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Thursday, 08 December 2011 12:16

Galvanic Table

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

Metallurgy

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 an s 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
Aluminium, wrought alloys of the 2000 Series 0.75

Iron, wrought, gray or malleable, plain carbon and low alloy steels 0.85

Aluminium, wrought alloys other than 2000 Series aluminium, cast alloys of the silicon type 0.90

Aluminium, 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

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

Published in Marine corrosion
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Thursday, 08 December 2011 11:43

European Electrolysis

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.

Published in Marine corrosion
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Thursday, 08 December 2011 11:24

Corrosion cells on ships

Simple anti corrosion measures.

When steel is immersed in sea water (e.g. a ships hull) small galvanic currents are initiated at anodic areas of the metal surface, causing corrosion. Such corrosion predominates at the stern of a ship, where the combined effects of increased turbulence and differential metals results in accelerated corrosion rates. The application of Cathodic Protection effectively suppresses these corrosion cells by applying an opposing current from external anodes and if the propeller is to receive the benefits of cathodic protection then there must be a continuous electrical circuit between the propeller and the ships structure. This circuit usually exists when the propeller is at rest, where a metal to metal contact is made between the shaft and the stern tube liners, or main engine bearings and journals.
However, whilst the shaft is turning the bearing lubrication creates an intermittent high resistance which effectively insulates the propeller from the hull structure and since the propeller presents a relatively large surface area of bare metal, it attracts cathodic protection currents, which tend to discharge by arcing across the lubrication film and in so doing, results in spark erosion which eventually leads to pitting and ‘striping’ of white metal bearing surfaces. It is generally accepted, that the effects of arcing are minimised when the potential across the shaft/hull interface is less than 50 mV.

Published in Marine corrosion
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Wednesday, 07 December 2011 16:53

Principals of marine corrosion

 What is galvanic corrosion? Galvanic corrosion or Bimetallic Corrosion or Dissimilar Metal Corrosion, as sometimes called, is defined as the accelerated corrosion of a metal because of an electrical contact (including physical contact) with a more noble metal  or nonmetallic conductor (the cathode) in a corrosive electrolyte.

The less corrosion resistant or the "active" member of the couple experiences accelerated corrosion while the more corrosion resistant or the "noble" member of the couple experiences reduced corrosion due to the "cathodic protection" effect.

The most severe attack occurs at the joint between the two dissimilar metals. Further away from the bi-metallic joint, the degree of accelerated attack is reduced, the dissimilar metals do not have to be widely different i.e. copper and steel, two different steel water pipes will produce the same effect.

What causes galvanic corrosion? Different metals and alloys have different electrochemical potentials (or corrosion potentials) in the same electrolyte. When the corrosion potentials of various metals and alloys are measured in a common electrolyte (e.g. natural seawater) and are listed in an orderly manner (descending or ascending) in a tabular form,  a Galvanic Series is created. It should be emphasized that the corrosion potentials must be measured for all metals and alloys in the same electrolyte under the same environmental conditions (temperature, pH, flow rate etc.), otherwise, the potentials are not comparable.

The potential difference (i.e., the voltage) between two dissimilar metals is the driving force for the destructive attack on the active  metal (anode). Current flows through the electrolyte to the more noble metal (cathode) and the less noble (anode) metal will corrode. The conductivity of electrolyte will also affect the degree of attack. It is generally accepted that the Mediterranean sea is the  saltiest with mid Atlantic and the Caribbean being second saltiest (All the popular yacht cruising grounds) the Arctic is the least salty. The cathode to anode area ratio is directly proportional to the acceleration factor.

How to prevent galvanic corrosion? Galvanic corrosion is best prevented by not putting your vessel in the sea! However this not practicable so some of the following will help.

  • Select metals/alloys as close together as possible in the galvanic series.
  • Avoid unfavourable area effect of a small anode and large cathode, this effect is utilised for protection with the typical zinc anodes used on ships where the anode is encouraged to corrode rather than the hull.
  • Insulate dissimilar metals wherever practical or bond them together to achieve close to zero potential difference a minimum of 16mm² copper cable is needed to achieve low Ω bonding.
  • Apply coatings with caution. Paint the cathode and keep the coatings in good repair on the cathode and make sure the anode is clean.
  • Avoid threaded joints for materials far apart in the galvanic series, better still avoid joints with greatly dis-similar metals.

 

Published in Marine corrosion
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Wednesday, 07 December 2011 16:40

Propeller slip rings

 

GadSolutions manufactures slip rings for propeller shafts to combat the electrolysis action of a spinning shaft (steel) within bearings (usually bronze/white metal) immersed in sea water (electrolyte).

Various combinations of slip-ring and carbon brush materials are available but time has determined that only high silver composition brushes running on a silver track or 0% Oxygen Copper, can provide the effective and sustained low conductivity necessary to ensure that the shaft bonding and its connections maintains a contact resistance no greater than 0.001 Ohms.

Current passing through the gearbox bearings can severely damage them see Bearing degradation through electric current

At 59.6×106 S/m copper has the second highest electrical conductivity of any element, just after silver. This high value is due to virtually all the valence electrons (one per atom) taking part in conduction. The resulting free electrons in the copper amount to a huge charge density of 13.6×109 C/m3. This high charge density is responsible for the rather slow drift velocity of currents in copper cable (drift velocity may be calculated as the ratio of current density to charge density). For instance, at a current density of 5×106 A/m2 (typically, the maximum current density present in household wiring and grid distribution) the drift velocity is just a little over ⅓ mm/s.

Voltmeter showing the efficacity of slip rings on propeller shafts.In practice very few ships engineers have ohm meters capable of reading such low values and it is therefore difficult to determine the conductivity of the system. If GadSolutions install the system we use our calibrated low ohm reading test meter to insure a sound electrical connection with all parts of the system. A voltmeter with a range of 75 Millivolt F.S.D is installed to constantly monitor the connections and ensure the level of potential difference between shafts and hull is kept below 50 Millivolt (believed to be the minimum at which electrolysis starts) in practice we usually achieve 3 Millivolt PD.

The  system of shaft bonding comprises a split slip-ring arrangement and ancillary brush gear, which is designed to facilitate ease of assembly by proficient technical personnel and without the need for specialist tools. We hone each slip-ring to ensure a precise fit and constant contact with the propeller shaft, coupled with the 4 grub screws that drill into the shaft ensuring a very good electrical connection.

The monitoring panel shows a working installation of the slip-rings, 1 millivolt was shown as the potential difference between propellers ,shafts and the hull. With such a low reading no electrolysis will take place.

The finished article, a gadsolutions slip ring installed on a propeller shaft

brush-gear

 

Published in Marine corrosion
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