1. basic corrosion
TRANSCRIPT
-
7/30/2019 1. Basic Corrosion
1/29
Basic corrosion
(Oxidation & Aqueous)
Prepared
Nasrizal Mohd RashdiLecturer
-
7/30/2019 1. Basic Corrosion
2/29
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
-
7/30/2019 1. Basic Corrosion
3/29
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
-
7/30/2019 1. Basic Corrosion
4/29
Unprotective porous film
O2 can continuously pass and react at metal-oxide
interface.
O2Atmosphere
Metal
Oxide
film
-
7/30/2019 1. Basic Corrosion
5/29
Non porous film (Protective againt O2
permeation).
Cations diffuse through the film reacting with
oxygen at the outer surface.
-
7/30/2019 1. Basic Corrosion
6/29
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-
-
7/30/2019 1. Basic Corrosion
7/29
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-
-
7/30/2019 1. Basic Corrosion
8/29
Mechanism
For (b) (d)
Metal-oxide interface
M Mn+ + ne-
Air-oxide interface
O2 + 4e- 2O2-
-
7/30/2019 1. Basic Corrosion
9/29
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)
-
7/30/2019 1. Basic Corrosion
10/29
Where c2 is a constant representing film
thickness at t=0.
This time dependence is appropriately termed
a linear growth rate law.
-
7/30/2019 1. Basic Corrosion
11/29
y
x
cCO
2
y
1
dt
dy12
2
ory
J
y
cD
x
cDJ
O
O
-
7/30/2019 1. Basic Corrosion
12/29
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)
-
7/30/2019 1. Basic Corrosion
13/29
Where c4 and c5 are two additional constants.
c4 = 2c3 , c5 is the square of the film thickness
at t=0
-
7/30/2019 1. Basic Corrosion
14/29
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
-
7/30/2019 1. Basic Corrosion
15/29
For R1, the oxide tends to be
protective.
-
7/30/2019 1. Basic Corrosion
16/29
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.
-
7/30/2019 1. Basic Corrosion
17/29
Problem 2
Given that the density of Cu2O is 6.00Mg/m3 ,
calculate the Pilling-Bedworth ratio for copper.
-
7/30/2019 1. Basic Corrosion
18/29
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.
-
7/30/2019 1. Basic Corrosion
19/29
Concentration cell
Anode
(corrosion)
Cathode
(electroplating)
High Fe2+
concentration
Low Fe2+
concentration
Fe Fe
Porous membrane
-
7/30/2019 1. Basic Corrosion
20/29
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.
-
7/30/2019 1. Basic Corrosion
21/29
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.
-
7/30/2019 1. Basic Corrosion
22/29
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.
-
7/30/2019 1. Basic Corrosion
23/29
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.
-
7/30/2019 1. Basic Corrosion
24/29
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.
-
7/30/2019 1. Basic Corrosion
25/29
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.
-
7/30/2019 1. Basic Corrosion
26/29
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.
-
7/30/2019 1. Basic Corrosion
27/29
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.
-
7/30/2019 1. Basic Corrosion
28/29
-
7/30/2019 1. Basic Corrosion
29/29