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Engineering Materials

2189101

Chedtha PuncreobutrDepartment of Metallurgical Engineering

Chulalongkorn University

Oxidation of Materials

http://pioneer.netserv.chula.ac.th/~pchedtha/

Things we've learnt about materials

22189101 – Engineering MaterialsChedtha Puncreobutr

• Price and availability of materials

• Stress, strain and modulus

• Bonding between atoms

• Elastic and non-elastic behaviours

• Yield strength• Plastic instability

Durability of materials


Degradation: Reduction in performance of componentduring lifetime

Mechanical processes

Chemical processes

Creep

Oxidation

Electrochemical Corrosion

Fatigue

Wear

What is Oxidation?


Ananta Samakhom PalaceThe Statue of Liberty

Oxidized copper often develops a greenish coating or patina after years of exposure to air and water.

What is Oxidation?


Metals become rusty

In the case of iron, the oxygen creates a slow burning process, which results in the brittle brown substance we call rust. We often used the words oxidation and rust interchangeably

rusty I-beamoxidized metal gears

Oxidation in nature


freshly-cut apple turns brown

Bauxite

The world's main source of Aluminium

Fresh Fruit

Ruby

Corundum is a crystalline form of aluminium oxide (Al2O3) with trace elements

(300 GJ/tonne to extract Al from Bauxite)

(skin usually provides a barrier against oxidation)

What is Oxidation?


• Oxidation is defined as the interaction between oxygen

molecules and all the different substances they may contact,

from metal to living tissue.

• More precisely, oxidation is defined as the loss of at least

one electron when two or more substances interact.

• Those substances may or may not include oxygen.

• The opposite of oxidation is reduction — the addition of at

least one electron when substances come into contact with

each other

Oxidation reaction


An oxidation reaction

Metal M becomes an n+ positively charged ion and in the process loses its n valence electrons e-

𝑀 → 𝑀𝑛+ + 𝑛𝑒−

The site at which oxidation takes place is called the anode.Oxidation is sometimes called an anodic reaction.

𝐹𝑒 → 𝐹𝑒2+ + 2𝑒−

𝐴𝑙 → 𝐴𝑙3+ + 3𝑒−

Stainless steel VS Plain-carbon Steel


Regular steel may be painted for protection against oxidation, but oxygen can still exploit any opening, no matter how small

1936 Deluxe Ford Sedan having a body that is made entirely of unpainted stainless steel

Which materials best resist oxidation?


• Turbine blades do oxidise in service and react with H2S, SO2

and other combustion products. • How can resistance to gas attack be improved?

Energy of oxidation


This tendency of many materials to react with oxygen can be quantified by laboratory tests that measure the energy needed for the reaction

• If this energy is positive -> the material is stable

• if negative -> material will oxidize

𝑀𝑎𝑡𝑒𝑟𝑖𝑎𝑙 + 𝑂𝑥𝑦𝑔𝑒𝑛 + 𝐸𝑛𝑒𝑟𝑔𝑦 → 𝑂𝑥𝑖𝑑𝑒 𝑜𝑓 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙

Energy of oxidation


Energies of formation of oxides at 273 K in kJ mol-1

of oxygen O2.

Rates of Oxidation


Vital to know how fast oxidation process is going to be when designing with oxidation-prone materials

Rates of Oxidation


Aluminium vs Iron

In fact, aluminium oxidised much more slowly than iron, why?

-1045 > -508

Rates of Oxidation


IronOxygen in air reacts with iron at the surface, creating a thin layer of iron oxide on the surface -> iron turns black

Rates of Oxidation


IronAt first, layer grows in thickness very quickly

Rates of Oxidation


IronThen more slowly.

Rates of Oxidation


IronThis is because iron atoms now have to diffuse through the film before they react with oxygen

Rates of Oxidation


IronDropping into liquid, shock of quenching breaks off the iron oxide layer

Iron

Rates of Oxidation


IronIf heats it up again, oxidization occur at the old (fast) rate

Iron

Therefore, oxide film acts as BARRIER which keeps oxygen and iron atoms apart and cut down rate at which these atoms react to form more iron oxide

Effective oxide barrier


Iron

• Aluminium and most other materials form oxide barrier layers in just the same sort of way as Iron

Aluminium

• The oxide layer on aluminium is a much more effective barrier than the oxide film on iron is. Therefore the aluminium oxidised much more slowly than iron

