CORROSION OF BOILER TUBES

CORROSION AND SCALING

Corrosion and scaling will occur when boiler water treatment does not practice. This will reduce the efficiency of the boiler and the performance is not is its maximal. The deposits will clog up the tubes and hence, this will increase the pressure in the tubes and in the end, the tubes will bust or leak.

These are the possible reasons that cause the tubes corrosion and scaling

TUBES OVERHEATED







Indication of tubes overheated-the colour of water tube. The tubes is dark in color at one section and another is light in color at another section.Hence, This indicate that the tubes at overheating.












The water tube is bulging. This shows the tube is overheating






LEAKING







This indicates that there is leaking to the boiler tubes. White deposit appeared at the bottom end of the tubes. It is a salt( SODIUM CARBONATE).







CAUSTIC CORROSION



The term of caustic gouging and ductile gouging refer to the corrosive interaction of sufficiently concentrated sodium hydroxide with the metal to produce distinct hemispherical or elliptical depressions. The depressions may be filled with dense corrosion products that sometime contain sparkling crystals of magnetite. At times, a crust of hard deposits and corrosion products containing magnetite crystals will surround and overlie the attacked region. The affected metal surface generally has a smooth, rolling contour.


This why boiler water treatment is so important so that it could get rid of sodium hydroxide in the feed water


OXYGEN CORROSION

One of the most frequently encountered corrosion problems results from exposure of boiler metal to dissolved oxygen. Since the oxides of iron are iron's natural, stable state, steel will spontaneously revert to this form if conditions are thermodynamically favorable. Generally, conditions are favorable if steel that is not covered by the protective form of iron oxide is exposed to water containing oxygen.The following reaction occurs:

2FE+H2O+O2=> FE2O3+2H

The fracture sites furnish anodic regions where oxygen containing moisture can react with bare, unprotected metal. The result may be deep, distinct, alomost hemispherical pits, which may be covered at times with caps of corrosion products.

HYDROGEN DAMAGE

Hydrogen damage may occur where corrosion reactions result in the production of atomic hydrogen. Damage may result from a high-pH corrosion reaction or low pH corrosion reaction. Damage resulting from a high-pH is simply caustic corrosion.

Concentrated sodium hydroxide dissolves the magnetic iron oxide according to the following reaction:

4NAOH+FE3O4=> 2NAFEO2+NA2FEO2+2H2O

With the protective covering destroyed, water is then able to react directly with iron to evolve atomic hydrogen:

3FE+4H2O=>FE3O4+8H

The sodium hydroxide itself may also react with the iron to produce hydrogen:

FE+2NAOH=>NA2FEO2+2H

If atomic hydrogen is liberated, it is capable of diffusing into the steel. Some of this diffused atomic hydrogen will combine t grain boundaries or inclusions in the metal to produce molecular hydrogen, or will react with iron carbides in the metal to produce methane.

FE2C+4H=> CH4+3FE

As microcracks accumulate, tube strength diminishes until stresses imposed by boiler pressure exceed the tensile strength of the remaining, intact metal. At this point a thick-walled, longitudianl burst may occur. Depending on the extent of hydrogen damage, a large, rectangular section of the wall frequently will be blown out, producing a gaping hole.