Electrode Reduction and Oxidation Potential Values

Electrode Reduction and Oxidation Potential

Corrosion, the degradation of metals as a result of electrochemical activity, requires an anode and a cathode in order to occur. The anode is the metal or site with a higher potential to oxidize (lose electrons) while the cathode is the metal or site with a higher potential for reduction (gaining of electrons). In other words, the cathode has a lower potential to oxidize than the anode.

The measure of a material to oxidize or lose electrons is known as its 'oxidation potential.' A difference between the oxidation potentials of two metals or sites can lead to corrosion that will consume the metal or site that is more anodic. This is assuming that the two other things needed for corrosion are also present: electrical connection between the two metals or sites with oxidation potential difference and the presence of an electrolyte (such as water) to conduct ions between them.

Table 1 presents the standard oxidation potential values of various elements. The values of the oxidation potential in this table are used relative to each other, to determine the tendency of a metal to become a cathode (or anode) with respect to another metal, for corrosion to occur.

Table 1. Standard Electrode Reduction and Oxidation Potential Values

Anodic - exhibits greater tendency to lose electrons
Reduction Reaction	E^o (V)	Oxidation Reaction	E^o (V)
Li⁺ + e^- → Li	-3.04	Li → Li⁺ + e^-	3.04
K⁺ + e^- → K	-2.92	K → K⁺ + e^-	2.92
Ba²⁺ + 2e^- → Ba	-2.90	Ba → Ba²⁺ + 2e^-	2.90
Ca²⁺ + 2e^- → Ca	-2.87	Ca → Ca²⁺ + 2e^-	2.87
Na⁺ + e^- → Na	-2.71	Na → Na⁺ + e^-	2.71
Mg²⁺ + 2e^- → Mg	-2.37	Mg → Mg²⁺ + 2e^-	2.37
Al³⁺ + 3e^- → Al	-1.66	Al → Al³⁺ + 3e^-	1.66
Mn²⁺ + 2e^- → Mn	-1.18	Mn → Mn²⁺ + 2e^-	1.18
2H₂O + 2e^- → H₂ + 2 OH^-	-0.83	H₂ + 2 OH^- → 2H₂O + 2e^-	0.83
Zn²⁺ + 2e^- → Zn	-0.76	Zn → Zn²⁺ + 2e^-	0.76
Cr²⁺ + 2e^- → Cr	-0.74	Cr → Cr²⁺ + 2e^-	0.74
Fe²⁺ + 2e^- → Fe	-0.44	Fe → Fe²⁺ + 2e^-	0.44
Cr³⁺ + 3e^- → Cr	-0.41	Cr → Cr³⁺ + 3e^-	0.41
Cd²⁺ + 2e^- → Cd	-0.40	Cd → Cd²⁺ + 2e^-	0.40
Co²⁺ + 2e^- → Co	-0.28	Co → Co²⁺ + 2e^-	0.28
Ni²⁺ + 2e^- → Ni	-0.25	Ni → Ni²⁺ + 2e^-	0.25
Sn²⁺ + 2e^- → Sn	-0.14	Sn → Sn²⁺ + 2e^-	0.14
Pb²⁺ + 2e^- → Pb	-0.13	Pb → Pb²⁺ + 2e^-	0.13
Fe³⁺ + 3e^- → Fe	-0.04	Fe → Fe³⁺ + 3e^-	0.04
Arbitrary Neutral : H₂
