power plant chemistry corrosion theory and its prevention

Power Plant Chemistry

Corrosion Theory & Its Prevention

Prepared By : -

Umar Farooq Chemist “A” SEC

MSC ( Chemistry ) MBA ( Marketing )

[email protected]

[email protected]

SEC Shoaiba Jeddah

Saudi Arabia 1

mailto:[email protected]





Power Plant Chemistry

Table Of Content Corrosion definition

Corrosion Theory

Corrosion Mechanism

Types Of Corrosion

Factor Of Corrosion

Corrosion Reaction

What Is Mean by De polarization

Effect of Various factor on Corrosion

Method Of Corrosion Control

Method Of Corrosion Prevention

How do Corrosion Inhibitor Work

Different Types of

Corrosion Attack

General Attack Localized or

Pitting Attack

Stress Corrosion

Cracking Crevic Corrosion

Filiform

Corrosion Inter granular

Corrosion

Microbial

Corrosion Galvanic

Corrosion

Corrosion Corrosion may be

defined as the destruction of

metal by chemical or electro

chemical with its environment.

In water system

the principal fact influencing

the corrosion of iron ( Fe )

metal .

Potential difference

Dissolved solid ( Electrolyte )

Depolarizer

CO2 , Free mineral acidity and

pH are also factor causing

corrosion .

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Corrosion Theory

Existence of potential difference with in a metal or

between two metal will cause chemical reaction at the anode and

cathode . for under standing this let us consider a sample cell in

which two electrodes one of iron and another of copper are placed in

water.

When the two electrodes are connected together by a wire a

closed circuit is completed . Electrons will flow from anode to

cathode by wire and from cathode to words by electrolyte .

The production of electrons is due to the following reactions.

Fe Fe ++ + 2e –

At cathode

H2O H+ + OH -

2H + 2e - H2



Corrosion Theory

In the above reaction iron ( Fe ) dissolved in (

H2O ) as ( Fe ) anodic direction. The Fe combine with

OH to form Fe ( OH )2. This process stop after some time

this is due to the formation of a thin film of H2 gas . If O2

is present in water it combine with H at the cathode to

form H2O.

4H + O2 2H2O

It is also present the formation of hydrogen

film or layer over the cathode this is the presence of O2 ,

the corrosion possess enhanced as the anodic or

Cathodic reaction continuous.



Corrosion Theory Another role of O2 increasing corrosion rate, is that

the corrosion product Fe (OH) 3 are formed by oxidation

of Fe (OH) 3 as shown below.

4Fe (OH) 2 + O2+2H2O 4Fe (OH)3

Basically corrosion is an

electro chemical process. For corrosion reaction to occur

there must be a corrosion cell, consisting of cathode ,

Anode and electrolyte . The electrons of metals flow to

words other point cathode where electron concerning

reaction occur.

The result of this activity is the loss of metal

and after the formation of a deposit.

Cathode has higher potential

Anode has lower potential. Umar Farooq Chemist “ A”


Corrosion Mechanism Simple Corrosion Cell

Corrosion is an electro Chemical

process in which a difference in

electrical potential develops between

two metals or between different parts

of single metal, This voltage can be

measured when a metal is electrically

connected to a standard electrode,

The electrical potential of a metal may

be more or less than a standard. In

which case the voltage is expressed

as either positive or negative . This

difference in potential allows current to

pass through metals causing reaction

at Cathodic and anodic sites. These

sites constitute the corrosion cell.



Types of Corrosion


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The anode is the reign of lower potential and conversely cathode is

the reign of higher potential.

Biological corrosion High temperature corrosion

Cold end corrosion Hydrogen attack corrosion

Cavitations Inter granular corrosion

Dezincification corrosion Stress corrosion

Fatigue corrosion Under deposit corrosion

Pitting corrosion ( Localized

corrosion

Galvanic corrosion

General corrosion ( uniform

corrosion )

Crevice Corrosion

Filiform Corrosion

Factor of Corrosion Dissolved solid electrolyte

Generally increase in dissolved solid concentration of water makes it

corrosive. Higher solid content increase the conductivity of water. Thus

favoring increase in corrosion . One logical theory involves the effect of ion

in destroying the coating of corrosive production the metal . Higher

concentration of Chloride destroy coating by inhibitors such as chromate.

De Polarizer: Dissolve Oxygen Concentration.

Oxygen react with hydrogen at the cathode surface forming depolarizing

surface. Thus permitting additional iron to dissolve . The solubility of oxygen

in water depends on the temperature and pressure. Oxygen produce is

identified easily and corrosion produces in the form of small pits or

depression.

pH and free mineral Acidity of oxygen.

If pH is below 4.3 approximately . The chief controlling factor in promoting

corrosion is pH . In the presence of oxygen pH has no controlling influence

an corrosion rate. In absence of oxygen pH has controlling influence on

corrosion rate.



Corrosion Reaction Anodic reaction.

