Stress Corrosion Cracking (SCC) and Pitting Corrosion of Low Alloy Steel (LAS), ASTM A516 G-70 in Sour Environment

Abstract

For the safe operation of plants, it is necessary to eliminate the causes of catastrophic failures. So, to eliminate the causes it is necessary to understand the reasons and mechanisms involved in the materials failure. In the light of such information the plant life can be assessed and operated in safe mode. As in the case of oil and power generation plants the one of the most important causes of failure is the corrosion problem of alloys in aggressive environments. Therefore, for the safe and economical use or alloys the selection of materials with sufficient corrosion resistance to a particular environment is of great concern. The environments usually encountered in oil field, power industry and nuclear industry contain substantial amount of hydrogen sulphide. Where low alloy steels (LAS) are widely used but mostly suffers the corrosion attack quite severely. Since all parts made of LAS cannot be simply replaced altogether with high alloy steels because of economy and other beneficial properties of LAS. It is, therefore, important to assess the rate of damage being encountered in sour environment which may restrict its usage at some selective sites or to control the environment at such places if possible and finally best choice of LAS be made. However, many investigations for various types of LAS have already been made by various researches but a very little is known about the behaviour of low alloy A5 l 6 G-70 in this environment. There are other areas of research where this particular alloy had already been studied i.e., its general corrosion behaviour, electrochemical corrosion, stress corrosion cracking (SCC) etc. but in sour environment (containing hydrogen sulphide), no research about this has ever been made. In the present case because of its importance in oil and power generating industries the SCC behaviour evaluations of alloy A516 G-70 in hydrogen sulphide atmosphere was undertaken. Since it is well known that hydrogen sulphide (H2S) is one of the most toxic, flammable and corrosive substances. So, arranging all protective measure it is very expansive to test the material in H2S environment. According to literature, solutions containing thiosulfate ions can be used as substitute of hydrogen sulphide and it needs no specific preventive measures. Therefore, all investigations were planned and the testing of alloy A516 G-70 in solutions containing thiosulfate ions were made as a substitute of H2S. The alloy was further investigated by electrochemical techniques to study its pitting behaviour in sour environments aggressive solution. because pitting corrosion is another most dangerous type of corrosion. Some sudden failures and perforations in structures are responsible by pitting in. The objectives of the research project were: • To study the susceptibility of LAS to SCC in simulated sour environments adding thiosulfate ions in various solution by slow strain rate tests (SSRT) method • To establish the standard test solutions for evaluating the SCC susceptibility of LAS in sour environment. • To establish limits on environmental variables to control cracking of components this could be useful for industrial safety. • Understanding of phenomenon of stress corrosion cracking of the steel and its remedy will lead to increase service life of the components. Thereby, contributing to economy and improved safety standards of the oil industry and power plants. • To study the electrochemical behaviour for determination of pitting potentials this, could be avoided at in-situ components in the industries. These project objectives were accomplished by using different corrosion testing methods, metallurgical tools, published literature, discussion with related field experts etc. Test specimens were fabricated according to ASTM standards. Slow strain rate tests (SSRT) were carried out to study the stress corrosion cracking (SCC) behaviour using electromechanical loaded SSRT machine. The mechanical properties of the material tested in different solutions were compared with the material tested without aggressive solution. The electrochemical studies were conducted at anodic polarization potential and open circuit potential by using a corrosion measurement system Amel Model 568 to study the pitting corrosion behaviour of the alloy. On the basis of the research results it is proposed that the solution containing thiosulfate ions can be used for evaluating the SCC susceptibility and pitting corrosion of low alloy steel in sour environments at elevated temperature. These results suggested the possibility of using thiosulfate ion as an alternate to H2S for evaluating the sour gas resistance of low alloy steel. From this research project, useful data and technical know-how regarding the SCC/Pitting behaviour of low alloy steel in sour environment are available, which can be helpful to improve the performance of the existing materials, other low alloy steels and the development of the new materials. This study also provides the information regarding careful selection of material. for oil field and heavy water plant.