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Engineering Materials
2189101
Chedtha PuncreobutrDepartment of Metallurgical Engineering
Chulalongkorn University
Corrosion of Materials
http://pioneer.netserv.chula.ac.th/~pchedtha/
Things we've learnt about oxidation
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• In dry condition, most materials that are unstable in oxygen tend to oxidized
• Oxidation is controlled by diffusion of ions or the conduction of electrons through oxide films
• Diffusion is thermally activated process thus rate of oxidation is much greater at high temperature
• Loss of materials by oxidation at room temperature under dry conditions is very slight
Durability of materials
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Degradation: Reduction in performance of componentduring lifetime
Mechanical processes
Chemical processes
Creep
Oxidation
Electrochemical Corrosion
Fatigue
Wear
Electrochemical (“wet”) Corrosion
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• It can simply occur due to water in atmosphere
• Also it can be promoted by• acids, alkalis, and salts• sea water• presence of other
materials
Comparing to oxidation, the only real difference is the medium that causes the process. Oxygen causes oxidation, while corrosion is the term applied to a similar electrochemical process caused by many other atoms and molecules.
Corrosion under wet conditions
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Electrochemical (“wet”) Corrosion
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Economic impact: ~3.5% of GDP in developed countries
Direct losses
Component replacement
Painting and other preventative measures
Use of expensive corrosion resistant materials
Indirect losses
Plant shutdown
Loss of product e.g. in pipes
Loss of efficiency
Contamination of product
Without prevention, corrosion can cause catastrophic
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1992 Guadalajara explosions
• Underground humidity caused these materials to create an electrolytic reaction, Steel gas pipe then corroded, creating a hole in the pipeline that permitted gas to leak into the ground and into the main sewer pipe.
• 252 people were killed• 500 injured• 15,000 were left
homeless. • Monetary damage
ranges between $300 million and $1 billion.
Wet Corrosion
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Materials in aqueous solutions tend to form ions
𝑀 → 𝑀++ + 2𝑒−
Fe Fe++2e-
Fe Fe++2e-
Abraded Iron Aerated water
Pass into the water Leaving behind electrons
Wet Corrosion
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Fe Fe++
2e-
Fe Fe++
2e-
Abraded Iron Aerated water
The electrons are then conducted through the metal to a place where oxygen reduction reaction can take place to consume electrons (cathodic reaction)
𝑂2 + 2𝐻2𝑂 + 4𝑒− → 4𝑂𝐻−Cathode(Reduction)
Anode(Oxidation)
This reaction generates OH- ions
Wet Corrosion
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𝑂2 + 2𝐻2𝑂 + 4𝑒− → 4𝑂𝐻−
Fe Fe++
Fe Fe++
Abraded Iron Aerated water
𝐹𝑒 𝑂𝐻 2
𝑖. 𝑒. 𝐹𝑒𝑂 ∙ 𝐻2𝑂
conductive solution(electrolyte)
The OH- ions then combine with Fe++ ions to form a hydrated iron oxide Fe(OH)2
Cathode(Reduction)
Anode(Oxidation)
Material + Oxygen (Hydrated) Material Oxide
Wet Corrosion
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• The Fe(OH)2 either deposits away or loosely deposits on the
surface, giving little or no protection
• Very rapid diffusion as M++ and OH- usually diffuse in liquid
• In conducting materials, electrons can move very easily as well
Much faster attack
found in wet corrosion
compared to dry air
Voltage differences as the driving force
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The tendency of a metal to oxidized in solution (wet corrosion) is described by using a voltage scale rather than energy
• Oxygen is reduced• Absorbing electrons• Metal becomes positively charged• Reaction continues until potential raise to +0.8V
• Fe++ forms• Leaving electrons behind• Acquires negatively charged• Reaction continues until potential fall to -0.6V
Voltage differences as the driving force
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The tendency of a metal to oxidized in solution (wet corrosion) is described by using a voltage scale rather than energy
• If the anode and cathode are connected, electron flow from one to the other.