The anodic reaction are typical by

dissolution of metal. Fe Fe ++ + 2e –

Cathodic reaction.

In Cathodic reaction evolving electrons

migrate through the metal to cathode ,

where they react in any one of the

several ways. Some typical cathode

reaction are as under .

Typical Cathodic reaction

Reduction of water occurs in natural

water . 2H2O +2e H2 +2(OH) -

Hydrogen ion reduction. Important in

acidic solution. 2H + 2e - H2

Oxygen reduction occurs aerated

acidic solution.

O4+4H+ + 4e 2H2O

Ferric ion reduction.

Fe +3 +e – Fe +2

Hence Fe +2 ( Ferrous ion ) produced

at anode combine with OH ions to

form ferrous hydroxide.

Fe + 2(OH)- Fe (OH) 2

Ferrous hydroxide produced has very

low solubility and is quickly

precipitated as Fe ( OH ) 3 or Fe2O3

as the condition prevail. Fe (OH) 2 O2+2H2O 4Fe (OH) 2

2Fe (OH) 3 Fe2O3+3H2O



What is mean by depolarization

It has been noted that some time Cathodic reaction is

reduced due to the formation of thin layer of hydrogen

gas is cathode . Which prevents the flow of electrons

from anode to cathode . This is only possible of oxygen

gas is not present in the media .

H2 + O2 2OH

Hydrogen gas there fore form the barrier and stop

corrosion reactions. Now if oxygen is present in the

system, it will react with hydrogen gas. And

depolarization occurs. This phenomena is called

depolarization and oxygen gas act as a depolarizer.



Effect of various factors on corrosion

The most important factor are .

Oxygen and other dissolved gases.

Dissolve or suspended solid

Alkalinity or acidity.

velocity and temperature .

Microbiological activity ( e.g. algae, fungi, bacteria )

These factors are discussed as .

Oxygen gas acts as depolarizer and effects directly on

corrosion. It is essential for Cathodic reaction to take

place. However other dissolved gases ( NH3,H2S,SO2 )

effect indirectly on corrosion.




How do dissolve solid or Suspended solid effect corrosion

Dissolved solid can effected the corrosion reaction by increasing the

electrical conductivity of water higher the concentration of dissolved

solid greater the conductivity and more like hood of corrosion

.dissolved chlorides and sulphate are practically corrosive by there

corrosive or behavior action they can settle on metal surface to set

localized corrosion cell.

How does water velocity effect corrosion .

Higher velocity of water increases corrosion by transporting oxygen

to the metal by carrying away corrosion products at faster rate.

Higher velocity can also cause corrosion of mater surface protective

oxide film. When water velocity is low, deposition of suspended solid

can establish localized corrosion cells , there by incensing corrosion

rate.




How does temperature effect corrosion .

Below 70C°every 15C° approximately increase in temperature

causes corrosion rate to double above 70C° additional temperature

increases has relatively little effect on corrosion rate in cooling water

system.

How dose acidity or alkalinity effect an corrosion .

Acidic and slightly alkaline water can dissolve metal and protective

oxide film on metal surface more alkaline water favor the formation

of protective oxide layer.

How does microbial growth effect corrosion .

Microbial promote the formation of corrosion cells. In addition the by

products of some organisms' such as H2S form anaerobic bacteria

are corrosive.



Different Types of Corrosion attack General Attack

When the corrosion is uniformly

distributed over the metal surface.

The conceder amount of iron

oxide produced by generalized

attack contribute to fouling

problem

It is recognized by a roughening of

the surface and usually by the

presence of corrosion products

The mechanism of the attack

typically is an electrochemical

process that takes place at the

surface of the material

.


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General attack corrosion deteriorates a steel lock.

The surface effect produced by most direct

chemical attacks (e.g., as by an acid) is a uniform

etching of the metal.

.

http://corrosion.ksc.nasa.gov/intercor.htm

http://corrosion.ksc.nasa.gov/images/uni1.jpg

Different Types of Corrosion attack

Localized or pitting attack

Exist when only small areas of

metal surface corrode.

Pitting is the most serious form of

the corrosion because action is

concentrated in small area

Pitting may perforate the metal in

a short time.


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Pitting corrosion is localized corrosion that occurs

at microscopic defects on a metal surface. The pits

are often found underneath surface deposits

caused by corrosion product accumulation.


Different Types of Corrosion attack Localized or pitting attack

Pitting is a very destructive, localized

corrosion that most commonly occurs

in water-filled systems

Pitting is caused by localized anodic

sites formed at surface cracks,

scratches, or at points of oxygen

differential on the metal surface.

Pitting is prevalent when there are very

low flow rates. Pitting can be a major

problem because after the pit initially

forms, flow rate in the pit decreases

further allowing for the acidity and the

oxygen differential of the fluid within

the pit to increase

This increased acidity and oxygen

differential within the pit accelerate the

corrosion process until it becomes self-

sustaining.


