Cyclic corrosion crack-growth resistance of 26KhN3M2FA steel

We present results of an investigation of the cyclic corrosion crack-growth resistance of 26KhN3M2FA steel under conditions of the water-vapor phase transition and plot invariant dependences (pH _t = const) of the corrosion-fatigue crack-growth rate on the stress intensity factors for various stress ratios and temperatures of the medium under extreme electrochemical conditions at the tip of a corrosion-fatigue crack and taking the influence of an inhibitor (NaH₂PO₄ · 12H₂O) into account. We also study the behavior of electrochemical characteristics on the specimen surface and at the tip of a propagating corrosion-fatigue crack, quantitatively determine the intensity of local anode dissolution and hydrogen embrittlement in the course of crack propagation, and compare cyclic corrosion cracking resistances of 26KhN3M2FA and 34KhN1M steels.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 5, pp. 19–27, September – October, 1994.     

Analysis of the contribution of local anodic dissolution and hydrogen embrittlement to the corrosion fatigue,of steels

On the basis of the results of the investigation of cyclic corrosion crack-growth resistance with regard for the electrochemical conditions at the tip of a corrosion-fatigue crack, we present a quantitative analysis of the mechanisms of local anodic dissolution and hydrogen embrittlement for 20N2M and 15Kh2NMF steels in distilled water. This analysis is based on the evaluation of the contributions of these mechanisms to the increase in the fatigue crack growth rate in the media. The maps of this analysis are constructed as functions of the range of the stress intensity factor and the frequency of loading. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 32, No. 6. pp. 23–32, November–December, 1996.     

Determining cyclic corrosion cracking resistance for titanium alloys with allowance for electrochemical conditions at the fatigue corrosion crack tip

Published data are examined on how various factors affect fatigue crack growth rates. Basic diagrams have been constructed for the cyclic cracking resistance in Ti-6AI-4V and Ti-6AI-6V-2Sn alloys in air, distilled water, and 3.5% NaCl for use in working-life calculations. Appropriate heat treatment can produce two microstructures in a titanium alloy, one of which has the largest cyclic cracking resistance, while in the second, the cracks grow at the lowest rate. The cyclic corrosion cracking resistance for a titanium alloy should be determined in relation to the state of stress and strain and to the electrochemical conditions at the corrosion fatigue crack tip, while the variations in fatigue crack growth rate for a given stress intensity factor in a corrosive medium are due to differing electrochemical conditions at the crack tip during the testing on different specimens. Basic diagrams can be derived for titanium alloys by using a physically sound methodology developed previously for steels, which is based on invariant diagrams for cyclic cracking resistance in air and in the corresponding medium, which can be constructed in relation to extremal working and electrochemical conditions at corrosion-fatigue crack tips.Translated from Problemy Prochnosti, No. 12, pp. 3–11, December, 1993.     

Inhibitor protection of metals at the stage of corrosion-fatigue crack growth

We suggest electrochemical methods for determining the intensity of local anodic dissolution and hydrogen embrittlement of metals at the tip of a corrosion-fatigue crack. Based on this, we develop a technique for estimating the efficiency of inhibitor protection of structural elements with cracks.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 17–25, January–February, 1994.     

