion potential also read at intersection
What’s on a Polarization Curve? Basic polarization curve with water decomposition and corrosion reaction
What’s on a Polarization Curve? Polarization curve with water decomposition, corrosion reaction and two mass-transfer-limited reactions.
What’s on a Polarization Curve? This is what is measured experimentally Transpassive region Passive region Corrosion Potential and Corrosion current Active Corrosion
What’s on a Polarization Curve? Transpassive Passive Active Polarization curve demonstrating a galvonostatic sweep. The arrows indicate how the potential is changing as one moves along the line.
There 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
The Polarization Curve for 304 SS in Deaerated Water Oxidation of water to O2 Measurable polarization curve Breakdown of water to H2 Peak Current density in the pit with the highest corrosion rate Open circuit potential and current density Corrosion of 304 ss
Kinetic 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 surface Active-passive transition influenced by Acid/base properties of passive oxide films Temperature Additional species that influence the dissolution kinetics of oxide layers Synthesis of the partial processes according to the mixed potential theory May 20, 1997 OLI Systems, Inc,
Application Time
Corrosion of 316 SS in Deaerated Water .00053 mm/yr @25 oC 1886 years to lose 1 mm of metal Much better corrosion rate than 304 ss
Corrosion of 316 SS in Deaerated Water Difference = 0.086 V
Application Time
Corrosion of 254 SMO in Deaerated Water Corrosion rate = 0.00033 mm/yr @ 25 oC > 3000 years to lose 1 mm of metal
Corrosion of 254 SMO in Deaerated Water Difference = 2.7 V
Summary in Deaerated Water Stainless Rate @ 25 oC (mm/yr) Potential difference (V) 304 0.0033 0.05 316 0.00053 0.086 254 SMO 0.00033 2.7
Adding Air/Oxygen The CorrosionAnalyzer allows you to add a gas phase based only on partial pressures You can set the water/gas ratio Species Partial Pressure (atm) N2 0.7897 O2 0.21 CO2 0.0003 WGR 0.01 bbl/scf
Application Time
304 SS in Aerated Solutions
304 SS in Aerated Solution The corrosion potential is greater than the passivation potential = .37 V at max O2 Pitting will occur
304 SS Polarization in Aerated Water 8 ppm O2 Corrosion potential shifted anodically of the repassivation potential. 0 ppm O2 The surface will couple galvanically with the pits to increase their rate of corrosion.
Application Time
316 SS Corrosion in Aerated Water Pitting occurs at higher oxygen concentrations = .21V at max O2
Application Time
S31254 Corrosion in Deaerated Water Pitting should not occur
Stability Diagram for 316L SS
Stability Diagram for 316 L Nickel Only
Mitigation Change Alloys Cathodic Protection S31254 seems the best at 25 oC S31254 increased potential for pitting at higher temperatures Cathodic Protection Shifting of potential to less corrosive potentials via a sacrificial anode. Analyzers do not model CP Polarization curves can help determine the change in potential.
Simulating Real World Corrosion Problems Welcome to the CORROSION TEACH-IN Simulating Real World Corrosion Problems
Dealloying of Copper Nickel Alloys Scope A 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.
Dealloying of Copper Nickel Alloys Purpose The 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.
Dealloying of Copper Nickel Alloys Objectives Input information into the software and perform calculations Use stability diagrams to display information about the alloy and the protective films Change the diagrams to view different aspects of the stability of the alloy
Application: 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.
Questions? 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?
Which Tools are Available? A Pourbaix diagram can help us determine where metals are stable. CorrosionAnalyzer
Creating the First Stability Diagram We will use the CorrosionAnalyzer to create a stability diagram for this system. Features of CorrosionAnalyzer diagrams Real-solution activity coefficients Elevated temperatures Elevated pressures Interactions between species and overlay of diagrams.
The Pourbaix Diagram
Application Time Time to start working with the OLI Corrosion Analyzer
The Pourbaix Diagram There are quite a few things to look at on this diagram. Stability field for water Stability fields for nickel metal and copper metal Stability fields for nickel and copper oxides Stability fields for aqueous species. We will now break down the diagram in to more manageable parts.
Stability Diagram Features Subsystems A base species in its neutral state and all of its possible oxidation states. Cuo, Cu+1, Cu+2 Nio, Ni+2 All solids and aqueous species that can be formed from the bulk chemistry for each oxidation state.
Stability Diagram Features For each subsystem Contact Surface Base metals Alloys Films Solids Solid Lines Aqueous Lines
Stability Diagram Features Natural pH Prediction based on the bulk fluid concentrations Displayed as a vertical line Solids All solids included by default The chemistry can be modified to eliminate slow forming solids.
Stability Diagram Features Passivity Thin, oxidized protective films forming on metal or alloy surfaces. Transport barrier of corrosive species to metal surface. Blocks reaction sites
Water Stability Water can act as an oxidizing agent Water is reduced to hydrogen, H2 Water can act as a reducing agent Water is oxidized to oxygen, O2 To be stable in aqueous solution, a species must not react with water through a redox process.
Water Stability
Water Stability – Natural Waters Surface water Ocean water Bog water Organic rich lake water Organic rich waterlogged soils Organic rich saline water
Copper Pourbaix Diagram Oxidized Species E Independent acid and base chemistry Predominant species pH dependent redox pH independent redox Reduced Species
Copper Pourbaix Diagram Aqueous species Stability field for passivating film Equilibrium between species in contact with a solid Equilibrium between species Natural pH Stability field for base metal or alloy
Copper 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.
Nickel Pourbaix Diagram No Nickel metal extends into the water stability field The solution pH is in a region where nickel is expected to corrode
Ni 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
Application Time
CorrosionAnalyzer Calculation
CorrosionAnalyzer Calculation The oxidation reduction potential is 0.463 V
Ni Overlaid on Cu The potential of 0.463 V lies above the passivating film. Dealloying can occur.
Conclusions 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 stability Nickel does not lie within the region of water stability The presence of Cu+ ions in equilibrium with copper metal promotes replating of copper metal driven by the oxidation of nickel.
Chemistry Standard OLI Chemistry 7400 components 9100 individual species 82 Elements of the Periodic Table fully covered 8 additional elements partially covered. Stability diagrams have access all of this chemistry
Chemistry Alloys 6 predefined classes supported User defined alloys Cu-Ni Carbon Steels – Fe, Mn, and C Ferritic Stainless steels – Fe, Cr, Ni, Mo and C Austenitic stainless steels - Fe, Cr, Ni, Mo and C Duplex stainless steels FCC phase - Fe, Cr, Ni, Mo, C and N User defined alloys
Limits to the Standard OLI Chemistry Aqueous Phase XH2O > 0.65 -50oC < T < 300oC 0 Atm < P < 1500 Atm 0 < I < 30 Non-aqueous Liquid Currently no Activity Coefficient Model (i.e., no NRTL, Unifaq/Uniqac) Fugacity Coefficients are determined from the Enhanced SRK
Limitations 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 pressure Oxide stability fields are calculated thermodynamically and may not provide an actual protective film. Dealloying cannot be predicted from the diagram alone.