University of Babylon/College of Engineering
Electrochemecal Engineering Dept./ 3rd Stage
Corrosion engineering
Dry corrosion (oxidation)
When metal M ,is exposed at room or elevated temperature to an oxidizing
gas e.g. oxygen, sulfur or halogen corrosion may occur in the absence of a
liquid electrolyte .This is some time called “dry” corrosion in contrast to wet
corrosion
which occurs when a metal to liquid electrolyte or soil.In
oxidation, an oxide layer or scale forms on the surface of metal , this
phenomenon is called frequently termed scaling , tarnishing or dry corrosion
Mechanism :
Initial stages. A clean reactive metal surface exposed to oxygen follows the
sequences
• Adsorption of Oxygen
• Formation of oxide nuclei
• Growth of an continuous oxide film
As in aqueous corrosion , the process of oxide layer formation is an
electrochemical reaction may express
M  O2  MO2
Or in general
xM 
1
( yO2 )  M x O y
2
Furthermore the above reacion consists of two reaction :
This reaction take place at metal-scale interface
M  M 2  2e 
And the other reaction take place at metal-scale interface
1
O2  2e   O  2
2
Figure (1) Schematic representation of processes that are involved in
gaseous oxidation at a metal surface.
For oxide layer increase in thickness , it is necessary that electrons be
conducted to scale –gas interface , in addition metal ions must diffuse away
from metal-scale interface and/ or oxygen ions must diffuse toward this
same interface. Thus the oxide scale serve both as an electrolyte through
which ions diffuse and as an electrical circuit for passage of electrons
• The scale may protect metal from rapid oxidation when is acts as a
barrier to ionic diffusion and /or electrical conduction ;most oxide s
are highly electrically insulation
Oxidation at elevated temperature
A metal M, with oxygen or other gases at high temperature by initial
adsorption of oxygen , chemical raection to form the surface oxide , oxide
nucleation and lateral growth in continoues film that may protect the
underlying metal
The film may also thicken into a non protected scale with various defects
including cavitation, microcracks and porosity.
Oxidation in air by oxygen proceeds according to a reaction such as
M  O2  MO2
More generally
(1)
1
xM  ( yO2 )  M x O y
2
(2)
A metal, M also can be oxidized similarly by either water vapor or carbon
dioxide according to:
xM  yH 2 O  M x O y  yH 2
(3)
xM  yCO2  M x O y  yCO
(4)
The MxOy oxide as formed on the metal surface , becomes a barrier between
the substrate metal and the oxidizing environment.the chemical and physical
properties of the oxide film are of paramount importance in determining
oxidation rate and life of equipment exposed to high temperature oxidizing
environments.
Thermodynamics of oxidation
∆G◦ must be negative in order for reaction to proceed spontonously from left
to right as wirren
G   H   TS 
(5)
When plot ∆G◦ versus temperature produce a straight line with changing
slope when phases form (i.e., at melting or boiling temperatures) to get
Ellingham diagram figure (2).
For reaction metal M with oxygen a departure from standard conditions
M O 
2/ y
gives: GO
2
/ MO
 G

O 2 / MO
 RT ln
x
y
M 2 x / y O2 
(6)
GO 2 / MO  RT ln pO2 , because activities of pure solids is unity
Consider copper oxidation at 900 ◦C , extending a line from O on thr left
scale through free energy line at 900 ◦C for copper to the scale marked
pO2,CuO (blue line in the figure (2)) and this lead he value for pO2,CuO equal 108
atm , and ∆G = - 190 kJ/ mole , that mean any oxygen partial pressure
above 10-8 atm will oxidize pure copper , any below it will reduce pure
copper oxide to pure copper at 900 ◦C.
Similar predictions can be made for atmospheres from mixture of water
vapor and hydrogen, and carbon monoxide and dioxide ,again using the
example of copper , extending a line from point H on left scale through freeenergy line of copper at 900 ◦C to scale H2/H2O ratio shows an equilibrum
ratio of hydrogen to water vapor of 10-4 for a higher ratio cuprous oxide is
reduced to pure copper;below it copper is oxidized , and same to get
CO/CO2 ratio by use point C
Figure (2) Ellingham tandard energy change diagram
Oxide properties
The oxidation rate of any alloy will be minimized if the oxide film has a
combination of favorable properties which include:
1. The films should have good adherence , to prevent flaking and
spalling.
2. The melting point for oxide should be high.
3. The oxide should have low vapor pressure to resist evaporation.
4. The oxide film and metal should have close to the same thermal
expansion coefficients.
5. The film should have low electrical conductivity and low diffusion
coefficients for metal ions and oxygen .
6. The film should have high temperature plasticity to accommodate
differences in specific volumes of oxide and parent metal.
Kinetics of corrosion in gases
Application of metals and alloys in high-temperature environments is of
technological importance since the rate of oxide layer formation,
mechanism, and degree of protectiveness are the subject matter to be
evaluated. Consider the model of oxide formation shown in below Figure in
which a metal is exposed to an oxygen-rich environment at high
temperatures .
The model in Figure predicts that
-The initial rate of oxide formation is determined by the reactions at the
metal/oxygen interface . Thus , MxOy forms as a thin layer.
- Once the thin layer is formed , it serves as a barrier separating or
inslating the metal substrate .The subsequent oxidation steps are controlled
by the diffusion of species M+2 and O-2 through layer
 If O -2 anions diffuse faster then M +2 cations ,the diffusion molar flux
is J O-2 > J M+2 , then an oxide inner layer forms as shown in above
figure (a) as oxide growth proceeds , the volume of oxide is VMxOy >
VM causing a high stress concentration and consequentlythe oxide
layer may rupture .Titanium falls into this category due to J O-2 > J
+2
Ti .
 If M +2 cations diffuse faster than O -2 anions , then J M+2 > J O-2 and
an outer oxide layer forms and proceeds through the concentartion
gradient as shown in figure (b) .In this situation the stress
concentration is reduced or relived .Thus , the oxide layer adheres to
metal surface and protects the metal from further oxidation .Nickel
falls into the category scince J Ni+2 > J O-2
Pilling-Bedworth ratio
Metal oxide scales can be defined as protective and non protective using
classical Pilling-Bedworth law as the ratio given by
PB 
 M M W ,O 
Vo

