3 cp corrosion control
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Transcript of 3 cp corrosion control
CATHODIC PROTECTION
Facts
• 1 Ton of steel rusts every 90 sec• Energy required to produce 1 ton of steel
can be consumed in a household family for 3 months
• 50% steel produced worldwide is to replace rusted steel
CORROSION
• Loss of metal due to its interaction with environment.
• Corrosion is an electrochemical process.• The corrosion is a complex process which
depends on so many factors such as physical ,chemical, metallurgical, electrochemical and thermodynamics .
FACTORS WHICH INFLUENCE CORROSION RESISTANCE
CORROSION RESISTANCE
ELECTROCHEMICAL
PHYSICALCHEMICAL
METALLURGICAL
THEMODYNAMIC
CORROSION• THE LESS NOBEL METAL BECOME ANODE (POSITIVE ELECTRODE)
WHILE MORE NOBLE METAL ACT A CATHODE( NEGATIVE ELECTRODE)
• ANODIC REACTION – DISSOLUTION OF METAL M++ + 2e-• CATHODIC REACTION – DEPOSITION OF IONS N++ + 2e- N• REACTION AT ANODE WHICH CAUSES DISSOLUTION OF METAL
LEADS TO CORROSION OF METAL M
Corrosion Cell
electrical path
electrolyte (soil)
cathode anode
Conventional current
e-
e-
e-
cathode
e-
e-
e-
Iron anode
Electron migrations (e-)
Conventional current flow ( + to –v)
e- e- H2H+
e- e- H2H+
Fe(OH)2 OH- Fe+
Fe(OH)2 OH- Fe+
HOW CORROSION TAKES PLACLE
• At Anode:• 2H2O (OH)- +H + + 2e-
FE + + 2(OH) - FE(OH)2 FE(OH)2+ FE + + (OH) - FE2(OH)3
At Cathode
2e- + H + H(atom)
H + H plarisation film
• Corrosion potential – Natural potential of object with no protection or interference current present
• Polarization -A non ohmic component that will apparently exist very close to electrode/ electrolyte surface of the cathode and anode
• Polarization films are very important factor in controlling the amount of current flow . In one sense , the film of hydrogen formed on the surface of cathode may be thought of as an insulating layer which introduced ohmic resistance and reduce the flow of ohmic current
CORROSION: degradation of material, unintended attack from environment
(electrochemical mechanism)Requirements:1. Anode (oxidation)
+ve for electrolytic cell, -ve for galvanic cell
2. Cathode (reduction)-ve for electrolytic cell, +ve for galvanic cell
3. Electrolyte4. Electrical path between anode and cathode
• The amount of metal that will be removed is directly proportional to the amount of current flow
• Effective resistance in the anode cathode circuit goverens the amount of current flow in the corrosion cell which is then the function of resitivity of soil & size of anodic and cathodic area
• Polarization film formed is important in controlling the amount of current flow
• Depolarization effect tend to remove the hydrozen polarized film either by mechanical effect or by supply of dissolved oxygen
Factors affecting corrosion
1. Activation energy/polarisation2. Concentration polarisation (concentration
of ions near electrode)3. Resistance polarisation (resistance to flow
of current, e.g., painting, coating, etc).Localised corrosion is more critical than uniform corrosion
TYPES OF CORROSION CELLS ON PIPELINE
• DISSIMILAR METAL CORROSION• CORROSION DUE TO DISIMILAR SOIL• DIFFERENTIAL AERATION
CORROSION CELL • NEW & OLD PIPE
Corrosion can also occur due to
• Difference in aeration (crevice corrosion)• Contact with a foreign body• Due to stress (stress corrosion), e.g., mild
steel in alkaline environment
Control of Corrosion
• Coating• Cathodic protection• Corrosion inhibitors
Cathodic Protection
A TECHNIQUE TO REDUCE THE CORROSION OF A METAL SURFACE BY MAKING THAT SURFACE THE CATHODE OF AN ELECTROCHEMICAL CELL
… NACE
Corrosion takes place at the anode only !
