LATENT PERIOD OF STRESS CORROSION CRACKING DEVELOPMENT AT THE MAIN PIPELINES

Abstract

One of the unsolved problems in pipeline transport is the insufficient study of the phenomenon of stress corrosion cracking, which is often observed at the main gas pipelines and practically does not appear at the main oil pipelines, despite the fact that these pipelines are built according to same construction standards, practically from the same pipes, are operated under close conditions. Firstly, it does not allow to find effective methods against cracking at main gas pipelines, and secondly, it does not give an opportunity to answer the important question whether further development of corrosion cracking at main oil pipelines should be feared in the future. Aims and Objectives The article poses a problem on the basis of analysis of various manifestations of pipe cracking, the study of known features and regularities, to find the mechanisms for the development of this phenomenon, to construct its physical model, and on its basis to try to answer the question posed. Results A physical model of stress corrosion cracking of underground pipelines has been developed, which makes it possible to explain all observed regularities of this type of destruction. The model is based on the determining role of hydrogen, indicates possible sources of hydrogen on underground pipelines, the patterns of interaction of hydrogen with the pipeline metal, shows the causes of increased internal stresses and cracking of the wall. According to this model, stress corrosion cracking occurs successively in several stages. The first stage is the latent period of development, when there is no cracking yet, but inside the metal of the pipes there are significant changes: the growth of internal stresses due to the accumulation of gas molecules H2 and CH4 at grain boundaries of the crystal structure. At the same time, structural changes occur in the crystals, decarburization and growth of the grains. The second stage - the birth of microcracks, their growth and integration into macrocracks. This is facilitated by stresses caused by working pressure and external forces. One of the components of external forces - the reaction of the soil, which is particularly important in areas with complex uneven terrain. The third stage is the growth of macrocracks before the destruction of the pipeline. Before the completion of the first stage, the remaining stages can not occur. In this lie the answers to the questions posed, as well as the methods of inhibition of cracking in general. For each of these stages, certain conditions are necessary. For the flow of the first stage, a source of atomic hydrogen is needed. Such source on underground pipelines is ground moisture (water) in the presence of electrochemical potential. This source only operates in places of insulation failure. Another necessary condition at the first stage is the presence of tensile mechanical stresses above a certain critical level - the stress corrosion limit. The proposed physical model has been experimentally confirmed by studying the patterns of introduction of hydrogen into the wall of the pipeline. On the basis of this physical model, it was concluded that on the main oil pipelines it is possible to develop stress corrosion cracking in local zones, where a concentration of stresses is created, and insulation in these zones either is absent or worn out.