Page 323 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 312 Wednesday, October 18, 2006 7:01 AM
312 Ship Construction
TABLE 27.1
Galvanic Series of Metals and Alloys in Sea Water

Noble (cathodic or protected) end
Platinum, gold
Silver
Titanium
--- ใช้เพื่อการศึกษาเท่านั้น---
Stainless steels, passive
Nickel, passive
High duty bronzes
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
Copper
Nickel, active
Millscale
Naval brass
Lead, tin
Stainless steels, active
Iron, steel, cast iron
Aluminium alloys
Aluminium
Zinc
Magnesium
ึ
Ignoble (anodic or corroding) end

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 equip-
ment 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 explosion-
bonded 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.