Corrosion Summary Notes || Chemistry & Materials Science PDF

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Summary

Corrosion is the degradation of a material due to an oxidation-reduction reaction with its environment - Huge economic impact - The term refers exclusively to processes in which a metallic material becomes oxidisedWhy Metals corrode - We use them in environments where they are chemically unstable - ...

Description

Corrosion is the degradation of a material due to an oxidation-reduction reaction with its environment • Huge economic impact • The term refers exclusively to processes in which a metallic material becomes oxidised Why Metals corrode • We use them in environments where they are chemically unstable • All metals exhibit a tendency to be oxidized, some more easily than other Common corrosion reaction • Fe --> Fe3+ + 3e- (oxidation) • O2 + 4e- --> 202- (reduction) • In a full balanced equation: • 4Fe + 302 --> 2Fe2O3 • • • •

The metallic element (Fe0) becomes oxidized (loses electrons to the oxygen) The oxygen becomes reduced (gains electrons) This is an example of a redox reaction The combination of positive cation and negative anion forms an ionic compound, Fe2O3

Galvanic Series:

Two main categories of corrosion

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Dry o o

Oxidation, tarnishing Formation of patinas

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Relative humidity > 60% Electrochemical process (a transfer of electrons from one chemical species to another

Wet

Dry corrosion is usually called oxidation because an oxide is being formed. It can also be called a sulphide compound. • Rate of oxidation is important o Linear or parabolic •

Parabolic oxidation slows with time because the oxide becomes a protective barrier between the metal and its environment

Wet corrosion

Corrosion occurs most where oxygen is lacking The surface where oxidation occurs is called the anode • Electrons generated at the anode flow around to a surface called the cathode where reduction occurs Anode and Cathode size factor • Small anode to large cathode results in large corrosion rate as the anode has to corrode more to compensate the large cathode area • A large anode reduces the rate of corrosion as the rate of penetration at the anode is spread over a wider area • Area Rule: o To avoid corrosion, avoid a small anodic area relative to cathodic area

Uniform corrosion • Most common • Equivalent attack over entire surface • Oxidation occurs randomly on surface • CPR = 10 (deltaM / pAt)

Crevice corrosion & pitting • Driven by oxygen concentration differences ration differences • A difference in concentration of ions or dissolved gases in the electrolyte Prevention of crevice corrosion

Galvanic corrosion: electrode potentials

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Not all metals oxidize to form ions with the same degree of ease To prevent separate the metals by another material such as rubber

Intergranular Corrosion • Preferentially along grain boundaries for some alloys in specific environments o Net result -->: macroscopic disintegration o Typical in stainless steels Selective leaching (de-alloying) • Alloy containing one noble element and one base element Stress corrosion cracking (SCC) • Can result in the sudden fracture of a component that is under load • Combined action of environment and tensile stresses e.g. stainless steels and solutions containing chloride ions • Stress can be external or residual Erosion corrosion • Occurs in areas where the diameter of the pipe or direction of flow is changing Preventing corrosion • For corrosion to proceed there must be an anode, cathode and an electrolyte joined by an external circuit • To prevent corrosion we have to break this tetrahedron by removing one of the legs • Or by reversing the flow of the current • Or by building resistance through painting or coating all surfaces Strategies to control corrosion o Design modifications to the system or component o Change to a more suitable material o Use of protective metallic or organic coatings to exclude the environment -Active protection through coating with something that is more active that will corrode preferentially and provide electrons to make the potential of the underlying structure more negative (making it a cathode) o o

Force the electrical current into reverse. Cathodic protection Or even, use it to your advantage

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An electrical insulator is a material in which the electron does not flow freely or the atom of the insulator have tightly bound electrons whose internal electric charges do not flow freely Galvanise is coating with a protective layer of zinc

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Summary

Corrosion Penetration Rate (mm/year) = (KW/pAt)

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K = constant (=87.6 with these units) W = weight loss after time (g) A = exposed area (cm^2) P = density (gcm^-3)...