The Galvanic Series and Corrosion

[GordMay]

The Galvanic Series & Corrosion:

The Galvanic Series is a list, sorted by corrosion potentials, for various alloys and pure metals in sea water. It should not be confused with the emf series. The emf series is a list of half-cell potentials for standard state conditions measured with respect to the standard hydrogen electrode, while the Galvanic Series is based on corrosion potentials in sea water.

Each metal or alloy has a unique corrosion potential (Ecorr), when immersed in a corrosive electrolyte. The most negative or active alloy is always attacked preferentially by galvanic corrosion, whereas the more noble metal becomes cathodic (where reduction of hydrogen ions or oxygen takes place) and is protected from corrosion. Generally, the closer one metal is to another in the series, the more compatible they will be (the galvanic effects will be minimal). Conversely, the farther one metal is from another, the greater the corrosion will be.

Often the relative areas of each metal exposed are more important than their position in the galvanic series. If the anode (more active metal) has a large area with respect to the cathode (more noble metal), the small area of the cathode will not provide enough current to support uniform corrosion of the anode. However, if the anode is small in comparison to the cathode, the rate of corrosion of the anode will be greatly accelerated and corrosion will be localized adjacent to the more noble metal. When using coatings to prevent galvanic corrosion, it is important to coat the more noble metal rather than the active metal, so that when defects are introduced to the coat, the effects are not catastrophic.

Listed below is the latest galvanic table from MIL-STD-889 where the materials have been numbered according to how they interact in a galvanic couple in a seawater environment. The table is the “Galvanic Series of Metals in Sea Water” from Army Missile Command Report RS-TR-67-11, "Practical Galvanic Series."

From: (#1) Corroded (Active) End - Most Anodic - Least Noble
To: (#92) Protected (Passive) End - Most Cathodic - Most Noble

ANODIC
1. Magnesium
2. Mg alloy AZ-31B
3. Mg alloy HK-31A
4. Zinc (hot-dip, die cast, or plated)
5. Beryllium (hot pressed)
6. Al 7072 clad on 7075
7. Al 2014-T3
8. Al 1160-H14
9. Al 7079-T6
10. Cadmium (plated)
11. Uranium
12. Al 218 (die cast)
13. Al 5052-0
14. Al 5052-H12
15. Al 5456-0, H353
16. Al 5052-H32
17. Al 1100-0
18. Al 3003-H25
19. Al 6061-T6
20. Al A360 (die cast)
21. Al 7075-T6
22. Al 6061-0
23. Indium
24. Al 2014-0
25. Al 2024-T4
26. Al 5052-H16
27. Tin (plated)
28. Stainless steel 430 (active)
29. Lead
30. Steel 1010
31. Iron (cast)
32. Stainless steel 410 (active)
33. Copper (plated, cast, or wrought)
34. Nickel (plated)
35. Chromium (Plated)
36. Tantalum
37. AM350 (active)
38. Stainless steel 310 (active)
39. Stainless steel 301 (active)
40. Stainless steel 304 (active)
41. Stainless steel 430 (active)
42. Stainless steel 410 (active)
43. Stainless steel 17-7PH (active)
44. Tungsten
45. Niobium (columbium) 1% Zr
46. Brass, Yellow, 268
47. Uranium 8% Mo
48. Brass, Naval, 464
49. Yellow Brass
50. Muntz Metal 280
51. Brass (plated)
52. Nickel-silver (18% Ni)
53. Stainless steel 316L (active)
54. Bronze 220
55. Copper 110
56. Red Brass
57. Stainless steel 347 (active)
58. Molybdenum, Commercial pure
59. Copper-nickel 715
60. Admiralty brass
61. Stainless steel 202 (active)
62. Bronze, Phosphor 534 (B-1)
63. Monel 400
64. Stainless steel 201 (active)
65. Carpenter 20 (active)
66. Stainless steel 321 (active)
67. Stainless steel 316 (active)
68. Stainless steel 309 (active)
69. Stainless steel 17-7PH (passive)
70. Silicone Bronze 655
71. Stainless steel 304 (passive)
72. Stainless steel 301 (passive)
73. Stainless steel 321 (passive)
74. Stainless steel 201 (passive)
75. Stainless steel 286 (passive)
76. Stainless steel 316L (passive)
77. AM355 (active)
78. Stainless steel 202 (passive)
79. Carpenter 20 (passive)
80. AM355 (passive)
81. A286 (passive)
82. Titanium 5A1, 2.5 Sn
83. Titanium 13V, 11Cr, 3Al (annealed)
84. Titanium 6Al, 4V (solution treated and aged)
85. Titanium 6Al, 4V (anneal)
86. Titanium 8Mn
87. Titanium 13V, 11Cr 3Al (solution heat treated and aged)
88. Titanium 75A
89. AM350 (passive)
90. Silver
91. Gold
92. Graphite
CATHODIC

See also:

PRINCIPLES OF GALVANIC CORROSION ~ by W.C. Johnson
University of Virgina Department of Materials Science and Engineering
http://www.tu-darmstadt.de/fb/ms/stu...10/mse10-0.htm
Specifically:
Galvanic Corrosion
http://www.tu-darmstadt.de/fb/ms/stu...10/mse10-3.htm

HTH,
Gord May

[BC Mike]

Exspensive stuff sinks and cheap stuff floats.
Michael

[Sonosailor]

I knew you'd be on this before I could get to it. Keep up the good work, Gord

[Alan Wheeler]

Now that is good Mike. I have a present, one socket ratchet, one adjustable wrench, five, yes five wood clamps, a drill bit, a pair of vice grips, a couple of screw drivers and I have lost count as to how many nuts, bolts and washers, are all now lying at the bottom under my boat in the marina. We have the marina dredge every now and then. That lot is sure going to make a noise up the spout of the machine.