Measurement of oxidation rates


• Oxidation proceeds by the

addition of oxygen atoms to

the surface of the material

• The weight of the material

usually goes up in proportion

to the amount of material

that has become oxidized

So we can measure the relationship between weight change (m) and time t

Oxidation behaviour


Two types of behaviour

• Linear oxidation

• Parabolic oxidation

∆𝑚 = 𝑘𝐿𝑡

∆𝑚 2 = 𝑘𝑃𝑡

where k = kinetic constant

where k = kinetic constant

Oxidation rate and Arrhenius’s law


Oxidation rates follow Arrhenius’s law

𝑘𝐿 = 𝐴𝐿𝑒−𝑄𝐿/ 𝑅𝑇

𝑘𝑃 = 𝐴𝑃𝑒−𝑄𝑃/ 𝑅𝑇

Rate of oxidation increases exponentially withincreasing temperature

Also, oxidation rates increase with increasing partial pressure of oxygen

Time for materials to be oxidized


Time in hours for material to be oxidized to a depth of 0.1 mm at 0.7 TM in air

No correlation between oxidation rate and energy required for reaction

Energies required to form Aluminium oxides (Al2O3) = -1045 kJ and Tungsten oxide (WO3) = -510 kJ. But Aluminium is oxidized at much slower rate.

Micromechanisms


Oxidation reaction

𝑀 + 𝑂 → 𝑀𝑂

The reaction goes into 2 steps :

M = oxidizing material O = oxygen

𝑀 → 𝑀++ + 2𝑒−

𝑂 + 2𝑒− → 𝑂−−

M forms an ion, releasing electrons

Electrons then absorbed by oxygen to give an oxygen ion

Metal Oxide Air

Parabolic oxidation behaviour


M++ diffuses very slowly in oxide. Oxide grows at metal–oxide interface. Examples: Ti, Zr, U

O-- diffuses very slowly in oxide. Oxide grows at oxide–air interface.Vacancies form between metal and oxide.Examples: Cu, Fe, Cr, Co

Electrons move very slowly. Oxide can grow (slowly) at metal – oxide interface or oxide–air interface depending on whether M++

diffuses faster than O-- or not.Example: Al

Fick’s law


• The concentration gradient of oxygen is simply the concentration in the gas, c, divided by the film thickness, x.

• The rate of growth of the film dx/dt is proportional to the flux of atoms diffusing through the film.

𝑑𝑥

𝑑𝑡𝛼 𝐷

𝑐

𝑥𝑥2 = 𝑘𝑃𝑡

𝑤ℎ𝑒𝑟𝑒 𝑘𝑃 = 𝑐𝐷0𝑒−𝑄/𝑅𝑇

• So, from Fick’s law

Growth rate of oxide film

concentration gradient of oxygen

diffusion coefficient

Protective film


• Protective films are those with low diffusion coefficients

and thus high melting points of the oxide films.

• That is one reason why Al2O3 protects aluminium, Cr2O3 protects

chromium, and SiO2 protects silicon so well

• Cu2O and even FeO (which have lower melting points) are

less protective

• Additional reason: electrons must also pass through the film

and thus protective films should be good insulators

(electrical resistivity of Al2O3 is 109 times greater than that of FeO)

𝑥2 = 𝑘𝑃𝑡 𝑤ℎ𝑒𝑟𝑒 𝑘𝑃 = 𝑐𝐷0𝑒−𝑄/𝑅𝑇

Linear Oxidation


• MoO3 and WO3 oxides evaporate as soon as they are formed (volatile) therefore no barrier to oxidation

Linear weight loss

• It is more complex. Basically, breakdown of oxide films leads to linear oxidation behaviour. Oxides are usually brittle.

• As the oxide film thickens, it develops cracks, or partly lifts away from the material. So the barrier between material and oxide does not become any more effective as oxidation proceeds.

Linear weight gain

crack

partly lift(breaking adhesion)

Example: Making stainless alloys


• Mild steel is an excellent structural material—cheap, easily

formed, and strong.

• But at high temperatures, it oxidizes.

