Low Frequency Corrosion Fatigue Research Articles

Low-Frequency Corrosion Fatigue Test Study of Sucker Rods under High-Salinity Well Fluids in Deep CBM Wells

Corrosion fatigue test is the most direct and effective method to study the corrosion fatigue characteristics of sucker rod. At present, the commonly used test method is the high frequency fatigue test, but the working state of sucker rod is typical low-frequency and high-cycle corrosion fatigue, and the test with high frequency will reduce the impact of corrosion. Alloy steel 4330 is widely used in coalbed gas well high strength sucker rod, but the research on its low frequency corrosion fatigue life is relatively few. Therefore, in this paper, the corrosion fatigue test method of axial low-frequency and high-cycle was adopted to study the corrosion fatigue characteristics of 4330 steel sucker rod through the corrosion fatigue test under different typical corrosion media, temperature, and stress levels. The results show that the fatigue life of 4330 sucker rod drops sharply when the Cl− concentration in high salinity well fluid exceeds the threshold value of 155 mg/L. When this threshold is exceeded, the downward trend slows down. It can be seen that the significant factor affecting the corrosion fatigue life of 4330 material is not the concentration of Cl−, but the existence of Cl−. The presence of HCO3− promotes a further decrease in the corrosion fatigue life of the 4330 sucker rod by Cl−. The corrosion fatigue life of 4330 sucker rod decreases with the increase of temperature. When the well fluid temperature is less than 50 °C, the impact is relatively significant. When the well fluid temperature is more than 70 °C, the decline trend of corrosion fatigue life slows down. Based on the fitted S-N curve (stress-fatigue life curve), it is calculated that the fatigue limit of 4330 sucker rod at the stress ratio of 0.6 is 196 MPa in the solution of 10,000 mg/L Cl− at room temperature. These could provide valuable theoretical and technical guidance for design and selection of high-strength sucker rod in high-salinity corrosion well fluid environment.

Study on fatigue crack growth rate of 15CrMo steel based on stress ratio and corrosion environment

In this paper, the fatigue crack growth rate of 15CrMo steel, a typical gas cylinder material, is studied under the condition of simulating the actual working condition of gas cylinder. The low-frequency corrosion fatigue tests with stress ratio of 0.1 and 0.5 are carried out in air and different concentrations of wet hydrogen sulfide environment. The relationships between da/dN and ΔK are expressed by Paris equation. The results show that the crack growth rate in hydrogen sulfide environment with concentration of 200 ppm is 27 times higher that in air. The addition of corrosion environment greatly fasts the crack growth rate of 15CrMo steel. With the increase of H2S concentration, the crack growth rate increases exponentially. With the increase of stress ratio, the average stress increases, the crack closure effect decreases, the crack tip is fully opened, and the contact area with corrosion medium increases, which results in the increase of crack growth rate.

Effect of oxygen on near-neutral pH stress corrosion crack initiation under a simulated tape coating disbondment

A test setup that enables investigating the synergistic effects of tape coating disbondment, cathodic protection (CP) and low-frequency cyclic loading is used to determine the effect of oxygen on near-neutral pH stress corrosion crack initiation. After low-frequency corrosion fatigue tests, results show that surface micro-cracks initiate under the simulated tape coating disbondments at regions with insufficient CP potentials, not more remote locations completely free from CP as commonly believed. The presence of 1% oxygen in the composition of the purging gas has binary effects on the crack initiation under the simulated tape coating disbondment. On one hand it increases the size of the surface micro-cracks initiated on the steel surface at locations with some but insufficient CP potentials; and on the other, it reduces the number of surface micro-cracks at regions with no CP because of an extension of the general corrosion region deep inside the simulated coating disbondment.

