Understanding the corrosion environment

1. The Corrosion Teach-in Understanding the corrosion environment

2. Different methods for corrosion control Coupons Online Monitors Inhibition programs Any method be made more effective…

3. …When you understand the effect of the corrosion environment Corrosion rates vary with process conditions

4. 5.5% NaCl

5. 5.5% NaCl, 5 atm

6. 5.5% NaCl, 85 °C

7. 5.5% NaCl, 10 °C, 15 atm

8. To interpret coupon and monitor data… It helps to know the effect of variations in the field

9. To locate where to place sensors & coupons… Wait for a failure…? Rely on past experience?

10. Coupons Online Monitors Tell you what has already happened, not what will happen

11. OLI getsthechemistryright OLI tools can help

12. ? Dew point pH Phase splits

13. Active Corrosion (dissolution) Protective Scale Passive Film pH Understand what’s happening in your system

14. Passive region Activation controlled Rate-limiting cathodic process Determine the rate limiting redox processes

15. Determine pitting potential and max growth rate Pitting No Pitting

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. Today’s seminar “Hands-on” and “How-To” • Using example problems • Examining plots and diagrams • Understanding the basis ofthe predictions

23. 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

24. Welcome to the CORROSION TEACH-IN Simulating Real World Corrosion Problems

25. 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

26. Gas Condensate Corrosion • Objectives • Determine the dew point of the acid gas • Remove the condensed phase and perform corrosion rate calculations • Mitigate the corrosion

27. Sour Gas Absorber Acid Gas Absorber liquor regenerator Gas Sweetening

28. Acid Gas Concentrations

29. Application Time

30. Dew Point • Dew Point = 37.6 oC • pH = 3.93 • ORP = 0.576 V

31. Corrosion Rates: Flow Conditions • Flow conditions have a direct effect on mass-transfer • Static • Pipe flow • Rotating disk • Rotating cylinder • Complete agitation

32. Application Time

33. H2CO3(aq)= ½ H+ + HCO3- - e H2S(aq)= ½ H2 + HS- - e HS-= ½ H2 + S2- - e H+= ½ H2 - e 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

34. Mitigation • Adjusting solution chemistry • Temperature profiling • Alloy screening • Cathodic protection

35. Adjusting the Solution Chemistry • Changing operating pH • Add acid or base

36. Application Time

37. Adjusting solution pH = 8.0

38. Screening Alloys • Select an alloy that has a preferential corrosion rate • 13% chromium • 304 Stainless

39. Application Time

40. H2CO3(aq)= ½ H+ + HCO3- - e HS-= ½ H2 + S2- - e 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

41. 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

42. Passivation is possible due to Cr2O3 304 Stainless Steel Stability @ Dew Point

43. Why Iron Rusts Explaining common observations using Stability Diagrams

44. 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

45. Start example

46. 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. White area is region of iron corrosion Fe(III)3+ is the dominant ion H2O is oxidized to O2 and H+ H2O is stable and aerated Fe(II)2+ is the dominant ion Water Oxidation Line Fe(II) oxidizes and precipitates as Fe2O3 Fe2O3 reduces and dissolves in water H2O is stable and deaerated FeO(OH), rust is stable in water at moderate to high pH’s H2O is reduced to H2 and OH- Fe3O4 coats the iron surface, protecting it from corrosion Water Reduction Line Elemental iron, Fe(0) oxidizes to Fe(II) in the presence of water Elemental iron, Fe(0)o, is stable and will not corrode in this region Explaining the EH-pH diagram using Fe, showing solid and dissolved species over range of pH’s and oxidation potentials

47. Q: We all know O2 is bad…But how much is bad? H2O is oxidized to O2 and H+ Water Oxidation Line H2O is stable and aerated Pure water is here… No air, no acid, no base H2O is stable and deaerated H2O is reduced to H2 and OH- Water Reduction Line 500 ppm O2 0.1 ppT H2 10 ppm O2 0.1 ppb H2 0.1ppm H2 3 ppb O2 0.1 ppT O2 80 ppm H2

48. Iron and water react because they are not stable together The reaction generates 2OH-, which increases the pH Region of instability The reaction generates H2, which puts the EH near the bottom line Elemental Iron (Feo)

49. Why is Stainless Steel stainless?

50. Cr will oxidizes, but the reaction goes through a tough Cr2O3 protective layer.