History of Cathodic Protection

History of Cathodic Protection

History of cathodic protection & factors determining current requirement in Oil & Gas, Chemicals, Fertilizers, Petrochemicals, Power Plants, Infrastructure, etc.

  • Cathodic protection was first described by Sir Humphry Davy in a series of papers presented to the Royal Society in London in 1824. Sir Humphrey Davy’s work on protecting the copper sheathing on wooden hulls in the British Navy by sacrificial zinc or iron anodes is generally considered to be the earliest example of practical cathodic protection.
  • Thomas Edison experimented with impressed current cathodic protection on ships in 1890 but was unsuccessful due to the lack of a suitable current source and anode materials.
  • In the USA by 1945, the use of CP was commonly applied to the rapidly expanding oil and natural gas industry. In the UK, CP was applied from the 1950s onwards and Cathodic Protection Company Limited was established in this period, pioneering its use in the UK.
  • CP is now well established on a large variety of immersed and buried metallic structures as well as reinforced concrete structures and provides effective corrosion control.
Important cathodic protection standards and guidelines
history of cathodic protection
 
Concept of cathodic protection
  • Cathodic protection is an electrochemical means of corrosion control.
  • The objective of cathodic protection is to ensure that the metal surface becomes cathodic (Electronegative) of an electrochemical cell.
  • The potential difference between anode and cathode that causes corrosion current to flow is neutralized by external current which eliminates the potential difference between anode and cathode and corrosion current ceases to flow.
  • The oxidation reaction at anode that releases electrons are consumed at cathode to produce hydrogen and OH-.Corrosion current is reduced as external current polarizes cathode in electronegative direction and as cathode potential becomes equal to or more of anode open circuit potential, the corrosion stops.
                      Oxidation reaction at anode:                     Fe → Fe +++ 2e-
                      Reduction reaction at cathode: 2e-+ 2H+2H→H2or H2O +2e-+½ O2→2(OH-)
  • Cathodic protection provides current through external anode to polarize the structure in electronegative direction. Thus external anode is consumed to protect the structure

factors determining current requirement of cathodic protection

cathodic protection system

Factors determining current requirement\
  1. Surface area: As metal surface exposed area is the more socurrent requirement is more
  2. pH: Low pH (acids) soil require more current than high pH(alkaline) or neutral soil
  3. Temperature: Increasing temperature has a higher current requirement than normal temperature @ 25ºc because reduction rate at cathode increases
  4. Oxygen concentration: More oxygen ingress increases current requirement while lower oxygen presence reduces current. Well-aerated, drained sandy soil or gravel require more current than less aerated soil such as clay
  5. Relative movement of structure and electrolyte: Any relative movement between structure and electrolyte will require more CP current than the stationary state. For example ships and their propellers, offshore structures, docks exposed to water flow or tides. This is due to increase in the reduction reaction at the cathode.

cathodic protection types

Current vs corrosion rate
 
impress current cathodic protection (iccp)
Relationship between corrosion and steel potential (Polarized potential) with difference electrodes
 
advantages and disadvantages of galvanic anode system
 
Effect of over protection
 
Cathodic protection current is required to polarize cathode potential equal to most electronegative anode potential to stop corrosion but more current or more electronegative potential is detrimental to the structure and/or its coating. Overprotection causes either dissipation of hydrogen gas or formation of hydroxyl ions at cathode depending upon acid or alkaline condition. The release of hydrogen gas may cause hydrogen embrittlement as hydrogen bubbles formed at the structure-electrolyte interface may induce hydrogen induced cracking (HIC). Further, the formation of OH? ions at the interface cause coating disbondment from the substrate. Hence overprotection or potential more negative than potential required for stopping corrosion is not required. Also more current is drainage of energy.
 
Criteria of cathodic protection
  • As per NACE SP 0169-2013 (Control of external corrosion on underground or submerged metallic piping system) section 6.2, criteria for cathodic protection of underground or submerged metallic piping system are as follows :

                     A minimum of 100 mV of cathodic polarization between the structure surface and a stable reference electrode contacting the electrolyte. The formation or decay of the polarization can be measured to satisfy this criterion.

