Electrochem

Question 1

Gibbs free energy change

Answer 1

ΔG’ = -nFE’

-negative values of delta G = thermodynamically favourable, for this we need a positive value of E’

Question 2

Calculating cell potentials from standard electrode potentials

Answer 2

• 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

Question 3

Ecell in terms of two cells

Answer 3

Ecell = E(RHS) - E(LHS)

Question 4

Ecell in terms of ox and red

Answer 4

Ecell = E(ox) + E(red)

Question 5

Cathodic reduction reactions

Answer 5

Low pH:
2H+ + 2e- = H2 (E=0.00v)

Normal pH:
02 + 2H2O + 4e- = 4OH- (E=+0.44V)

Question 6

Nernst equation

Answer 6

E = E’ + (RT/nF x ln[O]/[R])

Question 7

ln(x) to log10 conversion

Answer 7

ln(x) = 2.303 x log10(x)

Question 8

2.303RT/F =

Answer 8

0.0591 (recognise this doesn’t include n)

Question 9

pH

Answer 9

-log10[H+]

Question 10

Measurement of pH

Answer 10

• 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

Question 11

Cathodic Activator (Evans Diagram)

Answer 11

Icorr increases

Ecorr increases

Question 12

Anodic Activator (Evans Diagram)

Answer 12

Icorr increases

Ecorr Decreases

Question 13

Cathodic Inhibitor (Evans Diagrams)

Answer 13

Icorr decreases

Ecorr decreases

Question 14

Anodic Inhibitor (Evans Diagrams)

Answer 14

Ecorr increases

Icorr decreases

Question 15

Clarify the mechanism of corrosion inhibition

Answer 15

Comparing time-dependant Ecorr behaviour in presence/absence of inhibitor

Question 16

Tafel slopes for Ecorr and Icorr

Answer 16

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

Question 17

Mass loss per unit area

Answer 17

jcorr x t x Ar(M) / nF

Question 18

Penetration rate

Answer 18

If the density is known;

jcorr x t x Ar(M) / nFρ

Question 19

Potentiodynamic experiments

Answer 19

• 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

Question 20

Potentiodynamic methods for characterising pitting corrosion

Answer 20

• 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

Question 21

Potentiostatic mode for re-passivation of pitting corrosion

Answer 21

• 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

Question 22

Galvanostatic mode for thickness of coating

Answer 22

• 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