Galvanic Series

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.

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 5052-0 Al 5052-H12 Al 5456-0, H353 Al 5052-H32 Al 1100-0 Al 3003-H25 Al 6061-T6 Al A360 (die cast) Al 7075-T6 Al 6061-0 Indium Al 2014-0 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”

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.

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

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.

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.

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

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.

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.

Copyright © Cengage Learning. All rights reserved 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. Copyright © Cengage Learning. All rights reserved

Copyright © Cengage Learning. All rights reserved A Galvanic Cell Copyright © Cengage Learning. All rights reserved

Galvanic Cells anode oxidation cathode reduction spontaneous redox reaction

Copyright © Cengage Learning. All rights reserved 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. Copyright © Cengage Learning. All rights reserved

Two electrodes are connected by an external circuit.

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

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

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

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

Copyright © Cengage Learning. All rights reserved 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. Copyright © Cengage Learning. All rights reserved

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

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

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

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

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

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

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

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

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

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 = -0.40 V Cd is the stronger oxidizer Cd will oxidize Cr Cr3+ (aq) + 3e- Cr (s) E0 = -0.74 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) 3Cd (s) + 2Cr3+ (1 M) E0 = Ecathode - Eanode cell E0 = -0.40 – (-0.74) cell E0 = 0.34 V cell

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

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

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

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

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