CORROSION

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

High pH (> 9)

Salt

Preventing the Corrosion of Iron

(cathodic protection/sacrificial anode)

Sacrificial anode

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)

Design Generally speaking we want a design that: • Avoids entrapment of water.

• Allows access for surface preparation and coating operations.