                  Eon-E inst on > 100 mV                       Ecorr is open circuit potential
                  E off-Ecorr > 100 mV                                    E off is instant off potential
                  E off- E decay off > 100 mV          E decay off is potential after decay
  • A structure to electrolyte potential of -850 mV or more negative as measured with respect to a saturated copper/copper sulfate (CSE) reference electrode. This potential may be either a direct measurement of the polarized potential (Instant off potential which is Eon-IR) or a current-applied potential. Interpretation of current-applied measurement requires consideration of the significance of voltage drops in the earth and metallic parts.
  • In case of acid-producing or sulfate-reducing bacteria and elevated temperature, a polarized potential of -950 mV or more negative or as much as 300 mV of cathodic polarization may be required.
  • As per ISO 15589-1, polarized potential of -750mV CSE is sufficient for soil resistivity between 10000 Ω  cm and 100000 Ω  cm and -650mVCSE for soil resistivity is greater than 100000 Ω cm.
  • Polarized potential of more negative than -1200 mV with reference to CSE electrode may not be used as over protection is harmful to pipeline coating and not desirable
  • 100 mV potential shift is not recommended under conditions of high temperature, SRB, interference, telluric current, SCC and where more positive potential than -850 mV/cse is required.

Basis of criteria of cathodic protection

      1. -850 mVcse potential for steel:
           It has been established from experiment that the most negative corrosion potential of anode is at -800 to -850 mV cse, hence polarizing             the steel to -850 mV would satisfy the criterion of cathodic protection. If under certain conditions the most active corrosion potential is                   less negative than -850 mV, then polarizing potential of -850 mV is very conservative and difficult to achieve. Further if most active anode             potential is more negative than -850 mV, then polarized potential more negative than -850 mV is required.
2. 100 mV polarization shift criteria:
           In older pipelines where coating integrity is doubtful, most active anode potential is less negative than -850 mV that is corrosion potential             is also less negative than -850 mV , hence in such cases it has been established by experiment that 100 mV polarization shift is                           adequate for cathodic protection. In some cases as defined in criteria, 300 mV polarization shift is required. Corrosion potential is less                 negative in high resistivity well drained sandy soil, hence to obtain polarized potential of - 850 mV is difficult, hence 100 mV criteria can               be used.
The concept of corrosion potential, cathodic polarization, and polarized potential
  • Corrosion Potential Ecorr: The potential of a corroding surface in an electrolyte measured under open circuit conditions relative to a reference electrode also known as open circuit potential,
  •  Cathodic polarization: The change of electrode potential caused by a cathodic current across electrode/electrolyte interface or a forced active (negative) shift in electrode potential. E on- E instant on
  • Polarized potential: Polarized potential is the true measure of cathodic protection of a structure which as per NACE criteria is minimum -850 mV and maximum potential are maintained at -1200 mV. The polarized potential is ON potential - IR drop.
Polarization curve
pre-packaged magnesium anodes
 
Depolarization Curve

oil & gas training in India

Factors affecting polarization in cathodic protection
cathodic protection training in India
 
Measurement of pipe to soil potential
industrial safety training programs in Mumbai India
 
Current interruption circuit
history of cathodic protection & factors determining current requirement
 
Current interruption
history of cathodic protection
 
PSP graph
factors determining current requirement of cathodic protection
 
Type of cathodic protection
  1. Sacrificial(Galvanic) anode system
  2. Impress current cathodic protection (ICCP)
  • Components of galvanic anode system :
                1. The anode
                2. The anode backfill
                3. Connecting cable between anode and structure
                4. The structure to be protected
 
Cathodic Protection – Galvanic System
cathodic protection system
 
EFFECTS OF SOIL RESISTIVITY ON CURRENT DISTRIBUTION
cathodic protection types
 
Sacrificial (Galvanic) anode system
  • Based on the galvanic series of the metals, metals having more electronegative corrosion potential or active behave as an anode and more electropositive or noble metals acting as cathode. Thus anodes are sacrificed to protect metal surface acting as a cathode.
  • For example, Magnesium, zinc, and aluminum are active metals to steel and their alloys are used as sacrificial anode to protect the steel. For example, Magnesium connected to steel will have corrosion potential of -1. 70 V – (- 0.50 V) = -1.20 V
  • Sacrificial anode system has inherent limitation and is used for protecting a small stretch and/or during construction phase which is called temporary cathodic protection (TCP) as it is replaced by the permanent cathodic protection system
Practical Galvanic Series

