Reflections on Early Stages of Environmentally Assisted Cracking from Corrosion Pits

Abstract

A perspective is presented on the evolution of damage due to environmentally assisted cracking (EAC), from crack precursor development through to long crack growth. The variable nature of crack precursors is highlighted with an observation that uncontrolled chemistry excursions or fabrication defects could eliminate any significant delay associated with that step in the damage evolution process. Specimen preparation by machining and grinding can be critical in determining the apparent susceptibility of the metal to EAC and corrosion, and an example for 316L stainless steel is given to show how physical defects generated by the grinding wheel can become the dominant site for pitting attack relative to MnS inclusions. Corrosion pits are the most commonly observed precursor to cracks in aqueous chloride environments. The loci of sites of crack initiation around a pit are discussed and the inherent challenges in quantifying the growth of cracks smaller than the pit depth described with implications for modelling of the pit-to-crack transition. The remarkably enhanced stress corrosion crack growth rate data for short and small cracks in a 12Cr steam turbine blade in a simulated condensate environment are discussed in the context of crack electrochemistry modelling and the implications for engineering integrity.

Corrosion fatigue and damage tolerance in the nickel‐based superalloy RR1000 subjected to SO2 environments

“…22 Due to the complexity and scale of this mechanism, from an empirical perspective, the transition from corrosion pitting to crack growth will always be extremely difficult to detect in practice. Previous workers 10,11 have been able to quantify small crack growth rates from cracks emanating from pre-existing corrosion pits, notably under aqueous corrosion scenarios at ambient temperatures. This was an impossible task for the freely initiating cracks developed in the current plain cylindrical specimens at high temperature.…”

“…Under the present hot corrosion fatigue scenario and experimental setup, although a model invoking “small,” “short,” and “long” crack growth can be envisaged, 10 the characterization of the earliest stages of crack development was limited to post‐test fractography and metallographic sectioning. It was impossible to partition cyclic lifetimes apportioned to “initiation” and “growth” with precise accuracy, and indeed, this was not a prerequisite of the study.…”

“…Previous authors have monitored and evaluated the behavior of environmentally assisted short crack growth behavior in alternative alloys and in relatively benign aqueous environments at ambient temperatures. 10,11 However, the current investigation of high-temperature corrosion fatigue behavior was not intended to enable such characterization.…”

“…It is recognized that the transition from a corrosion pit to a true fatigue crack is highly complex, incorporating the effects of “small” and “short” scale growth during the early stages of crack development. Previous authors have monitored and evaluated the behavior of environmentally assisted short crack growth behavior in alternative alloys and in relatively benign aqueous environments at ambient temperatures 10,11 . However, the current investigation of high‐temperature corrosion fatigue behavior was not intended to enable such characterization.…”

Corrosion Inhibition by Sulfate after Surface Preparation

“…The application of a mechanical load to the surface during this treatment will also induce deformations deep within the material (>1 μm) which can decrease the lifetime of the material. 3,4 Electropolishing is an increasingly popular surface preparation technique in both corrosion studies, metallurgic studies, and in industrial applications. 2 After some form of mechanical grinding/ polishing, the sample is immersed in an aggressive media and a current or potential is applied.…”