CORROSION
Dec 14, 2015
What is Corrosion?
Definisi:
•Kerusakan pada material karena reaksi kimia atau
elektrokimia dengan lingkungan
•Korosi adalah proses oksidasi logam karena reaksi
elektrokimia Unsur oxidiser adalah O2 (atmospheric
corrosion) atau H+ (chemical corrosion) atau keduanya.
•Fenomena alamiah yang terjadi sepanjang waktu
•Happens at different rates with different metals and in
different environments
Why is it a problem?
Financial - $350 Billion Dollar Annual Problem in U.S.
(4.25% of GNP) Department of Defense spends $6 –
8 Billion
Example of Stress Corrosion
Aloha Flight 243 (28 APR 1988)
• 1988
• 19-year old Boeing 737
operated by Aloha Airlines
lost a major portion of the
upper fuselage in full flight at
24000 ft
The “pillowing” process in
which the faying surfaces are
forced apart
It has been estimated that at least 3000 people died as a result of this accident, while
figures for the number of people injured currently range from 200,000 to 600,000,
with an estimated 500,000 typically quoted.
Twelve persons who were camping under a concrete-decked steel bridge that
supported the pipeline across the river were killed and their three vehicles
destroyed. Two nearby steel suspension bridges for gas pipelines crossing the
river were extensively damaged with $1 million in property and other damages or
losses
Interconnecting pits were observed on the inside of the pipe in the ruptured area.
Typically, these pits showed the striations and undercutting features that are often
associated with microbial corrosion. A pit profile showed that chloride
concentration in the pits increased steadily from top to bottom.
Piping rupture caused by flow accelerated corrosion occurred at Mihama-3 at
3:28 pm on August 9, 2004, killing four and injuring seven
Although the carbon steel pipe carried the high temperature steam at high
pressure, it had not been inspected since the power plant opened in 1976. In April
2003, Nihon Arm, a maintenance subcontractor, informed Kansai Electric Power
Co., the plant owner, that there could be a problem, following which the power
company had scheduled an ultrasonic inspection for August 200
Recipe for corrosion
Active metal
Water
Oxygen
(atmospheric corrosion)
Acid
(chemical corrosion)
Salt
High temperature
Basics of Corrosion
Corrosion is essentially the oxidation of metal
Need:
1. An Anode (where oxidation is taking place)
2. A Cathode (where reduction is taking place)
3. Conductive electrolyte
4. Electrical contact between the Anode and Cathode
Source: Moore, J.J. Chemical Metallurgy
Electrochemistry
• Corrosion is an electrochemical reaction
– ½ reaction at the anode: M Mn+ + ne-
– Possible ½ reactions at the cathode:
2H+ + 2e- H2
Acid Solutions: H2O + e- ½ H2 + OH-
½ O2 + 2H+ + 2e- 2OH-
• Important thing to note is the flow of electrons
Thermodynamic Driving Force
• Like all chemical reactions – Thermodynamics
• What is the driving force for the reaction? (otherwise stated as what is the electrochemical potential for the reaction) – Dissimilar metals
– Different cold work states
– Different grain sizes
– Difference in local chemistry
– Difference in the availability of species for a reaction (concentration cells)
– Differential aeration cells
Tipe Korosi
• Uniform corrosion
• Nonuniform corrosion
• Stain corrosion
• Pitting corrosion
• Point corrosion
• Undersurface corrosion
• Selective corrosion
• Intergranular corrosion
• Transgranular corrosion
• Extraction corrosion
• Crack of corrosion
Uniform Corrosion
Uniform corrosion
• This type of corrosion includes the commonly
recognized rusting of iron and other metals.
Pitting Pitting corrosion
• 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. Depth of pitting is sometimes
expressed by the pitting factor , the ratio of deepest metal penetration to
average metal penetration as determined by the weight loss of the
specimen. A pitting factor of unity represents uniform attack.
Iron buried in the soil
corrodes with formation of
shallow pits, whereas
stainless steels immersed in
seawater characteristically
corrode with formation of
deep pits.
Erosion-CorrosionErosion corrosion
• when subjected to high - velocity liquids,
undergo a pitting type of corrosion called
impingement attack , or erosion corrosion .
• Copper and brass condenser tubes, for
example, are subject to this type of attack.
Fretting CorrosionFretting corrosion
• Fretting corrosion , which results from slight relative motion
(as in vibration) of two substances in contact, one or both
being metals, 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
Cavitation erosion Cavitation Erosion
• 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 — for example, in rotors of pumps or on trailing
faces of propellers. This type of corrosion causes a
sequence of pits
Selective corrosion Selective Corrosion
• 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 alloy so corroded often retains its original shape, but its tensile strength and ductility are seriously reduced. Dezincified brass pipe may retain sufficient strength to resist internal water pressures until an attempt is made to uncouple the pipe, or a water hammer occurs, causing the pipe to split open.
• Copper - base alloys that contain aluminum are subject to a form of corrosion resembling dezincification, with aluminum corroding preferentially.
• Selective corrosion can be
found in alloys and resulting
from the fact that the
components of the alloy
corrode at different rates.
Intergranular Corrosion Intergranular Corrosion
– corrosion along grain boundaries at microscopic level.Grain – boundary material of limited area, acting as anode, is in contact with large areas of grain acting as cathode. The attack is often rapid, penetrating deeply into the metal and sometimes causing catastrophic failures.
– stainless steels and heat treated high-strength steels
– carbides precipitate along grain boundaries leaving these areas with no alloyed Chromium
– Welds can have this same depletion effect
This is a localized type of attack at the grain boundaries of a metal, resulting in loss of strength and ductility.
