reduction ore = metal oxides metal oxidation

Transcription

1 Cathodic Protection and Interferences René Gregoor Madrid, June 18 th and 19 th

2 Cathodic protection and interferences Corrosion Cathodic protection Protection criterion ON potential measurements DC Interferences and mitigation measures AC Interferences and mitigation measures Conclusions 2

3 Corrosion process Corrosion, a natural process... reduction ore = metal oxides metal oxidation 3

4 Corrosion mechanism Cathodic area (Reduction) e- e- OH- OH- OH- OH- O O H H O Current entering the metal Anodic area (Oxidation) e- e- Fe++ Fe++ Cl- Fe Electrolyte Current leaving the metal 4

5 Electro-chemical chemical reactions Anode Cathode Fe Fe e - ( oxidation) O 2 + 2H 2 O + 4e - 4OH - (reduction O 2 ) 2H 2 O + 2e - H 2 + 2OH - (reduction H 2 O) 2H + + 2e - H 2 5

6 Measures against corrosion Electrical isolating coating to limit the metal surface area in contact with the electrolyte l the required cathodic protection currents interferences to third party infrastructure Cathodic protection to protect from corrosion at coating defects protect from DC stray currents 6

7 Cathodic protection installation Transformer-rectifier rectifier to produce DC current Anode bed CP test posts Reference electrodes 7

8 Cathodic protection installation Transformer-rectifier ~ - + pipeline pp FeSi or titanium oxide coated 8

9 Transformer-rectifier rectifier 9

10 Titanium oxide coated anode 10

11 Protection criterion (as OFF potential) European standard EN12954:2001 fixes the protection criterion for iron in soil as the potential more negative than -850 mv (Cu/CuSO 4 reference electrode) European standard EN13509:2003 defines the measurement techniques to be applied 11

12 Limits according to EN12954:

13 ON potential measurement V Reference electrode (half cell) electrolyte pipeline (half cell) 13

14 ON potential measurement 14

15 DC stray currents from cathodic protection installations, relatively constant, mitigated by direct bond (cable) resistive bond from DC traction systems (train, tramway, metro, ), highly variable, mitigated by unidirectional drainage in substations forced drainage on rails 15

16 Cathodic protection (CP system) transformer-rectifierrectifier anode protected pipeline 16

17 Stray currents from CP systems transformer-rectifierrectifier anode third party pipeline corrosion possible protected pipeline 17

18 Stray currents from CP systems - Remedies transformer-rectifierrectifier anode third party pipeline R = R ou R=0 protected pipeline 18

19 Stray currents from DC traction systems SS + - corrosion protection pipeline 0 potentiel 19

20 Stray currents from DC traction - Remedies SS + - protection pipeline 0 potential 20

21 Highly variable stray currents from DC traction systems 21

22 Forced drainage along the rails 22

23 Induced AC from High Voltage Power Lines Transformer effect between HV power lines (primary) and a metal structure in parallel (secondary) Interfering structures high voltage power lines pour transport of electricity high voltage power lines and catenaries of high speed trains Interfered structures long metal pipelines electricity cables 23

24 Induced AC from High Voltage Power Lines Amplitude of induced AC depends on length of parallelism and the distance between HV power line and the pipeline soil resistivity it electrical characteristics of the HV power line electrical characteristics of the pipeline type of isolating coating of the pipeline 24

25 Induced AC voltage Magnitude Some volt to some tens of volt under normal interference Limits within the framework of corrosion (Cu/CuSO 4 4) as recommended in CEN TS15280 < 4 V if specific soil resistivity < 25 ohm.m < 10 V if specific soil resistivity > 25 ohm.m m Remedies on existing pipelines Grounding of AC part using special electronic circuits 25

26 Conclusion Cathodic protection is simple as a principle but complex in its application Design of CP installations, follow-up of the polarization potential of the pipelines, evaluation of the interference situation is a matter for experts 26

27 Thank you for your attention 27