Review Sheet for Corrosion

Big picture ideas

Remember the mnemonic: LEO is the lion, and GER is his roar.

L oss of G ain of
E lectrons E lectrons
O xidation R eduction

You need to know the different parts of the electrochemical cell.

anode cathode oxidation reaction
electrolyte external circuit reduction reaction
galvanic cell    

 and what happens at each of the electrodes.

1. Oxidation occurs at the anode.  (If the oxidation product is soluble in solution, mass is lost from the anode.)

MÞMx+(aq)+xe-1(aq)                                                                                           (1)

2. Reduction occurs at the cathode.

My+(aq)+ye-1(aq)ÞM                                                                    (2)

3. Electrons are conserved, so the number of electrons produced at the anode must be equal to the number of electrons consumed at the cathode.

 

In many engineering applications where corrosion occurs on structural components, reaction (2) will not occur due to lack of reactants (metal ions in solution).  More often reactions of the following type will occur at the cathode.

2H+(aq)+2e-ÞH2 (g)­                                                                                                 (3)

O2+4H+(aq)+4e-Þ2H2O( l )   (Oxygen Reduction) Acid env.                 (4)

O2 + 2H2O + 4e-Þ 4(OH-) (Oxygen Reduction) neutral or basic solns. (5)

These are the main reactions that we will see in the lab.  The lab manual also has equations for change of oxidation states for ions and for electroplating reactions.

Driving Force for Corrosion

The voltage developed between two electrodes is a measure of the driving force for corrosion.  One way this is quantified is by developing the electropotential series.  This orders electrode materials in order from least reactive (most cathodic) to most reactive (most anodic).  These tables are constructed vs. a standard electrode.  For many tables this is hydrogen.  The voltages developed against this standard electrode are tabulated and placed in order from most cathodic to most anodic.  To determine the voltage from an electrochemical cell using two electrodes materials, the voltages in the table can be added.  For these potentials to be developed, you must have the standard cell (standard concentrations of reactants). 

 

Some motivating questions (stuff to think about)--

1.  What happens to the concentration of reactants at the anode as corrosion proceeds?

2.  What happens to the concentration of reactants at the cathode as the reaction proceeds?