• Anode: the electrode from which current leaves to the electrolyteAnode gets corroded
• Cathode: the electrode where current is collected from the electrolyteNo corrosion takes place at the cathode
IMPRESSED CURRENT GROUND BEDS
Insulated header cable
Cable trench
Insulating joint of header cable & pigtail of anode
High silicon cast iron anode
Carbonaceous backfill material, well temped
Angured hole for anode & backfill
Impressed Current CP
• Requires external power supply• TRU output can be controlled (automatic /
manual)• Can be used in high resistivity soil, can
protect uncoated structures• Wide choice on anode material; anodes
have long life
Anode
Protected Pipe Pipe
CP unit
Anode Cable
Cathode Cable
TYPICAL CP UNIT
+
-
Ground bed
Anodic Area
Half cell
• Copper / copper sulphate (used for land PSP)
• Silver / silver chloride (used in marine environment)
• Hydrogen (used in laboratory)• Calomel (used in laboratory)
On Potential
on potential
PSP
corrosion potential (550 mV)
days 0 2 4 6
CURRENT MEASUREMENTS
-V+-V+ ++ ++
BAA A
Buried Pipeline- Use of two electrode method
Foreign Pipelines close to Cathodic Protection Ground Bed and crossing
Protected line
Rectifier
Remote GB
Influence of ground bed surrounding foreign line
Current flow from foreign structure to protected line in crossing area
Foreign line
Foreign Pipelines close to Cathodic Protection Ground Bed but not crossing
Protected line
Rectifier
Remote GB
Foreign line
Current discharge from foreign line
+
-
Pipe-to-soil potential measurement
PSP
pipe earth
copperelectrode
CuSO4solution
porous plug
FOREIGN LINES CROSSING BARE CATHODICALLY PROTECTED LINE
Foreign line
Influence of GB
P/LineForeign line tends to become positive to soil. Current picked up by foreign line in electrically remote sections and discharge to the protected line in the crossing area
Most intense damage to foreign line
PEARSON TECHNIQUE
NB: 1Receiver indicates maximum as operator A passes directly over defect.2Receiver indicates full reading when both operators are equidistant from defect.3Receiver indicates maximum as operator & passes directly over defect.
Electrically connected
Coating defect
soil
transmitterOperator A Operator B
Receiver
EarthOperators use cleat shoes
CLOSE INTERVAL PIPE TO SOIL POTENTIAL SURVEYS (CIPS
Pipeline under investigation
Trailing wire
Long Half Cell
Mounting Board
Back pack
Protection criteriaNACE RP-0169-96
• A negative 850 mV PSP w.r.t. Cu/CuSO4 reference electrode with CP current applied
• A negative 850 mV polarised PSP w.r.t. Cu/CuSO4 reference electrode
• A minimum 100 mV cathodic polarisation• A net protective current• 300 mV shift (not per NACE)
IR Drop in On Potential
power supply
PSP half cell
anode pipe IR drop
Off Potential
IR drop PSP 100 mV min corrosion potential
time 0
On-Off PSP Survey
• Current interrupters are used• on:off time = 4:1• Off potential should be measured at least
after 0.5 sec.• Off period should be less than 3 sec to
avoid depolarisation.
