Galvanic or Bi-Metallic corrosion is one common form of corrosion in marine environments. It occurs when two (or more) dissimilar metals are brought into contact in an electrolyte. The corrosion rate of differing metals in seawater has been extensively studied and a galvanic series has been produced (see below).
Magnesium
Zinc
Aluminium
Steel/Iron
Nickel
Copper
Mill scale
Stainless steel
Silver
Gold
There are three conditions that must exist for galvanic corrosion to occur. First there must be two electrochemically dissimilar metals present. Second, there must be an electrically conductive path between the two metals (metals must be in contact). And third, there must be a conductive path or electrolyte (salt water) for the metal ions to move from the more anodic metal to the more cathodic metal. If any one of these three conditions does not exist, galvanic corrosion will not occur.
Both metals may corrode by themselves, but when in contact with a dissimilar metal however, the self corrosion rates will change: corrosion of the anode will accelerate while corrosion of the cathode will decelerate or even stop. The further apart the metals are in the galvanic table, the more the ignoble (anodic) metal will sacrificially corrode
Among the most common bi-metallic corrosion cell problems on ships are those produced by mild steel of the hull with the bronze propeller. Also problems exist when attaching aluminium superstructures or fittings to steel decks.
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
Corrosion protection may be afforded by the fitting of sacrificial anodes to any areas of the vessel that are regularly immersed in an electrolyte. The anodes consist of alloys of Zinc, Aluminium or Magnesium castings on a steel ‘strap’ which, after calculating size, weight, material specification and cost implications, are welded into position. The aforementioned alloys are chosen as they are higher in the Galvanic Series (less noble) than steel, and therefore will corrode sacrificially whilst protecting the steel.
Consideration should be taken with the placing of the anodes, particularly in cargo tanks, due to the spark hazard from Magnesium alloy anodes. There may be some height restrictions when fitting aluminium anodes for the same reason.
An alternative method of protecting the external sections of the hull is to install an impressed current system. This system consists of a rectifier, which distributes current through non non-consumable anodes to the hull. A reference cell fitted to the hull detects the potential difference and supplies the required voltage without overprotection.
dariusz.lipinski