Corrosion is degradation of a metal by an electrochemical reaction with its environment
2 types:
1. wet corrosion (aqueous corrosion)
2. dry corrosion
high temperature oxidation
high temperature corrosion
Cost of corrosion:-
In UK: 3-4% of GNP
£10 billion per year
£300 – 400 per person per year
Corrosion cost society, inevitable but still controllable.
For a spontaneous reaction to occur, delta G must be negative:
Delta G = G(corrosion product) – G(metal)
Delta G is free energy change must be negative for a spontaneous reaction
Delta G represents the Tendency of the metal to corrode
Most metals have an inherent tendency to corrode.
Cu and Mg are expected to corrode naturally in this condition, while Au is not.
Standard Electrode Potentials of Metals
(volts vs. Standard Hydrogen Electrode, in 1M solution of its ions at 25C)
H2 à H+ + 2e- 0.000
Definition: voltage of metal with respect to solution
Tendencies of elements to ionise, i.e. to lose electrons
Electrode potential: measurable (with respect to Ref. electrode)
Electrochemical Cell (Galvanic Cell)
Driving force: potential difference
The cathode is at a higher potential than the anode
The most negative or active metal tends to lose electrons (to be oxidized – dissolved – corrosion) – Anode
The most positive or noble metal tends to consume electrons (to be reduced) – Cathode
Galvanic Cell I: two dissimilar metals each immersed in a solution of their own ions (1M)
Galvanic Cell II: in acid or alkaline electrolytes
Galvanic Cell III: Single electrode
Galvanic Cell IV (microscopic): created by differences in composition, structure, and stress
Grain – grain boundary
Precipitated phases in solid solution
Corrosion resistance: 1) very pure metals > commercial materials 2) solid solution > alloys with two or more phases
Galvanic Cell V: Oxygen Concentration Cells
Summary of Electrochemical Reactions
Anode: produce electrons, oxidation reaction
Cathode: consume electrons, reduction reaction
Zn in hydrochloric acid (HCl)
Fe in seawater (aerated NaCl solution)
Applications of Thermodynamics
-Polarisation
-Passivation
corrosion rate is not dependant on the electronegativeness of the ion
Polarization: - The change in potential caused by the passage of a current
Reasons for polarisation:
- activation polarisation refers to an electrochemical process that is controlled by a slow step in a reaction sequence of steps at the metal-electrolyte interface
- refer to an electrochemical reaction which is controlled by the diffusion of ions in the electrolyte
Passivity: -
Passivity phenomenon
Passivity – loss of chemical reactivity under certain environmental conditions
Reasons – formation of a protective film on a metal surface, which causes a drop in current density due to the resistance of the film and its effect as a barrier to diffusion.
Galvanic Series
For practical applications, many engineering metals form passive films, so they do not behave in galvanic cells as the standard electrode potentials would indicate.
Galvanic series: determined experimentally for every corrosive environment.
Comparison of standard emf to galvanic series in seawater.
Pourbaix Diagram of Zn-H20 System.
Immunity: a metal is thermodynamically stable species, and therefore cannot corrode.
Use of Pourbaix Diagrams: what might happen, three regimes:
Corrosion, Immunity, Passivity.
Can only be plotted for pure metals and simple solutions, not for alloys.