VM x O M W .M 
Where :
V◦ =volume of oxide scale (cm3)
VM = volume of solid metal (cm3)
ρ M ,ρ ◦= density of metal and metal oxide respectively ( g / cm3)
M wM, Mwo= metal and metal oxide molecular weight respectively (g / mol )
x = is coefficient of metal species in equation (2) , in case of divalent metals
the equation becomes: PB 

M
M W ,O 
 O M W .M 
The PB ratio is used to characterize several oxidation conditions thus
♦ If PB < 1 or PB > 2, the oxide scale is nonprotective (NP) and
noncontinuous due to insufficient volume to cover the metal surface
uniformly. Thus, weight gain is usually linear.
♦ If 1≤ PB ≤ 2 the oxide scale is protective (P), adherent and strong due to
compressive stress, refractory due to high melting temperature, low
electrical conductor, and nonporous. Because of these factors, diffusion
proceeds in the solid state at low rates. Some oxides may not develop
compressive stresses, invalidating PB law .
♦ If PB = 1, then the oxide scale is ideally protective
Table 1 lists several metal / oxide PB ratios and quality
In general , thermal degrdation of metals and alloys , and oxides scales is a is
a major concern in practical applications such as furnce parts , heaters tubes
and like .Degradation at high temperatures may occur isothermally or
cyclically under operating conditions. Therefore, metallic parts must be
protected using a lesser aggressive environments if possible; otherwise,
applying a protective thin film prior to high temperature applications can be
beneficial. on the isothermal and cyclic-oxidation behavior of austenitic
stainless steel, such as AISI 316, 321, and 304, in dry air.
Oxidation rate
One of the primary concerns relative to metal oxidation is the rate at which
the reaction progresses. Inasmuch as the oxide scale reaction product
normally remains on the surface, the rate of reaction may be determined by
measuring the weight gain per unit area as a function of time. When the
oxide that forms is nonporous and adheres to the metal surface, the rate of
layer growth is controlled by ionic diffusion. A parabolic relationship exists
between the weight gain per unit area W and the time t as follows:
W 2  K1t  K 2
where K1 and K2 are time-independent constants at a given temperature.
This weight gain–time behavior is plotted schematically in the below Figure.
The oxidation of iron, copper, and cobalt follows this rate expression.
In the oxidation of metals for which the scale is porous or flakes off (i.e., for
P–B ratios less than about 1 or greater than about 2), the oxidation rate
expression is linear; that is,
W  K 3t
where K3 is a constant. Under these circumstances oxygen is always
available for reaction with an unprotected metal surface because the oxide
does not act as a reaction barrier. Sodium, potassium, and tantalum oxidize
according to this rate expression and, incidentally, have P–B ratios
significantly different from unity (Table 1). Linear growth rate kinetics is
also represented in the figure
Still a third reaction rate law has been observed for very thin oxide layers
(generally less than 100 nm) that form at relatively low temperatures. The
dependence of weight gain on time is logarithmic and takes the form
W  K 4 log( K 5 t  K 6 )
Again, the K’s are constants. This oxidation behavior, also shown in Figure
has been observed for aluminum, iron, and copper at near-ambient
temperatures.
Figure (2) Oxidation film growth curves for linear, parabolic and logarithmic
rate laws
Lecturer
Hameed Hussein