Mechanisms of corrosion fatigue below K Iscc

Information on corrosion-fatigue crack growth in engineering structures is necessary for the prediction of service lives of structures subjected to both fatigue loading and an aggressive environment. The rate of crack growth in corrosion fatigue is governed by the interaction between the chemical mechanisms and the mechanical mechanisms occurring at the crack tip. Thus, as part of a long-range program aimed at establishing the necessary relations for predicting the corrosion-fatigue behavior of structural steels, the crack-tip mechanisms in corrosion fatigue were studied by using controlled-potential techniques, pH measurements at the crack tip, and fractographic analysis of the crack surface. The tests were conducted on 12Ni-5Cr-3Mo maraging steel at a cyclic-stress frequency equal to 6 cycles per minute in a room-temperature pH 7, 3 percent solution of sodium chloride. The results showed that the controlled-potential technique is inadequate for isolating the crack-tip mechanisms in thick specimens subjected to fatigue loading and an aggressive environment. At the crack tip the pH was 3. In addition, localized neighboring regions of basic and acidic solutions were observed in the proximity of the central portion of the crack tip. Hence, it is concluded that the effectiveness of cathodic protection decreases as the crack front moves away from the free surface. The fractographic tests showed that hydrogen embrittlement is the primary mechanism responsible for acceleration of fatigue cracks in 12Ni-5Cr-3Mo steel in 3 percent sodium chloride solution. Finally, by using the corrosion-fatigue crack-propagation law $$\frac{{{\text{d}}a}}{{{\text{d}}N}} = D(t){\text{ [}}\Delta {\text{(}}\delta \sigma {\text{,)]}}$$ 10 where da/dN is crack-growth rate per cycle, δ is the crack-opening displacement, σ _y is the yield strength, and D(t) is a time- or frequency-dependent function, it is shown that, for the environment-material system investigated, hydrogen embrittlement accelerates corrosion-fatigue crack propagation by changing the ductility of the material (or δ).     

Estimating cyclic corrosion cracking resistance for titanium alloys

Conclusions 1. Cyclic cracking resistance tests for titanium alloys, as for steels, are affected not only by the state of stress and strain but also by the electrochemical conditions at the crack tip.2. Cyclic corrosion cracking tests require one to consider the electrochemical conditions at the crack tip.3. These methods have been tested in corrosion crack stability tests, which involved monitoring the electrochemical conditions at the surface of the specimen, which do not unambiguously define the corrosion cracking resistance for titanium alloys in a particular medium.4. It is possible to derive invariant fatigue failure diagrams for titanium alloys in a given medium only if one provides constant conditions at the crack tip during growth.5. The electrochemical conditions at the crack tip can be characterized reasonably fully (integrally) for titanium alloys, as for steels, by means of the pH and the electrode potential.6. The type of alloy governs the electrochemical characteristics at the tip of a stationary statically loaded crack and a growing one.7. To select basic fatigue-failure diagrams for a corrosive medium, one can use the methodology developed previously for steels, which is based on invariant diagrams in air and in the medium, where one examines the effects of extremal working conditions and the electrochemical conditions at the crack tip.Karpenko Physicomechanics Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 2, pp. 33–42, March–April, 1993.     

Corrosion and Corrosion Crack Resistance of the PT3V Titanium Alloy in Aqueous Solutions of Ammonia

We describe some results of experimental investigations into the influence of the structure of the PT3V titanium pseudo--alloy, formed by plastic deformation and heat treatment, on its mechanical properties, corrosion, and cyclic crack resistance in aqueous ammonia media of various concentrations and temperatures. The kinetics of electrode processes on a smooth surface and at the tip of a fatigue crack is studied. By using the methods of the dropping mercury electrode and a disk rotating electrode, we identified the main partial electrochemical reactions and determined the limiting stage of the cathodic process (diffusion of molecular oxygen), the hydrodynamic conditions of the course of these reactions, and the diffusivity of oxygen in aqueous ammonia media at 298 K (D = 4.8·10^–8m²/sec). The kinetics of fatigue and corrosion-fatigue fracture of the alloy under study (crack initiation and growth) was investigated as well. We have established that, at the stage till crack initiation in air, alloys with a globular structure have the maximum lifetime, which is 1.5–2 times as much as that of lamellar structures. In corrosive media, the time to crack initiation increases with the ammonia concentration and solution temperature. A corrosive medium strongly affects fatigue crack growth in the alloy for K below 5–7 MPa m^1/2, i.e., if V > (2·10^–8–8·10^–9) m/cycle, especially for the initial (thin-lamellar) and -transformed structures. With the help of investigations of electrochemical processes in a corrosion-fatigue crack, we established the linear dependence of the stabilized value of the electrolytic potential E _st on K. Specific features of the fatigue fracture of alloys having different structures in air and aqueous ammonia media were discovered on the basis of metallographic analysis. Finally, we analyzed the kinetics and the mechanisms of corrosion and corrosion-fatigue crack growth.     

