This can occur when two different metals are placed in contact with each other and is caused by the greater willingness of one to give up electrons than the other. Three special features of this mechanism need to operate for corrosion to occur:
- The metals need to be in contact electrically
- One metal needs to be significantly better at giving up electrons than the other
- An additional path for ion and electron movement is necessary.
Metals have to be significantly different in terms of their nobility in the specific environment that they are exposed to. This can be looked up in a table.
Ranking in sea water (top of table is most noble)
Noble
Platinum
Gold
Titanium
Stainless Steel 316 (passive state)
Brasses
Tin
Lead
Stainless steel 316 (active state
Carbon steel
Aluminium alloys
Zinc
Magnesium
Prevention
Prevention of this problem is based on ensuring that one or more of the three features do not
exist.- Break the electrical contact using plastic insulators or coatings between the metals.
- Select metals close together in the galvanic series.
- Prevent ion movement by coating the junction with an impermeable material, or ensure environment is dry and liquids cannot be trapped.
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