Gibbs free energy change
ΔG’ = -nFE’
-negative values of delta G = thermodynamically favourable, for this we need a positive value of E’
Calculating cell potentials from standard electrode potentials
- Measure the potential difference between two electrodes, one being the electrode of interest and the other being a SHE.
- All species at standard conditions (1mol/dm or 1 atm)
- Salt bridge used to prevent the solution of 2 half cells from mixing
- LHS oxidation
- RHS reduction
Ecell in terms of two cells
Ecell = E(RHS) - E(LHS)
Ecell in terms of ox and red
Ecell = E(ox) + E(red)
Cathodic reduction reactions
Low pH:
2H+ + 2e- = H2 (E=0.00v)
Normal pH:
02 + 2H2O + 4e- = 4OH- (E=+0.44V)
Nernst equation
E = E’ + (RT/nF x ln[O]/[R])
ln(x) to log10 conversion
ln(x) = 2.303 x log10(x)
2.303RT/F =
0.0591 (recognise this doesn’t include n)
pH
-log10[H+]
Measurement of pH
- Measured using a glass membrane electrode
- 2 reference electrodes separated by the membrane
- If [H+] differs across membrane, potential difference is set up
- Internal sol [H+] is fixed and so potential difference between internal and external electrodes (Eint - Eext) = A - 0.0591pH
- 59mV potential difference change per pH unit
Cathodic Activator (Evans Diagram)
Icorr increases
Ecorr increases
Anodic Activator (Evans Diagram)
Icorr increases
Ecorr Decreases
Cathodic Inhibitor (Evans Diagrams)
Icorr decreases
Ecorr decreases
Anodic Inhibitor (Evans Diagrams)
Ecorr increases
Icorr decreases
Clarify the mechanism of corrosion inhibition
Comparing time-dependant Ecorr behaviour in presence/absence of inhibitor
Tafel slopes for Ecorr and Icorr
extrapolation of the slopes, where they intersect determines the Ecorr and Icorr, the latter provide the corrosion rate. As such mass loss per unit area or penetration rate can be worked out from Icorr, without the need to run a long term immersion experiment
Mass loss per unit area
jcorr x t x Ar(M) / nF
Penetration rate
If the density is known;
jcorr x t x Ar(M) / nFρ
Potentiodynamic experiments
- Employ a 3 electrode cell
- potential of working electrode will be measured relative to SCE (standard/ saturated calomel electrode)
- Current passed between working electrode and counter electrode (inert Pt)
- If dilute electrolyte used, lugging capillary used to minimise significant potential drop, due to high solution resistivity
Potentiodynamic methods for characterising pitting corrosion
- log[j] is plotted as a function of applied potential
- Plot characterised by area of passive region, more positive potentials than Ecorr
- further increase in applied potential do not give increase in anodic current density
- however at sufficiently high app pot, sharp increase in current density
- This potential is termed pitting potential (Eb) and is a measure of corrosion resistance
- The more conc the chloride, the more negative the Eb value
Potentiostatic mode for re-passivation of pitting corrosion
- Fixed potential versus a ref, measuring current/current density with respect to time
- When surface is scratched, sharp increase in current which decays as it repassivates
- However at a critical potential Ec, stable pit growth, current will increase with time after scratch and wont decay
Galvanostatic mode for thickness of coating
- Applied potential vs ref is monitored with respect to time
- Following a change in potential vs time indicates when the coating has been removed substrate is exposed
- From the applied current density and time taken, thickness can be worked out
