Corrosion in Metals

      

        

   Steel (Fe)

                   

Will readily corrode by the reaction with oxygen in the water primarily by galvanic action.

The Iron reduces to Iron ions at anode, the oxygen is reduced by combining with water and electrons passed from the anode (by iron changing to ions) to hydroxyl ions. Temperature, pH and the concentration of oxygen all affect the rate of corrosion.

The oxygen reacts with the Fe²⁺ to form Hematite (Fe₂O₃) . This is a reddish brown loose deposit. With reduced oxygen content the formation of Magnetite (Fe₃ O₄) will occur. This is a more tenacious layer and forms a protective boundary on the metal preventing further corrosion. This layer may be removed in low pH or high pH conditions.

Scabs and tubercles of Ferric oxides and Ferric hydroxides form over an active pit. 

Stainless Steels

These are alloys of steel with high chromium content (around 11%). The alloying process results in a material with excellent corrosion resistance. Oxygen combines with the chromium and iron to form a tenacious self healing oxide layer.

The disadvantage of stainless steel is that in low oxygen environments, such as boil feed, the corrosion resistance is actually reduced. In addition stress corrosion cracking and pitting can occur when in the presence of chlorine ions. In this way stainless steel is not recommended in situations were stagnant sea water might exists at it could perforate quicker than mild steel. The chlorine ions are the correct right size to enter the atomic matrix of the metal and their concentration accelerates corrosion by the propogation of cracks. Catastrophic failure can occur. 

Copper(Cu) and Copper alloys

Used in heat exchangers due to there high heat conductivity. Copper corrosion in oxygenated water is slow due to the time taken for oxygen to diffuse throught the oxide layer.

As copper is a relatively soft metal water velocities must be kept low. Its presence can lead to heavy pitting if deposited in steel systems. Ammonia in the water can remove the oxide layer and promote rapid corrosion

Aluminium (Al)

Is essentially inert in neutral water up to about 180'C. It is ampoteric meaning it will corrode rapidly in high and low pH conditions. In the presence of Sodium Carbonate or sodium hydroxide at pH above 9 this corrosion is particularly severe. These conditions may exist were boiling occurs concentrating hydroxyl ions. 

Zinc (Zn)

Is anodic to steel and is often used as a protective coating on steel in a process called galvanising. It is ampoteric, corrosion increases in high and low pH conditions.

If galvanised pipes are to be fitted in a cooling circuit the coating must first be removed by controlled acid washing.