MATS1101 Materials Page | 1 MATS1101 Final Exam â Materials ...
MATS1101 Materials MATS1101 Final Exam → Materials Paper A Oxidation Categorising oxidisation resistance Material + 02 + 𝐸𝑛𝑒𝑟𝑔𝑦 → 𝑂𝑥𝑖𝑑𝑒 If energy is positive then material is stable If energy is negative it will oxidise How fast does oxidisation occur? Aluminium oxidises much slower than iron. The Oxide film acts as a barrier, which keeps oxygen and metal apart. I.e. oxide film on aluminium is much more effective barrier to oxidation then the oxide film on iron Oxidation →weight gain (usually). Therefore rates of oxidation can be monitored by measuring weight gain as a function of time. Types of behaviour observed 1. Linear oxidization, KL = kinetic constant and is positive (unless the oxide evaporates as it forms; material then loses weight, and KL is negative) 2. Parabolic oxidation, Kp = constant and is always positive
Oxidation rates follow Arrhenius’ law → kinetic constants KL and Kp increase exponentially with temperature.
Oxidation rates also increase with increase in partial pressure of O2 Micro-mechanisms Parabolic oxidation M + O → MO occurs in two steps 1. M → M++ + 2e2. O + 2e- → O- Either M++ and 2e- diffuse outward through the film to meet the O- - at the outer surface Or the O - - diffuses inwards (with two electron holes) to meet the M+ + at the inner surface The rate of growth of the film dx/dt is proportional to the flux of atoms diffusing through the film Oxidation rate depends on diffusion coefficient Protective films are those with low diffusion co-efficient
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MATS1101 Materials Oxide films →electrons must also pass through the film. Protective films must also be insulators. They must also avoid oxide skin from splitting or wrinkling
Wet Corrosion Wet corrosion → loss of material by oxidation at room temperature under dry conditions is very slight, under wet conditions, oxidisation can be very fast. This is due to: 1. Fe atoms pass into solution in the water as Fe++, leaving behind 2e- each (anodic reaction) 2. E- conducted through metal to a place where ‘oxygen reduction’ can take place to consume e(cathodic reaction) 3. Reaction generates OH- ions which then combine with Fe++ ions to form a hydrated iron oxide Fe(OH)2. Instead of forming on the surface to give protection it often forms in water Reaction is analagous to dry oxidation. However attack is much faster in wet corrosion because: • Fe(OH)2 deposits away from corroding metal, or as a loose deposit on the surface giving giving little little or no protection • M++ and OH- usually diffuse in liquid state and therefore diffuse very rapidly • In conducting materials electrons can also move very easily -
The driving force for wet corrosion is volatage difference between catodic and anodic reactions Law 1: the greater the driving force the more potential for oxidation Law 2: anode correde bimetals, this is known as galvanic corrosion Law 3: corrosion takes place more easily in a dilute solution
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Wet corrosion attacks metals selectively instead of uniformly →may lead to component failure more rapidly than indicated from corrosion rates Types of corrosion cracking that lead to unplanned failure: 1. Stress corrosion cracking→ cracks grow steadily under constant stress of intensity 2. Corrosion fatigue → corrosion increases the rate of growth of fatigue cracks 3. Intergranular attack →grain boundaries can have different compositions and therefore different corrosion properties to grains. Therefore may corrode preferentially, giving crack that then propagate by stress corrosion or corrosion fatigue 4. pitting→ preferential attack can also occur at localised breaks in oxide film or at precipitated compounds in some alloys
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Oxidation rates follow Arrhenius’ law → kinetic constants KL and Kp increase exponentially with temperature.
Oxidation rates also increase with increase in partial pressure of O2 Micro-mechanisms Parabolic oxidation M + O → MO occurs in two steps 1. M → M++ + 2e2. O + 2e- → O- Either M++ and 2e- diffuse outward through the film to meet the O- - at the outer surface Or the O - - diffuses inwards (with two electron holes) to meet the M+ + at the inner surface The rate of growth of the film dx/dt is proportional to the flux of atoms diffusing through the film Oxidation rate depends on diffusion coefficient Protective films are those with low diffusion co-efficient
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MATS1101 Materials Oxide films →electrons must also pass through the film. Protective films must also be insulators. They must also avoid oxide skin from splitting or wrinkling
Wet Corrosion Wet corrosion → loss of material by oxidation at room temperature under dry conditions is very slight, under wet conditions, oxidisation can be very fast. This is due to: 1. Fe atoms pass into solution in the water as Fe++, leaving behind 2e- each (anodic reaction) 2. E- conducted through metal to a place where ‘oxygen reduction’ can take place to consume e(cathodic reaction) 3. Reaction generates OH- ions which then combine with Fe++ ions to form a hydrated iron oxide Fe(OH)2. Instead of forming on the surface to give protection it often forms in water Reaction is analagous to dry oxidation. However attack is much faster in wet corrosion because: • Fe(OH)2 deposits away from corroding metal, or as a loose deposit on the surface giving giving little little or no protection • M++ and OH- usually diffuse in liquid state and therefore diffuse very rapidly • In conducting materials electrons can also move very easily -
The driving force for wet corrosion is volatage difference between catodic and anodic reactions Law 1: the greater the driving force the more potential for oxidation Law 2: anode correde bimetals, this is known as galvanic corrosion Law 3: corrosion takes place more easily in a dilute solution
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Wet corrosion attacks metals selectively instead of uniformly →may lead to component failure more rapidly than indicated from corrosion rates Types of corrosion cracking that lead to unplanned failure: 1. Stress corrosion cracking→ cracks grow steadily under constant stress of intensity 2. Corrosion fatigue → corrosion increases the rate of growth of fatigue cracks 3. Intergranular attack →grain boundaries can have different compositions and therefore different corrosion properties to grains. Therefore may corrode preferentially, giving crack that then propagate by stress corrosion or corrosion fatigue 4. pitting→ preferential attack can also occur at localised breaks in oxide film or at precipitated compounds in some alloys
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