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                  hydrogen  embrittlement  caused  by  evolution  of  hydrogen  due  to  the  reduction

                  reaction of bicarbonate ions.

                         The  influence  of  long-term  operation  of  the  welded  joint  of  the  17Г1С

                  pipeline  steel  on  the  characteristics  of  corrosion  resistance,  resistance  to  stress

                  corrosion cracking and corrosion fatigue has been analysed. It has been found that

                  long-term operation of the butt weld of the 17Г1С pipeline steel reduces resistance

                  of the metal of different zones to hydrogen cracking in NS4, which simulates the

                  soil environment solution, by 2.3-13 times. The threshold stress intensity factor for

                  corrosion fatigue crack growth in the welded joint of this steel in NS4 solution is

                  most  intensively  reduced  by  a  combination  of  a  number  of  factors:  long-term

                  operation, high loading-cycle asymmetry R = 0.9 and cathodic protection potential

                  E cp (-0.98 V). Under such conditions, values of corrosion fatigue crack growth rate

                  in the plateau area for the operated base metal and weld metal are 1.5 and 2.6 times

                  higher, respectively, compared to the as-received condition.

                         Scientific novelty of the obtained results.

                         It was established that cavitation under combined action with cyclic loading

                  significantly  reduces  the  characteristics  of  corrosion  fatigue  durability  of  the  35

                  steel in tap water: the corrosion fatigue limit is reduced by 2.4 times, and limited

                  durability for loads close to the fatigue limit – by an order of magnitude.

                         For the first time, the effectiveness of inhibitor protection of the 30ХГСНА

                  steel  against  corrosion  cavitation  damage  in  tap  water  with  triethylammonium

                  iodide,  which  provides  a  degree  of  protection  of  90-97%  depending  on  the

                  intensity of cavitation loading, was revealed.

                         It was established that the resistance of the 17Г1С steel to fracture at a constant

                  deformation rate of 10  s  in 0.01 N aqueous solution of NaHCO 3 with the addition
                                           -7 -1
                                                    -
                                             -
                                                                     3-
                                                            2-
                  of  anions  (0.005 N)  Сl ,  NO 3 ,  CO 3   і  PO 4   decreases  with  increasing  cathodic
                  polarization,  does  not  depend  on  intensity  of  local  corrosion  and  increases  with
                  improvement of barrier properties of the corrosion products inhibiting hydrogenation
                  of  the  metal.  In  deaerated  aqueous  bicarbonate  solution,  the  main  mechanism  of


                  corrosion-mechanical  fracture  of  the  steel  is  hydrogen  embrittlement  caused  by
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