Cathodic Protection Presentation

CATHODIC PROTECTION

What is Corrosion ??Why Corrosion occurs ??How Corrosion occurs??

WHAT IS CORROSION ??Gradual destruction or alteration of a metal and its mechanical properties caused by direct chemical attack or by electrochemical reaction.

WHY CORROSION OCCURS ??A significant amount of energy is put into a metal when it is extracted from its ores, thus placing it in a high energy state. As per the principle of thermodynamic, material always seeks the lowest energy states. Hence all metals are unstable because of their high energy levels. Metals tend to revert to their natural lowest energy steady states through process of corrosion.

HOW CORROSION OCCURS ??Corrosion cell consists of following elements1. Anode2. Cathode3. Electrical contact between both Anode and Cathode4. ElectrolyteTwo basic mechanisms by which metals in electrolyte corrode1. Galvanic corrosion2. Electrolytic corrosion

HOW CORROSION OCCURS ??Corrosion of metals in aqueous environment is almost electrochemical in nature.It occurs when two or more electrochemical reactions occur on metalAs a result some of the elements change from metallic to non metallic state.Electrochemical reactions occur either uniformly or non uniformly on the surface

HOW CORROSION OCCURS ??Corrosion cell consists of following elements1. Anode2. Cathode3. Electrical contact between both Anode and Cathode4. Electrolyte

NextOxidation reactions at anodeFe ----------> Fe++ + 2 e-Reduction reactions at cathodeH + + e- ----------> HH + H ---------> H2 (Highly acidic)O2 + 4 H - ----------> 2 H20O2 + 2 H2O + 4 e- -----> 4 (OH)- (Neutral solution)

HOW CORROSION OCCURS ??Galvanic corrosionWhen two metals (having different potentials) are connected electrically and are immersed in an electrolyte, current will be generated and the metal which is less noble will corrode. Current from the corroding metal will flow into the electrolyte, over to a non corroding metal and back through the connection between the two metals.The metal where current leaves will get corroded and is known as anodeThe metal which receives current is known as cathode.

HOW CORROSION OCCURS ??Galvanic corrosionThe differences in potential can be due to 1. Coupling of dissimilar materials ( CS and SS)2. Non homogenity in same material (Holidays in coatings/inclusion in a metal)3. Non homogenity in electrolyte (different salt concentrations in soil)

HOW CORROSION OCCURS ??Electrolytic corrosionThis is a result of direct current entering from outside sources leaving a particular metal structure through electrolyte. The location where current enters the structure is provided with protection and is called cathode.

The location where current leaves the structure is known as anode and corrosion is occurred.This type of corrosion is often referred as stray current corrosion and is a result of currents entering the ground from sources of DC as street railways or DC machinery.

HOW TO MITIGATE CORROSION ??

1. COATINGS (Primary means of preventing corrosion)2. CATHODIC PROTECTION (Secondary means of preventing corrosion)

HOW TO MITIGATE CORROSION ??Coatings are normally intended to form a continuous film of an electrically insulating material over the metallic surface to be protected. The function of such coating is to isolate the metal from direct contact with the surrounding electrolyte.Coatings impose high electrical resistance so that electrochemical reactions do not occur readilyHowever coatings will develop discontinuities or holidays during paint application or material transportation or material installation which is unavoidableHolidays in coatings can also develop during service due to degradation of coating, soil stresses or movement of pipe in the ground.

HOW TO MITIGATE CORROSION ??A good coating should have following characteristics1. Effective electric insulator2. Effective moisture barrier3. Ability to resist development of holidays with time4. Good adhesion to pipe surface5. Ability to maintain constant electrical resistivity with time6. Resistance to disbonding7. Ease of repair

IntroductionCathodic protection is an electrical method of preventing corrosionIt operates by passing direct current continuously from electrodes which are installed in the electrolyte, to the structure to be protected.Corrosion is arrested when the current is of sufficient magnitude and is properly distributedIt is used for protecting wide variety of metallic structures which are submerged in electrolytes such as water/soil.

