Chemical and electrochemical conditions within stress corrosion and corrosion fatigue cracks

Abstract

In the area of environment assisted cracking, literature aimed at understanding the chemical and electrochemical conditions at/ near the crack tip establishes that the crack tip is occluded and not well represented by bulk conditions. A review of the relevant literature, both modeling and experimental, is presented here and shows that crack tip conditions are determined by the balance between high metal ion concentrations resulting from crack tip anodic reactions and subsequent hydrolysis, mass transport (including ion migration, diffusion, and advection), and electrochemical polarization of the bold surface, which determines the extent of anodic and cathodic reactions occurring in the crack environment. Under both freely corroding conditions and anodic polarizations, the crack tip pH decreases with increasing polarization above the freely corroding condition, most often leading to a very acidic crack environment. Under sufficient cathodic polarization, the crack tip pH increases. Because of high-anion and -cation concentrations in the crack environment, an IR drop down the crack exists, leaving the crack tip relatively unpolarizable. Ion migration enhances the occluded nature of the crack tip by supplying anions from the bulk solution to maintain electroneutrality at the crack tip. Diffusion to counteract this concentration gradient is minimal and only plays a role in crack tip conditions at very small crack lengths. When cyclic loading conditions are encountered, the occluded nature of the crack tip can be counteracted by advection; although, the role decreases with decreasing f and increasing R, essentially as corrosion fatigue conditions approach those of stress corrosion cracking.npj Materials Degradation (2017) 1:12 ; doi:10.1038/s41529-017-0015-0 INTRODUCTION Structural alloys must withstand both demanding mechanical loads and corrosive environmental conditions when utilized in many applications. Consequently, these materials can be susceptible to premature failure by environment assisted cracking (EAC) when the loading and environmental conditions are sufficiently aggressive. The influence of EAC on the performance and life of structural alloys can be seen in many applications such as oil and gas, nuclear power, nuclear waste storage, and aerospace. For example, several reviews of aircraft structural failures and teardown inspections concluded that fatigue and corrosion dominate aerospace component failures 1, 2 with 78% of the corrosion damage sites identified during teardown having initiated fatigue cracks.2 In addition, the United States Nuclear Regulatory Committee (NRC) released a report in 2014 concluding that one of two top priority needs is understanding stress corrosion cracking (SCC) of spent nuclear fuel storage canisters.

Stress Corrosion Cracking Probability of Selective Laser Melted 316L Austenitic Stainless Steel under the Effect of Grinding Induced Residual Stresses

“…Over the past decade, many researchers have investigated different aspects of the effect of machining on SCC and pitting susceptibility of ASS. It is well-reported that a higher magnitude of RS makes the surface more susceptible to SCC occurrence [28][29][30][31][32]. On the other hand, recent investigations reported that tensile RS has no direct effect on SCC of ASS made by manufacturing methods such as hot or cold rolling and it only makes the surface more active compared to annealed specimens.…”

Toward Understanding the Effects of Strain and Chloride Deposition Density on Atmospheric Chloride-Induced Stress Corrosion Cracking of Type 304 Austenitic Stainless Steel Under MgCl2 and FeCl3:MgCl2 Droplets

“…25,26,85 Moreover, the crack chemistry can be far more acidic and reach pH down to 2-3 where hydrogen evolution reactions then dominate. 99 The oxygen concentration at the crack tip usually depletes quite rapidly which promotes anodic activities, with the crack wall and crack mouth becoming cathodic sites. 99 When the crack gets sufficiently long, the IR drop between the crack tip and the free surface where more electrolyte is available, can reach high values, so that the crack tip is practically disconnected from the electrolyte volume.…”

“…99 The oxygen concentration at the crack tip usually depletes quite rapidly which promotes anodic activities, with the crack wall and crack mouth becoming cathodic sites. 99 When the crack gets sufficiently long, the IR drop between the crack tip and the free surface where more electrolyte is available, can reach high values, so that the crack tip is practically disconnected from the electrolyte volume. 99 A site-specific corrosion cell, with the crack tip forming the anode and the crack walls at some distance behind the crack tip forming the cathode, is established, both favoring metal dissolution at the crack tip and also producing atomic hydrogen near the crack tip at some distance to it at the crack walls.…”

“…99 When the crack gets sufficiently long, the IR drop between the crack tip and the free surface where more electrolyte is available, can reach high values, so that the crack tip is practically disconnected from the electrolyte volume. 99 A site-specific corrosion cell, with the crack tip forming the anode and the crack walls at some distance behind the crack tip forming the cathode, is established, both favoring metal dissolution at the crack tip and also producing atomic hydrogen near the crack tip at some distance to it at the crack walls. 99 Regions adjacent to the crack walls, in particular the crack tip, are highly strained (as shown in the EBSD results in Figure 11 and Figure 12), and some of the hydrogen formed can be absorbed into the material and diffuse to the sites with high strain.…”

Stress Corrosion Cracking in an Extruded Cu-Free Al-Zn-Mg Alloy

“…It is not unlikely that several of these mechanisms occurs simultaneously and affect one another, as suggested by Najjar et al [10]. In the vicinity of the crack tip the pH is low and the dominating cathodic reaction is proton reduction [11]. This results in hydrogen evolution and diffusion ahead of the crack tip, promoting conditions for local embrittlement [9,10].…”