reaction is expected but the metal shows no sign of corrosion. It is generally
agreed that passivation results from the formation of a current barrier on
the metal surface, usually in the form of an oxide film. This thin protective
film forms, and a change in the overall potential of the metal occurs when a
critical current density is exceeded at the anodes of the local corrosion cells
on the metal surface.
Among the more common bimetallic corrosion cell problems in ship
hulls are those formed by the mild steel hull with the bronze or nickel alloy
propeller. Also above the waterline problems exist with the attachment of
bronze and aluminium alloy fittings. Where aluminium superstructures are
introduced, the attachment to the steel hull and the fitting of steel equipment
to the superstructure require special attention. This latter problem is
overcome by insulating the two metals and preventing the ingress of water
as illustrated in Figure 27.2. A further development is the use of explosionbonded
aluminium/steel transition joints also illustrated. These joints are
free of any crevices, the exposed aluminium to steel interface being readily
protected by paint.
STRESS CORROSION Corrosion and subsequent failure associated
with varying forms of applied stress is not uncommon in marine structures.