Practical Galvanic Series

Choice of sacrificial anode and backfill material
  • Alloys of magnesium, zinc, and aluminum are used to remain active as pure metals undergo self-corrosion in the environment and hence not active.
  • Magnesium alloy material: Magnesium alloy is used in both high resistivity and low resistivity soil where high corrosion potential of -1.75 V and low corrosion potential of - 1.55 V are used respectively. These anodes are used in soil and fresh water
  1. Anode consumption rate 7.9 Kg/ Amp-year, Efficiency-50 %
  • Zinc alloy material: Zinc alloy materials are used in soil and for sea water application. At higher temperature more than 54ºC and particularly in presence of carbonates, the passive film is formed on the surface of zinc and becomes nobler than steel, leading to corrosion of steel.
  1. Zinc corrosion potential is -1.10 V
  2. Anode consumption rate 11.24 Kg/ Amp-year, Efficiency 90 %
  •  Aluminum alloy material:
  1. Aluminum alloy material is used in the marine environment. Its corrosion potential is – 1.15 V
  2. Consumption rate- 3.46 Kg/ Amp-year, Efficiency 85 %
Anode backfill material
  • Properties of Backfill Material:
          1. Prevents direct soil contact to reduce localized corrosion to the anode
          2. Prevents passivation (reduction of the anode reaction rate) of the anode caused by reactions with the soil salts which prevent function                  of the anode (Anode loses its property of being active)
          3. Provides low resistivity environment around the anode
          4. Eliminates air void when wet and expands thus creating a conductive medium
  • Magnesium Anode:
  1. Gypsum 75 % ( For supplying sulfate ions and prohibiting polarization of anode),
  2. Bentonite 20 % (For absorbing moisture and conducting path to ions)
  3. Sodium sulfate 5 % (For lowering backfill resistance and earth contact resistance)
  • Zinc Anodes:
  1. Gypsum 50 %,  Bentonite 50 % .
  2. No sodium sulfate is used as zinc anode is used for low soil resistivity
Selection of galvanic anodes based on soil resistivity
impress current cathodic protection (iccp)
 
Advantages and disadvantages of galvanic anode system
  • Advantages
  1. Easy to design, install and maintain
  2. High reliability
  3. Low maintenance cost
  4. Steady current output hence dependable
  5. Economical for smaller structure
  6. No external power is required
  7. Well suited for electrically isolated structures due to limited current
  8. Due to low potential, stray current interference is minimal
  • Disadvantages
  • Limited driving potential
  • Short span of life due to passivation
  • Limited current output
  • Limitation due to soil resistivity restrictions and seasonal variation
  • Cannot be used for poorly coated pipes due to high current requirement
Application of galvanic anode system
  • Small isolated coated structures
  • Small isolated fittings such as valves, couplings and risers
  • Internal surfaces of small vessels
  • Structures where electrical continuity poses problem like in tank farm or station piping
  • Structures in seawater environment where zinc or aluminum are used
  • Structures in close vicinity where current distribution is a problem
  • For mitigation of AC and DC interference
  • For carrier pipe protection inside casing and casing protection
  • For valves body, at HT line crossings and across IJ
  • Where power resources are not available
Pre-packaged magnesium anodes

advantages and disadvantages of galvanic anode system

Impress current cathodic protection system (ICCP)
  • Impress current cathodic protection system (ICCP) is called permanent cathodic protection system as it is designed for life time of pipeline and current is impressed upon pipeline through an external power source and anode ground bed.
  • The advantage of ICCP system is this that it can be applied for long stretch of pipeline and more reliable and effective.
  • Protective current can be controlled in auto mode to provide adequate cathodic protection of pipeline
  • The important components of ICCP systems are power source such as TRU or CPPSM, anode ground bed, Anode backfill material, anode and cathode header cable, anode lead junction box, cathode junction box in case of multiple pipelines, cathode and anode cable, permanent reference cells etc
  • The CP power source data can be monitored at central place through SCADA and corrective measures may be taken.
Characteristics of impressed current system
  • Impressed current anodes are relatively inert material, hence anode materials corrode but at a very low rate
  • Impressed current anodes are surrounded by electronically conducted carbon backfill (which allows only electrons to flow) and minimize ionic current from an anode, the primary reaction takes place at the outer surface of the carbon backfill, hence anode simply functions as the electrical contact to carbon backfill
  • Oxidation of carbon produces gases and acids
Cathodic Protection – Impressed Current System

pre-packaged magnesium anodes

Application of Impress current system
  • For large current requirement and high resistivity environment
  • For mitigation of stray current interference
  • For underground storage tanks
  • For cross country pipelines
  • For LPG bullets
  • For underground plant piping network
  • For bottom of aboveground storage tank
  • For underwater offshore structures and subsea pipeline
Advantages and disadvantages of ICCP System
 