Current Interrupters and Synchronisation
• GPS- Costly
• SCADA- Not available everywhere
• Quartz clock- Can loose synchronisation by 0.5 sec/day
Sacrificial System CP
• No external power requirement• Suitable in low resistivity soil / marine
environment only• Choice on anode material is limited, e.g.,
magnesium, zinc, aluminium, etc.;anodes get corroded fast;
• Can protect well coated structures only, protected area is small
CP Design
• Impressed current system• Sacrificial anode system• Hybrid system
Designing CP Installation-impressed current
• Establish soil resistivity• Estimate total current requirement• Select suitable anode material• Calculate anode bed size, shape and
configuration / calculate total mass of anode for design life
• Select location of CP / test stations• Consider facilities required for control
monitoring
Span of protection for CP Stations
CP stn 1 CP stn 2 CP stn 3
l l l l
pipeline
Required Protective Current
• 2I0 = 2πDJL
I0 = current to one side of CP station(A)
D = diameter of pipeline( m)J = protective current density(A/m2)L = spread of protection on one side of CP station
SPREAD PROTECTION• 2L= 8 U L/ 3.14 D J R’
I0 = current to one side of CP station(A)D = diameter of pipeline( m)J = protective current density(A/m2)L = spread of protection on one side of CP station
R’= Resistance of the pipe (ohm/m) ^UL= 0.30 Volts(max . Potential diff between
drain point and end protection)
Types of Anode Bed
• Horizontal anode bedLower resistanceR = (ρ/2πL)ln(L2/tD)
• Vertical anode bedR = (ρ/2πL)ln(2l/d×(4t+3l)½/(4t+l) ½)
Sacrificial Anode Materials
• Zinc Alloy (C-Sentry)Sea water, low resistivity soil
• Aluminium Alloy (Galvalum I, Galvalam II, Galvalum III, Alanode)Sea water
• Magnesium Alloy (Galvomag)soil
Anode Life
L = Ecm/IL = anode lifeE = efficiencyc = capacity(Ah/Kg)m = anode weight (kg)I = anode output current(A)
Anode Life
L = Ecm/I (HOURS)L = anode lifeE = 50% say for Mg anode, 90% for ZnC = 2200Ah/Kg for Mg & 820 Ah/ Kg Zn
m = anode weight (kg)I = anode output current(A)
Impressed Current Anode Materials
• Platinum and platinised metals• High silicon iron• Lead-platinum• Lead-silver• Graphite• Iron• Cast iron
GALVANIC ANODE CURRENT OUTPUT
The output current from a sacrificial anode can be calculated by Ohm’s law as follows:
• Ia = ^V / R• Ia = anode out current (A)• V= driving voltage (V)• R = Current resistance • V is the voltage difference between the potential of the
cathodically protected surface and the potential of the anode. The circuit resistance comprises resistance of anode to earth, resistance of protected structure to earth and resistance of cables and structure itself.
Anode Bed
anode lead
anodebackfillcanister
Backfill
• Effective size of anode increases• Anode loss partly shared by bedding
material• Porous surrounding for gases to escapeCarbonacious backfill – coal coke breezeCalcined petroleum coke breeze
Ground Bed
– A system of anode beds• Swallow / deep ground bed• Horizontal / vertical type ground bed• Distributed / closely spaced ground bed• Close / remote ground bed
CP Field Surveys
• Pearson survey• Current attenuation Test (CAT) survey• Direct Current Voltage Gradient survey• Close Interval Potential Logging (CPL)
survey
Sources of Power Supply
• AC power grid• Solar power• Wind power• Thermo electric generator• CCVT (closed circuit vacuum turbine)
Controls
• DC output controlManual output voltage controlConstant voltage controlConstant current controlConstant PSP control
• Multi reference control
Annunciations
• Reference fail• Overprotection• Under protection
Stray Current Interface
Stray current: current through path other than the intended circuitSources:– DC traction system– Geomagnetic / telluric current– HVDC interference– EHV AC interference
Anodic Interference
power supply
anode design current p/l
stray currentmetallic object
Cathodic Interference (more critical)
power supply
anode design current p/l
stray current metallic
object
Geomagnetic Current
• Also called telluric current• Caused by the effect of solar winds• Varies from time to time
Safe Potential
Etouch = Ib×{Rb+Rfeet}/duration of shock
Rb = 1000 Ω human body (adult)
Ib = Safe current 15mA let go
117mA ventricular fibrillation
CP of Offshore Structures
• Resistivity is extremely low (16-40 Ω-cm) • CP current requirement is very high
(thousands of amperes)• Depolarisation due to temperature, oxygen,
sea currents• Steel corrodes at the rate of 0.13 mm/year
under laboratory conditions
Galvanic Anode material
• AluminiumLow driving potentialExtensively used for marine applications
• MagnesiumLow efficiencyCan affect rapid polarisation of offshore structures in combination with aluminium
Galvanic Anode Material
• ZincLow driving potentialHigh efficiencySuitable for low resistivity soils, fresh water and marine applications (widely used)Protection of ship hulls
Choice of Half Cell
• Silver / Silver Chloride half cell is most common0.8V w.r.t. Ag/AgCl electrode in aerated seawater, 0.9V under anaerobic conditions
• Zinc / Zinc Chloride half cell can also be used
• Cu/CuSO4 half cell will polarise due to the presence of chloride ions
Thank You