BASIC DIAGRAMS OF CYCLIC CRACK GROWTH RESISTANCE OF TITANIUM ALLOYS

Abstract— Basic diagrams of the cyclic crack growth resistance of two of the most investigated titanium alloys, namely Ti-6A1–4V and Ti-6A1–6V-2Sn, are presented. Diagrams are plotted for, in-air, distilled water and 3.5% NaCl solution, which are necessary for lifetime calculations of structural elements made of these metals. The dependency of cyclic crack growth resistance on the yield strength is established. It is shown that cyclic crack growth resistance of titanium alloys in corrosive environments is determined not only by the stress-strain state but also by the electrochemical conditions at the corrosion fatigue crack tip, which for aqueous environments can be characterized integrally by the hydrogen index of the environment and the electrode potential of the metal. Therefore, cyclic corrosion crack growth resistance testing should be performed under constant electrochemical conditions at the corrosion fatigue crack tip or these conditions should be taken into account. A new method of plotting the basic cyclic corrosion crack growth resistance diagrams of titanium alloys is considered.     

Effect of sulfate-reducing bacteria on the electrochemical conditions at a crack tip

We study the effect of sulfate reducing bacteria on the electrochemical conditions at the tips of stationary and growing corrosion fatigue cracks in 40Kh steel specimens in Postgate medium. Sulfate-reducing bacteria change significantly the pH and the electrochemical potential at the crack tip and greatly decrease the efficiency of KXO inhibitor. The effect of sulfate-reducing bacteria and KXO inhibitor on the intensity of local anodic dissolution and hydrogen embrittlement in the vicinity of a corrosion fatigue crack tip is estimated.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 29, No. 5, pp. 72–78, September–October, 1993.     

Description of the kinetics of growth of a fatigue crack on the basis of the criterion of current crack tip opening displacement

We suggest a microprocessor-based data-processing and measuring system and a special device aimed at the determination of current values of the crack tip opening displacement. They are used to establish current values of the crack tip opening displacement at working frequencies of loading of the specimen and describe the kinetics of fatigue crack growth. A decrease in the loading frequency of the specimen from 15 to 0.1 Hz in the course of measuring the current values of the crack tip opening displacement does not affect the reliability of determination of the characteristics of cyclic crack-growth resistance. The reliability of the results is affected only by the inertia of the mass of the gauge case. Institute for Problems of Strength, National Academy of Sciences of Ukraine, Kiev, Ukraine. Translated from Problemy Prochnosti, No. 4, pp. 60–67, July–August, 1998.     

Low-temperature cyclic crack-growth resistance of hydrogenated welds

We study the influence of chemical and phase composition on the cyclic crack-growth resistance of non-hydrogenated and hydrogenated welded joints in low-alloy steel at normal and low (–70°C) temperatures. It was discovered that the increase in the nickel content from 0.06% to 3.27% induces an increase in impact toughness and cyclic crack-growth resistance at low temperatures which can be explained by the increase in the content of needle ferrite and by the substitution of uniformly distributed residual austenite for the pearlitic component in the zone characterized by the columnar structure of heat treated beads. Hydrogen saturation of the weld metal leads to a decrease in its cyclic crack-growth resistance at normal temperatures and produces almost no effect on this parameter at low temperatures (–70°C). The presence of the second austenitic phase in the low-alloy weld metal decreases its susceptibility to hydrogen embrittlement in the case where finely divided austenite is uniformly distributed over the weld.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 2, pp. 62–68, March – April, 1995.     

