| Corrosion of the water side of the boiler may occur in three ways, by
oxidation, by electrolytic dissolution and by acid attack.
The boiler corrosion process is essentially an oxidationl
reduction reaction. It may occur in various forms including the special form
known as caustic cracking. The most usual forms, however, are pittings and
general wastage.
When a metal dissolves in water, whether the water is alkaline or acid, the
metal is oxidized and the oxidizing agent is usually the hydrogen ion. Thus,
iron will dissolve more readily in acid solutions which have a high hydrogen
ion concentration. While boiler corrosion is an electrochemical process it may
be represented simply as: Fe —> Fe+ + 2 electrons.
The reaction continues in the presence of oxygen as
follows: 4 electrons + 02 + 2H20
—> 4 OH
This indicates that iron corrodes in pure water (such as condensate) hydroxide
ions are liberated and the pH increases at the site of attack until metal,
water and corrosion products (rust of metal oxide which tends to stifle the
reaction) reach equilibrium.
The corrosion reactions occurring in a boiler are complex but may be summarized
by saying that they are caused directly or indirectly by oxygen, carbon dioxide
or certain salts, e.g. magnesium chloride, in the boiler water.
Boiler corrosion can proceed undetected under a layer of scale. Boiler
corrosion fatigue occurs when the metal is in contact with a corrosive medium
and the metal is subjected to fluctuating stresses.
Caustic cracking results from the contact of concentrated
caustic soda solutions with steel which has not been stress relieved.
The main cause of boiler corrosion results from the dissolved gases, oxygen and
carbon dioxide found in feed water. The presence of oxygen and carbon dioxide
can cause corrosion of the feed water, boiler or condensate systems.
For all boilers it is essential to reduce the concentration of dissolved gases,
particularly oxygen in the feed of lowest value possible by mechanical means
followed by supplementary chemical treatment to remove the last traces of
oxygen. CO2 gases, however, will not be scavenged into
the boiler but will follow the steam into the system and decrease the
condensate pH value.
Sea water is a constant potential source of acid because of its content of
salts, particularly magnesium chloride, which hydrolize to yield acid. Upon
concentration in solution, magnesium chloride reacts with water in the
following manner:
MgCI2 + 2H20 --- Mg(OH2) + 2HC1
Since magnesium hydroxide is relative insoluble, it separates as sludge,
leaving hydrochloric acid in solution. Prevention of this acid type of boiler
corrosion is simply a matter of maintaining the boiler water alkaline.
Electrolytic corrosion is a process whereby, under the
influence of an electric current, metal is dissolved at one pole, transported
through the solution and deposited at the other pole. Such a process will
result in corrosion and pitting of the less noble, anodic metal.
The electric currents necessary for this process may be stray currents from
grounds in the ship, in which case the attack is usually localized and quite
severe. More commonly the electrical currents are generated by the chemical
forces resulting from dissimilarities and nonhomogeneities in the metals from
which the boiler is made.
It is impossible to eliminate this type of boiler corrosion by insuring a
perfectly homogeneous boiler structure, because there will always remain some
residual stresses in welded joints and not all mill scale can be removed.
The best protection against this type of boiler corrosion is to insure that the
boiler water is always alkaline.
The least destructive type of boiler corrosion is that called
"general", in which the entire exposed surface is evenly attacked.
Localized pitting, with its more rapid weakening and penetration of the metal
wall, is the more severe form of boiler corrosion.
If the alkalinity of the boiler water is maintained between the prescribed
limits, and care is exercised in preventing the introduction of air to the feed
water, the occurrence of serious pitting of this type should be very rare.
NOTE!
In open feed water systems, keep the feed water temperature as near the boiling
point as possible.
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