[GordMay]

Another
GALVANIC SERIES OF METALS IN SEAWATER (TABLE 4-1)
from: USCG NVIC 7-95
“Guidance on Inspection, Repair, and Maintenance of Wooden Hulls”
http://www.uscg.mil/hq/g-m/nvic/7_95/n7-95.htm

Voltages are those measured against a silver/silver chloride (Ag/AgC1) reference electrode.

Noble or Cathodic Metals (Designation) = Voltage Potential

Graphite (C) = + 0.27 V
Platinum (Pt) = + 0.24 V
Titanium (Ti) = + 0.02 V
Incoloy 825 ( ) = + 0.02 V

Ag/AgCl Reference Electrode = 0.00 V

Active or Anodic Metals (Desig. = Potential

316 Stainless Steel (passive) = - 0.03 V
Monel 70 %, 30 % cu/ (400, K-500) = - 0.06 V
304 Stainless Steel (passive) = - 0.06 V
Silver (Ag) = - 0.10 V
Nickel (Ni) = - 0.13 V
Silver Brazing Alloys ( ) = - 0.13 V
Inconel 600 (passive) = - 0.13 V
Ni-Al Bronze (C63x, C954-8) = - 0.16 V
Cu-Ni 70-30 (C715-9, C964) = - 0.18 V
Lead (Pb) = - 0.20 V
Cu-Ni 80-20 and 90-10 (C710, C706) = - 0.22 V
"Nickel Silver" (C745-70, C97x) = - 0.25 V
Phosphor (Tin) Bronze (C524, C903-5, C92x) = - 0.26 V
Silicon Bronze (C655, C872) = - 0.25 V
Manganese Bronze (C675, C86x) = - 0.29 V
Admiralty Brass (C443-5) = - 0.30 V
Aluminum Brass (C687-90) = - 0.30 V
Lead-Tin solder ( ) = - 0.30 V
Copper (C10x, Cllx, C12x) = - 0.31 V
Tin (Sn) = - 0.31 V
Naval Brass/"Bronze" Tobin Bronze (C464) = - 0.33 V
Yellow and Red Brass (C23x-27x, C83x-85x) = - 0.33 V
Aluminum Bronze (C606-24, C952-3) = - 0.34 V
Stainless Steel 316 (active) = - 0.39 V
Stainless Steel 304 (active) = - 0.49 V
Low Alloy Steels = - 0.58 V
Steel, Cast Iron = - 0.63 V
Aluminum Alloys = - 0.87+/-.10 V
Zinc ( Zn) = - 1.00 V
Magnesium (Mg) = - 1.60 V

Notes on the Use of the Galvanic Series Table

All values are for sea water at room temperature.

Average variability is +/-.04 Volts for alloys containing nickel or iron, +/-.02 V for copper alloys without nickel.

Sign of corrosion potential assumes that the "COMMON" or negative (Black) terminal of the voltmeter is connected to the reference electrode and the "VOLTS-OHMS", or positive (Red) terminal is connected to the metal to be measured. The reference electrode must be immersed in the same body of electrolyte as the metal being measured, preferably in close proximity.

To use Zinc as a reference electrode instead of Ag/AgCl add +1.00 volts to the potentials listed in this table. For example, low alloy steel should measure -.58V +1.00 V, or +0.42V against zinc, and magnesium should measure - 1.60V + 1.00V,or -0.60V against zinc. Extremely accurate measurements should not be attempted with zinc as a reference, since it isn't as stable as the Ag/AgCl electrode.

Metals are receiving cathodic protection when their measured potentials are more negative than their natural corrosion potentials listed here, and are generally completely protected from corrosion when their potentials measure .20V to .25V more negative than the values listed in this chart.

Metals are receiving stray current or are the anode of a galvanic system (these are equivalent situations) when their potentials measure more positive than the values listed in this chart. Metals in this situation are generally suffering accelerated corrosion.

Copper alloy designations: Alloys numbered C100 to C799 are wrought alloys, those numbered C800 to C999 are casting alloys. "x" indicates a range of alloys sharing the preceding digits.

[Richhh]

The electromotive series is important but just as important is that all such dissimilar metals must be electrically bonded so that the least noble of the series is the actual anode. If not electrically bonded then you may experience localized corrosion ... and wonder why the 'anodes' are not working.

[vittorio6]

galvanic corrosion is a function of several factor and the galvanic series of metals is just one.. you have also to consider the area ratio between the metals.. large cathode area and small anode will increase the corrosion rate

Vitt

[GordMay]

Greetings and welcome aboard the CF, Vittorio.

Thanks for reminding us of the importance of the relative arrea (and mass) of anodic/cathodic couples.

Originally Posted by GordMay

... Often the relative areas mass of each metal exposed are more important than their position in the galvanic series. If the anode (more active metal) has a large area or mass with respect to the cathode (more noble metal), the small area of the cathode will not provide enough current to support uniform corrosion of the anode. However, if the anode is small in comparison to the cathode, the rate of corrosion of the anode will be greatly accelerated and corrosion will be localized adjacent to the more noble metal ...

[delmarrey]

So this must be why the zincs on my prop shaft go so quickly?

However, if the anode is small in comparison to the cathode, the rate of corrosion of the anode will be greatly accelerated and corrosion will be localized adjacent to the more noble metal ...