Cyclic corrosion cracking resistance in 34KhN1M steel at the phase transition in water

Summary Tests have been done on cyclic corrosion cracking resistance in 34KhN1M steel used for making steam turbine disks in distilled water with the addition of 1.2 mg/liter of NaCl and KOH. to pH 9.3 with loading frequency f of 0.017-1 Hz and temperatures of 293–368 K under constant electrochemical conditions at the crack vertices or with allowance for changes occurring there. Measurements have been made on the trends in the electrochemical characteristics at the vertices of static and growing cracks during corrosion cracking tests. The extremal working and electrochemical conditions have been determined and a quantitative evaluation is given for the extents of the local anodic dissolution and hydrogen embrittlement occurring at the vertices of static and growing cracks.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 26, No. 5, pp. 40–47, September–October, 1990.     

Question of determining the electrochemical condition in a developing crack in investigating the crack resistance in a corrosive medium

Conclusions The proposed method of crack resistance testing of constructional materials in corrosive media provides determination of the electrochemical condition at the tip of not only a steady but also of a propagating crack. The values of pH and at the tip of the crack change with growth of it.For an investigated material-medium system at the tip of a developing crack in cyclic loading the minimum pH of the medium pH _min ^tc is –0.4, which is much lower than the corresponding parameter for a statically loaded steady crack (pH _min ^{ts 2.3} ). Together with the stress intensity factor, the electrochemical condition at the crack tip determines the fatigue crack growth rate in a corrosive medium.Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 18, No. 3, pp. 42–49, May–June, 1982.     

Cyclic fracture toughness of structural alloys with surface cracks at low temperatures

In the present paper, we study specific features of crack propagation from surface flaws in full-scale sheets used in manufacturing pressure vessels for cryogenic applications. The process of crack propagation consists of several stages and terminates in stages of surface through or central through cracks under conditions of low-frequency repeated tension. The effect of a decrease in temperature from 292 to 77°K on crack growth behavior was studied for sheets with a thickness of 2, 8, and 12mm. We describe a procedure for testing for crack-growth resistance at cryogenic temperatures and construct fatigue crack growth diagrams. It is shown that zones of influence of the front and rear faces of the specimen on the stress and strain fields near the crack front arise in the plane of a semielliptic crack. The shape of the interface of these zones can be approximated by a second-order curve. Variations in the thickness of the specimen and the test temperature affect the slope of the curve, i.e., the interface of the zones of influence. Specific features of the fracture process in the material of the plate with surface cracks manifest themselves most adequately at points of the crack front located on the indicated interface. We suggest a procedure for estimating the cyclic crack growth resistance of highly ductile stainless steels that is based on the use of the cyclic J-integral. We propose to regard the lengthl of the interface of the zones of influence of the front and rear faces of the specimen as a geometric parameter of the crack. It is used to construct kinetic fatigue crack growth diagrams for specimens with semielliptic surface cracks.Published in Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 31, No. 1, pp. 9–19, January – February, 1995.     

Electrochemical model of local corrosion at the tip of a loaded crack

A model of electrochemical processes near a crack tip in a stressed metal subjected to corrosion with hydrogen depolarization is suggested. It is shown that, in order to describe the kinetics of hydrogenation of the prefracture area, it is necessary to take into account the type of passivation layer on the newly formed metal surface near the crack tip and the mechanism of its formation. The model was applied to an iron-water system whose pH value is lower than that of hydrate formation. As a result, we determined the nonstationary potential of the metal-electrolyte interface as a function of the parameters of the stress-strain state near a crack tip and characteristics of the metal.Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 1, pp. 25–29, January–February, 1994.     

