Corrosion within boilers

Corrosion within boilers

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.

In open feed water systems, keep the feed water temperature as near the boiling point as possible.