1. basic corrosion

7/30/2019 1. Basic Corrosion

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Basic corrosion

(Oxidation & Aqueous)

Prepared

Nasrizal Mohd RashdiLecturer


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Oxidation

Direct atmospheric attack

Generally metals and alloys form oxide

compounds under expose to air at elevated

temperature.

Reactivity of metal with atmospheric oxygen

(oxidation) is different.

Some active and other passive


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There are four mechanisms commonly identify

with metal oxidation.

(a) Unprotective porous oxide film

Non porous film that are protective against O2

(b) Cations diffuse

(c) Anions diffuse

(d) Both cations and anions diffuse


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Unprotective porous film

O2 can continuously pass and react at metal-oxide

interface.

O2Atmosphere

Metal

Oxide

film


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Non porous film (Protective againt O2

permeation).

Cations diffuse through the film reacting with

oxygen at the outer surface.


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Non porous film

Which O2- ions diffuse in order to react with the

metal at the metal-oxide interface.

O2Atmosphere

Metal

Oxide film

2O2-

4e-


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Non porous film

Which both cations and O2- anions diffuse at

roughly the same rate.

Oxidation reaction occur within the oxide film

rather than interface.

O2Atmosphere

Metal

Oxide film

2O2-

Mn+

(4 + n)e-


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Mechanism

For (b) (d)

Metal-oxide interface

M Mn+ + ne-

Air-oxide interface

O2 + 4e- 2O2-


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Growth Rate

Case (a) unprotective oxide

Where yis the thickness of the oxide film, tthe

time and C1 a constant.

Integration of equation (1)

1C

dt

dy (1)

21ctcy (2)


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Where c2 is a constant representing film

thickness at t=0.

This time dependence is appropriately termed

a linear growth rate law.


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y

x

cCO

2

y

1

dt

dy12

2

ory

J

y

cD

x

cDJ

O

O


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For film growth that is limited by ionic

diffusion, the growth rate diminishes as the

film thickness grows. Growth rate;

Where C3

is constant different, from those in

eq. 1 and 2. Integration gives

54

2ctcy (4)

yc

dt

dy 13

(3)


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Where c4 and c5 are two additional constants.

c4 = 2c3 , c5 is the square of the film thickness

at t=0


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Pilling-Bedworth

The tendency of a metal to form a protective

oxide coating is indicated by an especially

simple parameter known as the pilling-

Bedworth ratio, R, given as

M is molecular weight of the oxide, D density of oxide

m is a atomic weight of the metal, d density of metal

amD

MdR


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For R1, the oxide tends to be

protective.


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Problem 1

A nickel based alloy has a 100 nm thick oxide

coating at time (t) equals zero, upon being

placed in an oxidizing furnace at 600C. After

1 hour, the coating has grown to 200 nm inthickness. What will be the thickness after 1

day, assuming a parabolic growth rate law.


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Problem 2

Given that the density of Cu2O is 6.00Mg/m3 ,

calculate the Pilling-Bedworth ratio for copper.


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Aqueous

Dissolution of a metal into an aqueous

environment.

The metal atoms dissolve as ions.

A simple model of this aqueous corrosion is

given in figure below.

An electrochemical cell in which chemical

change is accompanied by an electrical

current.


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Concentration cell

Anode

(corrosion)

Cathode

(electroplating)

High Fe2+

concentration

Low Fe2+

concentration

Fe Fe

Porous membrane


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The metal bar on the left side of the

electrochemical cell is an anode.

Anode will dissolves or corrodes and supplies

electrons to the external circuit.

The anodic reaction as below;

Fe Fe2+ + 2e-

The reaction is driven by an attempt to

equilibrate the ionic concentration.


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The porous membrane allows the transport of

Fe2+ ions between the two halves of the cell

while maintaining a distinct difference in

concentration level.

A metal bar on the right side of

electrochemical cell is a cathode.


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Cathode accepts the electrons from the

external circuit and neutralizes ion in a

cathodic reaction;

Fe2+ + 2e- Fe

At cathode, metal builds up as opposed to

dissolving.

This process is known as electroplating.


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An electrochemical cell consisting of iron and copper electrodes,

each of which is immersed in a 1M solution of its ion. Iron corrodes

while copper electrodeposits.


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Pure iron immersed in a solution containing

Fe2+ ions of 1M concentration.

The other side of the cell consists of a pure

copper electrode in a 1M solution of Cu2+ions.

The cell halves are separated by a membrane,

which limits the mixing of the two solutions.


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If the iron and copper electrodes areconnected electrically, reduction will occur forcopper at the expense of the oxidation of iron,

as follows:Cu2+ Fe Cu Fe2+

Cu2+ ions will deposit (electrodeposit) as

metallic copper on the copper electrode, Iron dissolves (corrodes) on the other side of

the cell and goes into solution as Fe2+ ions.


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Thus, the two half-cell reactions are

represented by the relations

Fe Fe2+ + 2e-

Cu2+ 2e- Cu

When a current passes through the external

circuit, electrons generated from the oxidation

of iron flow to the copper cell in order that

Cu2+ be reduced.


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The driving force for the overall cell is the

relative tendency for each metal to ionize.

The net flow of electrons from the iron bar to

the copper bar is a result of the tendency of

ionize.

A voltage of 0.777 V is associated with the

overall electrochemical process.


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1. basic corrosion

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