• There is a large demand of oxidation-resistant steel for high-

temperature applications ranging from chemical reactors to

superheater tubes

Stainless steels


• Stainless steels (SS) are alloy steels containing at least 10-12%wt. of Chromium. Normally 18% Cr

• The Cr elements dissolved in SS oxidizes preferentially (Cr has larger negative energy of oxidation compared to Fe)

• Oxidation resistance is imparted by the formation of a passivation layer characterized by:

– Thin layer of chromium oxide film on the surface of the metal - (Cr2O3) is more stable than FeO

– The film develops when exposed to oxygen and impervious to water and air. Also, it quickly reforms when damaged

– As this film is protective so it stifles further growth and protects the steel

Applications of stainless steels


Gateway Arch in St Louis – 304 series SS

F-35 Joint Strike Fighter (JSF) Lightning II, built by Lockheed Martin – airframe 17-7 PH – 600 series SS

Making other stainless alloys by alloying


• Silicon carbide (SiC) and silicon nitride (Si3N4) both have large negative energies of oxidation -> meaning that they oxidize easily

• Silicon in them turns to SiO2 which quickly forms a protective skin and prevents further attack.

Protection by alloying has one great advantage over protection by surface coating (like chromium plating or gold plating) -> it repairs (protective oxide layer is reformed) itself when damaged

• In Steels, other alloying elements such as Aluminium and Silicon can cut down the rate of oxidation too. (Al2O3 and SiO2 form in preference to FeO)

• In Copper, sufficient amount of Aluminium alloying element gives a range of stainless alloys called “Aluminium Bronzes”

• In Silver, we can prevent tarnishing (reaction with sulfur) by alloying it with aluminium or silicon, giving protective Al2O3 or SiO2 surface films.

Protecting Turbine blades (Ni-base Superalloys)


Nickel loses 0.1 mm of metal from its surface by oxidation in 600 hr


• The thickness of the metal between the outside of the blade

and the integral cooling ports is about 1 mm

• So losing 0.1 mm in 600 hr, the blade would lose about 10%

of its cross-section in service

• Serious loss in mechanical integrity and makes no

allowance for statistical variations in oxidation rate

• Due to large cost of replacing a set of Ni blades

• They are expected to last for more than 5000 hr



Nickel oxidizes with parabolic kinetics

Obviously this sort of loss is not OK to last for more than 5000 hr


𝑥2

𝑥1=

𝑡2𝑡1

1/2

𝑥2 = 0.15000

600

1/2

= 0.29 𝑚𝑚

But, as we have seen in the earlier lecture class, superalloys used for turbine blades contain large amount of Chromium it can lead to formation of protective Cr2O3 oxide layer


• At 20%Cr level in the Ni-based alloy, Cr2O3 oxide will form in preference to NiO on the surface of the alloy (more negative energy) -> The alloy behaves only partly as if it were protected by Cr2O3

• We find experimentally that the time taken metal to loss 0.1mm is 6000 hr rather than 106 hr

Protecting Turbine blades – Cr2O3 film

𝑡2𝑡1

=𝑒− 𝑄/𝑅𝑇1

𝑒− 𝑄/𝑅𝑇2= 0.65𝑥103

𝑡2 = 0.65𝑥103𝑥1600 ≈ 106 ℎ𝑟

• In an alloy, some of foreign elements contained in it can greatly increase diffusion coefficients or electrical conductivity thus increasing oxidation rate

1504K (0.7Tm of Cr) 1208K (0.7Tm of Ni)

Cr will loss 0.1 mm in 1600 hr at 1504K


Protecting Turbine blades by sprayed-on aluminium

• We saw that the 20%Cr alloy loss 0.1mm in 6000 hr at 1208K

• Better than pure Nickle, but still not good enough

• In commercial alloys, Cr content is reduced to 10% to improve creep

properties thus making the oxidation resistance worse

• So we need to coat the blades with a protective layer by sprayed-on aluminium

Spraying molten droplets of aluminium

Heating in furnace

Al elements oxidize giving Al2O3 which is a good protective film

Al diffuse into Ni surface and form AlNi compound which is good barrier to oxidation and it helps insulate the blade (poor thermal conductivity)

Influence of coatings on mechanical properties


Although oxide films can reduce the rate of oxidation, they have some disadvantages

• Oxides are quite brittle

• They can crack due to thermal

stresses and act as initiation sites

for thermal fatigue cracks

• Crack can spread into the alloy

itself as the oxide and alloy are

well bonded

• Therefore, the properties of oxide films are important in affecting

the fatigue properties of the whole component

Concerns regarding refractory metals & joining


Refractory metals

• Nb, Ta, Mo and W have very high melting points and thus have very good creep properties. But they oxidized very rapidly

• Unsafe because catastrophic oxidation can take place if the break occurs in the coating

Joining operations and Sintering

• Protective films can be problem in material joining as they create poor

electrical contacts

• Stainless steel is hard to braze and almost impossible to solder.

Even spot welding and diffusion boding become difficult.

• Sintering of powdered material is also made difficult by protective surface films

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