str654 - lecture 7b 2019

Cairo University STR654 Faculty of Engineering

Fall2020

Sherif A. Mourad - Lecture 7b 1

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STR654Inspection, Repair, and

Maintenance of Steel Structures

Lecture 7b, December 1, 2020

Sherif A. Mourad

Professor of Steel Structures and Bridges

Faculty of Engineering,

Cairo University

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Lecture 7b: Cathodic Protection

Topics Introduction / History.

Principles of Cathodic Protection.

Advantages of Cathodic Protection.

Basic Requirements.

Monitoring and Maintenance


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Why Cathodic Protection

One of the most widely used methods.

Works almost all the time on all metals in all environments.

First proposed 1982.

Used for pipelines, oil and gas wells, offshore structures, ship hulls, marine piling, water tanks,...

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Historical Background

In 1820, Sir Humphry Davy was asked to investigate the corrosion of copper sheeting used for cladding the hulls of naval vessels

He found that attaching small quantities of iron, zinc, or tin. The copper became “cathodically protected”.

By protecting the copper, its anti-fouling properties became retarded, reducing the streamline of the ship and collecting marine growth.


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Historical background

Rapid development followed in the US, especially in the oil and natural gas industry.

The method has been successfully used to protect miles of fuel-line networks.

It is currently used for steel, as well as steel reinforcement in concrete.

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Why Corrosion Occurs

Metals that has been extracted from its primary ore (metal oxides or other free radicals) has a natural tendency to revert to that state under the action of oxygen and water.

This action (corrosion) is typically noted in steel.


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Why Corrosion Occurs Corrosion is an electro-chemical

process that involves the passage of electrical currents on a micro or macro scale. The change from the metallic to the combined for occurs by an anodic reaction.

Metal =

M+ (soluble salt) + e- (electron)

Fe = Fe++ + 2e-

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This reaction produces free electrons, which pass within the metal to another site on the metal surface (the cathode), where it is consumed by the cathodic reaction.

In acid solutions: 2H+ + 2e- = H2(gas)

In neutral solutions: O2+2H2O+4e-= 4OH-


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Corrosion thus occurs at the anode, but not at the cathode (unless the metal of the cathode is attacked by alkali).

The anode and cathode in a corrosion process may be on two different metals connected together forming a bimetallic couple.

The anode and cathode may be close together on the same metal surface.

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How is it initiated

Difference in natural potential in galvanic (bimetallic) couples.

Metallurgical variations in the state of the metal at different points on the surface.

Local differences in the environment, such as variations in the supply of oxygen at the surface (oxygen rich areas become the cathode and oxygen depleted areas become the anode.


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How is it initiated

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How is it initiated


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How is it initiated

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How is it initiated


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Principle of CP By connecting an external anode to

the metal to be protected and passing an electric current, all the areas of the metal become cathodic and do not corrode.

The external anode may be:

• a galvanic anode, where the current is a result of potential difference between the two metals.

• An impressed current anode, where the current is impressed from an external dc power source.

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Galvanic (Sacrificial) System

Employs reactive metals as auxiliary anodes that are directly electrically connected to the steel to be protected.

The difference in the natural potentials between the anode and the steel causes positive current to flow in the electrolyte, from the anode to the steel.

Metals commonly used as anodes are aluminum, zinc, and magnesium, alloyed to improve the long-term performance.


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Galvanic (Sacrificial) System on a

Coated Pipeline

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Impressed Current System

Employ inert (zero or low dissolution) anodes.

Uses an external source of dc power (rectified ac) to impress a current from an external anode onto the cathode surface.


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Impressed Current System

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Advantages

The main advantage over other forms of anti-corrosion treatment is that it is applied simply by maintaining a current.

Its effectiveness may be monitored continuously.

Avoids the need for corrosion allowance to thin sections.


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Use

Commonly applied to coated structures to provide corrosion control to areas where the coating may be damaged.

Can be applied to any metallic structure in contact with a bulk electrolyte (including concrete).

Cannot be used to prevent atmospheric corrosion of metals.

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Typical Structures

Exterior surfaces of:

• Pipelines

• Ships hulls.

• Storage tank base.

• Jetties and harbor structures.

• Steel sheet, tubular and foundation piling.

