Corrosion

Question 1

Requirements for corrosion to occur

Answer 1

Two electrodes (anode and cathode) with different electromechanical potentials

Electrical/physical connection between anode and cathode

Electrolytic connection between anode and cathode

Question 2

Anode

Answer 2

Electrode from which electrons flow into the electric circuit

Question 3

Cathode

Answer 3

Electrode into which electrons flow from the electric circuit

Question 4

In corrosion cells

Answer 4

The anode gets eaten while the cathode is protected from corrosion

Question 5

Corrosion resistant materials

Answer 5

May be corrosion resistant either because they are naturally noble/cathodic or they are reactive/anodic but have a protective passive layer

Question 6

Way of creating a difference of electrochemical potential between two electrodes

Answer 6

Two different materials (this is the most common corrosion cell)

Same material, one in oxygen rich environment, one oxygen deprived (oxygen deprived part becomes anode)

Same material, one under applied or residual stress, the other unstresses (stressed part becomes anode)

Same material, one in more aggressive solution, one in less aggressive solution (one in more aggressive solution becomes the anode)

Same material, but connected to a DC electric source, e.g. a battery. (Part connected to + terminal of DC source becomes the anode)

Question 7

Design against corrosion

Answer 7

Avoid using mixed materials where possible

Better to have a large anode and small cathode

Break the electric circuit by inserting an electric insulator between electrodes

Avoid oxygen deprivation cells

Use annealing heat treatments to remove unnecessary residual stresses

Coatings that provide a barrier between corrosion env and part

Cathodic protection

Anodic protection