Effect of cyclic frequency on fracture mode transitions during corrosion fatigue cracking of an Al-Zn-Mg-Cu alloy

Abstract There are complex issues remaining to be resolved before environment-assisted cracking models can be included in structural mechanics programs that are currently used to analyze mechanical fatigue crack growth. Considered here is the effect of cyclic frequency on fracture mode transitions that occur during corrosion fatigue of high-strength Al-Zn-Mg-Cu alloys. These alloys are used in civilian and military aircraft applications where they are exposed to detrimental aqueous saline environments. A previously developed “critical hydrogen at a critical distance” crack growth model is used to rationalize the observed transitions in crack-path fracture-modes, from intergranular (IG), to brittle transgranular (BTG), to ductile transgranular (DTG), as the alternating stress intensity factor and cyclic frequency increase beyond critical values. Corrosion fatigue crack growth rate data obtained on Al-Zn-Mg-Cu alloy 7017-T651 base metal and heat-affected “white zone” metal tested in aqueous NaCl solutions over a frequency range of 0.01–70 Hz are analyzed. For the white zone metal, dependence of the critical crack velocity on the critical frequency, at which the IG-BTG transition occurs, undergoes an abrupt reversal as the critical frequency increases above about 0.1 Hz. Mechanisms potentially responsible for this change in frequency dependency are discussed in the context of the critical hydrogen model. The transition from low to intermediate frequency behavior is speculated to be due to a change in the critical distance from microstructural control, for frequencies at or below 0.1 Hz, to control by the critical hydrogen criterion at higher frequencies. The low frequency behavior is discussed relative to the transition from static load stress corrosion cracking to low frequency corrosion fatigue, which occurs as cyclic frequency increases above zero.

Effect of corrosion potential on the corrosion fatigue crack growth behaviour of low-alloy steels in high-temperature water

The low-frequency corrosion fatigue (CF) crack growth behaviour of different low-alloy reactor pressure vessel steels was characterized under simulated boiling water reactor conditions by cyclic fatigue tests with pre-cracked fracture mechanics specimens. The experiments were performed in the temperature range of 240–288 °C with different loading parameters at different electrochemical corrosion potentials (ECPs). Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by SEM were used to quantify the cracking response. In this paper the effect of ECP on the CF crack growth behaviour is discussed and compared with the crack growth model of General Electric (GE). The ECP mainly affected the transition from fast (‘high-sulphur’) to slow (‘low-sulphur’) CF crack growth, which appeared as critical frequencies ν crit = f(Δ K, R, ECP) and Δ K-thresholds Δ K EAC = f( ν, R, ECP) in the cycle-based form and as a critical air fatigue crack growth rate d a/d t Air,crit in the time-domain form. The critical crack growth rates, frequencies, and Δ K EAC-thresholds were shifted to lower values with increasing ECP. The CF crack growth rates of all materials were conservatively covered by the ‘high-sulphur’ CF line of the GE-model for all investigated temperatures and frequencies. Under most system conditions, the model seems to reasonably well predict the experimentally observed parameter trends. Only under highly oxidizing conditions (ECP ⩾ 0 mV SHE) and slow strain rates/low loading frequencies the GE-model does not conservatively cover the experimentally gathered crack growth rate data. Based on the GE-model and the observed cracking behaviour a simple time-domain superposition-model could be used to develop improved reference CF crack growth curves for codes.

Evaluation of the mitigation effect of hydrogen water chemistry in BWRs on the low-frequency corrosion fatigue crack growth in low-alloy steels

The mitigation effect of hydrogen water chemistry (HWC) on the low-frequency corrosion fatigue crack growth behaviour of low-alloy steels was investigated under those critical boiling water reactor (BWR) system conditions, where fast corrosion fatigue crack growth significantly above the ASME XI ‘wet’ reference fatigue crack growth curves was observed under normal water chemistry conditions (NWC). The experiments were performed under simulated BWR conditions at temperatures of 250, 274 or 288°C. Modern high-temperature water loops, on-line crack growth monitoring (DCPD) and fractographical analysis by scanning electron microscope were used to quantify the cracking response. HWC resulted in a significant drop of low-frequency corrosion fatigue crack growth rates by at least one order of magnitude with respect to NWC conditions and is therefore a promising and powerful mitigation method.