ApplicationsUnder ground pipe linesBuried vesselsSub sea pipe linesTank bottom plates (Internal/External)Ship hullsLock gates and damsSteel pilings

Principle of cathodic protection The principle of cathodic protection is to make the potential of the whole surface of the steel structure sufficiently negative with respect to the surrounding medium to ensure that no current flows from the metal into the medium.This is done by forcing the electric current to flow through the electrolyte towards the surface of the metal to be protectedThe current may be obtained from any convenient, external source, such as battery, rectified alternating current supply,DC generator or by galvanic action.

Methods of cathodic protectionCathodic protection to immersed and buried surfaces can be achieved by following two methods: Sacrificial anodeThis method makes use of galvanic action to provide required cathodic protection current. The surface of the structure is made cathodic by connecting it electrically to a mass of more active metal buried or immersed in the common electrolyte, the more active metal then become anode.Magnesium, Aluminium, Zinc etc. are commonly used for this purpose.

Object to be protectedGalvanic AnodeProtective currentGalvanic Anode system

Methods of cathodic protectionImpressed current cathodic protection (ICCP)This method uses anodes/ground beds which are energized by an external DC power source. The ground bed may consist of titanium, platinum, graphite, high-silicon iron rods, cast ion scrap, old steel pipe etc.In this anodes are installed in the electrolyte and are connected to the positive terminal of the DC source, the structure which is to be protected is connected to the negative terminal of that source.DC power source can either be a battery, a Transformer Rectifier unit, solar cells, DC generators etc.

Methods of cathodic protectionImpressed current cathodic protection (ICCP)Anode beds used in ICCP1. Shallow anode beds Anode beds are located at depth of 3-10 m from ground surfaceLand surface requirement is very highThis causes interference problem to the greater extentNot suitable to the protection of the in plant pipelines.

Methods of cathodic protectionImpressed current cathodic protection (ICCP)2. Distributed anode beds Anodes are installed along the length of pipelines in the whole plantLesser interference problemsLarge quantity of CP cables requiredDifficult to maintain

Methods of cathodic protectionImpressed current cathodic protection (ICCP)3. Deep Well anode beds Used for protection of underground/buried facilities in congested plantsMore interference problems with foreign structuresAnodes are installed in deep well iron casing of 8/10 dia in vertical configuration upto the depth of 100 meters

COMPARISION BETWEEN SACRIFICIAL AND ICCP

SACRIFICIAL CP SYSTEM

1. Independent of electrical power source2. Limited current is available. Hence restricted to protection of well coated pipelines3. Impractical except with soils or water with low resistivity4. Easy to install5. Replacement is required whenever anodes get consumed6. No/very less interference effects7. No control over the current output8. They cannot be wrongly connected, so that polarity is reversed

COMPARISION BETWEEN SACRIFICIAL AND ICCP

ICCP

1. External source is required to supply current2. Can be used to protect un coated structures3. Not restricted due to high resistivity of electrolyte4. Need careful design and difficult to install5. Less number of anodes are required and anodes do not consume6. Interference effects are high7. Polarity to be checked before commissioning, misconnection can accelerate the corrosion.

CP SYSTEM AT JAMNAGAR COMPLEX

1. Tank bottoms ( soil side) ICCP2. Tank bottoms (Product side)SA3. Underground vesselsSA4. Underground pipingSA5. Sub sea pipelinesSA6. 48 U/G Crude pipelinesICCP7. Jetty trestle piles & SPMSA

Protection of tank bottom plates (Soil side)

Protection of underground pipe lines < 8

Protection of underground pipelines > 8

Protection of tank bottom plates (product side)

Protection of underground vessels

MONITORING OF CP SYSTEMPOTENTIAL MEASUREMENTSPRINCIPLEIn measuring the pipe to soil potential principle used is to form an electrochemical cell, the second half cell being a standard reference electrode.(Cu-CuSo4 or Ag-AgCl)

Reference electrodePotential requiredCu/CuSo4-850 to -2000 mV Ag/AgCl/sea water-800 to -1950mV Zinc 0 to 250

DESIGN CRITERIA OF CP SYSTEMPre-design surveys Soil resistivityThis is one of the primary design parameter.It is required to determine the corrosivity of the soil for the buried metal structure.It is required to identify the suitable locations for the anode ground beds and to select suitable T/R ratings.It varies greatly with its water content and with the electrolyte dissolved in the water.It vary with the season of the year and the rainfall.

