Mechanisms of corrosion fatigue below K Iscc

Abstract

Information on corrosion-fatigue crack growth in engineering structures is necessary for the prediction of service lives of structures subjected to both fatxgue loading and an aggressive environment. The rate of crack growth m corrosion fatigue is governed by the interaction between the chemical mechamsms 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 techmque is inadequate for isolating the crack-tip mechamsms 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 acldac solutions were observed in the proximity of the central portion of the crack tip. Hence, it is concluded that the effectiveness of cathodic protectmn decreases as the crack front moves away from the flee surface. The fractographac 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 solutmn.Finally, by using the corrosion-fatigue crack-propagation lawwhere da/dN is crack-growth rate per cycle, b is the crack-opening displacement, ay is the yield strength, and D(t) is a time-or frequency-dependent functmn, 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 6). Introd~'tionSubcritical crack growth in corrosion fatigue may be divided into two components: one caused by cyclic stresses and the other caused by the environment. The cyclic-stress component has been thoroughly investigated, and it is generally accepted that it is caused by the mechanisms of plastic deformation at the crack tip [14]. However, the chemical mechanisms at the crack tip that increase crack growth in corrosion fatigue are yet to be investigated in detail. Most investigations on the influence of the environment on subcritical crack growth in highstrength steels have been designed to determine the existence of environmental effects in a given environment-material system [5][6][7][8][9][10][11][12][13]. Furthermore, in these investigations, the effect of the environment on crack growth was studied in terms of either time to failure or in terms of the change in crack-growth rates.In a recent investigation on the corrosion-fatigue behavior below K1~c of 12Ni-5Cr-3Mo maraging steel in a room-temperature 3 percent solution of sodium chloride, Barsom [14,15] showed (Figur...

Advanced testing methods for studying the mechanical behavior of materials

“…17 The reason for this strong frequency dependence of da/dN is a superposition of cycle-dependent FCG and time-dependent SCC during each load cycle. Although the principal importance of cyclic frequency for FCG rate measurements in metals immersed in an environment was emphasized by Barsom, 20 Gallagher, 21 and Meyn 22 as early as 1971, up to now many corrosion FCG tests have been carried out at frequencies far above 1 Hz. The crack growth rate increases signifi cantly with increasing the aggressiveness of the environment.…”

Mechanisms for corrosion fatigue crack propagation

“…In the case of high‐strength low‐alloy steels and titanium alloys, for example, no firm consensus exists on the corrosion FCG mechanism in any one material‐environment combination. For these materials, some authors propose that HIC plays a dominant role in corrosion fatigue crack propagation 5–15 . As evidence of HIC they point to the deleterious effect of increasingly negative cathodic potential on corrosion FCG rates 5–13 and the similarity between features of the fracture surfaces produced by crack growth in aqueous solutions and in hydrogen gas 5,12 , 14,15 .…”

“…For these materials, some authors propose that HIC plays a dominant role in corrosion fatigue crack propagation 5–15 . As evidence of HIC they point to the deleterious effect of increasingly negative cathodic potential on corrosion FCG rates 5–13 and the similarity between features of the fracture surfaces produced by crack growth in aqueous solutions and in hydrogen gas 5,12 , 14,15 . Others provide evidence that SAD is the dominant mechanism for corrosion FCG in high‐strength steels 16–18 and titanium alloys 19, 20 .…”

“…In 1971, Barsom 5 and Gallagher 6 pioneered the application of the controlled‐potential technique to distinguish between HIC and SAD during corrosion fatigue crack propagation. In addition, Barsom, 5 Meyn, 14 and, more recently, Rungta and Begley 16 first examined in greater detail the features of the fracture surfaces produced by corrosion FCG under applied potentials by scanning electron microscopy (SEM) and transmission electron microscopy of plastic/carbon replicas. These examinations were conducted to provide further information concerning the corrosion FCG mechanisms.…”

“…These examinations were conducted to provide further information concerning the corrosion FCG mechanisms. However, none of the above‐mentioned attempts, 5,6 , 14,16 nor numerous attempts of other authors 7–13,35 who employed the controlled‐potential technique and fractographic evidence, allowed for the possibility of quantifying the role of any one mechanism in corrosion fatigue crack propagation.…”