corrosion protection of steel

8/13/2019 Corrosion Protection of Steel

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CONTENT What is corrosion and its process?

Corrosion in steel

What affects the rate of Corrosion?

Types of corrosion

Corrosion protection methods

Bibliography


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The corrosion process

What is it? The chemical or

electrochemicalreaction between amaterial and itsenvironments thatproduces adeterioration of thematerial and its

properties is calledCorrosion.


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Corrosion in Steel

Most corrosion of steel can be considered as an electrochemicalprocess that occurs in stages.Initial attack occurs at anodic areas on the surface, whereferrous ions go into solution.Electrons are released from the anode and move through themetallic structure to the adjacent cathodic sites on thesurface where they combine with oxygen and water to formhydroxyl ions.These react with the ferrous ions from the anode to produceferrous hydroxide which itself is further oxidised in air toproduce hydrated ferric oxide; red rust.

The sum of these reactions can be described by the followingequation:-

4Fe + 302 + 2H20 = 2Fe203.H20

(Iron/Steel) + (Oxygen) + (Water) = Rust


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Two important points emerge from this:-

For iron and steel to corrode it is necessary tohave the simultaneous presence of water andoxygen. In the absence of either, corrosion does notoccur.

All corrosion occurs at the anode; no corrosionoccurs at the cathode.

From this it can be seen that for iron and steel to

corrode it is necessary to have the simultaneouspresence of water and oxygen. In the absence ofeither, corrosion does not occur.


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What affects the rate of corrosion? The principle factors that determine the rate of

corrosion of steel in air are the time of wetness andthe presence of atmospheric pollution typicallypresent as suplhates and chlorides.

Both sulphates and chlorides increase corrosionrates. They react with the surface of the steel toproduce soluble salts of iron that can concentratein pits and are themselves corrosive.


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Types of Corrosion

Uniform Corrosion Pitting Corrosion Crevice Corrosion Galvanic Corrosion Fretting Corrosion Stress Intergranular Bacterial Erosion


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Uniform Corrosion

It is characterized by a corrosive attack proceedingevenly over the entire surface area or a large fractionof the total area. A general thinning of the sectiontakes place until the failure.


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Pitting Corrosion It is a localized form of corrosion resulting in the

cavities in the surface of steel. It is more dangerousthan Uniform corrosion since it is more difficult todetect.

Crecive Corrosion It is a localized form of corrosion, usually associated

with stagnant water in crevices(shielded areas)such as those formed under gaskets, washers,insulation materials, etc.


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Galvanic Corrosion

Refers to the corrosion damage that occurs at theinterface of two highly loaded surfaces that are notdesigned to move against each other. Most

commonly caused because of vibration.

Fretting Corrosion

Corrosion damage occurs when two dissimilarmetals are coupled in a corrosive electrolyte.


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Corrosion ProtectionMethods


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Coated Steel Galvanized

Galvanizing structural steel is another popularmethod used to protect the steel. During the galvanizing process, the steel is

immersed in pickling acid, flux and finally in molten

zinc. Common recommended procedures to avoid the

build up of pressure are to either seal the liquidsout of crevices and pockets or provide vent holesin the steel assembly to permit liquids to drain.


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Weathering Steel

The use of weathering steel is a popular outdoorcorrosion protection method With weathering steel, a protective patina forms on

the surface of the steel that prevents corrosion.

Weathering steel works well in most environmentalconditions. However, it is not as effective in areas of aggressive

chemistry, in tunnel like conditions (constant

moisture in the air), immersed water conditions(consistently wet), and routine salt environments(coastal conditions).


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Weathering steels are high strength, low alloy

weldable structural steels that possess goodweather resistance in many atmospheric conditionswithout the need for protective coatings.

They contain up to 2.5% of alloying elements, e.g.chromium, copper, nickel and phosphorous.

On exposure to air, under suitable conditions, theyform an adherent protective rust patina. This acts asa protective layer that, with time, causes thecorrosion rate to reduce until it reaches a lowterminal level, usually between 2-5 years.


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This photo providesan example of twopopular corrosionprotectionsystems paintingand the use ofweathering steel.


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Coated Steel Painted Painting is the principle method of protecting structural

steelwork from corrosion. Paints are made by mixing, pigments (the coloured

part), binders (the film forming component) and thesolvent (which dissolves the binder).

Paints are usually applied one coat on top of anotherand each coat has a specific function or purpose. The primer is applied directly onto the cleaned steel

surface. Its purpose is to wet the surface and to providegood adhesion for subsequently applied coats. In the

case of primers for steel surfaces, these are also usuallyrequired to provide corrosion inhibition.

The finishing coats provide the first line of defenceagainst the environment and also determine the finalappearance in terms of gloss, colour, etc.


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Impressed Current Method

A low voltage direct current is applied to the steelstructure from a rectifier. The positive terminal of the rectifier is connected to

an electrode installed underwater or underground,and the negetive terminal is connected to the steelstructure to be protected.

The electric current density can be controlledaccording to the rate of corrotion.

The electrodes do not need frequent replacements.The impressed current method is thus well suited tolarge piers that require semi-permanent protection.


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BIBLIOGRAPHY http://www.steelconstruction.info/Corrosion_protec

tion http://www.engineering.uiowa.edu/~swan/courses

/53086/Corrosion_Protection.pdf http://www.cortecvci.com/Publications/Papers/Vp

CIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_ 2_.pdf

Guide-to-protection-of-steel-against-corrosion (PDF)
http://www.steelconstruction.info/Corrosion_protectionhttp://www.steelconstruction.info/Corrosion_protectionhttp://www.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdfhttp://www.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.cortecvci.com/Publications/Papers/VpCIPolymerAdditives/DAFLOU%20BATIS%20VpCI%20_2_.pdfhttp://www.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdfhttp://www.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdfhttp://www.engineering.uiowa.edu/~swan/courses/53086/Corrosion_Protection.pdfhttp://www.steelconstruction.info/Corrosion_protectionhttp://www.steelconstruction.info/Corrosion_protectionhttp://www.steelconstruction.info/Corrosion_protection


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corrosion protection of steel

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