Bi-Metallic corrosion Galvanic Corrosion is the additional corrosion that occurs when dissimilar metals are in contact in the presence of an electrolyte. The corrosion of a metal, the anode, results from the positive current flowing from the anode to the less reactive (more noble) metal, the cathode, through the electrolyte. This process is similar to the conventional corrosion of a single, uncoupled metal but generally proceeds at a higher rate depending on the difference in electrochemical reactivity of the anode and cathode metal. The requirements for bi-metallic corrosion are as follows:
An electrolyte bridging the two metals
Electrical contact between the two metals.
A difference in potential between the metals to enable a significant galvanic current
A sustained cathodic reaction on the more noble of the two metals.
Electrolyte
The degree of bi-metallic corrosion is affected by the electrolyte pH and conductivity. The intensity of the corrosion can increase with the conductivity of the electrolyte. Typical values of conductivity of various fluids are listed below;
Distilled Water0.5-2 μS/cmStored Distilled Water2-4 μS/cmSupply Water50-1500 μS/cmSea Water50,000 μS/cmSat. Sodium Chloride250,000 μS/cmSulphuric Acidup to 800,000 μS/cm
Bi-metallic corrosion is seldom a problem when the metals are immersed in pure water.
Methods of Reducing Corrosion resulting from Galvanic Corrosion
Where contact between dissimilar metals cannot be avoided the following steps should be considered
Select metals that are close together in the galvanic series for the relevant environment
Avoid relatively small areas of the less noble metal and large areas of the more noble metal
Insulate the metals from each other
Exclude electrolyte from around the bimetallic junction e.g painting
Paint both metals where possible: if impractical paint the most noble metal
Provide additional corrosion allowance on the less noble metal
Apply compatible metal or sacrificial metal coatings
If electrical insulation is used to minimise the risk, then test for the insulation quality as part of maintenance regime
Galvanic Series
Reference Oxidation Reduction Galvanic corrosion is driven by the voltage potential between two electrically connected conductors ( To minimize this form of attack, materials in electrical contact, if required, should be selected so as to minimize their relative potential. The galvanic series of metals lists common materials in order of their electrical potential relative to a recognized standard. Materials widely separated on this list will rapidly corrode in the presence of electolyte (e.g. Seawater) when in electrical contact, the anodic material suffering rapid material loss. Materials close together on this list will suffer less damage due to corrosion.
Anodic – Least Noble
Magnesium
Magnesium Alloys
Zinc
Cadmium
Aluminum
Mild Steel , Wrought Iron
Cast Iron, Low Alloy High Strength Steel
Chrome Iron (active)
Stainless Steel, 430 Series (active)
Stainless Steel 302, 303, 321, 347, 410,416, (Active)
Ni – Resist
Stainless Steel 316, 317, (Active)
Aluminum Bronze
Hastelloy C (active) Inconel 625 (active)
Titanium (active)
Lead – Tin Solders
Lead
Tin
Inconel 600 (active)
Nickel (active)
Hastelloy B (active)
Brasses
Copper
Manganese Bronze , Tin Bronze (
Nickel Silver
Copper – Nickel Alloy 90-10
Copper – Nickel Alloy 80-20 s
Stainless Steel 316, 430
Nickel, Aluminum, Bronze
Monel
Silver Solder
Nickel (passive)
60 Ni- 15 Cr (passive)
Inconel 600 (passive)
80 Ni- 20 Cr (passive)
Chrome Iron (passive)
Stainless Steel 302, 303, 304, 321, 347,(PASSIVE)
Stainless Steel 316, 317,(PASSIVE)
Incoloy 825nickel – Molybdeum – Chromium
Iron Alloy (passive)
Silver
Titanium (pass.) Hastelloy C (passive)
Inconel 625(pass.)
Graphite
Zirconium
Gold
Platinum
Source: wilsonpipeline Pipe Industry Co., Limited (www.wilsonpipeline.com)
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