Nayak Dilipkumar New 29-09-2011

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SEMINAR TOPIC: HIGHTEMPERATURE CORROSION & ITS

PROPERTIES

NAME : NAYAK DILIPKUMAR SOMABHAI

ROLL NO: 930FACULTY OF TECH & ENGG M.S.U

MET & MAT ENG DEPARTMENT


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TOPICS

INTRODUCTION

CLASSIFICATION

TYPES OF HIGH TEMPERTURECORROSION

MECHANISM

CORROSION PREVENTIONS.


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INTRODUCTION

Damage from corrosion & metallurgicalmechanisms often leads to failures in various equipment, which interrupt

the routine operations create safety hazards.

The existence as well as the degree of damage isdependent on the particular process operating conditions & contaminantspresent in the process stream. Everyone in the refining industry today,mechanical engineer, metallurgist, &process engineer is looking for waysto prevent the effects of corrosion. Corrosion control is paramount to the

safe & productive operation of a facility.


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INTRODUCTION OF CORROSION

The deterioration of a material, usually a metal, because of areaction with its environment, is termed as corrosion.

The definition is very general & recognizes that some forms ofcorrosion are not chemical or electrochemical in nature. The definition also

recognizes that materials other than metals may corrode. These materialsinclude concrete, wood,ceramics, & plastics. In addition , in some forms ofcorrosion the properties of the material as well as the material itself deteriorate.A material may experience no weight change or visible deterioration yet, due toproperty changes promoted by corrosive action , the material may failunexpectedly


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CLASSIFICATION OF CORROSION

1. LOW TEMPERATURE CORROSION - Occurs at temperaturesbelow 260 c & in the presence of water.

Example: Corrosion of steel by water.

LOW temperature refinery corrosion is also called aqueouscorrosion, wet corrosion, or electrochemical corrosion. It requires thepresence of an aqueous solution , including water even in very smallamounts, or an electrolyte in hydrogen stream.

S


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High temperature corrosion

2.High temperature corrosion :

Occurs at temperatures above 260c , with nowater present.

Example: Corrosion on steel by furnaces gases.

High temperature corrosion is referred to as dry corrosion ordirect chemical combination. It occurs above the environment dew pointand is normally associated with high temperatures. Gasses are the typicalcorrosive agents.

Although oxidation generally refers to an electron producingreaction, this term is also employed to designate the reaction between ametal & air or oxygen in the absence of water or an aqueous phase.Scaling tarnishing and dry corrosion are also sometimes used to describethis phenomena of HIGH TEMPERATURE CORROSION.

Since virtually every metal & alloy reacts with air at elevated temperatures,

oxidation resistance must be considered in most metallurgical engineeringa lications.


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HIGH TEMPERATURE CORROSIONMECHANISM

As with low temperature corrosion , high temperature corrosionis an electrochemical process consisting of two or morepartial (oxidation or reduction) reaction. When metal isexposed to air , it is oxidized to an ion at the metal scale

interface according to the following equation:

M M+n + ne-

At the same time ,oxygen is reduced at thescale surface as shown in the equation below

1/2 O2 + 2e- O-2

The overall corrosion reaction is obtained bycombining the oxidation & reduction reaction to form a metal

oxide as follow:


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HIGH TEMPERATURE CORROSIONCONDITIONS

High temperature corrosion is related to nature of scalethat is formed. Many processes at elevated temperatures involvevapor or liquid streams at high flow velocities, high temperaturecorrosion often results in fatigue,erosion, & cavitations damage.

Carbon steel may be used in high temperature conditionswithout excessive scaling up to a temperature of about 565 'C .

Above this temperature ,various alloys must be used toincrease oxidation resistance & to provide adequate mechanicalproperties.


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TYPES OF HIGH TEMPERATURECORROSION

1. HIGH TEMPERATURE OXIDATION

2. HIGH TEMPERATURE SULPHIDECORROSION

3. HIGH TEMPERATURE HYDROGENATTACK


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HIGH TEMPERATURE OXIDATION

OXIDATION IS ATYPE OF CORROSION INVOLVING THEREACTION BETWEEN A METAL & OXYGEN AT HIGHTEMPERATURE IN THE ABSENCE OF WATER OR ANAQUEOUS PHASE.

It is also called dry corrosion .At nomaltemperatures, the oxides of the metals are more stable thanmetals. Metals being in the metastable state are bound to formoxides.

The rate of oxidation o a metal at high temperaturesdepends on the nature of the oxide layer that forms on the surfaceof metal.


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MECHANISM OF OXIDATION

When a fresh surface of metal like gold comes incontact with oxygen, the latter forms a loosely bound layer on the

surface of gold due to secondary van der waals attractive forcesarising between the atoms of the metal surface & the oxygenmolecules. This layer is said to be adsorbed oxygen layer, & nofurther chemical bonding occurs. This layer is easily removed byapplying ultra high vacuum.

The first oxygen molecules coming in contact with theclean surface of metal. Dissociate into oxygen atoms & then, theseoxygen atoms bond chemically with the atoms of the metal surface.


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High Temperature Sulfide corrosion

As the areas of energy conversion & environmental protectionbecome increasingly concerned with processing & cleanup of sulfurbearing fossil fuels & their combustion products , the attack of metal

by sulfer compounds at high temperatures has become a majorcorrosion problem. the term sulfidation is used to describe thegaseous attack of metals by H2S, S2, SO2, & other gaseous

sulfur species.High Temperature Sulphide corrosion with Hydrogen present is

moresevere than high temperature sulfide corrosion without hydrogen

present hydrogen converts organic sulfur compounds to hydrogen

sulfide & corrosion becomes a function of H2S concentration orpartial pressure.


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HIGH TEMPERATURE HYDROGENATTACK

HTHA is primarily a problem in downstreamoperations in which carbon & low alloy steels are exposed to 221'c&m partial pressure above 200 psi. when damaged, the steel

losses tensile strength & ductility & if under stress , can crack.

With in steel , the hydrogen reacts with other elements, suchas carbon , to form gases , primarily methane. The reaction follows

Fe3C + 2H2 3Fe + CH4


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CORROSION PREVENTIONS

1.High temperature corrosion prevention.

2.High temperature sulphide corrosion prevention.

3.High temperature hydrogen attack prevention.


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HIGH TEMPERATURE CORROSIONPREVENTION

1.High resistance to oxidation is possible if the oxide film has pillingbedworth ratio slightly greater than one so that it is continuous,

compact & adhering.

2.Chromium , silicon & aluminium when added as alloying agentsgive good corrosion resistance.

3.A resistance alloy containing sufficient Cr can be used.


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HIGH TEMPERATURE SULFIDE CORROSIONPREVENTION

1. Alloying elements usually contains Fe, Ni & Cr as the

major components with Al, Mn,Si,Mo & c as importantconstituents.

2.High temperature scaling resistance in oxidizing gases isgenerally provided by the formation & maintenance of a slow

growing , protective layer of Cr2O3 or Al2O3.


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HIGH TEMPERATURE HYDROGEN ATTACKPREVENTION

1. HTHA is usually not uniform throughout an affectedcomponent.

Attack is first initiated in areas where Hydrogen diffuses.

2. Alloying elements like Mo, Cr, W are used to limit the amount ofC available for the formation of methane.


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Nayak Dilipkumar New 29-09-2011