• The difference in voltage of 1.4V is driving force for oxidation reaction
Bigger voltage difference leads to bigger tendency to corrode
Pourbaix (Electrochemical equilibrium) Diagram
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The diagrams are maps that show the conditions under which a metal:
• Cannot corrode (immunity)—because there is no voltage driving force, or a negative one
• May corrode (corrosion)—because there is a voltage driving force, and a stable oxide film does not form on the surface
• May not corrode (passivation)—although there is a voltage driving force, a stable oxide film forms on the surface (this may or may not be an effective barrier to corrosion)
Corrosion in Copper
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• A protective film has
formed which is why the
colour is darker than
freshly polished copper
• Bright green colour in old
architecture because the
carbon dioxide in the air
forms a copper carbonate
film
• Copper is used as water pipe because corrosion rate is extremely low when
water is neutral or alkaline (oxide film acted as an effective barrier)
• If the water is mildly acidic, corrosion “pinholes” can form in the tube wall
Copper cladding
Corrosion in Copper alloys
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• Brass (copper and zinc) and Bronze (copper and tin) alloys
are resistant to corrosion
Brass fire hydrant
Corrosion in Steel
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• To have stable surface film, the pH of the water needs to be above 9• so in most applications
(neutral or slightly acidic water) steel will rust unless it is protected.
• Making steel resistant to corrosion is to alloy it with foreign elements• Stainless steel - adding
18% chromium produces an invisible film of Cr2O3
• Another way is to keep any moisture and/or oxygen away from the surface of the steel (e.g., using paint or epoxy coatings).
Corrosion in Aluminium
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• Between pH 4 and 8.5, a thin
and very stable film of
hydrated aluminium oxide
forms, protecting the metal
• The corrosion rate of
aluminium in pure water is
extremely low but over time
there is a tendency for attack
to occur at weak points in the
oxide film
Protecting aluminium by anodizing it
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A surface treatment that artificially thickens the oxide film to make it even more protective
The film can even be coloured for decorative purposes, by adding colouring agents to the bath toward the end of the process
Standard Electrode Potentials
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• The voltage when the metal is in equilibrium with a solution of its ions having a concentration of 1 mol per liter
• The same voltages as those shown in the Pourbaix diagrams for the horizontal lines at the top of the immunity field when there is no oxide film
Standard Electrode Potentials
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The driving force for corrosion can be estimated by subtracting the potential for the metal from that for the oxygen reduction reaction.
Gold, platinum, and palladium are not corroded as there is no voltage driving force.
Corrosion Type
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Pitting Corrosion
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Preferential attack starts at breaks or weaknesses in the oxide film
Intergranular attack and stress corrosion cracking
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Intergranular attack can occur when the grain boundaries have a lower corrosion resistance than the grains themselves.
stress corrosion cracking, a critical combination of stress, material, and corrosiveenvironment can lead to cracks forming and growing under static stress
Protecting ships’ hulls from corrosion
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Magnesium sacrificial anode bolted to the steel hull just above the keel to prevent corrosion
Sacrificial protection (Cathodic protection)
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The magnesium becomes the anode and corrodes. The iron becomes the cathode, where the oxygen reduction reaction takes place, and does not corrode.
Typical materials used are Mg, Al, and Zn.
Corrosion in reinforced concrete
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Spalled reinforced concrete
Corrosion in reinforced concrete
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Volume of rust produced is greater than the volume of steel that is lost.
• density of iron oxide is less than that of steel
• rust deposits are hydrated (FeO•H2O) and full of voids, their density is usually much less again
So if the steel reinforcement bars inside the concrete rust, they “expand,” and crack open the surrounding concrete.
This occurs when there is not enough thickness of concrete over the steelwork (“cover”) to protect it from the environment, or when there is a lot of chloride ion around (immersion in seawater or salt-water spray in the air)