Boiler Corrosion

Galvanic Corrosion
Can occur due to Oxygen attack or contact of two dissimilar metals in an electrolyte (eg: Copper + Steel).

Oxygen Attack (Pitting Corrosion)

At the Anode:

Metal goes into solution               Fe ⟶ Fe²⁺ + 2 e^-               OXIDATION

At the Cathode:

Oxygen is reduced           ½ O₂ + H₂O + 2 e^- ⟶ 2 OH^-            REDUCTION

The variables pH, temperature and the concentration of oxygen affect the rate of corrosion.
To avoid this alkaline conditions are maintained in the boiler.
Oxygen reacts with iron to give ferric oxide (rust) Fe₂O₃ which will not protect the metal from further attack and metal is continuously dissolved.

4Fe + 3O₂ → 2 Fe₂O₃ Hematite (ferric oxide)

Oxygen corrosion is usually observed as localized pitting on a metal surface. (Pitting Corrosion)

This form of corrosion can be reduced by:

1.by reducing the level of oxygen as far as possible using mechanical means which include deaeration and/or judicious heating, coupled with good feed-system design (Cascade Tank)

2. by ensuring that there is an adequate and controlled reserve of alkalinity in the boiler water at all times.

3. by the application and maintenance of an adequate reserve of a chemical oxygen scavenger.

Acidic Corrosion

When the pH is reduced the rate of corrosion increases.
Usually occurs when there are acidic compounds present in the boiler water. (eg: decomposition of MgCl gives HCl).

Acidic corrosion – Condensate lines

Most commonly due to CO₂. Mostly given off by thermal breakdown of bicarbonates and carbonates present in the feedwater.

Ca/Mg(HCO₃)₂ → Ca/MgCO₃ + CO₂ + H₂O

This CO₂ is carried over with steam and dissolves with the condensate. As it dissolves it forms Carbonic Acid.

CO₂ + H₂O → H₂CO₃

Corrosion due to acid attack usually shows thinning of metal.

Caustic Embrittlement

This type of corrosion mainly occurs in high pressure boilers where high heat transfer rates are present.
At the metal surface the high rate of steam evolution causes high concentration of salts and alkalinity at the heat
transfer surface.

High concentrations of alkalinity will begin to attack the boiler metal.

1. 4 NaOH + Fe₃O₄ → 2 NaFeO₂ + Na₂ FeO₂ + 2 H₂O Magnetite film breaks down

2. Fe + NaOH + H₂O → Na₂FeO₂ + H₂ Tube-wall attack and wastage of metal & Hydrogen evolution.

3. 4 H + C → CH4 (methane)

Here inter-granular penetration by hydrogen causes de-carbonisation of the metal(Steel) along grain boundaries which can cause catastrophic failure.