marine environment : - corrosive environmentat

7/28/2019 Marine Environment : - Corrosive Environmentat

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ENVIROMENTAL CORROSION


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INTRODUCTION

Corrosion is the destruction of materials by chemicalinteraction with their environment. The termcorrosion is sometimes also applied to the

degradation of plastics, concrete and wood, butgenerally refers to metals. The most widely usedmetal is iron (usually as steel)


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THE CONSEQUENCES OF CORROSION


The consequences of corrosion are many and variedand the effects of these on the safe, reliable and

efficient operation of equipment or structures areoften more serious than the simple loss of a mass ofmetal. Failures of various kinds and the need forexpensive replacements may occur even though the

amount of metal destroyed is quite small. Some ofthe major harmful effects of corrosion can besummarised as follows:


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1. Reduction of metal thickness leading to loss ofmechanical strength and structural failure or

breakdown.

2. Hazards or injuries to people arising from structuralfailure or breakdown (e.g. bridges, cars, aircraft).

3. Loss of time in availability of profile-making industrialequipment.

4. Reduced value of goods due to deterioration ofappearance.

5. Contamination of fluids in vessels and pipes


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6. Perforation of vessels and pipes allowing escape oftheir contents and possible harm to the surroundings.

7. Loss of technically important surface properties of ametallic component. These could include frictional and

bearing properties, ease of fluid flow over a pipe surface.

8. Mechanical damage to valves, pumps, etc, or blockageof pipes by solid corrosion products.

9. Added complexity and expense of equipment whichneeds to be designed to withstand a certain amount ofcorrosion, and to allow corroded components to beconveniently replaced


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CORROSION IN THE MARINE ENVIRONMENT

oCHEMICAL CORROSION

oELECTRO-CHEMICAL CORROSION


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CHEMICAL CORROSION

OXIDATION AND REDUCTIONEg:fe+h2o=(ferrous oxide)rust

BY THE ACTION OF SALT& ACIDS


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ELECTRO-CHEMICAL CORROSION

ANODE GET CORRIED

LOCAL CORROSION AT A FASTER RATE

SEA WATER ACT AS ELECTROLYTE

MORE NOBLE METAL GET PROTECTED LESS NOBLE METAL GET CORROIED


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Uniform Corrosion

Galvanic Corrosion

Crevice corrosion

Pitting corrosion

Intergranular corrosion

Selective corrosion (dealloying)

Graphitic corrosion


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Uniform corrosion of metal pipe occurs, it can be typically due to an

adaptation creating a polyelectrode at the surface of the pipe.

The electrochemical reaction whereby any one particular site on the metal

surface can become anodic one instant and cathodic the next.

The anodic sites shift or creep about the surface, the rate of loss of metal isrelatively uniform over the metal surface.


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Galvanic corrosion is either a chemical or an electrochemical corrosion. The

latter is due to a potential difference between two different metals connected

through a circuit for current flow to occur from more active metal (more

negative potential) to the more noble metal (more positive potential).
http://en.wikipedia.org/wiki/File:Galvanic_corrosion_of_aluminum_and_steel_in_seawater.jpg


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Electrochemical reaction.mp4.wmv


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Crevice corrosion is an electrochemical oxidation-reduction

process.

Crevice corrosion is considered much more dangerous than uniformcorrosion since its rate is 10-100 times higher.

Crevice corrosion is highly accelerated if chloride, sulphate orbromide ions are present in the electrolyte solution.
http://www.substech.com/dokuwiki/lib/exe/detail.php?id=crevice_corrosion&cache=cache&media=crevice_corrosion.png


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Pitting corrosion is anelectrochemical oxidation-reduction process,which occurswithin localized holes (cells) on thesurface of metals coated with a

passive film. Pitting corrosion is considered much

more dangerous than uniform

corrosion since its rate is 10-100 times

higher.

Pitting corrosion is highly acceleratedif chloride, sulphate or bromide ions

are present in the electrolyte solution.
http://www.substech.com/dokuwiki/lib/exe/detail.php?id=pitting_corrosion&cache=cache&media=pitting_corrosion.png


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Intergranular corrosion is an electrochemical oxidation-reduction process, which is localized in the zones adjacent to thegrain boundaries.


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Its a preferential attack of a particular component of an alloy inpresence of electrolyte as a result of an electrochemical oxidation-reduction process.

Selective corrosion may occur in a single-phase and in multi-phase

alloys. Alloys prone to selective corrosion are composed of components

with a great difference between their Electrode potentials (eg.Cu-Zn).

Selective corrosion either uniform or plug type (localized).


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CORROSION CONTROL METHODS

o PAINTING

o CATHODIC PROTECTION

o SACRIFICAL ANODE TYPE

o IMPERESSED CURRENT TYPE


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PAINTING

oFINISHED OFF WITH SUITABLE COAToVERY EFFECTIVE &COMPARITIVELY CHEAP


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Cathodic protection is the method of corrosion control.

Its Provides suppression of the metal oxidation due to electrons flowing to

the metal from an external source.

The external source of such electrons may be either DC power supply

(impressed current cathodic protection) or sacrificial anode. Impressed current cathodic protection is widely used for protection of large

underground equipment


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SACRIFICAL ANODE TYPE

Metals such as Al,Zn etc..are used as anode

Anode are to be fitted to the ship side or in the tank at strategic

position

Sacrificial have 2-4years life

This system is suitable for ship protection as well as in cargo and

blast

tanks

Anode can be typically seen in vicinity of the propeller


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IMPRESSED CURRENT TYPE

This systems are fitted for protection of the immersed portion of extent hull

only.

Voltage of difference is maintained between hull and fitted anodes which will

protect the hull against corrosion

The system consist of a source of direct current, anode & apparatus for

measuring and controlling current

The amount of immersed current will vary with the area to be protected, ship

speed. Salinity and condition of the paint work


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References

en.wikipedia.org/wiki/corrosion.

http://www.youtube.com

www.ariesmar.com/knowledge bank
http://www.youtube.com/http://www.ariesmar.com/knowledgehttp://www.ariesmar.com/knowledgehttp://www.youtube.com/


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