current practices for corrosion protection ceocor …€¦ · •bs 7361: part 1 (1991) •en 12954...

dc STRAY CURRENTS

CURRENT PRACTICES FOR CORROSION

PROTECTION

CEOCOR 2016 – LJUBLJANA Presented by Chris Lynch

Content

• Historical Journey to Present Day Thinking

• Criteria through Standards adopted (for Iron and Steels)

• Mitigation

• Options

• Industry Approaches

• Operators

• Consultants

• Contractors

1

Criteria & The Criteria Debate

2

• Originally recommended as +20mV anodic shift based on a polarised

potential for pipelines in soils

• How, where and why?

• Some have postulated due to meter sensitivity, some results

reproducibility and others rely on experience

• Others, a (part)understanding of advice in earliest standards

• So what is under discussion………….?

Criteria & The Criteria Debate

• +20mV maximum positive potential shift excluding IR drop – Is this

too high?

• Maximum positive potential shift including IR drop

• Surface films / electrochemical interfaces still not (if ever) considered

• Permitted corrosion rates

• Steel in buried concrete

• Is a single criterion suitable………….?

3

Classic Scenarios

4

Criteria – General Acceptance

5

Limits on cathodic

shift

>+20mV if CP still

achieved

Case by case

+20mV between ON and

OFF potential

Time averaged values -

Not short duration

peaks

Acceptable

Stray Current Effects – A Timeline in History

6

• The Joint Committee for Coordination of the Cathodic Protection of Buried

Structures (1957) & the work of Hoar and Farrer (1960/61)

• CP 1021 (1973/79)

• BS 7361: Part 1 (1991)

• EN 12954 (2003)

• ISO 15889-1 (2003)

• EN 10562 (2004 & Present)

• Other Guidance Documents & Reports

7

Technical Instruction – JC for the Coordination of CP Key Points

• Lack of complete information

• 20mV maximum change in the positive direction

• -2.5V maximum change in the negative direction

• Chapter included on the research work of Hoar & Farrer

• In reference to 20mV rise - ….tolerance depends on “natural”

corrosion rate and if small such increases are probably tolerable, if

large then not.

8

Paper – Hoar and Farrer Key Points

• Provided estimations for the increase in corrosion rates of steel in soil

solutions resultant from changes in potentials. It did not define a

criterion

• A change of potential of +20mV produced a two-fold (most

favourable) and three-fold (least favourable) increase in anode

current density

• Goes on to say ΔE of 20mV is “far too liberal” and Δ1mV “too

conservative”. Suggesting a general criterion of ≤5mV. …provided ΔE

can be measured

• Accepts the issues are complex and other variables figure

Standard – CP 1021

• Maximum positive shift of 20mV….but a single criterion is “over

simplification”

• Reasonable on the basis of evidence at present available…

• ….where there is evidence to suggest a secondary structure is

corroding at an appreciable rate no change should be permitted

• Steel In Concrete – may be impossible to make firm

recommendations due to oxygen evolution at +0.5V and with the

effects of chlorides

• Temporary operation at +50mV for a max. 3 months

9

Standard – BS 7361

• Despite further consideration no change either positive or negative or

temporary

• No evidence of damage

• Variable potential changes from dc traction systems considered and

larger maximum positive changes may be accepted

10

Standards – 12954 / 13509 / 15589-1

• Less prescriptive in terms of criteria (12954)

• Detailed in terms of testing and equipment specifications (13509)

• No criteria but should not affect pipe to soil potential such that

minimum protection criteria cannot be met (15589-1)

11

Standard – 50162

• Criteria under review

• Other metals

• Search for anodic sites if ΔE more negative than -500mV

12

ρ (Ω.m) Max +ve Shift (inc.

IR Drop) (mV)

Max +ve Shift (exc. IR

Drop) (mV)

≥ 200 300 20

15 - 200 1.5 x ρ 20

< 15 20 20

International Guidance - Various

• +10mV anodic shift provided -0.850V IR free potential achieved

• ρ is critical – more so than proximity

• +10mV when ρ ≥ 100Ω.cm

• 0mV when ρ < 100Ω.cm

• Difficult to measure anodic shift so test before and after and measure

conventionally switching on and off and considering as a coating

defect

• Avoid bonds

13

International Guidance - Various

• In higher resistivity soils the majority of the 20mV shift will be

measured as voltage drop in soil whilst in low resistivity soils it will be

the measurement across the metal soil interface

• A +70mV shift increases corrosion rate by a factor of 10

• A cathodic shift more negative than -200mV may have a remote

anodic shift > +20mV

• Consider also achieving min pipe to soil potential for CP and

corrosion rates to be <5µm/yr

14

Mitigation - Bonding

• Early days thoughts were to “use” the beneficial effects of stray

currents to provide CP by bonds. OK in principle for non-fluctuating

stray current NOT for dc transit systems

• Electric drainage or Direct drainage

• So inserted rectifiers to tolerate reverse currents but only if interfering

structure was always more negative

• Polarised electric drainage or Uni-directional drainage

• Bonds became more robust and secure by adding resistances,

diodes, sensors, fuses and if more current required a transformer

rectifier

• Forced electric drainage

15

Mitigation - Bonding

• Bonds became more robust and secure

by adding resistances, diodes, sensors,

fuses and if more current is required a

transformer rectifier

• Forced electric drainage

16

Mitigation - Other

• 2ry structure galvanic anodes (small) or ICCP systems (large)

