Welcome, Guest

• Forgot your password?
• Apply for membership

1. • CommentAuthormatco
   • CommentTimeJan 20th 2007

    permalink
   Hydrogen damage-Hydrogen damage in boilers is a phenomena associated with methane formation. The reaction may occur at the surface, resulting in decarburization with a loss in strength, or it may occur internally, resulting in both decarburization and the formation of fissures and cracks, which result in a loss in both strength and ductility. Some of the hydrogen atoms adsorbed on the metal surface, diffuse into the metal. Carbides in the grain interior dissolve and, the carbon migrates to the grain boundaries where it reacts with hydrogen to form hydrocarbon intermediates and ultimately methane molecules which start to form and can not diffuse through the lattice. The reaction and nucleation sites are at grain boundaries. The reaction is:
   
   C + 2H2 -----> CH4
   
   Methane molecules in principal can precipitate at grain boundaries, voids and interfaces within the metal lattice causing very high pressures. In particular, prolonged exposures to a hydrogen producing environment at temperatures in excess of 200'C can result not only in severe decarburization, but also in the formation of methane gas. The equilibrium methane pressure increases as the temperature increases. Under certain conditions these voids link up, usually at grain boundaries (nucleation sites) leading to fissures . Hydrogen diffusion largely depends on the characteristics and integrity of the oxide film, and the size and density of defects in the protective film. Whether or not steel deteriorates by surface decarburization, or methane fissuring and internal decarburization, depends on temperature and the barrier properties of the oxides and hydrogen fugacity(effective pressure) at the surface. At relatively high temperatures and low fugacity, surface decarburization proceeds faster than internal fissuring. Both types of attack will occur if both fugacity and temperature are sufficiently high.
   
   Acid cleaning operations are another category that may introduce hydrogen into the steel with a resultant loss in ductility. The percentage loss in ductility depends directly on the hydrogen content of the steel. The fracture strain may also be lowered. Furthermore, if the acid contains hydrogen recombination poisons(As, P, Te and Sb compounds), the effects will be more severe.
   
   The failure of a carbon steel pipe in a high pressure line of a steam generator resulted in a portion of the tube wall becoming detached. Cross-section microscopy revealed that the ruptured area contained a discontinuous network of intergranular cracks with decarburization along the cracks. Thick internal corrosion deposits were found in the failed area which had not been removed in a chemical cleaning effort.