Mechanical redesign of the rotating biological contactor

The principle of the Rotating Biological Contactor (RBC) was originated in the early part of the century and today there are many thousands of units operating world wide. However, the RBC has been plagued with mechanical deficiencies since its conception. This paper presents a brief insight into some of the major mechanical defects associated with RBCs. These findings have been accumulated over the past three years during a mechanical design audit, and encompass over two hundred and sixty RBC units visited operating on site. The reasons for mechanical failures are attributed to poor engineering design, low frequency corrosion fatigue and microbiologically influenced corrosion; evidence supporting these claims is presented. Having the benefit of a thorough understanding of the mechanisms and reasons for mechanical failure, a new approach to designing RBCs is encouraged. This has resulted in a new generation of RBC designs with a guaranteed operational life of twenty years.

The relationship between crystallographic texture and salt water cracking susceptibility in Ti6Al4V

Stress corrosion cracking (SCC) and low-frequency corrosion fatigue were investigated for the α + β alloy Ti6Al4V in 3.5% aqueous salt solution. Both coarse lamellar and nominally equiaxed microstructures with distinctly different textures were developed by thermomechanical processing. Despite their inherent SCC sensitivity, lamellar microstructures have K ISCC values equal to or greater than those of equiaxed microstructures, owing to the superior fracture toughness. The relative SCC sensitivity, K ISCC/ K IQ, correlates well with the relative intensity of basal poles parallel to the tensile axis, suggesting that brittle fracture, rather than slip on basal or near-basal planes, is responsible for enhanced cracking. When the relative basal pole intensity parallel to the loading axis is significantly less than unity, K ISCC is equal to K IQ and corrosion fatigue crack growth is frequency independent. Otherwise, K ISCC can be as low as one-half of K IQ, and the “typical” frequency dependence involving “cyclic SCC” is observed. The relative basal pole intensity and, in turn, the elastic modulus can therefore be used to predict or prevent environmentally enhanced cracking in this type of alloy.

A study of corrosion fatigue behaviour of anodized and unanodized 2024-T3 aluminium alloy

The corrosion fatigue (CF) behaviour, under constant deflection bending conditions with a pulsating tension stress form, of 2024-T3 aluminium alloy, unanodized and anodized to form a thick porous film, in 3.5% NaCl solution has been investigated. It was found that E corr varies very little until specimen fracture under low frequency CF conditions, whereas E corr drops rapidly when approaching the later fracturing stage of the CF process under high-frequency conditions for unanodized specimens. However, a slow drop in E corr was detected from the commencement of the CF process, and lasted up to a much more rapid drop at a later fracturing stage for the anodized specimen. This behaviour presumably can be explained by the cracking of the anodic film and the theory of imperfect recovery of the surface film. It is suggested that the E corr monitoring technique may be useful for determining the remnant CF life for existing structural parts of this alloy or other aluminium alloys regardless of whether or not they are anodized. Furthermore, the T3 temper provides a microstructure which may retard main-crack formation and penetration in the CF process of the anodized alloy, thus mitigating partly the negative effect of the readily crackable anodic film.

The influence of environment and stress ratio on the low frequency fatigue crack growth behaviour of two medium-strength quenched and tempered steels

The results of low frequency corrosion fatigue crack growth tests on HY 130 and Q1N steels are reported and compared. The similarities and differences in the response of the two steels to variations in environment and stress ratio are highlighted and discussed in terms of some recent theories of corrosion fatigue crack growth and the impurity element embrittlement of grain boundaries.

Low Frequency Corrosion Fatigue of a Carbon Steel with Stress Concentrations

A low speed constant strain type of reverse bend corrosion fatigue machine using U-shaped specimens of rectangular section is described. The air-fatigue and corrosion-jatigue behaviour of a carbon steel at the fixed frequency of 1·7 Hz is reported in specimens tested with and without stress concentrations being present in the form of drilled holes. Measurements of corrosion potential and metallographic details of the fatigue cracks are presented. The possibility of establishing a ‘corrosion factor’ for use in conjunction with the well defined stress concentration factor is examined on the basis of the experimental data derived.