Cathodic Protection: Pipelines and Other Components

Transcription

1 Cathodic Protection: Pipelines and Other Components By Dr. W.J.D. (Bill) Shaw Professor & Director, Pipeline Engineering Center Schulich School of Engineering University of Calgary 1

2 Presentation 1. Perspective 2. Basics - theory 3. Design 4. Evaluation & criteria 5. DC stray current 6. AC stray current 7. Summary 2

3 Corrosion Protection = * cathodic protection + coating 3

4 Cathodic Protection Pipelines, Buried Tanks - external Pressure Vessels - internal Offshore Platforms - exterior 4

5 Alternates to protection of Components - no protection - protective coating - CP of bare component - CP of coated component - corrosion resistant material - above ground installation 5

6 1. Basics - theory Cathodic protection governed by basic corrosion theory a) thermodynamic equilibrium - Pourbaix diagram ( potential vs ph) b) kinetics - Evans diagram ( potential vs log current) 6

7 Basics of Pourbaix Diagram Nernst Equation E = E Ө + (R T / z F) Ln (a reactants Faraday s s Equations m = I t A o / z F reactants /a products ) ΔG G = - E z F 7

8 Basics of Evans Diagram Faraday s s Equations as previous m = f(i ) Arrhenius Rate equation r = C exp(q / R T) r = m / t r = f(i ) / t 8

9 Basics of Corrosion Reactions Oxidation Partial Reaction (anodic) Fe Fe e Reduction Partial Reaction (cathodic) O 2 + 4H + + 4e 2H 2 O 2H + + 2e H 2 9

10 Elements Required - Corrosion 1. Anode (dissolution of metal) 2. Cathode (reduction reaction) 3. Conducting electrolyte 4. Conducting path - joins anode to cathode 10

11 Laboratory Elements 11

12 Field Elements 12

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15 Pourbaix Diagram Iron-water 15

16 Pourbaix Diagram - Steel 16

17 Soil Reduction Reaction Normally in soil - very little oxygen present Therefore controlling reduction reaction is Hydrogen reduction 2H + + 2e H 2 17

18 Pipe - without protection 18

19 Reference electrodes - unchanging voltage - every potential measured against (an effective ground) - by convention, zero voltage set as hydrogen - standard hydrogen electrode, SHE - V SHE 19

20 Standard hydrogen electrode 20

21 Field electrode copper - copper sulfate Cu/CuS0 4 V SHE = V cu/cus

22 Evans Diagram 22

23 Conventional Evans Diagram 23

24 2. CP Design - based upon driving the potential downwards on Pourbaix diagram - drive to a position of near zero corrosion - material is then thermodynamically stable without corrosion occurring (or minimal corr) - to do this one must provide current to offset the corrosion current 24

25 CP system types - impressed DC current - sacrificial (Mg, Zn) 25

26 CP Applications 26

27 ph sensitive CP requirements 27

28 Evans Diagram- CP 28

29 Coating characteristics - pin holes - resistivity - water / gas diffusivity * effective area for current flow 29

30 Soil parameters - water content - deaeration /aeration condition - ph - salts, type and amount - microbiological presence - soil resistivity 30

31 Soil voltage attenuation Major design - anodic bed - soil voltage attenuation 31

32 New Trends - computer controlled CP systems - solar photovoltaic systems - permanent buried monitoring systems - pulsed cathodic systems 32

33 3. Evaluation of CP system - measurement of pipe potential - Potential survey, along pipeline - criteria (NACE RP-0169) a) Potential with CP on = -850 mv Cu/CuS04 b) Potential with CP off = -850 mv Cu/CuS04 c) 100 mv polarization 33

34 Complexity complex interactive system - soil type & condition - coating - anode location - reference measuring location - microbiological corrosion - temperature of pipe - shielding from other pipe or structures 34

35 CP on criteria Potential with CP on = -850 mv Cu/CuS04 - most commonly used criteria - overprotects for most systems - under protects for high potential drops - used with coated pipelines 35

36 CP off criteria Potential with CP off = -850 mv Cu/CuS04 - becoming more popular - accounts for potential drop (thus more accurate) 36

37 Polarization criteria 100 mv polarization - for normal soils gives good protection - very good for uncoated pipelines - difficult to apply (depolarize pipe) - not as much overprotection 37

38 Over voltage CP effects Preferable limit to V Cu/CuS04 PROBLEMS OVER CATHODIC PROTECTED - coating disbondment (excessive hydrogen generation) - hydrogen embrittlement - wasted electrical energy 38

39 Temperature effects on CP Temperaure, o C off mv Cu/CuS04 polarization, mv 20 to

40 Other effects on CP MIC mv Cu/CuS04 SCC mv Cu/CuS04 off off 40

41 4. DC Stray Current - sources - galvanic couples (soil changes) - pipe located within current path (train) - close to anode of another system - welding with earth ground damage - where stray current exits structure 41

42 Approach for stray currents - identification of path - if possible minimize magnitude - install corrective items - monitor 42

43 Control of stray currents - remove source - current drainage connections - tuned resistive shunts - cathodic shield - sacrificial anodes at hot spots - install insulating joints on pipeline - electrical isolation - special coatings 43

44 Typical stray current problem 44

45 5. AC Stray Current - presence of AC field (overhead power line) - geomagnetic activity (telluric) - grounding fault from AC equipment - detected by potential surveys - grounding pads to dissipate - overcome by extra CP 45

46 General AC criteria - less than 1% of DC of similar magnitude - ignored if current < 30 A/m 2 or 3 ma/cm 2 46

47 Summary Cathodic Protection - well defined - adjusted in field - applies to all structures Evaluation Critera - near maturity DC Stray Current (always important) - controlled by CP AC Stray Current (important sometimes) 47

48 48