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14 Cards in this Set

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3 Types of Corrosion
- Gas Phase (oxidation)
- Chemical (reaction, dissolution
- Electro Chemical (metals)
Gas Phase Corrosion
- Oxygen and Ozone in the atmosphere attack surfaces of materials. Rate of oxidation increases with growing temperatures, at room temperature, oxidation hardly occurs, where at high temps happens rapidly. Also reffered to as dry corrosion.
- Sometimes oxide layer protects agains further corrosion
Chemical Corrosion
- Metals attacked by acids
- Polymers attacked by oxidising acids
- Glass attacked by alkalis
- Dissolution is when a material is in contact with a chemically similar material, and may soften or disolve
Galvanic Corrosion (electrochemical)
- Two metals in electrical contact, and in presence of an electrolyte.
- More reactive metal takes on role of Anode, and oxidation occurs
- Less reactive (Noble) metal takes on role of cathode, and reduction occurs. The cathode is protected.
Anodic Process
Electons liberated in anode into the electrolyte. Anode oxidises (corrodes), and loses material. Liberated electons travel to the cathode and take part in the reaction there.
Cathodic Process
Electrons consumed in the process, thus reduction. Negative ions go into solution. Each reaction produces a voltage, The Half Cell Potential (Electrode Potential)
Galvanic Series
Appropriate Galvanic series need to be used, as electrode potentials vary with enviroment (eg sea water)
Corrosion Rate - Can be assessed by:
- Weight Change
- Depth increase
- Property Change (eg reduction in yield strength)
- Corrosion current measurement
Faraday's Law of Electrolosis
The quantity of a substance converted at an electrode depends on the number of electrons involved in its half cell reaction.
Polarization
Rate of Corrosion may be determined by the affect of polarization.
- Concentration polarisation
- Activation polarisation
Passivation
The material forms a protective layer, usually the metals oxide. May be formed by exposition to strong oxidising enviroment, but may change with change in enviroment.
Porbaix Diagram
Shows how corrosion rate changes with pH in enviroment.
- Electrode potential vs pH
- Regions: Immunity: where corrosion impossible, Passivation: where corrosion doesn't occur, Corrosion: where it occurs
Galvanic Cells
Electrochemical cells formed when there is variation in:
- Composition (between metals)
- Concentration of Electrolyte
- Lattice energy in metal
Galvanic corrosion Prevention
- Ensure good drainage to prevent electrolyte forming.
- Sacraficial anode (Magnesium anode on hull of ship)
- Zinc protection layer (Galvanised steel)
- Make protected material into cathode, impress a voltage
- Anodic: encourage material to form oxide layers, Occasionally use maintanance current