Corrosion Control and Paint Systems

Nature and Forms of Corrosion

There is a natural tendency for nearly all metals to react with
their environment. The result of this reaction is the creation of
a corrosion product which is generally a substance of very
similar chemical composition to the original mineral from
which the metal was produced. It is a natural process of
materials, usually metals, moving towards their lowest
possible energy state, resulting in a spontaneous reaction
between the material and its environment which results in the
degradation of that material. The word derives from Latin
‘corrodere’ which translates to ‘gnaw to pieces’.
For marine applications, mild steel remains the number one
metal for constructional purposes by virtue of its relatively
low cost, mechanical strength and ease of fabrication. Its
main drawback is that it corrodes easily in seawater and
unless adequately protected, rapidly loses strength which
may result in structural failure.
• ATMOSPHERIC CORROSION Protection against
atmospheric corrosion is important during the
construction of a ship, both on the building berth
and in the shops. Serious rusting may occur where
the relative humidity is above about 70 per cent;
the atmosphere in British shipyards is
unfortunately sufficiently humid to permit
atmospheric corrosion throughout most of the
year. But even in humid atmospheres the rate of
rusting is determined mainly by the pollution of the
air through smoke and/or sea salts.
• CORROSION DUE TO IMMERSION When a ship is in
service the bottom area is completely immersed and
the waterline or boot topping region may be
intermittently immersed in sea water. Under normal
operating conditions a great deal of care is required to
prevent excessive corrosion of these portions of the
hull. A steel hull in this environment can provide ideal
conditions for the formation of electro-chemical
corrosion cells.
• ELECTRO-CHEMICAL NATURE OF CORROSION Any
metal in tending to revert to its original mineral state
releases energy. At ordinary temperatures in aqueous
solutions the transformation of a metal atom into a
mineral molecule occurs by the metal passing into
solution. During this process the atom loses one or
more electrons and becomes an ion, i.e. an electrically
charged atom, with the production of an electric
current (the released energy).
• This reaction may only occur if an electron acceptor is present
in the aqueous solution. Thus any corrosion reaction is always
accompanied by a flow of electricity from one metallic area to
another through a solution in which the conduction of an
electric current occurs by the passage of ions. Such a solution
is referred to as an electrolyte solution; and because of its high
salt content sea water is a good electrolyte solution.
• A simple corrosion cell is formed by two different metals in an
electrolyte solution (a galvanic cell) as illustrated in the figure. 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 connection the
corrosion reaction on each plate would be small. Once the two
plates are connected externally to form an electrical path then
the corrosion rate of the iron will increase considerably, and the
corrosion on the copper will cease.
• When 2 dissimilar metals are in contact with each other in
the presence of a corrosive medium (electrolyte), the
more active metal in the galvanic series acts as an anode
and undergoes corrosion. This means, in a galvanic
series of metals, the more active metal acts as anode and
undergoes corrosion and the less active metal acts as a
cathode and stays protected.
• If these two metals are placed in seawater and are in
direct electrical contact, a current will pass through the
electrolyte from the more active metal (anode) onto the
least active metal(cathode). This electrical current is
referred to as Corrosion Current and is nothing but a
metal ion and electron transfer process from the anode,
which dissolves and passes into the solution. This simple
cell where the corrosion process takes place is called a
Galvanic Cell.
•
• 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 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 potentials as the
result of physical differences on the metal surface. The
latter is typified by steel plate carrying broken millscale in
sea water 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 moisture 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 as below:
• Noble (cathodic or protected) end
• Platinum, gold
• Silver
• Titanium
• Stainless steels, passive
• Nickel, passive
• High duty bronzes
• 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
• The positions of the metals in the table apply only in a sea water
environment; 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 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 the figure . 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. Internal stresses
produced by non-uniform cold working are often more
dangerous than applied stresses. For example, localized
corrosion is often evident at cold flanged brackets. A
particular case of stress corrosion in marine structures
has occurred with early wrought aluminium magnesium
alloy rivets. With a magnesium content above about 5 per
cent stress corrosion failures with cold driven rivets were
not uncommon. Here the corrosive attack is associated
with a precipitate at the grain boundaries, produced by
excessive cold working, which is anodic towards the
solid solution forming the grains of the alloy. Failure
occurs along an intergranular path. Specifications for
aluminium/magnesium alloy rivets now limit the
magnesium content.
• CORROSION/EROSION Erosion is essentially a mechanical
action but it is associated with electro-chemical corrosion in
producing two forms of metal deterioration. Firstly, in what is
known as ‘impingement attack’ the action is mainly electro-
chemical but it is initiated by erosion. Air bubbles entrained in
the flow of water and striking a metal surface may erode away
any protective film that may be present locally. The eroded
surface becomes anodic to the surrounding surface and
corrosion occurs. This type of attack can occur in most places
where there is water flow, but particularly where features give
rise to turbulent flow. Sea water discharges from the hull are a
particular case, the effects being worse if warm water is
discharged.
• Cavitation damage is also associated with a rapidly flowing
liquid environment. At certain regions in the flow (often
associated with a velocity increase resulting from a contraction
of the flow stream) the local pressures drop below that of the
absolute vapour pressure.
• Vapour cavities, that is areas of partial vacuum, are formed
locally, but when the pressure increases clear of this region
the vapour cavities collapse or ‘implode’. This collapse
occurs with the release of considerable energy, and if it
occurs adjacent to a metal surface damage results. The
damage shows itself as pitting which is thought to be
predominantly due to the effects of the mechanical damage.
However it is also considered that electrochemical action may
play some part in the damage after the initial erosion.