Pitting corrosion is a direct result to the presence of OXYGEN dissolved gas in boiler water.
Pitting corrosions a form of extremely localized corrosion that leads to the creation of small holes in the metal the characteristic pitting attack can cause metal failure if allowed to progress unabated .

Pitting is considered to be more dangerous than uniform corrosion damage because it is more difficult to detect, predict and design against.

Corrosion products often cover the pits the most universally accepted technique to prevent this problem is the removal of dissolved oxygen.

 

   Mechanism

That a small cavity exists in the surface of the metal into which oxygen cannot diffuse quickly.

A current will be produced between the anaerobic area within the cavity, which will become anodic, and the aerated part of the surface outside, which will be the cathode; soluble salt will be formed at the anodic surface within the cavity, but this will not, of course, interfere with further anodic attack.

At the mouth of the cavity where the soluble metallic salt from the interior mixes with the alkali from the cathodic part outside, hydroxide may be precipitated, but it will not put a stop to the anodic attack proceeding within.

Since the rate of attack is determined by the supply of oxygen to the whole surface outside the pit, and since it is all concentrated on the small area within the pit, the rate at which the corrosion bores into the metal will be very great.

   Factors affecting pitting corrosion rate

    Water chemistry such as acidity, high dissolved oxygen concentrations and high concentrations of chloride.
    Localized damage to, or poor application of, a protective coating.
    The presence of non-uniformities in the metal structure of the component.

Caustic corrosion occurs when caustic is concentrated and dissolves the protective magnetite (Fe3O4) layer, causing a loss of base metal and eventual failure.

Caustic conditions caused scabbing and pitting of these heat exchanger tubes in a shell boiler.

Attention to the correct control of boiler water chemistry is crucial in preserving the health of the waterside.

STEAM BOILER SCALE Scale deposits are formed by precipitation and crystal growth on a surface which is in contact with water, Precipitation occurs when solubility’s are exceeded either in the bulk water or the surface. The most common scale-forming salts that deposit on heat transfer surfaces are those that exhibit retrograde solubility with temperature. Scaling is not always related to temperature, Calcium carbonate and calcium sulphate scaling occur on unheated surfaces when their solubility’s are exceeded in the bulk water .

     Factor effecting scale formation :

1- Heating.
2- Availability of hardness ions.
3- Solubility of ions.

     Scale problems in steam boiler :

1- Increased fuel consumption and decreasing the operating efficiency.
2- Thermal damage due to overheating.
3- Unscheduled down-time.
4- Increased cleaning time and cleaning costs.
5- Under deposit corrosion.
6- Reduced working life of a boiler.

Theory Dispersion of particles :

            Synthetic polymers control deposition in three specific ways :

    Crystal modification: adsorption of the charged group of the polymer on to the surfaces of nuclei resulting in                slowed crystal growth rates, crystal distortion and the very small particles.
    Dispersion: Adsorption of charged polymers on to the surface of insoluble particles and metal surfaces and                      prevention of particle agglomeration and deposition.
    Complexation: formation of a soluble complex with an ion (such as calcium) and prevention of precipitation & deposition.

The primary purpose of condensate treatment is to protect condensate equipment and piping. In addition, it is important to prevent corrosion products from returning to the boiler.

      The mechanisms of condensate corrosion are mainly based on :

1-Carbonic acid attack :

The carbon dioxide being issued from :

    The dissociation of natural alkalinity.

    Free CO2.

2-Dissolved oxygen.

3-Organic acid.

NOTE

The degree of acid corrosion protection afforded by a neutralizing amine depends on :

The basicity, neutralizing capacity and distribution ratio of the amine.
The amine concentration.
The level of carbonic and organic acid.
The presence of other contaminants such as oxygen.
The system design.

Taking into account the information we have got, mixture of amines is judged preferable for wider protection of different condensation zones.