an investigation on the role of hydrogen in metals

recently, grave explosions in the oil and gas transport tubes, as well as, the rapid failure of the machine parts in service, were caused by hydrogen cracking. during solidification of metals, in casting ,welding, cathodic charging, etc., the hydrogen can enter in metals and cause premature cracking. the hydrogen embrittlement in metals is accompanied with a charge in the type of fracture surface ( intergranular or interaganular ) and a decrease or in certain casesan increase in the ductility of metals. the embrittlement by hydrogen depends on two factors: (1) when the amount of hydrogen is in the order of 1-2 ppm , the decohesion, between atoms in the crystal lattice is the predominent factor, (11) when the hydrogen concentration in the metal is more than 1-2 ppm, the very high pressure of h molecules in microcracks plays an essential role to cause brittle fracture. but, hydrogen cannot pin the dislocations, as it was not able to increase the elastic limit in many studied cases. in this article, it has been tried to compare the results and suggestions obtained by the author with the more recent works in this field. consequently, attempts have been made to clear some of the questions in this subject by deducing the common points and contradictions in these works.