Pollution of the Heat Carrier and Its Influence on the Service Life of Power-Generating Units of Nuclear Power Plants

We present the data of autoclave testing of 10GN2MFA and 08Kh18N10T steels for cyclic crack-growth resistance in reactor borated water at a temperature of 300°C. We tested standard 25-mm-thick specimens for two values of the load ratio equal to 0.2 and 0.7. The loading frequency of specimens in the autoclave did not exceed 0.0167 Hz. We propose a model and a procedure of numerical evaluation of the increments of crack length according to the mechanisms of corrosion-fatigue and static corrosion cracking under the conditions of their combined action based on the analysis of principal characteristics of structural materials. Unlike the method based on the use of the Paris equation proposed by foreign standards, the method developed in the present work is more general and informative. Moreover, it includes the method used in standard specifications as a special case and enables one to explain the premature fracture of collectors of steam generators at the South-Ukrainian Nuclear Power Plant and other elements of power-generating units of the nuclear power plants. It is shown that, in the absence of corrosion-fatigue cracking, power-generating units of nuclear power plants can operate without emergency shutdowns for the entire service life. In the case where the heat carrier is polluted to a level sufficient for the initiation of the mechanism of static corrosion cracking, the service life of units to the time where the crack attains its critical size sharply decreases. We propose to improve the systems of water supply and perform continuous monitoring of the controlled parameters of the heat carrier and, in particular, of the contents of admixtures promoting the static corrosion cracking of steels.     

Nucleation and growth of corrosion-fatigue cracks from semicircular notches

The initial stages of corrosion fatigue on the surfaces of semicircular notches are studied for the system of low-strength carbon steel and a 3% NaCl solution under different loading and electrochemical conditions on the notch surface. It is shown that the process of electrochemical dissolution of the metal plays the predominant role in the first stages of growth of short corrosion-fatigue crack. We suggest an expression for the prediction of the characteristic density of corrosion-fatigue cracks as a function of the synergistic action of cyclic stresses and parameters of the process of electrochemical dissolution on the notch surface. Karpenko Physicomechanical Institute, Ukrainian Academy of Sciences, L'viv. Published in Fizyko-Khimichna Mekhanika Materialiv. Vol. 34, No. 5, pp. 53–60. September–October, 1998.     

Protective Action of Phosphate and Calcium-Containing Pigments under the Conditions of the Stress Corrosion Fracture of Steels

We have established that strontium chromate and a mixture of modified zinc phosphate and calcium-containing pigment inhibit substantially the corrosion of low-carbon steel in slightly acid rain solution. We observed mainly the anodic control of corrosion in the extract of chromate pigment and mixed control in the extract of a mixture of nonchromate pigments. Zinc ions in corrosive media enhance significantly the protective action of this pigment mixture. Here, a crystal-like protective film, consisting of calcium and zinc phosphates, is formed on the surface of steel. The extracts of chromate pigment and a mixture of nonchromate pigments in a slightly acid medium increase the resistance of low-carbon steel to corrosion fatigue in the stages of crack initiation and growth. Thus, the necessary prerequisites are created for enhancing the service life of steel constructions with organic coatings, containing inhibiting pigments, under conditions of the joint action of a corrosive medium and cyclic loading.__________Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 40, No. 5, pp. 31–36, September–October, 2004.     

Factors influencing the electrochemical conditions at the tip of a corrosion-fatigue crack

Summary Electrochemical characteristics have been measured at crack tips during tests on rectangular specimens having edge cracks, which are used for stress corrosion cracking tests in pure bending, where studies have been made on how the pH and electrode potential vary at the tip of a growing crack as a function of the number of loading cycles and the interaction time; measurements have been made on the initial pH at the tip of a stress corrosion crack, the steel composition, the loading frequency, and the oxygen concentration. Estimates are made of how those factors affect the rate of hydrogen depolarization at the tip, which governs the hydrogen embrittlement there.Translated from Fiziko-khimicheskaya Mekhanika Materialov, Vol. 27, No. 1, pp. 17–26, January–February, 1991.     

Electrochemical situation in corrosion-mechanical cracks

It is shown that the electrochemical situation in corrosion cracks is determined by the electromotive force of local galvanic cells at the crack tip and the polarization resistance of anodic processes.Lomonosov Odessa Technological Institute of the Food Industry, Odessa. Translated from Fiziko-Khimicheskaya Mekhanika Materialov, Vol. 30, No. 4, pp. 54–60, July – August, 1994.