Corrosion can be broadly classified into two.

Dry Corrosion

Metals have a tendency to return back to their natural state of metal oxides. These metal oxides form a thin layer over them and it prevents further corrosion attack.

Wet Corrosion

Every metals have a specific potential when placed in an electrolyte. When two metals of different potential are placed in an electrolyte, it forms an electro-chemical cell. This results in flow of electrons to and from the metals. Even different parts of a single metal can have different potentials in certain conditions. This also may set up an electro-chemical cell or galvanic cell or corrosion cell.

Whatever be the case a cell means there is an anode and cathode, in which anode loses electrons and deteriorates with time. While at the cathode two conditions are there:

• If electrolyte is acidic, hydrogen gas is released.
• If electrolyte is alkaline, polarizing layer of hydrogen is formed, which prevents further corrosion.

Another thing to be noted is that when there is dissolved oxygen in the electrolyte, the polarizing layer of hydrogen is reacted with oxygen to form water. So no more protection for the metal.

Hence all types of corrosion in marine boilers can be minimized if boiler water is alkaline and it contains no dissolved oxygen.

See Also: How to Eliminate Oxygen and Introduce Alkalinity in Boiler Water

The variation in rate of corrosion with alkalinity at different dissolved oxygen content is shown below.

 

Where,

1. High Dissolved Oxygen
2. Medium Dissolved Oxygen
3. Low Dissolved Oxygen
4. Nil Dissolved Oxygen

The graph clearly explains the effect of high dissolved oxygen content and alkalinity of boiler water in rate of corrosion.