Reduction Reaction	E^o (V)	Oxidation Reaction	E^o (V)
2H⁺ + 2e^- → H₂	0.00	H₂ → 2H⁺ + 2e^-	0.00
Cathodic - exhibits greater tendency to gain electrons
Reduction Reaction	E^o (V)	Oxidation Reaction	E^o (V)
S + 2H⁺ + 2e^- → H₂S	0.14	H₂S → S + 2H⁺ + 2e^-	-0.14
Sn⁴⁺ + 2e^- → Sn²⁺	0.15	Sn²⁺ → Sn⁴⁺ + 2e^-	-0.15
Cu²⁺ + e^- → Cu⁺	0.16	Cu⁺ → Cu²⁺ + e^-	-0.16
SO₄²⁺ + 4H⁺ + 2e^- → SO₂ + 2H₂O	0.17	SO₂ + 2H₂O → SO₄²⁺ + 4H⁺ + 2e^-	-0.17
AgCl + e^- → Ag + Cl^-	0.22	Ag + Cl^- → AgCl + e^-	-0.22
Cu²⁺ + 2e^- → Cu	0.34	Cu → Cu²⁺ + 2e^-	-0.34
ClO₃^-+ H₂O + 2e^- → ClO₂^- + 2OH^-	0.35	ClO₂^- + 2OH^- → ClO₃^-+ H₂O + 2e^-	-0.35
2H₂O + O₂ + 4e^- → 4OH^-	0.40	4OH^- → 2H₂O + O₂ + 4e^-	-0.40
Cu⁺ + e^- → Cu	0.52	Cu → Cu⁺ + e^-	-0.52
I₂ + 2e^- → 2I^-	0.54	2I^- → I₂ + 2e^-	-0.54
O₂ + 2H⁺ + 2e^- → H₂O₂	0.68	H₂O₂ → O₂+ 2H⁺ + 2e^-	-0.68
Fe³⁺ + e^- → Fe²⁺	0.77	Fe²⁺ → Fe³⁺ + e^-	-0.77
NO₃^- + 2H⁺ + e^- → NO₂+ H₂O	0.78	NO₂ + H₂O → NO₃^- + 2H⁺ + e^-	-0.78
Hg²⁺ + 2e^- → Hg	0.78	Hg → Hg²⁺ + 2e^-	-0.78
Ag⁺ + e^- → Ag	0.80	Ag → Ag⁺ + e^-	-0.80
NO₃^- + 4H⁺ +3 e^- → NO+ 2H₂O	0.96	NO + 2H₂O → NO₃^- + 4H⁺ +3 e^-	-0.96
Br₂ + 2e^- → 2Br^-	1.06	2Br^- → Br₂ + 2e^-	-1.06
O₂ + 4H⁺ + 4e^- → 2H₂O	1.23	2H₂O → O₂ + 4H⁺ + 4e^-	-1.23
MnO₂+ 4H⁺ + 2e^- → Mn²⁺ + 2H₂O	1.28	Mn²⁺ + 2H₂O → MnO₂+ 4H⁺ + 2e^-	-1.28
Cr₂O₇^2- + 14H⁺ + 6e^- → 2Cr³⁺ + 7H₂O	1.33	2Cr³⁺ + 7H₂O → Cr₂O₇^2- + 14H⁺ + 6e^-	-1.33
Cl₂ + 2e^- → 2Cl^-	1.36	2Cl^- → Cl₂ + 2e^-	-1.36
Ce⁴⁺ + e^- → Ce³⁺	1.44	Ce³⁺ → Ce⁴⁺ + e^-	-1.44
Au³⁺ + 3e^- → Au	1.50	Au → Au³⁺ + 3e^-	-1.50
MnO₄^- + 8H⁺ + 5e^- → Mn²⁺ + 4H₂O	1.52	Mn²⁺ + 4H₂O → MnO₄^- + 8H⁺ + 5e^-	-1.52
H₂O₂ + 2H⁺ + 2e^- → 2H₂O	1.78	2H₂O → H₂O₂ + 2H⁺+ 2e^-	-1.78
Co³⁺ + e^- → Co²⁺	1.82	Co²⁺ → Co³⁺ + e^-	-1.82
S₂O₈^2- + 2e^- → 2SO₄^2-	2.01	2SO₄^2- → S₂O₈^2- + 2e^-	-2.01
O₃ + 2H⁺ + 2e^- → O₂ + H₂O	2.07	O₂ + H₂O → O₃ + 2H⁺ + 2e^-	-2.07
F₂ + 2e^- → 2F^-	2.87	2F^- → F₂ + 2e^-	-2.87

For example, if tin is deposited over copper, then there is a possibility for corrosion to occur. From Table 1, copper has a lower oxidation potential (-0.34 V) than tin (0.14 V), so Cu can serve as the cathode while Sn can serve as the anode, creating the potential difference necessary for corrosion to occur.

Reliability Models

Table 1 References:

1) http://www.physchem.co.za

2) http://hyperphysics.phy-astr.gsu.edu

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