Page 22 - Avtoreferat Voloshyn 2020

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Thesis for the Candidate’s degree in engineering sciences by speciality 05.17.14 –
Сhemical resistance of materials and corrosion protection. – Karpenko Physico-Mechanical
Institute of the National Academy of Sciences of Ukraine, Lviv, 2020.
An actual scientific and technical task has been resolved in this work devoted to
establishment of the regularities of influence of chemical composition of aqueous
environments and type of mechanical loading on the resistance of steels of pipeline systems
and their welded joints to corrosion-mechanical fracture and development of methods for
predicting their corrosion fatigue durability, as well as application of inhibitor protection
during cavitation in aqueous environments.
The main factors reducing corrosion fatigue durability of carbon steel under cavitation,
cyclic loading and their combined action have been established. It is shown that the
combined action of cavitation and cyclic loading significantly affects 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 - an order of
magnitude. A triethylammonium iodide inhibitor has been proposed to protect pipeline steels
against cavitation-corrosion damage in tap water, which provides protection degree of 90–
97% depending on the cavitation loading.
It has been established that the main mechanism of corrosion-mechanical fracture of the
17Г1С pipeline steel in bicarbonate aqueous solutions is hydrogen embrittlement caused by
additional reduction of bicarbonate ions with hydrogen evolution, and not local anodic
dissolution of metal. It is shown that the resistance of the 17Г1С pipeline steel to fracture at
a constant deformation rate in 0.01 N aqueous NaHCO 3 solution increases with increasing

cathodic polarization, does not depend on the intensity of local corrosion and decreases with
the formation of surface films of corrosion products.
The influence of various additives to deaerated bicarbonate aqueous solution on the
indicators of electrochemical relaxation of freshly deformed surface of the 17Г1С pipeline
steel has been quantitatively evaluated. The correlation between resistance to corrosion-fatigue
crack initiation in the steel and charge of oxidation of freshly deformed steel surface under
corrosion potential has been revealed. On this basis, an electrochemical rapid method of
predicting corrosion-fatigue durability of ferritic-pearlite pipeline steels in bicarbonate
environments depending on their composition has been developed.
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. The higher sensitivity of the weld metal to hydrogen
assisted cracking compared to that of the base metal has been revealed. It is found that the
threshold stress intensity factor for corrosion fatigue crack growth in the welded joint of the
17Г1С pipeline steel in NS4 solution, which simulates the soil environment, is most
intensively reduced by a combination of a number of factors: long-term operation, high
loading-cycle asymmetry and cathodic protection potential. 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.
Keywords: pipeline steel, electrochemical properties, stress corrosion cracking,
corrosion fatigue, hydrogenation, cavitation, operation.