Corrosion in boilers

Corrosion in Steam Boilers

Corrosion in steam boilers is caused by chemical and electrochemical reactions of the metal with its surrounding environment. The most important corrosion factors are as follows:

1. Oxygen corrosion
2. Carbon dioxide corrosion
3. Corrosion caused by pH drop
4. Alkalinity corrosion
5. Corrosion of hydrolyzable salts
6. Corrosion of organic materials

Oxygen corrosion

Oxygen dissolved in water causes metal to oxidize. This type of corrosion intensifies at temperatures above 60 and causes the metal to corrode. Corrosion caused by oxygen is very dangerous because it causes holes in pipes and tanks in a short time. If the free oxygen in the boiler water reaches even less than 0.01 mg/liter, the risk of corrosiveness cannot be ruled out. Most of the spots that are in contact with water become corrosive, especially the spots where mechanical or welding work has been done.

Special Mitreh Solutions for Steam Boilers

Carbon dioxide corrosion

If carbon dioxide, resulting from the decomposition of bicarbonates in the water, is not removed from the boiler feed water with the help of an air separator, it will cause corrosion in the steam consumption lines, because carbon dioxide dissolved in water has acidic properties. This acid dissociates in water and releases hydrogen ions:

CO2+ H2O = H2CO3
H2CO3 = HCO3 – + H+

The released hydrogen ion lowers the pH of the water and makes it acidic. Another source of carbonic acid is bicarbonate in the feed water. The bicarbonate in the water is converted to carbonate by heating, releasing carbonic acid. If conditions are favorable, the carbonate ion hydrolyzes in the water, in turn producing carbonic acid. The reactions are as follows:

2HCO3 – = CO3– + H2O + CO2
CO3– + H2O = 2OH – + CO2

The released carbon dioxide, along with the steam, travels to various points in the system and, in areas where the temperature drops below the dew point, dissolves in the water and makes the water corrosive. The result is corrosion, metal decay, and a reduction in the wall thickness of pipes and tanks.

Corrosion caused by pH drop

A drop in pH makes the water corrosive. The result of the corrosion process either remains on the metal surfaces in the form of rust or is transferred to other parts with the water. An increase in temperature accelerates this process. One of the factors that causes pH drop is the entry of hard water into the boiler and the high concentration of water in the boiler.

Alkalinity corrosion

The bicarbonate-containing water in the boiler, on the one hand, causes the condensate water to become acidic by releasing carbon dioxide from the boiler, and on the other hand, by creating hydroxide ions, it increases the alkalinity of the boiler water. Increased alkalinity causes embrittlement of the weld lines and the ends of the pipes. High alkalinity will cause the boiler water to foam and transfer salts to the steam lines and traps.

Corrosion of hydrolysable salts

Some salts are hydrolyzed in an aqueous environment by heat. Magnesium chloride is one of this group. Hydrolysis of magnesium chloride in water produces hydrochloric acid, which attacks the metal surface.

MgCl2 + 2H2O = Mg(OH)2 + 2HCl
Fe + 2HCl = FeCl2 + H2

Corrosion of organic materials

The decomposition of some oils and organic materials in steam boilers can produce acidic and corrosive compounds.

Preventing corrosion in steam boilers

1. The best method of preventing corrosion is to add appropriate chemicals to create a protective layer on the thermal surfaces during equipment startup. Of course, this method requires some expense, but with its role in preventing corrosion and, consequently, preventing damage to the equipment, it can be even more economical than other methods in the long run.
2. Choosing the right metal with effective coatings for water treatment equipment
3. Accurately treating the boiler's water supply by installing a softener and deaerator
4. Continuously injecting appropriate chemicals into the boiler feed water to absorb residual oxygen(Oxygen scavenger)
5. Controlling the pH of the feed water and the boiler at the appropriate level
6. Controlling the concentration of chemicals in the boiler