• Additional coating on 2ry structure / Repair coating defects

• Isolation joints

• Improved insulation / restriction of current flow

• Adjust CP settings

• Improved continuity of return rail

• Computer modelling

• 1ry structure auto-potential transformer rectifiers – no bond to 2ry structure

17

Auto Potential Control Schematic

18

Pipeline Operators Perspective – National Grid Gas

19

• Dedicated procedure for management of stray current from dc transit

system – ECP/3 (2008)

• Acknowledges the issue arising from 1000’s km of dc light rail transit

systems since the 1980’s

• Install coupons where min CP criteria not achieved

• Pipelines should not exceed

• +/- 20mV from energising a dc traction system

OR

• +/- 60mV for more than 2.5hrs / day

OR

• +/- 20mV and < +/- 60mV for more than 7hrs / day


20

• Access to rail network is not permitted directly and most testing is

undertaken by the Railway Operators so cannot record rail IR free

data therefore permanent reference electrode and coupons are

installed as close as possible to tracks

• Isolation joints and galvanic anodes as required

• Report no historical corrosion issues with either

ductile or cast iron mains (replacing with plastic)

• 24 hour monitoring and test points at substations,

vehicle stops, inclines and intermediate locations

• Record current flow to coupons

• Install ER probes & data loggers


21

• Prefer 24 hour monitoring “fingerprint” data 1 year before energising

in each of the 4 seasons

• Posts energising data logging every 3 months for 2 years

• Auto potential control TRs that are suitably rated and sensitive to

reference electrodes, position of the controlling electrodes and have

adequate reaction response times

• Supports use of uni-directional bonds at dc substations but not

retrospectively

• Telluric Interference provides unreliable data and so requires re-

testing

• Other HVDC sources e.g. Solar Farms, require case by case

specialist considerations

Pipeline Operators Perspective – British Pipelines Agency

22

• Determines areas of stray current interference from close interval

potential survey data

• Pipelines should not exceed

• +/- 20mV from a dc interference source

• Mitigation using magnesium anodes or resistive

bonds

• Install remote monitoring

• Log data once per year at

• Solar Farms

• Rail Crossings

• Pipeline Crossings with known issues

BPA – Probe Data

23

BPA – Probe Data

24

Railway Consultants Perspective – AECOM

25

• Follow EN 50162 guidance

• Design stage is key

• Rely on robust insulation as a successful scheme as opposed to

direct voltage measurements

• Design ‘short’ structures to minimise volt drop

Railway Consultants Perspective – AECOM

26

• Install monitoring points, sacrificial elements and occasionally

drainage bonds

• Approach is to limit interference damage to ALARP by ensuring civils

details are sensible to the risk

• Little incentive to install monitoring and perform measurements in

case there is a problem that cannot be dealt with reasonably

Railway Consultants Perspective – Intertek

27

• Test pipe to soil ON potentials for levels of stray current & coupon

instant OFF with permanent reference electrode for levels of CP.

Sometimes coupon current measurements

• Where no CP is applied try to measure effects with permanent

reference electrodes

• Night-time tests and baseline average values assessed

• Consider % time when >20mV, >60mV and >100mV

Railway Consultants Perspective – Intertek

28

• Install coupons and reference electrodes at pipe invert and <150mm

separation. Typical separation of ~10m from structure to portable

reference electrode

• No recent known failures directly attributable to dc transit stray current

(historically 1-2 cable failures)

• Data is often difficult to acquire as not freely shared, records are

sometimes poor and there is an urban network replacement

programme

CP Contractors Perspective – Corrpro Companies Europe

29

• Tend to follow guidance in BS 7361 Section 9

• Generally select crossings or nearest points to test

• Original switching period was 3 mins ON and 2 mins OFF (still used

today) with all TRs switching synchronously

• Apply the simple +20mV limit between ON and OFF pipe to soil

potential

• Greater positive shift accepted if both structures are fully protected by

the CP systems


30

• Select multiple test locations

• Generally do not perform interaction testing on

galvanic anode systems (unless in seawater)

• Avoid direct bonds, have installed resistive bonds,

galvanic anodes, TR smoothing

• Experience of telluric activity effects

• 24 hour before and after testing using coupons and portable and

permanent reference electrodes plus DCVG and PCM surveys

Test Post dc and ac Voltage Measurements

31

Test Post dc and ac Voltage Measurements

32

Coupon dc Current Density Measurements

33


34

• Installed specific monitoring equipment

• Operation of auto-potential control systems

• Use of ER ground probes to calculate corrosion

rates

• Ship-to-shore and ship-to-ship testing

Suggested further testing requirements and understanding

•ᵝ - plots

• Intensive monitoring (EN13509)

• Rail potential monitoring

• Weight loss coupons

• Auto Potential Control Requirements

35

Summary

36

• Single maximum positive shift criterion cannot be universally applied

despite the low numbers of known corrosion failures directly

attributed to dc stray current

• More testing and a greater understanding required of the surface

film / electrochemical interfaces

• Serious consideration of reducing the upper limit especially to

structures without CP, known to be freely corroding and in known

low resistivities

• Better training for field measurements and understanding

particularly of intensive monitoring techniques and ᵝ-plot surveys

• Improved interfaces and data sharing between 1ry and 2ry bodies

Acknowledgements

• A big thank you to:

• Ade Wathen, Corrosion Integrity Engineer– National Grid UK Gas Distribution

• Bill Whittaker, Senior CP Engineer - BPA

• Erik Bates, Lead Engineer Railway Electrification and Power - AECOM

• David Buxton, Business Manager Principal Engineer – Intertek Production

and Integrity Assurance

• …..and special thanks to all those good people who actually do put their work

out there and on record for folk like me to benefit.

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