DESIGN CRITERIA OF CP SYSTEMSoil resistivity is commonly measured by following two methodsa. Shepard Cane Soil-Resistivity Meter.In this method two terminals are placed in the soil 0.3m apart, and inserted to a depth of 0.3m. Current from a three volt battery is passed through the soil between the electrodes. Current flow is measured by a milli-ammeter graduated directly to read the soil resistivity in ohm-cm.Measurements made in this way indicate the conditions near the ground surface only. To measure the resistivity at greater depths, test pits should be excavated which is not practically possible. Electrodes should be placed in undisturbed soil,otherwise inaccurate results will be obtained. The push button switch for closing the circuit shall be pushed momentarily, otherwise results will be affected by polarization.

DESIGN CRITERIA OF CP SYSTEMb. Four-Terminal Soil-Resisitivity meters.In this method four bare metal rods equally spaced along a straight line are driven into the ground. AC is passed between outer pair of rods and the potential difference between the inner pair of rods is measured.Soil resistivity is measured using the formula p = 2 * 3.14 * a * Rwherep = average soil resistivity in ohm-cma = Electrode separation in cmR = measured resistance between the electrodes in ohmsInterpretation of soil-resistivity readingsLess than 1500 Ohm-cmVery corrosiveBetween 1500 to 3500 Ohm-cmModerately corrosiveover 3500 Ohm-cmSlightly corrosive

DESIGN CRITERIA OF CP SYSTEMCURRENT DRAINAGE TESTSThey are carried out when it is intended to use ICCP for determining the current required for providing protection This involves installation of one or more ground beds in a direct current circuit and maximizing the output. The current is then interrupted in a sequence of 40 seconds on and 20 seconds off.The effect is then checked by measuring the PSP potential variation at various places. The location with least swing determines the required minimum output of the final installation in such a way that the minimum acceptable swing is 300 mV negative once the final installation is activated.

Insulating flanges: Pipelines which are to be cathodically protected should have an insulating flange at both ends to prevent current loss to other installations. In general gaskets and sleeves will be of phenolic products.

Anode Characteristics

Current output is of primary importance in selection of anodes. This depends on the driving force available and the circuit resistance.The driving force is the difference in potential difference betweenanode and protected steel. Greater the potential difference greater will be current output.Circuit resistance largely depends on cable resistance and anode-in-medium resistance.

Anode Efficiency

It is defined as the ratio of ampere-hours actually supplied (at site) to the theoretical ampere-hour output per unit weight of metal consumed.

Efficiency of magnesium alloy anodes is about 50%Efficiency of Zinc anodes is about 90%Efficiency of Aluminium anodes is about 94%.

Anode CharacteristicsAnodes used as sacrificial anodes are Zinc, Magnesium and AluminiumMagnesiumUsed for higher resistivity electrolytes Where Zinc or Aluminium are not recommendedMg used for CP should be of high purityGives High current output as its potential difference with steel is more than Zinc or aluminiumEnergy capacity : 1100 Ah/kgEfficiency : 50 %


ZincZn used for CP should be of high purityGives small current output as its potential difference with steel is less.Not recommended to use with electrolyte resistivity greater than 1500 ohm-cmWidely used to protect submarine pipelinesCan be used at higher water temperatures upto 60 deg CEnergy Capacity is 820 Ah/kgAnode efficiency is 90 %


AluminiumUsed only for sea service.It is never used in its pure form as it tends to passivate with time resulting in under protection of the system.Widely used are Al-Zn-Hg, Al-Zn-In, Al-Zn-SnEnergy capacity varies from 930-2760 Ah/kg for various alloys.Efficiency of anode is 94 %.

FCC work continues for 1 more week while doing HMR.Distillation and some other areas being finalised - still some PFIsSome Capex values still with RPL to confirm.




























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Cathodic Protection Presentation