Corrosion Fatigue Corrosion Fatigue
• In the absence of a corrosive environment, the metal stressed similarly, but at values below a critical stress, called the fatigue limit or endurance limit , will not fail by fatigue even after a very large, or infinite, number of cycles. A true endurance limit does not commonly exist in a corrosive environment: The metal fails after a prescribed number of stress cycles no matter how low the stress.
If a metal cracks when subjected to repeated or alternate tensile stresses in a corrosive environment, it is said to fail by corrosion fatigue.
Galvanic Corrosion Galvanic Corrosion
• Galvanic corrosion (also called “dissimilar metal corrosion”) refers to corrosion damage induced when two dissimilar materials are coupled in a corrosive electrolyte. In a bimetallic couple, the less noble material becomes the anode and tends to corrode at an accelerated rate, compared with the uncoupled condition and the more noble material will act as the cathode in the corrosion cell.
Galvanic Corrosion
(contact between unlike metals;
opposite of cathodic protection)
Copper Iron
H2O O2
Cathode:
O2 + 2H2O + 4e- → 4OH-
Anode:
Fe → Fe2+ + 2e-
Crevice Corrosion Crevice Corrosion
• narrow crevice filled with ionized solution
• Oxygen-rich on the outside, oxygen-poor on the
inside metals oxidize with salt anions
• FeCl2 and pH rises in cathodic zone
• H+ may destroy passivity
STRESS CORROSION CRACKING
• Stress Corrosion Cracking
– tensile stress and corrosive environments
– cracks are initiated at corrosion areas
– tensile stresses propagate the crack
– corrosion further deteriorate crack
– etc.....
Steel Corrosion
2 2 22 2 2Fe O H O Fe OH ( )
21
222 2 2 3Fe OH O H O Fe OH( ) ( )
Initial Oxidation Reaction
Secondary Oxidation Reaction
Rust
• Passivity barrier breaks down
• Presence of Oxygen
• Moisture
Chemical vs. Atmospheric Corrosion (H+ vs. O2)
Anodic Reaction:
Fe0(s) Fe2+
(aq) + 2e- Deterioration of metal
Cathodic Reaction:
2H+(aq) + 2e- H2 (g) Chemical
O2 (g) + 2H2O (l) + 4e- 4OH-(aq) Atmospheric
O2 (g) + 4H+(aq) + 4e- 2H2O (l) Combination
Which of these will oxidize copper? Silver? Gold?
Overall Reaction:
Fe0(s) + 2H+
(aq) Fe2+(aq) + H2 (g) Chemical
2Fe0(s) + O2 (g) + 2H2O (l) 2Fe2+
(aq) + 4OH-(aq) Atmospheric
2Fe0(s) + O2 (g) + 4H+
(aq) 2Fe2+(aq) + 2H2O (l) Combination
Eo red
(V)
Eocell (V)
0.00
+0.40
+1.23
+1.67
+0.84
+0.44
Cu(s) Cu2+(aq) + 2e-
- 0.44
+0.80
Why won’t iron corrode in pure (degassed) water?
Anodic Reaction:
Fe0(s) Fe2+
(aq) + 2e-
Cathodic Reaction:
Eo (V)
-0.44
-0.83
Fe0(s) + 2H2O (l) Fe2+
(aq) + H2 (g) + 2OH-
(aq)
Eocell (V)
-0.39
What metals will corrode in pure (degassed) water?
Any sufficiently active metal Eored < -0.83 V
(alkali metals, alkaline earth metals, aluminum, manganese)
Overall reaction:
2H2O (l) + 2e- H2 (g) + 2OH-(aq)
Example of Atmospheric Corrosion
Corrosion on wing of Navy aircraft
Why does corrosion of an airplane
occur primarily while the plane is on
the ground?
How might this corrosion be
minimized?
F/A-18C Hornet
Example of Chemical Corrosion
Nuclear Reactor Vessel Head Degradation
• February 16, 2002, Davis-Besse Nuclear Power Station in Oak Harbor, Ohio
Boric Acid leak from control rod
drive mechanism led to chemical
corrosion of reactor vessel head
Serious potential for loss of
reactor coolant access
Corrosion of a Ship’s Hull Anodic and Cathodic Regions
O2
O2 + 2H2O + 4e- 4OH-
OH-
Fe2+
Fe2+ + 2OH- Fe(OH)2
4Fe(OH)2 + O2 2(Fe2O3·H2O) + 2H2O
Fe Fe2+ + 2e-
Hull of ship
Cathodic Region
RUST
Anodic Region
Electrons Migrate from
Anodic to Cathodic Region
e-
Avoiding Corrosive Situations
• Choose couple metals close on the galvanic
series
• Use large anode, and small cathode areas
• Electrically insulate dissimilar metals
• Connect a more anodic metal to the system
• Avoid turbulent flow and impingements in pipe
systems
Preventing Corrosion
Applications of Cathodic Protection
• Galvanized Steel
Zinc coating
• Sacrificial Anodes
Ship Hulls
Subs (free flooding areas)
Los Angeles Class Sub
Arleigh-Burke Destroyer
Impressed Current Cathodic
Protection
Power Supply
Shipboard Power
Controller
Shipboard Power
Insulation
Pt Anode
Reference
Electrode
Paint
Layer
Hull
e- e-
Surface treatment
• katodic protect (e.g. zinc coatings in steel)
• we can enrich surface of metal by alloying elements (e.g. hot chromizing, alumining)
• coating isolate protect surface against corrosion medium (e.g. corrosion protective paint and plastics, coatings from tin in steel)
• synthetic compound layer of protect metal has better protective properties (e.g. phosphated coatings in steel, oxidized coatings in aluminium)
• layer of matter has inhibic effect for protective metal (e.g. chromate treatment of zinc, inhibic basic paint)