 ADVANTAGES    DISADVANTAGES 
LARGE DRIVING VOLTAGES.       HIGHER COST OF INSTALLATION.
HIGHER CURRENT OUTPUTS. HIGHER COST OF MAINTENANCE.
APPLICABLE IN ANY RESISTIVITY. RECURRING POWER COST.
FLEXIBILITY IN CURRENT CONTROL. INTERFERENCE WITH EXISTING OR UPCOMING ADJACENT UTILITIES.
APPLICABLE IN BOTH POORLY & WELL COATED PIPES. NEEDS IMMACULATE DESIGN BASED ON SITE DATA.
PROTECTS LARGER STRUCTURES FOR LONGER DURATIONS.


Comparison between 
a Sacrificial anode and ICCP System
Sacrificial Anode System
Impressed Current System
Independent of any source of electrical power
Requires a mains supply or other source of electric power.
Generally used for protection of bare or partially protected structures where electricity is not available or costly
Can be applied to a wide range of structures including preferably coated.
Their use may be impracticable except with soils or waters with low resistivity.
Use is less restricted by the resistivity of the soil or water.
Simple to install, additions may be made until the desired effect is obtained.
Needs careful design although the ease with which output may be adjusted
allows unforeseen or changing, conditions to be catered for.
They are less likely to affect any nearby neighboring structures because the output at any one point is low.
Requires the effects on other structures that are near the ground structures to be assessed but interaction is often easily corrected, if neccessary.
 
Impress current anode material and their properties
oil & gas training in India
 
Mixed metal oxides anodes
Mixed metal oxide anode surface consists of solid state solution of rare metal oxides like Iridium, Tantalum and titanium oxides baked onto a titanium substrate. The consumption rate of active surface coating is very low due to pre-oxidized surface.
cathodic protection training in India
High silicon iron anodes
High-silicon iron anode is a chemically resistant alloy containing silicon, chromium, and iron. HISI is very brittle and forms a SiO?film on the surface in underground applications that can increase the resistance of the anode in dry environments. It has also got end effect or penciling effect which is metal loss on the end of the anode
industrial safety training programs in Mumbai India
Type of anode ground bed
  • Anodes beds in the ICCP system are of the following types –
  • Shallow ground bed :
  • Horizontal shallow ground bed
  • Vertical shallow ground bed
  • Deep ground bed

 anode ground bed

Carbon backfill for anodes
  • Calcined (heat treated coke of increased density and low resistivity of particles) coke breeze is used as a backfill material around impressed current anodes for increasing the life of anodes. The conduction of current from anode takes place through carbon backfill only and anodes are consumed very less. The resistivity of carbon backfill is very less.
The specification of calcined coke breeze is as follows:
 
Power sources
  • The following power sources may be used 1. Transformer rectifier units (TRU)
  1. Cathodic protection power supply modules (CPPSM)
  2. Solar power units (SPU)
  3. Grid power
  4. Battery power back up
Transformer-rectifier units
  • AC power for transformer/rectifier units can be either single-phase or three-phase. Especially for high power units, three-phase units are preferred because they normally provide a smoother DC output
  • Further four diodes, full wave rectifier are commonly used in single phase and full wave six anodes bridge rectifier are commonly used in three phase supply

Basic circuit of single phase 4 diodes rectifier

Transformer rectifier units

Mode of operation of TRU
  • Three modes of operation
  1. Constant current
  2. Constant voltage
  3. Constant potential
  1. Constant current: TRU is set in AVCC mode that is automatic voltage constant current mode. The current is preset to a value to maintain potential. When circuit resistance changes due to seasonal variation then the voltage is automatically adjusted to supply constant current output and required potential.
  2. Constant voltage: Constant voltage is essentially a manual mode of operation where the voltage is kept constant and current varies as circuit resistance changes. Voltage is manually adjusted to take care of resistance changes and meet the current requirement to maintain potential
  3. Constant potential: In this mode of operation the structure to soil potential is controlled in auto mode through a feedback circuit and output voltage is controlled automatically to maintain a preset value.

For Training and Consultancy Services Contact us on 022-62210100 or Email us on techsupport@marcepinc.com

Share this Post:

Related Posts: