1. Galvanic Series

2. Galvanic Series of Some Commercial Metals and Alloys in Seawater
The following galvanic table lists metals in the order of their relative activity in seawater environment.
The list begins with the more active (anodic) metal and proceeds down the to the least active (cathodic) metal of the galvanic series.
In a galvanic couple, the metal higher in the series represents the anode, and will corrode preferentially in the environment.

4. Latest Galvanic Table from MIL-STD-889.
Start - Active (Anodic) “loose electrons”
Magnesium
Mg alloy AZ-31B
Mg alloy HK-31A
Zinc (hot-dip, die cast, or plated)
Beryllium (hot pressed)
Al 7072 clad on 7075
Al 2014-T3
Al 1160-H14
Al 7079-T6
Cadmium (plated)
Uranium
Al 218 (die cast) Al
Al 5052-H12
Al , H353
Al 5052-H32 Al
Al 3003-H25
Al 6061-T6
Al A360 (die cast)
Al 7075-T6 Al
Indium Al
Al 2024-T4
Al 5052-H16
Tin (plated)
Stainless steel 430 (active)
Lead
Steel 1010
Iron (cast)
Stainless steel 410 (active)
Copper (plated, cast, or wrought)
Nickel (plated)
Chromium (Plated)
Tantalum
AM350 (active)
Stainless steel 310 (active)
Stainless steel 301 (active)
Stainless steel 304 (active)
Stainless steel 430 (active)
Stainless steel 17-7PH (active Tungsten
Niobium (columbium) 1% Zr
Brass, Yellow, 268
Uranium 8% Mo
Brass, Naval, 464
Yellow Brass
Muntz Metal 280
Brass (plated)
Nickel-silver (18% Ni)
Stainless steel 316L (active)
Bronze 220
Copper 110
Red Brass
Stainless steel 347 (active)
Molybdenum, Commercial pure
Copper-nickel 715
Admiralty brass
Stainless steel 202 (active)
Bronze, Phosphor 534 (B-1)
Monel 400
Stainless steel 201 (active)
Carpenter 20 (active)
Stainless steel 321 (active)
Stainless steel 316 (active)
Stainless steel 309 (active)
Stainless steel 17-7PH (passive)
Silicone Bronze 655
Stainless steel 304 (passive)
Stainless steel 301 (passive)
Stainless steel 321 (passive)
Stainless steel 201 (passive)
Stainless steel 286 (passive)
Stainless steel 316L (passive)
AM355 (active)
Stainless steel 202 (passive)
Carpenter 20 (passive)
AM355 (passive)
A286 (passive)
Titanium 5A1, 2.5 Sn
Titanium 13V, 11Cr, 3Al (annealed)
Titanium 6Al, 4V (solution treated and aged)
Titanium 6Al, 4V (anneal)
Titanium 8Mn
Titanium 13V, 11Cr 3Al (solution heat treated and aged)
Titanium 75A
AM350 (passive)
Silver
Gold
Graphite
End - (Less Active, Cathodic) “Gain electrons”

5. Galvanic Compatibility – Anodic Index
For harsh environments, such as outdoors, high humidity, and salt environments fall into this category. Typically there should be not more than 0.15 V difference in the "Anodic Index". For example; gold - silver would have a difference of 0.15V being acceptable.
For normal environments, such as storage in warehouses or non-temperature and humidity controlled environments. Typically there should not be more than 0.25 V difference in the "Anodic Index".
For controlled environments, such that are temperature and humidity controlled, 0.50 V can be tolerated. Caution should be maintained when deciding for this application as humidity and temperature do vary from regions.

6. Anodic Index Metallurgy Index (Volt)
Gold, solid and plated & Gold-platinum alloy
0.00
Rhodium plated on silver-plated copper
0.05
Silver, solid or plated; monel metal & High nickel-copper alloys
0.15
Nickel, solid or plated, titanium and alloys & Monel
0.30
Copper, solid or plated; low brasses or bronzes; silver solder & German silvery high copper-nickel alloys; nickel-chromium alloys
0.35
Brass and bronzes
0.40
High brasses and bronzes
0.45
18% chromium type corrosion-resistant steels
0.50
Chromium plated; tin plated; 12% chromium type corrosion-resistant steels
0.60
Tin-plate; tin-lead solder
0.65
Lead, solid or plated; high lead alloys
0.70
Aluminum, wrought alloys of the 2000 Series
0.75
Iron, wrought, gray or malleable, plain carbon and low alloy steels
0.85
Aluminum, wrought alloys other than 2000 Series aluminum, cast alloys of the silicon type
0.90
Aluminum, cast alloys other than silicon type, cadmium, plated and chromate
0.95
Hot-dip-zinc plate; galvanized steel
1.20
Zinc, wrought; zinc-base die-casting alloys; zinc plated
1.25
Magnesium & magnesium-base alloys, cast or wrought
1.75
Beryllium
1.85
Anodic Index

7. Polarity Reversal
The normal polarity of some galvanic couples under certain conditions may reverse with the passage of time.
Polarity reversal is invariably caused by the change of surface conditions of at least on of the coupled metals, such as formation of a passive film.

8. Polarity Reversal
This phenomenon was first reported by Schikorr in 1939 on a zinc-steel couple in hot supply water with iron becoming anodic to zinc, which has been a serious problem for galvanized steel hot water tanks.
Polarity reversal of an aluminum-steel couple has also been found to occur where aluminum alloys are used as anodes for cathodic protection of steel.

9. Preventive Measures
The essential condition for galvanic corrosion to occur is two dissimilar metals that are both electrically and electrolytically connected.

10. Practical Approaches to Prevent Galvanic Corrosion
(a) Avoid combinations of dissimilar metals that are far apart in the galvanic series applicable to the environment. (b) Avoid situation with small anodes and large cathodes (c) Isolate the coupled metals from the environment (d) Reduce the aggressiveness of the environment by adding inhibitors.

11. Practical Approaches to Prevent Galvanic Corrosion
(e) Use cathodic protection of the bimetallic couple with a rectifier or a sacrificial anode. (f) Increase the length of solution path between the two metals. This method is beneficial only in electrolytes of low conductivity, such as freshwaters, because strong galvanic action exists several meters away in highly conductive media, such as seawater.

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Galvanic Cell
Device in which chemical energy is changed to electrical energy.
Uses a spontaneous redox reaction to produce a current that can be used to do work.
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A Galvanic Cell
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14. Galvanic Cells anode oxidation cathode reduction spontaneous
redox reaction

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Galvanic Cell
Oxidation occurs at the anode.
Reduction occurs at the cathode.
Salt bridge or porous disk – devices that allow ions to flow without extensive mixing of the solutions.
Salt bridge – contains a strong electrolyte held in a Jello–like matrix.
Porous disk – contains tiny passages that allow hindered flow of ions.
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16. Two electrodes are connected by an external circuit.

17. Voltaic Cells The two half-cell reactions, as noted earlier, are:
(oxidation half-reaction)
(reduction half-reaction)
The first reaction, in which electrons are lost, is the oxidation half-reaction.
The electrode at which oxidation occurs is the anode. 2

18. Voltaic Cells The two half-cell reactions, as noted earlier, are:
(oxidation half-reaction)
(reduction half-reaction)
The second reaction, in which electrons are gained, is the reduction half-reaction.
The electrode at which reduction occurs is the cathode. 2

19. Voltaic Cells Note that the sum of the two half-reactions
is the net reaction that occurs in the voltaic cell; it is called the cell reaction
Note that electrons are given up at the anode and thus flow from it to the cathode where reduction occurs. 2

20. Voltaic Cells Note that the sum of the two half-reactions
is the net reaction that occurs in the voltaic cell; it is called the cell reaction
The anode in a voltaic cell has a negative sign because electrons flow from it.
The cathode in a voltaic cell has a positive sign 2

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Galvanic Cell
All half-reactions are given as reduction processes in standard tables.
Table 19.1 (next slide)
1 M, 1atm, 25°C
When a half-reaction is reversed, the sign of E° is reversed.
When a half-reaction is multiplied by an integer, E° remains the same.
A galvanic cell runs spontaneously in the direction that gives a positive value for E°cell.
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22. E0 is for the reaction as written
The more positive E0 the greater the tendency for the substance to be reduced
The half-cell reactions are reversible
The sign of E0 changes when the reaction is reversed
Changing the stoichiometric coefficients of a half-cell reaction does not change the value of E0

23. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
Consider a cell constructed of the following two half-reactions. 2

24. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
You will need to reverse one of these reactions to obtain the oxidation part of the cell reaction. 2

25. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
This will be Cd, because has the more negative electrode potential. 2

26. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
Therefore, you reverse the half-reaction and change the sign of the half-cell potential. 2

27. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
We must double the silver half-reaction so that when the reactions are added, the electrons cancel. 2

28. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
This does not affect the half-cell potentials, which do not depend on the amount of substance. 2

29. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
Now we can add the two half-reactions to obtain the overall cell reaction and cell emf. 2

30. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
Now we can add the two half-reactions to obtain the overall cell reaction and cell emf. 2

31. Cd is the stronger oxidizer
What is the standard emf of an electrochemical cell made of a Cd electrode in a 1.0 M Cd(NO3)2 solution and a Cr electrode in a 1.0 M Cr(NO3)3 solution?
Cd2+ (aq) + 2e Cd (s) E0 = V
Cd is the stronger oxidizer
Cd will oxidize Cr
Cr3+ (aq) + 3e Cr (s) E0 = V
Anode (oxidation):
Cr (s) Cr3+ (1 M) + 3e-
x 2
Cathode (reduction):
2e- + Cd2+ (1 M) Cd (s)
x 3
2Cr (s) + 3Cd2+ (1 M) Cd (s) + 2Cr3+ (1 M)
E0 = Ecathode - Eanode
cell
E0 = – (-0.74)
cell
E0 = 0.34 V
cell

32. Calculating Cell emf’s from Standard Potentials
The emf of a voltaic cell constructed from standard electrodes is easily calculated using a table of electrode potentials.
Note that the emf of the cell equals the standard electrode potential of the cathode minus the standard electrode potential of the anode. 2

33. A Problem To Consider
Calculate the standard emf for the following voltaic cell at 25 oC using standard electrode potentials. What is the overall reaction?
The reduction half-reactions and standard potentials are 2

34. A Problem To Consider
Calculate the standard emf for the following voltaic cell at 25 oC using standard electrode potentials. What is the overall reaction?
You reverse the first half-reaction and its half-cell potential to obtain 2

35. A Problem To Consider
Calculate the standard emf for the following voltaic cell at 25 oC using standard electrode potentials. What is the overall reaction?
To obtain the overall reaction we must balance the electrons. 2

36. A Problem To Consider
Calculate the standard emf for the following voltaic cell at 25 oC using standard electrode potentials. What is the overall reaction?
Now we add the reactions to get the overall cell reaction and cell emf. 2