Page 321 - Ship Construction.DJ Eyres 6Ed

Ch27-H8070.fm Page 310 Wednesday, October 18, 2006 7:01 AM
310 Ship Construction
A simple corrosion cell is formed by two different metals in an electrolyte
solution (a galvanic cell) as illustrated in Figure 27.1. It is not essential to
have two different metals as we shall see later. As illustrated a pure iron
plate and a similar pure copper plate are immersed in a sodium chloride
solution which is in contact with oxygen at the surface. Without any connec-
tion the corrosion reaction on each plate would be small. Once the two
plates are connected externally to form an electrical path then the corro-
--- ใช้เพื่อการศึกษาเท่านั้น---
sion rate of the iron will increase considerably, and the corrosion on the
copper will cease. The iron electrode by means of which the electrons leave
the cell and by way of which the conventional current enters the cell is the
งานห้องสมุด ศูนย์ฝกพาณิชย์นาวี
anode. This is the electrode at which the oxidation or corrosion normally
takes place. The copper electrode by means of which the electrons enter
the cell and by way of which the conventional current leaves the cell is the
cathode, at which no corrosion occurs. A passage of current through the
electrolyte solution is by means of a flow of negative ions to the anode and a
flow of positive ions to the cathode.
Electro-chemical corrosion in aqueous solutions will result from any
anodic and cathodic areas coupled in the solution whether they are metals
of different potential in the environment or they possess different poten-
tials as the result of physical differences on the metal surface. The latter is
ึ
typified by steel plate carrying broken millscale in sea water (Figure 27.1)
or corrosion currents flowing between areas of well painted plate and areas
of defective paintwork.
In atmospheric corrosion and corrosion involving immersion both oxygen
and an electrolyte play an important part. Plates freely exposed to the
atmosphere will receive plenty of oxygen but little moisture, and the mois-
ture present therefore becomes the controlling factor. Under conditions of
total immersion it is the presence of oxygen which becomes the controlling
factor.

BIMETALLIC (GALVANIC) CORROSION Although it is true to say
that all corrosion is basically galvanic, the term ‘galvanic corrosion’ is usually
applied when two different metals form a corrosion cell.
Many ship corrosion problems are associated with the coupling of metallic
parts of different potential which consequently form corrosion cells under
service conditions. The corrosion rates of metals and alloys in sea water
have been extensively investigated and as a result galvanic series of metals
and alloys in sea water have been obtained.
A typical galvanic series in sea water is shown in Table 27.1.
The positions of the metals in the table apply only in a sea water environ-
ment; and where metals are grouped together they have no strong tendency
to form couples with each other. Some metals appear twice because they
are capable of having both a passive and an active state. A metal is said
to be passive when the surface is exposed to an electrolyte solution and a