Understanding Fatigue Crack Growth Behavior at Low Frequencies for a Mn–Ni–Cr Steel in 3.5 % NaCl Solution Under Controlled Cathodic Potential

Abstract: Accurate and reliable life prediction is one of the challenges faced by engineers working in safety-critical domains such as power plants, transportation, and offshore structures. This paper presents the developments to estimate the fatigue properties of materials using small volume of sample material—similar to scooped samples. Cyclic ball indentation and cyclic small punch testing methods have been developed over a period of nearly two decades and have been demonstrated to predict the fatigue properties of in-service materials. Some salient results are discussed in this paper. In the case of offshore structures, the synergistic effect of mechanical loading at a low frequency combined with the corrosive environment accelerates the damage. Life prediction for such structures requires data on corrosion-fatigue crack growth at low frequencies. This is a time-consuming effort and hence there is a need to estimate the properties through novel test methods. Frequency shedding method is proposed to estimate the fatigue crack growth rate behavior in corrosive environments. Further, designers and operators of offshore equipment resort to avoidance of free corrosion through the use of electrode potentials. However, the choice of electrode potential is dependent on the stress state. Through a systematic study, the optimum electrode potential for a fatigue crack growth study has been identified and the same is discussed here. It is hoped that the results of this study and the directions shown to carry out the data generation under more realistic conditions would help the life prediction and life extension community at large.

Abstract: A mathematical model has been developed to describe the mass transport and electrochemical conditions in a corrosion fatigue crack in steel in 3.5% NaCl and in sea water for both freely corroding and anodic polarization conditions. Mass transport by advection (fluid flow induced by the movement of the crack walls), diffusion and ion migration was considered. Anodic and cathodic processes, hydrolysis reactions (including hydrolysis of alloying elements) and buffering reactions were included in the model. The pH value developed within the crack at a temperature of 5°C was between 7.0 and 8.5 for a wide range of conditions, with the maximum value controlled by the buffering associated with deposition of ferrous hydroxide. The lower pH values corresponded to relatively high ferrous ion concentration and were obtained for combinations of high R values (minimum/maximum load) and low frequencies for which convective mixing with the bulk solution was minimized. The presence of chromium in the steel at the 1 wt% level had only a small effect on the crack tip pH value in deep cracks but could lower the pH considerably (to about 4.0) in very shallow cracks (2.5 × 10 −2 cm) if the potential was about −600 mV(SCE). The potential drop in the crack was relatively small (

Abstract: Fatigue crack growth rates over wide ranges of stress intensities (from threshold to nearly critical stress intensity) were measured on HY130 steel in 3.5% sodium chloride aqueous solution at several cyclic frequencies, stress ratios, and potentials. The growth rates are compared with reference data measured in laboratory air. The growth rate curves for both environments can be approximated by two linear sections converging at low stress intensity ranges to the same threshold, which depends only on the stress ratio. The upper parts of the lines with lower slopes converge again to the point where maximum stress intensity approaches its terminal value. As a result, the maximum environmental acceleration of crack growth appears at intermediate stress intensities, and it increases with decreasing frequency and potential. Data indicate a relatively low susceptibility of HY130 steel to corrosion fatigue. With increasing stress ratio R the fatigue crack growth threshold is shifted to lower stress intensity ranges by the same amount in both air and salt water environments. The effect of R on growth rate can be introduced into a power law expression.

Abstract: The results of recent experiments on measuring corrosion fatigue crack propagation rates in structural steel immersed in seawater with and without cathodic polarisation are reported. These include measurements of electrochemical potentials near the growing crack tip. The experiments have been designed to learn more about the mechanisms and rate-determining processes influencing the rate of crack growth. From these and other results in the literature, deductions are made about the relative importance of mechanical and electrochemical limitations on the rate of crack propagation.

Abstract: Fatigue tests were carried out on 4140 steel heat treated to various hardness levels. Environmental effects are reported in dry and moist air, aerated and deaerated 3 pet NaCl solution, and under conditions of cathodic protection and of passivity. Critical corrosion rates were measured below which the environment does not affect fatigue life. The results are interpreted in terms of environmentally-induced plastic deformation.

Abstract: Crack growth rates of small fatigue cracks in a high strength steel tested in 3.5% NaCl solution with cathodic protection are analysed in the Paris regime through the comparison with the corresponding results obtained in air or in high vacuum. Environmental effects in the saline solution are due to hydrogen produced by cathodic polarisation, which causes intergranular and transgranular brittle fracture surfaces. By comparison to fatigue crack growth rates obtained in air, it could be concluded that hydrogen effects are negligible at low Delta K and then increase with Delta K. But in fact, when compared to results obtained in a non-active media such as high vacuum, hydrogen effects in the saline solution are very high at low Delta K and decrease slightly when Delta K increases. In air, adsorption of the different gaseous species as well as hydrogen effects due to water vapour dissociation strongly enhance crack growth rate compared to vacuum especially for low Delta K. Consequently, a comparison with fatigue crack growth results obtained in air does not allow to quantify properly environmental effects due to another active environment such as the saline solution with cathodic protection considered in the present work.