Studio della NN-SCC di acciai per tubazioni interrate tramite prove di flessione in tre punti

Near Neutral Stress Corrosion Cracking (NN-SCC) is a particular form of stress corrosion cracking that takes place on coated buried pipelines It occurs under particular circumstances, in presence of slow plastic straining, in areas of disbondment in which the coating remains intact but a crevice is formed between it and metallic surface Disbonded coating prevents the CP current from reaching deep inside the crevice and allows the accumulation on steel of a dilute bicarbonate solution with CO(2) content from ground moisture, having pH 6-7 Thus, critical conditions are achieved [1]. NN-SCC phenomena are usually reproduced in laboratory experimental study by SSR tensile tests on cylindrical specimens. Cracking is found after the onset of necking, at high plastic strain, i.e. under conditions that are not representative of real loading. Another limitation is related with the corroding surface, as NN-SCC only initiates after several years of service, from localized attacks. In order to reproduce the nucleation sites that promote NN-SCC on real structures an electrochemical technique for pre-corrosion of has been Improved [2]. This work deals with the initiation and growth of crack for NN-SCC. Initiation conditions were investigated by means of 3 point slow bending tests on full thickness specimens and, for comparison purpose, slow strain rate tests on cylindrical specimens the NN-SCC propagation was investigated through low frequency corrosion-fatigue tests on single notch three point bend beam specimens. Tests were carried out on a API 5L X65 controlled rolled ferritic-pearlitic steel for pipelines (Table I) in NS4 solution (0 483 g/L. NaHCO(3), 0 122 g/L KCl, 0 18 g/L. CaCl(2) and 0 1 g/L MgSO(4)). This solution was proposed by Parkins [4] and represents the average composition of the water found on Canadian pipelines, where NN-SCC phenomena were observed NS4 solution show 8 3 pH that reduces to 7 1 saturation with CO(2)/N(2) gas mixture at 0.05 atm CO(2) partial pressure The electrochemical pre-corrosion procedure involves 10 cycles of cyclic voltammetry from -1.8 to +1.8 V vs SCE (scan rate 20 mV/s) in NS4 solution modified by increasing the CO(2) and bicarbonate content (1 atm and 12.4 g/L, respectively) to obtain neutral pH. Near Neutral Stress Corrosion Cracking (NN-SCC) is a particular form of stress corrosion cracking that takes place on coated buried pipelines It occurs under particular circumstances, in presence of slow plastic straining, in areas of disbondment in which the coating remains intact but a crevice is formed between it and metallic surface Disbonded coating prevents the CP current from reaching deep inside the crevice and allows the accumulation on steel of a dilute bicarbonate solution with CO(2) content from ground moisture, having pH 6-7 Thus, critical conditions are achieved [1]. NN-SCC phenomena are usually reproduced in laboratory experimental study by SSR tensile tests on cylindrical specimens. Cracking is found after the onset of necking, at high plastic strain, i.e. under conditions that are not representative of real loading. Another limitation is related with the corroding surface, as NN-SCC only initiates after several years of service, from localized attacks. In order to reproduce the nucleation sites that promote NN-SCC on real structures an electrochemical technique for pre-corrosion of has been Improved [2]. This work deals with the initiation and growth of crack for NN-SCC. Initiation conditions were investigated by means of 3 point slow bending tests on full thickness specimens and, for comparison purpose, slow strain rate tests on cylindrical specimens the NN-SCC propagation was investigated through low frequency corrosion-fatigue tests on single notch three point bend beam specimens. Tests w