Hydrogen degradation of steels in gas mains after long periods of operation

We study the degradation of the mechanical and corrosion properties of 17G1S steel in gas mains as a result of operation for 28–40 yr. We reveal the simultaneous decrease in its strength and hardness, on the one hand, and impact toughness and crack resistance, on the other hand. The indicated parameters prove to be most sensitive to the in-service degradation of the metal. The degradation of steel leads to the increase in the strain-hardening coefficient and the appearance of the yield plateau in the tensile stress-strain diagram and causes the appearance of sensitivity to hydrogen embrittlement increasing in the process of operation. We analyze the role of absorbed hydrogen affecting the mechanism of strain aging and responsible for the appearance of defects. It is shown that the polarization resistance can be used not only for monitoring of the corrosion state of gas pipelines but also for the prediction of changes in their mechanical properties in the process of operation. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 43, No. 5, pp. 97–104, September–October, 2007.     

Computation model of corrosion-fatigue crack growth in thin metallic plates

A computation model of corrosion-fatigue crack growth in thin metallic plates is suggested based on well-known theoretical concepts of fatigue and corrosion fracture and some assumptions. A physicochemical model of this process is constructed, and mathematical relations that describe the kinetics of crack propagation are established. The model agrees with well-known experimental data.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 30–34, January–February, 1994.     

Plasma Electrolytic Oxidation of Arc-Sprayed Aluminum Coatings

Different posttreatment methods, such as heat treatment, mechanical processing, sealing, etc., are known to be capable to improve microstructure and exploitation properties of thermal spray coatings. In this work, a plasma electrolytic oxidation of aluminum coatings obtained by arc spraying on aluminum and carbon steel substrates is carried out. Microstructure and properties of oxidized layers formed on sprayed coating as well as on bulk material are investigated. Oxidation is performed in electrolyte containing KOH and liquid glass under different process parameters. It is shown that thick uniform oxidized layers can be formed on arc-sprayed aluminum coatings as well as on solid material. Distribution of alloying elements and phase composition of obtained layers are investigated. A significant improvement of wear resistance of treated layers in two types of abrasive wear conditions is observed. This article is an invited paper selected from presentations at the 2007 International Thermal Spray Conference and has been expanded from the original presentation. It is simultaneously published in Global Coating Solutions, Proceedings of the 2007 International Thermal Spray Conference, Beijing, China, May 14-16, 2007, Basil R. Marple, Margaret M. Hyland, Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and Ghislain Montavon, Ed., ASM International, Materials Park, OH, 2007.     

Realization of synergism in 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate corrosion inhibitors

The inhibiting action of new 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate compounds is explained by the presence of intramolecular synergism as a result of which the coefficient of inhibition of the corrosion rate of steel in water becomes much higher than the total inhibition coefficient of the rate of corrosion by compositions based on benzotriazole and the corresponding oxoanions. The 1-, 2-, 3-benzotriazole trimolybdate, tungstate, and chromate compounds are inhibitors of mixed action inhibiting both electrode reactions and shifting the compromise potential to less negative values. According to the data of impedance tests and the results of evaluation of the degree of packing of these compounds on the steel surface, we conclude that their inhibiting effect is caused by chemisorption satisfactorily described by the Frumkin-Temkin isotherm. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 5, pp. 21–29, September–October, 2006.     

Specific features of the influence of hydrogen on the properties and mechanism of fracture of the metal of welded joints of steam pipelines at thermal power plants

We study the standard tensile mechanical properties of a live-steam pipeline operated for about 2 · 10⁵ h and model (repair) welded joints. As a result of the degradation of weld metal under service conditions, its strength becomes lower than that of base metal. We show that the unregulated orientation of specimens along the axis of a welded joint is the most acceptable for detecting the degradation of weld metal. Cathode polarization facilitates evaluating the state of the degraded weld metal and that of the base metal. We have also disclosed the specific features of the influence of internal (preliminary hydrogenation) and external (electrolytic hydrogenation in the course of tests) hydrogen on the properties and mechanisms of the fracture of weld metal. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 4, pp. 26–35, July–August, 2006.     

Influence of Hydrogen on the Formation of Fatigue Thresholds in Structural Steels

We analyze certain phenomena related to the influence of gaseous hydrogen and hydrogen dissolved in a metal on the near-threshold growth of fatigue cracks. The significant decrease in crack growth resistance due to the action of hydrogen as compared with that in vacuum is caused mainly by the adsorption decrease in strength due to adsorption. We established three factors of the ambiguous influence of hydrogen on the effective fatigue threshold K _theff, for which a positive influence is replaced by a negative one, namely: the strength level, temperature of testing, and high-temperature degradation of the metal. The following fractographic peculiarities of the near-threshold growth of cracks in a degraded metal are revealed: the local tunneling along the front of a crack and the presence of fatigue grooves. We propose a mechanism of crack closure due to both roughness and the component of longitudinal shear at the tip of the crack. We analyze the scale effect of fatigue thresholds, determine the conditions for invariance of the parameter K _theff under conditions of plane deformation, and established the dependence of K _theff on the thickness of specimens in the case of tests of a hydrogenated degraded metal.     

Percolation methods for the construction of curves of hydrogen-assisted corrosion of chromium-molybdenum steels

We present a new procedure developed to calculate the curves of hydrogen corrosion for moderately alloyed steels. It is based on approaches of percolation theory, allows one to consider the distribution of the parameters of a material structure (the initial radius of voids on grain boundaries, distances between them, the percentage composition of carbides), and is justified by construction of the Nelson curves for 2.25Cr-1 Mo steel. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 35, No. 6, pp. 32–36, November–December, 1999.