Basics on Corrosion in Wastewater Collection and Treatment Systems
Basics of Corrosion 2
-
Upload
mohammed-suleman -
Category
Documents
-
view
25 -
download
6
description
Transcript of Basics of Corrosion 2
WELCOME TO ME 575(Advanced Corrosion Engineering)
(6.30~7.45 PM / MW)
Instructor: Dr. Ihsan-Ul-Haq Toor
Office: 63-358/Phone:7493
Office Hours: 12.30~1.30 (UTR)
or appointment via E-mail:
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion process requires a complete corrosion cell/circuit, which
includes;
How Corrosion Takes Place?
Electrochemical Cell
1. Anode
2. Cathode
3. Electrolyte
4. Electrical path
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Anodic reactions/Oxidation reaction
General Reaction (metal oxidation)
MMn+ + ne-(gives off electrons))
Zn Zn2+ + 2e- (Zn corrosion)
Fe Fe2+ + 2e- (Fe corrosion)
Al Al3+ + 3e- (Al corrosion)
Fe2+ Fe3+ + e- (Ferrous ion oxidation)
How Corrosion Takes Place?
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Cathodic reactions/reduction reaction:
How Corrosion Takes Place?
O2 + 2H2O + 4e- → 4OH- (oxygen reduction in water/bases)
O2 + 4H+ + 4e- → 2H2O (oxygen reduction in acids)
2H2O + 2e- → H2 + 2OH- (hydrogen evolution in
water/bases)
2H+ + 2e- → H2 (hydrogen evolution in acids)
Cu2+ + 2e- → Cu (metal deposition=>copper plating)
Fe3+ + e- → Fe2+ (metal/ferric ion reduction)
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
How Corrosion Takes Place?
Fig. 2.4 Coupled electrochemical reactions
occurring at different sites on the same metal
surface for iron in an acid solution. The
electrons lost by the oxidation of Fe atoms
are consumed in the reduction of two H+
ions to form hydrogen gas (H2)
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
How Corrosion Takes Place?
Fig. 2.5 The heterogeneous nature of a metal surface
showing various types of imperfections
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
How Corrosion Takes Place?
Fig. 2.6 Coupled electrochemical reactions occurring at different sites
on the same metal surface for iron in a neutral or a basic solution
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Consider what happens when Zn is placed in aerated dilute HCl solution (acidic solution):
Electrochemical nature of Corrosion
CR↑ or ↓?
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
How Corrosion Takes Place?
Atmospheric Corrosion & Relative Humidity
Fig. 2.8 Corrosion of iron in air containing 0.01% SO2 after 55
days of exposure showing the effect of a critical relative
humidity (approximately 60%). Redrawn from Vernon [3] by
permission of the Royal Society of Chemistry
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
How Corrosion Takes Place?
Atmospheric Corrosion & Relative Humidity
Fig. 2.9 Adsorption isotherms for water vapor on α-Fe2O3
[4] showing that multi molecular layers of adsorbed water
are formed at a relative humidity of 60% and higher
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Secondary Effects of Cathodic Reactions
How Corrosion Takes Place?
During SCC=> hydrolysis of Fe ions decrease in pH=>adsorption of
hydrogen atoms and Hydrogen embrittlement
Fig. 2.10 Crack tip reactions can produce hydrogen atoms
available for migration into the metal at stressed regions ahead
of the crack tip
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Secondary Effects of Cathodic Reactions
How Corrosion Takes Place?
CR in basic solutions=> O2 reduction OH- formation=>accumulation will
increase pH more alkaline solution more corrosion of some metals
Fig. 2.11 Aluminum has high corrosion rates for both acidic (low pH) and basic
(high pH) solutions [5]. Reproduced by permission of ECS – The
Electrochemical Society
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Types of Cells
1: Dissimilar Electrode Cells/Galvanic cell
A potential difference exists when two dissimilar metals, electrically connected, are
immersed in a corrosive solution. A cell that produces electricity as a result of the
spontaneous cell reaction.
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Types of Cells
2: Concentration Cells
-Difference in the composition/salt content of the soil or solution
-Change in oxygen concentration
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Types of Cells
3: Electrochemical Cells
A cell in which non spontaneous reaction is
driven by an external power source.
For example: Electrolysis
M+ M
m+
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
ActivePassive
Behavior of Active and Passive metals
Corrosion of Metals
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion Types
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion Types
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion Types
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion Types
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Corrosion Types-Uniform/General
Rates of uniform attack are reported in various units;
millimeters penetration per year (mm/y); grams per
square meter per day (gmd), inches penetration per
year (ipy), mils (1 mil = 0.001 inch) per year (mpy),
and milligrams per square decimeter per day (mdd).
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Metals are classified into three groups according to their corrosion rates and
intended application.
A. < 0.15 mm/y ( < 0.005 ipy) — Metals in this category have good corrosion
resistance to the extent that they are suitable for critical parts, for example,
valve seats, pump shafts and impellors, springs.
B. 0.15 to 1.5 mm/y (0.005 to 0.05 ipy) — Metals in this group are satisfactory
if a higher rate of corrosion can be tolerated, for example, for tanks, piping,
valve bodies, and bolt heads.
C. > 1.5 mm/y ( > 0.05 ipy) — Usually not satisfactory.
Corrosion Types-Uniform/General
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
This is a localized type of attack, with the rate of corrosion being greater at
some areas than at others.
If appreciable attack is confined to a relatively small, fixed area of metal, acting
as anode, the resultant pits are described as deep.
If the area of attack is relatively larger and not so deep, the pits are called
shallow.
Corrosion Types-Uniform/General
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
FC is the result of “slight relative motion (as in
vibration) of two substances in contact, one or
both being metals.
It usually leads to a series of pits at the metal
interface.
Metal - oxide debris usually fills the pits so that
only after the corrosion products are removed do
the pits become visible.
Corrosion Types- Fretting
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Cavitation – erosion is the loss of material
caused by exposure to cavitation, which is the
“formation and collapse of vapor bubbles” at a
dynamic metal – liquid interface.
Example, in rotors of pumps or on trailing faces
of propellers.
Corrosion Types- Cavitation
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
Dealloying is the “selective removal of
an element from an alloy by corrosion”.
One form of dealloying, dezincification,
is a type of attack occurring with zinc
alloys (e.g., yellow brass) in which zinc
corrodes preferentially, leaving a porous
residue of copper and corrosion
products.
The tensile strength and ductility are
seriously reduced.
Corrosion Types- Dealloying
Copyright @ Dr. I. H. ToorADVANCED CORROSION ENGINEERING
This is a localized type of attack at the grain boundaries of a metal, resulting in
loss of strength and ductility.
Improperly heat - treated 18 - 8 stainless steels or Duralumin - type alloys (4%
Cu – Al) are among the alloys subject to intergranular corrosion.
Corrosion Types- Intergranular