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MARINE CATHODIC PROTECTION

MUZZAMIL AHMAD EATOO163110059IIT BombayMEMS

Contents:

Introduction Anode materials in marine applications. Cathodic protection of marine pipelines Cathodic protection of offshore structures Cathodic protection of ship hulls.

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Cathodic protection can be defined as a technique of reducing or eliminating the corrosion of a metal by making it the cathode of an electrochemical cell and passing sufficient current through it to reduce its corrosion rate. ICCP, SACP

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Introduction

Anode Materials The choice of anode material depends on whether

active (ICCP) or passive (SACP) systems are under consideration.

Sacrificial anode :Should be anodic to steel.Must corrodeinexpensive and durableCommercial sacrificial anodes are magnesium,

aluminum, or zinc or their alloys like Al-Zn-In alloy etc.

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IC ANODES: insoluble and corrosion resistant, sustain high current density, exchange current density, lower power consumption examples: High-silicon caste iron, precious

metals, Mixed-metal oxide, polymer anodes.

Cathodic Protection of Marine Pipelines

• Protective coatings • supplemental CP.

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Effective coating resistance in ohmsfor one average square foot

Current required in amperes

Bare Pipea 500

10,000 14.91

25,000 5.964

50,000 2.982

100,000 1.491

1,000,000 0.1491

5,000,000 0.0298

Perfect coating 0.000058

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Coating & Cathodic Protection

Cost

Coating

CathodicProtection

0 % 100 %

Total Cost

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2.Supplemental CP The CP system supplements coatings and is intended to

control corrosion at holidays in coatings. Large pipelines : ICCP at one/ both ends. Most marine pipelines are protected by SACP.

Zn, or Al sacrificial anodes Electrical contact by insulated copper cables.

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Design Considerations. The average cathodic protection current density

required to protect a marine pipeline will depend on: the type of coating applied,the amount of damage,whether or not burial is specified, and the location of the pipeline.

Large-diameter pipe lines can be protected by installing an ICCP system atone or both ends of the pipeline.

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Bracelet-type zinc or aluminum /alloys sacrificial anodes

Spacing between anodes

o Small dia pipeline(≤14in.) = 150mo Large dia pipeline = 300m.

Design procedure

Calculation of current: Surface area of pipe segment Fraction of steel assumed to be bare.

Anodes are then sized to fit the condition: W/C > IL ;

where W is the anode weight (kg), C is the alloy consumption rate in kilogram/amp year (kg/Ayr), I is the anode current output (A), and L is the desired design life in years.

I=E/R and R=0.315r/√A (McCoy’sEquation) ; where r is the electrolyte resistivity (Ω cm), and A is the anode area (cm2), R= anode –electrolyte resistance, E= net driving voltage.

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Cathodic Protection of Offshore Structures

• Most plateforms are not painted below the waterline, CP system causes change in pH near cathode which causes precipitation of minerals and formation of protective scale.

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• Problems with new technology:

Offshore structures are now being built in deeper, colder water where mineral deposits are less likely to form. mineral deposit require current densities 750 to 1000 mA/m2 (70 to 93 mA/ft2).

Under-protection as current requirement change with depth (Gulf of Mexico and Santa Barbara Channel).

carbonate scales are harder to deposit in deep, cold waters.

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Methods used for protection

SACP systems: simple and rugged, do not depend on external electric power supplies.

the weight of sacrificial anodes can be a serious consideration for deep-water platform.

Hybrid designs: ICCP (primary CP) + SACP. Example Murchison Platform

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Cathodic protection of ship hulls

Ships normally have protective coatings as their primary means of corrosion control.

Cathodic protection systems are then sized so that an adequate electric current will be delivered to polarize the structure to the desired level.

Bare steel Holidays in coatings.

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Anode material

Aluminum anodes are available for ship hulls, but they can passivate and become inactive on ships that enter rivers or brackish estuaries. For this reason, zinc anodes are almost universally used in commercial service.

ICCP systems are used on very large ships.

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ICCP of ship hulls

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Anode surrounded by di-electric shield

Impressed-current cathodic protection systems can produce overprotection in some cases. Organic coatings can disbond because of the formation of hydrogen gas bubbles underneath coatings. Coating disbondment can produce increased surface areas that require more cathodic protection and is controlled by placing dielectric shields between the impressed-current anode and the hull.

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Thank you