The Corrosion

Corrosion in power plant equipment produce pits, grooves, and crack or a general wastages of the wall material. Allowed to continue, corrosion ultimately makes metal parts fail. Corrosion mechanisms are under close study. While the results are not conclusive, the mechanisms appear to act as follows:

Dissolved gasses are the most prevalent cause corrosion, oxygen may be the most active, but CO2, NH4 and H2S are also detrimental. Dissolved O2 may enter in the make up water or through leaks in the condensate system around the hot well and low pressure heater. The figure shows outlines the action of dissolved O2 in water in contact with iron.

Fe++ + 2OH- = Fe(OH)2
2Fe(OH)2 + O2 + 2H2O = 2Fe(OH)4

The material formed is ferric hydroxide Fe(OH)4 or common rust, ferric oxide
Fe2O3 + 3H2O may also form. Keeping oxygen out of the water prevent the dissolved gas type corrosion. This can be done mechanically with de-aerating heater or chemical treatment. The de-aerator leaves traces of oxygen and supplemental chemical treatment removes it. The treatment produces the reaction

2Na2SO2 + O2 = 2Na2SO4

The sodium sulphate is soluble. A newer and promising method uses hydrazine with the reaction

N2H4 + O2 = 2H2O + N2

Hydrazine solution is a clear, water white, alkaline liquid with an order like ammonia. It contributes alkalinity with end product of water and no solids. The nitrogen evolves with the steam.

Carbon dioxide may be in the make up water or may be formed by decomposition of sodium bicarbonate in the water at boiler temperature. Corrosion by CO2 appears as general wastages or grooving of the metal in the steam space of a drum. The gas evolves with the steam. Boiler corrosion of this type is not common.

Corrosion from ammonia and hydrogen sulphide is not common either,. Because these gas are rarely found in feed water. But ammonia tends to attack copper or its alloys, and hydrogen sulphide causes acidic corrosion similar to that caused by carbon dioxide.

Since corrosion proceeds by metal ions replacing hydrogen ions, waters that are relatively acidic (low pH) tend to be corrosive. This appears as a general thinning and corrosion of the metal and happens frequently in economisers. Soda ash or sodium hydroxide is injection for alkalinization and keep pH at about 10.5. Acidity raises the rate or corrosion of water.

Two dissimilar metals in equipment handling water cause galvanic action. This makes the metal of higher electronegative potential in the electrochemical series pass into solution. This should be avoided because chemical treatment can not counteract this type of corrosion.

Stray electrical currents through piping or equipment may also cause electrolytic action. Stray current may be caused by high voltage lines closely paralleling steam or water piping or from improper grounding of electrical equipment. These conditions can be corrected only by eliminating the causes. The corrosion from these sources resembles that caused by oxygen, but few cases these types occur in power station.

Steam above about 750F (398C) is corrosive to iron, the chemical reaction producing black magnetic oxide Fe3O4 and hydrogen. This can be avoided by using corrosion resistance alloys, usually of high chromium content. These alloys are expensive. In laying out a super heater the designer must make an economic choice between thicker walled conventional alloys and high cost corrosion resistance alloys.

Sometimes corrosion of internal and external surface may be avoided by applying protective coating. If the coating should fail, the metal underneath may suffer acceleration local corrosion.

Skrotsi, Vopat POWER STATION ENGINEERING AND ECONOMY Tata McGraw-Hill Publishing company LTD Bombay 1960.