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Different Types of Corrosion attack Galvanic attack Galvanic corrosion occurs when two dissimilar

metals come in contact with each other. This

corrosion is caused by the potential difference that

develops between the metals. Galvanic corrosion

occurs when the contact between dissimilar metals

is bare metal to bare metal and they are in the

same physical environment.

A common area of galvanic corrosion is in cooling

systems that contain both steel and copper

alloys. As the copper components corrode, copper

is released into the water. This dissolved copper

plates out on the steel surfaces and induces

galvanic corrosion on the steel.

This is an area of concern in power plants, which

use copper alloys in the condenser or the feed

water system. Corrosion of copper alloys in the

feed water components will plate out on the inside

of the steel water wall tubes; eventually leading to

water wall tube failures.

Common example in water system are

Steel & brass , Zinc & brass , Aluminum & steel ,

Zinc & steel ,

If galvanic attack occurs the metal named first will

corrode


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Stress Corrosion Cracking Stress corrosion cracking in stainless steel is

normally seen in the presence of a high

chloride concentration. Ammonia is the main

cause of stress corrosion cracking in brass

components. Crevices and low flow sites are

the most likely locations for stress corrosion

cracking.

Stress corrosion cracking is a type of localized

corrosion that occurs in a corrosive

environment where metal stress is

present. This type of corrosion causes

cracking of the metal.

The problem and indications of stress

corrosion cracking are most often seen in

cooling systems that use brass and/or

stainless steel components. These alloys are

more susceptible to this type of corrosion.


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Crevice Corrosion

Crevice or contact corrosion is the

corrosion produced at the region of

contact of metals with metals or metals

with nonmetals. It may occur at

washers, under barnacles, at sand

grains, under applied protective films,

and at pockets formed by threaded

joints.

The mechanism of crevice corrosion is

similar to pitting except that the

corrosion takes place in crevices.

As with the case of pitting, the solution

in the crevice becomes acidic and an

oxygen differential may

develop. Eventually the corrosion

process becomes self-sustaining.





Microbial corrosion Microbial corrosion (also called

microbiologically -influenced corrosion or

MIC) is corrosion that is caused by the

presence and activities of microbes. This

corrosion can take many forms and can be

controlled by biocides or by conventional

corrosion control methods

Filiform Corrosion

This type of corrosion occurs on painted or

plated surfaces when moisture permeates

the coating. Long branching filaments of

corrosion product extend out from the

original corrosion pit and cause

degradation of the protective coating.

Inter granular corrosion

Inter granular corrosion is an attack

on or adjacent to the grain

boundaries of a metal or alloy.



http://corrosion.ksc.nasa.gov/images/microb1.jpg

Method of Corrosion Control

Changing the environment of the metal is

a more practical method of corrosion

control. Three methods of changing the

environment to which a metal is exposed

include removing oxygen, using corrosion

inhibitors, or promoting the formation of a

protective scale.

Removing oxygen from the water is one way

to alter the environment of the

metal. Oxygen maintains and promotes the

cathodic reaction of the corrosion process.

Metal surfaces in contact with a higher

oxygen concentration will become cathodic

compared to the surfaces in contact with

the lower oxygen concentration. The

surfaces exposed to the lower oxygen

concentration will act as the anode and

suffer accelerated corrosion in what is called

an oxygen concentration cell.



Method of Corrosion Control

Oxygen removal will normally reduce the

corrosiveness of water. The oxygen may

be removed through de aeration or by

addition of chemicals called oxygen

scavengers.

Sodium sulfite is a common oxygen

scavenger used with cooling water

systems. For small, closed-loop systems,

this is an acceptable method. However, for

large systems such as circulating water

systems, chemical costs would be too high.

As a maintenance consideration, once the

deposits become severe enough to cause

problems, the piping and equipment must

generally be cleaned, removed, replaced, or

repaired.



Method of Corrosion Prevention Corrosion can be prevented or minimized by one

or more of the following methods.

When designing a new system ,

chose corrosion resistance material to minimize the

effect of aggressive environment.

Adjust pH

Apply protective coating , such as paints metal plating or

plastics.

Protect catholically, using sacrificial anodes ( Cathodic

protection )

Add protective film forming chemicals inhibitors that the

water can be distributed. To all wetted part of the

system.



How do Chemical Corrosion Inhibitors work

Chemical inhibitors reduced or stop corrosion by

interfering with the corrosion mechanism. Inhibitors usually effect

either anode or cathode and so are called as anodic and Cathodic

inhibitors.

Anodic Inhibitors

The inhibitors' which stop or minimize anodic corrosion

are called anodic inhibitors.

Sodium chromate ( Na2CrO4 ) sodium nitrate ( NaNO2 )

Potassium nitrate, poly phosphate , all hydroxide ( NaOH , KOH )

etc.

Cathodic Inhibitors.

The inhibitors which stop or minimize Cathodic corrosion are called

Cathodic inhibitors e.g. salt of zinc and nickel

ZnCO3,NiCO3,ZnO,NiO etc.



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