Corrosion Rates. OLI model basics Polarization curves

Transcription

1 Corrosion Rates OLI model basics Polarization curves

2 Bulk Aqueous Phase E 0 = E (RT/zF zf) Π a νi i e.g. Fe Fe Fe e - Equilibrium Activities of species potential OLI thermo and Databank Fe +2 Fe +2 H + H 2 Boundary Layer H 2 O OH - H 2 H + Fe 2e - H 2 Anodic process (Oxidation) Cathodic process (Reduction) Fe 2e - H 2 O H 2 Metal

3 Bulk Aqueous Phase Diffusion of Species to and from the bulk fluid and the metal surface Fe +2 Fe +2 H + H 2 Boundary Layer H 2 O OH - H 2 H + Fe 2e - H 2 Fe 2e - H 2 O H 2 Metal

4 Bulk Aqueous Phase Other cathodic processes H 2 O CO 2 Fe +2 H 2 CO 3 HCO 3 - H 2 Fe +2 O 2 H 2 O OH - Boundary Layer Fe 2e - H 2 CO 3 H 2 Fe 2e - O 2 :H 2 O OH - Metal

5 Predicting Corrosion Rates Corrosion Kinetics has 2 parts Chemical kinetics (Activation control) Surface adsorption/complexation (anodic processes in both active dissolution and passive regime) Dissolution of the surface complexes Mass Transfer (diffusion control) Diffusion of oxidizing agents to the surface of the metal (cathodic processes) Diffusion of oxidized and reduced species away from the metal surface Homogeneous reactions that influence mass transfer, e.g., hydration of CO 2

6 Predicting Corrosion Rates Activation controlled reactions Butler-Volmer kinetics i = i o exp [ αf (E -E o ) / RT ] Exchange current density depends on concentrations of active species: i o = i * a Kk a L l a Mm. Reaction orders depend on various mechanisms Electrochemical transfer coefficient α, i*, and reaction orders are regressed from experimental data for specific chemical-metal systems Activities a K a L a M from OLI thermo and Databank

7 Predicting Corrosion Rates Activation controlled reactions Butler-Volmer kinetics i = i o exp [ αf (E -E o ) / RT ] Can be plotted on a graph of potential current density, E vs log I Applies to both cathodic and anodic processes E i i H Fe 0 α H F, a = ih exp RT F = i 0 Fe Fe exp α RT log i 0 ( E E ) H 0 ( E E ) Fe

8 Predicting Corrosion Rates Diffusion controlled mass transfer Limiting current density due to the diffusion of species X to the interface: i lim = z k m Fa X, bulk The mass transfer coefficient k m depends on Diffusivity of species X Viscosity Flow geometry (pipe, rotating cylinder, etc.) Flow velocity Transport properties (diffusivity and viscosity) from OLI models and regressed data E i H 0 α H F, a = ih exp RT Limiting current density 0 ( E E ) H log i

9 Predicting Corrosion Rates Diffusion controlled mass transfer Mass transfer can also be reduced by solid scales that form at the surface, accounted for by a fraction surface coverage model Applies to cathodic and anodic processes E i H 0 α H F, a = ih exp RT Limiting current density 0 ( E E ) H log i

10 Predicting Corrosion Rates Passivation Models developed to predict the active-passive transition, and the passive current density. Model parameters regressed from experimental data for specific chemical-metal systems Elements of the approach: active dissolution current, passive current, and fraction of surface covered by passive film E Activepassive transition Passive region log i Passive current density

11 Polarization Curve for a System Maps the Potential-Current behavior over a broad range Experimentally generated various ways, e.g. by imposing a current and measuring the potential Assists in characterizing the corrosion process and understanding the mechanisms and variables for the corrosion process E log i

12 Putting it all together OLI can produce a theoretical polarization curve for a system by adding up all of the partial processes for a specific system The mixed potential is the corrosion potential The corresponding corrosion current density is proportional to the corrosion rate E cp E i Corrosion current (rate) log i

13 Putting it all together The theoretical curves can be produced for all systems over the range of the OLI thermo and transport models, and even for conditions that are difficult to measure experimentally You can learn a lot about your system from the polarization curves. E cp E i Corrosion current (rate) log i

14 Reduction of H + i H 0 α, a = ih exp H F ( 0 E E ) RT H Limiting current density (diffusion limited) Reduction of H 2 O Water in carbon steel

15 i Fe = i 0 Fe exp α Fe 0 ( E E ) F RT Fe oxidation Fe Water in carbon steel

16 Passive region Activepassive transition Passive current density

17 Water in carbon steel

18 Water in carbon steel

19 150 C 30 C 30 C 150 C 30 and 150 C Water in carbon steel

20 150 C 30 C 30 C 150 C Water in carbon steel

21 30 C 150 C Water in carbon steel

22 Water in carbon steel

23 30 C Water in carbon steel

24 Now with 0.5 molal NaCl, 30 C

25 H2O reduction H+ reduction ph effect with NaCl, ph 7 and 0, this is 7, (same as before) At 30 C

26 ph 0 Now at ph 0 with NaCl At 30 C

27 At ph = 7 At 30 C

28 ph 0 ph 7 At 30 C

29 At 30 C

30 30 C without O2 (same as previous set of slides with NaCl)

31 30 C with and without O2, static conditions (no mixing)

32 30 C with and without O2 with complete agitation

33 30C with NaCl as before, no CO2, no mixing

34 H+ 30C w/ CO2, carbonic acid reduction (CO2 hydration) is rate limiting

35 30C without CO2

36 65C without CO2

37 30C with CO2

38 65C with CO2

39 130C with CO2 and with effect of scale (normal expected case)

40 130C with CO2 without effect of scale