Electrochemistry and Corrosion

Question 1

What is the basis of electrochemistry

Answer 1

In solution metal ions detach from the surface leaving free electrons at the interface. These electrons attract metal ions to the surface. This process continues until an equilibrium is reached

Question 2

Define the Anodic Reaction

Answer 2

The metal with the lower standard potential is oxidised - loses electrons

Question 3

Define the Cathodic Reaction

Answer 3

The metal with the higher standard potential is reduced - gains electrons

Question 4

Define standard potential

Answer 4

The standard potential is defined as the electrode potential of the metal in solution of its ion at a concentration of 1 mol/litre

Question 5

Describe the mechanism of the redox reaction

Answer 5

The redox reaction occurs due to the differential affinity between two compounds. The lower the potential the more likely the material will be oxidised.

Question 6

What conditions are needed for a reduction reaction involving metal ions

Answer 6

Metal ions will only be formed in a reduction reaction if an electrolyte containing metal ions is present

Question 7

What are the anodic and cathode reactions for Iron in an acidic environment

Answer 7

Iron oxidises

Hydrogen reduces

Question 8

What are the anodic and cathodic reactions of Iron in neutral environments

Answer 8

Iron oxidises

Oxygen reduces

Question 9

What are the conditions needed for Rust

Answer 9

1. Water

2. Oxygen

Question 10

What is the issue with the formation of Rust

Answer 10

Rust (hydrated ferric oxide) does not adhere to the surface of the metal and thus expose further metal for corrosion

Question 11

What are the three known elements that form protective oxide layers

Answer 11

1. Aluminium
2. Silicon
3. Chromium

Question 12

Why do metals with higher standard electrode potentials not corrode like Iron

Answer 12

In principle these materials should corrode more extensively. However their oxide layers and non-porous and form a protective barrier protecting the underlying metal from corrosion.

Question 13

Where does localised corrosion occur in materials?

Answer 13

Localised corrosion occurs at sites that promote the anodic reaction:

1. Defects in protective coating.
2. Electrolyte present in crevices
3. Cracks

Question 14

What are the necessary conditions for a corrosion reaction

Answer 14

1. Anodic Reaction
2. Cathodic Reaction
3. Conductive Path
4. Electrolyte

Question 15

Why does Galvanic Corrosion occur

Answer 15

Galvanic corrosion occurs due to the differential affinity for electrons. This leads to material with the lower electrode potential being oxidised. However these electrons are collected by the metal with the higher electrode potential.

Question 16

Outline Galvanisation

Answer 16

Galvanisation involves coating Zinc (a metal with a high electrode potential) on other metals and alloys such that the Zinc is preferentially corroded.

Question 17

Outline Galvanic Protection (Sacrificial Anodes)

Answer 17

Sacrificial anodes are a block of a more reactive metal that protects the surrounding metal from corrosion provided its electrically connected.

Question 18

What conditions are needed for oxide layers form

Answer 18

Materials that form oxide layers on their surface must have anodic and cathodic reactions occurring at the same location

Question 19

How can we determine the thermodynamic feasibility of a reaction

Answer 19

The change in free energy can be used to determine whether the reaction will be spontaneous.
dG = G(products) - G(reactants)

Question 20

How can we use the change in Gibbs to determine if the oxide layer is stable or not

Answer 20

If the change in Gibbs is negative the oxide layer is stable and will form spontaneously. If the Gibbs is positive the layer is unstable and will not form spontaneously.

Question 21

What are the three types of mass variation when forming oxide layers

Answer 21

1. Linear loss: when the oxide does not stay on the surface or is volatile (thickness = -AT)
2. Parabolic again: oxide adheres to the surface and acts as a barrier to the diffusion of reactants. (thickness = AT^1/2)
3. Linear gain: when the oxide stays on the surface but peels off allowing for further oxidation (thickness = AT)

T - time