• Offshore platforms, floating and sub sea structures.


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Typical Structures

Interior surfaces of:

• Large diameter pipelines

• Ships tanks.

• Storage tank (oil or water).

• Water circulating systems.

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Basic Requirements

For a galvanic system:• Sacrificial anodes.• Direct welding to the structure or

a conductor connecting the anode to the structure.

• Secure and minimum resistance connections between conductor and structure, and between conductor and anode.


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Sacrificial Anode

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Basic Requirements

For an impressed current system:• Inert anodes.• DC power source• Electrically well insulated,

minimum resistance and secure conductors between anodes and power source.

• Secure and minimum resistance connections between power source and structure.


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Impressed Current

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Impressed Current


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Other Requirements

Electrical continuity (minimum resistance to minimize the potential drop).

Coating (reduce current demand).

Structure isolation.

Test facilities.

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Design Considerations

Initial considerations:• Best results when considered at the early

design stage.

• Survey and study to establish economical justification.

• For existing structures, measure structure-to-soil potential.

• Specifications.

• Study cable routs, transformer-rectifier location, electricity supply, …


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Design Considerations

Potential level & distribution:• Structure-to-electrolyte potentials are

measured to assess protection criteria.

• Potentials are most negative at points nearest to anode, and may attenuate towards the natural corrosion potential as the distance from the anode increases.

• A single CP installation may protect up to 150 km of well coated pipeline, and only 2 km of bare pipes.

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Design Considerations Economic Decisions:

Galvanic Impressed-current

Simple to install

Independent of a source of current.

Suitable for localized protection.

Less liable to interaction with neighboring struct.

Able to supply large current.

Enable to supply high driving voltage, to be used in most types of electrolytes.

Provide flexible output to accommodate changes.


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Economics of Decision

Generally, galvanic systems have found favor for small well-coated, low current demand structures, or localized protection.

Impressed current schemes are utilized for large structures, which may be bare metal or poorly coated.

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Problems to be Avoided

Excessive negative potentials can cause accelerated corrosion, and may be detrimental to certain coatings.

Hydrogen evolution at the cathode surface may result in hydrogen embrittlement of steel, and loss of strength.

Introduction of electrical current may create spark hazard.


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Equipment

Galvanic anode alloys are available in block, rod, or wire forms. They are typically cast around steel inserts to enable fixing of the anode and maintain continuity.

Impress current groundbeds in soil usually consist of high-silicon cast iron.

Most popular dc power source is the selenium plate or silicon-diode rectifier.

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Equipment In remote areas, power sources include

thermo-electric generators, closed-cycle vapor turbines, and solar or wind generators.

The choice is a function of power requirements, maintenance capabilities, and environmental conditions.

Automatic control units can adjust current output in accordance with potential changes at a half-cell.


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Monitoring and Maintenance Monitoring can be performed by measuring the

structure-to-electrolyte potential using a high input impedance voltmeter and suitable half-cell.

Maintenance includes the mechanical maintenance of power-supply equipment and maintenance of the painted surfaces of equipment.

It is good practice to inform all owners of cathodic protection systems and infrastructure in the area of influence of any new cathodic protection system so that the effect on these facilities may be assessed.

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References

J.H. Morgan, Cathodic Protection, NACE, 1987.

H.H. Uhlig, Corrosion and Corrosion Control, Wiley, 1985.

W.V. Baeckmann, Handbook of Cathodic Corrosion Protection, Gulf Pub., 1997.


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Standards

BS 7361 Part 1, 1991, Cathodic Protection –Code of Practice for Land and Marine Applications.

BS EN 12473, General Principles of Cathodic Protection in Sea Water.

BS EN 12696, Cathodic Protection of Steel in Concrete – Part 1: Atmospherically exposed concrete.

BS EN 12954, Cathodic Protection of Buried or immersed metallic Structures – General Principles and Application for Pipelines.

BS EN 13173, Cathodic Protection of Steel Offshore Floating Structures.

BS EN 13174, Cathodic Protection for Harbour Installations.

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SUMMARY

Topics Introduction / History.

Principles of Cathodic Protection.

Advantages of Cathodic Protection.

Basic Requirements.

Monitoring and Maintenance

str654 - lecture 7b 2019

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