Potential relative to saturated calomel half cell (volts) +0.2
0
Magnesium Zinc
Aluminium alloys Mild steel, cast iron Low alloy steel
Austenitic nickel cast iron Aluminium bronze
Naval brass, yellow brass, red brass Copper
Admiralty brass, aluminium brass Manganese bronze Silicon bronze Tin bronze
Stainless steel -types 410, 415 90/10 Copper-nickel Stainless steel -type 430 70/30 Copper-nickel Nickel aluminium bronze Nickel-chromium alloy 600 Silver braze alloys Nickel 200
Stainless steel -types 302, 304, 321, 347 Nickel-copper alloys 400, K-500 Stainless steel -types 316, 317 Nickel-iron-chromium alloy 825 Nickel-molybdenum alloy B Titanium
Nickel-chromium-molybdenum alloy C Platinum Graphite
Figure 7 - A galvanic series in seawater(2)
Problems with galvanic corrosion can usually be avoided by following the rules below:
1. Use materials of similar electrode potential in seawater.
2. Where this is not possible, make the key component of a more noble material; for example, use copper base alloy trim in a cast iron valve body.
3. Ensure that the material of lower potential is present in a much larger area than the more noble material so that the accelerated corrosion of the anode is spread over a large area.
4. Paint the more noble material. This can be beneficial as it reduces the cathode area even when the paint film is incomplete. Imperfect paint film, if only the anode is coated, would intensify the attack at breaks in the paint film.
5. Insulate to prevent galvanic current flowing between the metals, e.g. by insulating sleeves, washers, flange gaskets and spool pieces.
Marine Fouling or Biofouling
Growth of marine plant and animal life on surfaces exposed to seawater can give rise to significant corrosion problems. For example, shell growth in intake systems can become detached and shells carried into a heat exchanger where they generate local turbulence and erosion corrosion. Attachment of hard fouling during low velocities can lead to crevice corrosion in stainless steels but can act as a protective barrier to steel. In severe cases, fouling can cause complete blockage of heat exchangers necessitating shutdown for its removal. Chlorination can
be used to control fouling but in some cases this is impractical or environmentally unacceptable.
Copper and copper alloys are well known for their inherent behaviour towards macro fouling (Figure 8) whereas materials such as plastics, concrete, carbon steel and titanium foul readily. Copper-nickels have a high resistance to macrofouling, showing similar behaviour to copper itself. The 90-10 alloy has found several applications using its combined corrosion and biofouling resistance such as intake screens and sheathing on boats and offshore structures. Best resistance is achieved when the copper alloy is freely exposed and not galvanically coupled to less noble materials in any way.
Stainless steels, nickel-chromium- molybdenum alloys, and austenitic cast irons are all susceptible to fouling.
Active State -0.2 -0.4 -0.6 -0.8
-1.0
-1.2
-1.4
-1.6
40 | The Report • June 2018 • Issue 84
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68 |
Page 69 |
Page 70 |
Page 71 |
Page 72 |
Page 73 |
Page 74 |
Page 75 |
Page 76 |
Page 77 |
Page 78 |
Page 79 |
Page 80
