Hydrogen attack 

  • Both acid attack and caustic attack of steel initially causes the formation of atomic hydrogen. Hydrogen damage may result . The atomic hydrogen diffuses into the steel and reacts with iron carbide, with formation of methane.

  • The pH can be controlled by means of appropriate boiler water conditioning and daily testing of the feedwater and boiler water composition, which will prevent this form of corrosion. 

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Refer 5823

Hydrogen damage occurs in boilers operating usually above  (6.9 MPa) and under heavy deposits or other areas where corrosion releases atomic hydrogen. Concentrated sodium hydroxide beneath the deposits can remove the protective magnetite film by the following reactions.

4NaOH + Fe3O4- 2NaFeO2 + Na2FeO2 + 2H2O              

Concentrated sodium hydroxide can then react with freshly exposed base metal to yield sodium ferroate and atomic hydrogen:

Fe + 2NaOH - Na2 FeO2 + 2H                       

The hydrogen produced at the metal surface can diffuse into the steel where it can combine to form molecular hydrogen or react with iron carbide to form methane and iron:

4H+ + Fe3C - CH4 + 3Fe                           

Upsets in phosphate treatment programs or residual acid from chemical cleanings can also cause hydrogen damage, especially if the acids remain trapped beneath the deposits.

These failures are typically characterized as thick-lipped, brittle type ruptures.
 Sometimes, thick-walled "windows" can be completely blown out of the tube wall. Microstructural examination at the area of failure typically reveals short discontinuous intergranular cracks accompanied by decarburization.