Galvanic corrosion

Galvanic series relationships are useful as a guide for selecting metals to be joined, will help the selection of metals having minimal tendency to interact galvanically, or will indicate the need or degree of protection to be applied to lessen the expected potential interactions. In general, the further apart the materials are in the galvanic series, the higher the risk of galvanic corrosion, which should be prevented by design. Conversely, the farther one metal is from another, the greater the corrosion will be. However, the series does not provide any information on the rate of galvanic corrosion and thus serves as a basic qualitative guide only.

The use of the galvanic series has to be done with caution and a basic knowledge of the environments that is a necessary part of this serious form of corrosion. The following documents provide different points of view regarding the ranking of metals and coatings in practical schemes for preventing galvanic corrosion.

Galvanic Table

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. A "galvanic series" applies to a particular electrolyte solution, hence for each specific solution which is expected to be encountered for actual use, a different order or series will ensue. In a galvanic couple, the metal higher in the series (or the smaller) represents the anode, and will corrode preferentially in the environment. Listed below is the latest galvanic table from MIL-STD-889 where the materials have been numbered for discussion of characteristics. However, for any combination of dissimilar metals, the metal with the lower number will act as an anode and will corrode preferentially.

Active (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

End - Noble (Less Active, Cathodic)

Galvanic Compatibility

Often when design requires that dissimilar metals come in contact, the galvanic compatibility is managed by finishes and plating. The finishing and plating selected facilitate the dissimilar materials being in contact and protect the base materials from corrosion.

• 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

Galvanic Series in Seawater

A galvanic series has been drawn up for metals and alloys in seawater, which shows their relative nobility. The series is based on corrosion potential measurements in seawater. The relative position of the materials can change in other environments. The further apart the materials are in this series, the higher the risk of galvanic corrosion.

Most cathodic, noble, or resistant to corrosion

• Platinum
• Gold
• Graphite
• Titanium
• Silver
• Chlorimet 3
• Hastelloy C

1. 18-8 Mo stainless steel (passive)
2. 18-8 stainless steel (passive)
3. Chromium steel >11 % Cr (passive)
4. Inconel (passive)
5. Nickel (passive)
6. Silver solder
7. Monel
8. Bronzes
9. Copper
10. Brasses

• Chlorimet 2
• Hastelloy B
• Inconel (active)
• Nickel (active)
• Tin
• Lead
• Lead-tin solders
• 18-8 Mo stainless steel (active)
• 8-8 stainless steel (active)
• Ni-resist
• Chromium steel >11 % Cr (active)

1. Cast iron
2. Steel or iron
3. 024 aluminum
4. Cadmium
5. Commercially pure aluminium
6. Zinc
7. Magnesium and its alloys

Most anodic or easy to corrode

It would appear that the European Union has a significant electrolysis problem. It turns out that the 1 and 2 Euro coins that are bimetallic are leaching higher than recommended levels of Nickel into people that have an acidic sweat, the sweat acts as an electrolyte and promotes the leaching of Nickel into the skin.

Corrosion

Electrolysis is a form of corrosion. This is not to be confused with the breakdown of metal by mechanical means such as wear, galling, erosion, or cavitation. Electrolysis is the alteration, decomposition or breakdown of metals or alloys from persistent electrochemical reactions (or by direct chemical attack).

Simplistically Corrosion can be broken down to two possible causes: Galvanic Action or Electrolysis. It is impossible to tell which was responsible after the fact.

Galvanic Action: This is the most common cause of electrolysis in the marine environment. Galvanic corrosion is the interaction of two dissimilar metals connected in an electrolyte (salt-water). In this situation the least noble becomes the anode and corrodes. The more noble is the cathode which in extreme cases can actually be coated with the anode metal. Metals can be connected through a bonding system or directly such as Bronze prop to Stainless shaft. Thus by adding a shaft zinc the zinc deteriorates before the prop.

Electrolysis on the other hand is corrosion caused by stray current. Generally this is much more aggressive causing a lot of damage in a relatively short amount of time.

Typically when Nickel-Aluminium-Bronze (Nibral) corrodes the aluminium leaches out of the alloy leaving irregular shallow pitting in the blade surface. In advanced cases the blade edges become scalloped.

Manganese Bronze on the other hand turns copper red as the zinc leaches out of the alloy. In advanced cases the blade edges actually begin to peal apart much like the leaves of a book. Prior to this the propeller will cease to ring and only thud when tapped with a hammer.
In copper based alloys once corrosion has set in it becomes almost impossible to weld the propeller. The metal has changed sufficiently that attempting to weld the affected areas only results in creating a larger hole.

Corrosion can be aggravated by cavitation. Cavitation is the mechanical breakdown of the propeller material through the implosion of small air bubbles on the surface of the blades as a result of the water “boiling” from low pressure. If the structure of the metal is compromised by corrosion than cavitation erosion occurs much more readily.

The chromium, nickel, and molybdenum content of stainless steel are major contributors to it’s corrosion resistance. Stainless steel is interesting because it is one of the metals that, under certain conditions becomes more noble. The oxygen in the atmosphere and in moving water is sufficient to allow stainless steel to form or repair a tough transparent film of chromium-oxide that renders the metal non-corrodible. When this film is damaged under conditions where there is insufficient oxygen to repair it stainless becomes active and corrodes freely. Typically this corrosion is localized but can be very aggressive. This type of corrosion is often referred to as crevice corrosion.

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