PE/EP 318 CORROSION
ENGINEERING
(3CR)
By:.Dr.Mohamed Saad
Dr. Fathy Shokry
(2 L, 1 T, 2 P)
LECTURE (6) WEEK 3
CORROSION RATE
• There are three main methods that are used to express the
corrosion rate:
a)Thickness reduction of the material per unit time.
• In the metric system this measure is usually expressed in
mm/year.
b) Weight loss per unit area and unit time.
• Weight loss per unit area and unit time was commonly used in
earlier times, mainly because weight loss was usually the
directly determined quantity in corrosion testing. Here the
test specimens were weighed before and after the exposure
to the corrosion medium.
CORROSION RATE
C) Corrosion current density.
• Corrosion current density is a particularly suitable measure of corrosion rate when
treating corrosion theory and in connection with electrochemical corrosion testing.
• Common Corrosion Rate Units
• – gmd (grams of metal lost per square meter per day)
• – mm/y (average millimeters penetration per year)
• – mpy (avg. milli inches penetration per year, 1 mili = 0.001 in)
EXAMPLE 2
A carbon steel test specimen of dimensions 2-in × 3-in × 0.125-in with a 0.25-in hole
for suspending in solution is exposed for 120 hours in an acid solution and loses 150
milligrams. Calculate the corrosion rate in gmd, mpy and mm/y. Given that the density
of carbon is 7.87 g/cm3
SOLUTION
• Area = 2* (A1+A2+A3)+A4.
• A1 = 0.125*2 = 0.25 in2
• A2 = 0.125*3 = 0.375 in2
• A3 = (2*3) – 3.14 *(0.25)2/4=5.95 in2
• A4 = 3.14 *(0.25)*0.125= 0.0981in2
Total area = 13.24 in2
• Corrosion rate = weight loss/ area. Time
• C.R =
150∗39.372 ∗24
13.24∗120∗1000
= 3.51 gm/m2.day
• Assignment repeat for mpy and mm/y(6.42 mpy
&0.1625 mm/y)
• The relationship between inch and cm is that one inch is exactly
equal to 2.54 cm
CALCULATING CORROSION RATE USING
FARADAY’S LAW
• The flow of charge (i.e., electrons) is a measure of the reaction rate (metal
dissolution or corrosion rate).
• Thus, if the corrosion “current” can be measured, the corrosion rate is directly
evaluated through Faraday’s Law
FARADAY’S LAW
M wt  I
m
nF
•
•
•
•
•
•
•
•
•
m = mass losses or metal oxidized (g)
Mwt = atomic or molecular weight (g/mol);
I = Icorr* t *A (coulomb) or (ampere.sec);
Where Icorr=corrosion current density (ampere /m2)
m M wt  I corr
A = the area of the specimen (m2)
CR 

t = time
t. A
n  F.
n = electrons transferred in the half-cell reaction;
F = Faraday constant (96485 Amps.sec/mol).
FARADAY’S LAW
• Say the imeasured = 10-5 (amp/cm2)
• Using Faraday’s Law the corrosion rate is calculated as:
56 𝑔 𝑚𝑜𝑙 × 10−5 𝐶/(𝑠. 𝑐𝑚2
𝐶𝑅 =
= 2.9 × 10−9 𝑔 𝑐𝑚2 𝑠
2 × 96485 𝐶 𝑚𝑜𝑙
• Columbus is Amp.sec
• Or in a more useable unit … (divide by the density of Fe which = 7.86 g/cm3; convert
cm to mm and seconds to years)
CR  116 m / yr  0.12 mm / yr
POURBAIX DIAGRAMS
• Pourbaix diagram (Electrode potential / pH
diagram) is a graphical presentation of the thermodynamic
equilibrium states of a metal-electrolyte system.
• Pourbaix diagrams are plotted in the axes Electrode potential
of the metal vs. pH of the electrolyte.
• Oxidizing conditions are described by the top part of the
diagram (high positive electrode potential).
• Reducing conditions are described by the bottom part of the
diagram (high negative electrode potential).
POURBAIX DIAGRAMS
• Acidic solutions are presented in the left side of the diagram
(pH lower than 6).
• Alkaline solutions are presented in the right side of the
diagram (pH higher than 6).
• The lines of the diagrams dividing different zones of the
equilibrium states are calculated by the Nernst equation.
• Pourbaix diagrams is a potential vs. pH diagram allow to
determine the corrosion behavior of a metal in water
solutions i.e. the direction of electro-chemical processes and
the equilibrium state of the metal at a certain electrode
potential in a water solution at a certain value of pH.
POURBAIX DIAGRAMS
As shown in the opposite Fig., there
are four regions in the diagram
corresponding to:
•
oxidizing (acidic),
•
oxidizing (alkaline),
•
reducing (acidic) and
•
reducing (alkaline)
environments.
POURBAIX DIAGRAM FOR WATER
• Thermodynamic stability of
water, oxygen, and hydrogen.
2H2O = O2 + 4H+ + 4eEquilibrium potential
falls as 2.0
pH increases
• (A is the Equilibrium line for
the reaction:
O2 is stable
1.2
0.8
Potential
• H2 = 2H+ + 2e-.
1.6
0.4
H2O is stable
0.0
2H+ + 2e- = H2
Equilibrium
potential falls as
pH increases
-0.4
• B is the equilibrium line for
the reaction: 2H2O = O2 + 4H+
+ 4e-.
-0.8
H2 is stable
-1.2
-1.6
0
7
14
POURBAIX DIAGRAM FOR IRON
• The diagram defines the following
zones of the equilibrium states:
• below the line a-b-j: Solid iron
(immunity zone).
• The electrochemical reactions in this zone
proceed in the direction of reduction of iron
ions. No corrosion occurs in this zone.
• a-b-n-c-d-e: Aqueous solution of ion
Fe2+ (corrosion zone). Metallic iron
oxidizes in this zone.
• e-d-f-g-k: Aqueous solution of ion
Fe3+ (corrosion zone). Metallic iron
oxidizes (corrodes) in this zone.
POURBAIX DIAGRAM FOR IRON
• h-f-g-m: Aqueous solution of ion FeO42(corrosion zone).
• c-d-f-h-i: Solid ferrous oxide Fe2O3 (passivation
zone). Iron oxidizes (corrodes) in this zone however
the resulted oxide film depresses the oxidation
process causing passivation (corrosion protection of
the metal due to formation of a film of a solid
product of the oxidation reaction).
• n-c-i-p: Solid oxide Fe3O4 (Fe2O3*FeO)
(passivation zone). The oxide film causes
passivation.
• b-n-p-j: Solid hydroxide (II) Fe(OH)2 /
FeO*nH2O / green rust (passivation zone).
LIMITATIONS OF POURBAIX DIAGRAMS:
• The diagrams provide no information about the kinetic parameters of
the corrosion reactions.
• The diagrams consider pure metals and aqueous solutions at standard
conditions (temperature 298K, pressure 1 bar, ion concentration 10-6M).
Thermodynamic conditions of corrosion for alloys and for non-standard
conditions differ from those described by Pourbaix diagrams.
• The diagram do not take into account non-ideal behavior of aqueous
solutions.