Stress corrosion cracking (SCC) in power generation, petrochemical and chemical plants is a perplexing problem to detect with non-destructive testing due to the unique requirements to initiate and propagate SCCs. Stress corrosion crack initiation and propagation require a material susceptibility, a corrosive environment, and tensile stress. For a given metal alloy, a sweet spot of these three factors may initiate and propagate SCC. SCC which may manifest as intergranular corrosion stress corrosion cracking (IGSCC) and/or transgranualar stress corrosion cracking (TGSCC) may occur at stresses much lower than design stresses and lead the equipment and structures to premature failure. In fact, the standard fracture toughness used for design is replaced with K1SCC for the specific material and corrosive environment. From a non-destructive testing perspective, SCC are typically more difficult to detect and size due to their dendritic like propagation pattern. The article presents three different options for non-destructive testing for stress corrosion cracking: acoustic emission testing, eddy-current testing, and phased array ultrasonic testing.
Figure 1: Stress corrosion cracking in austenitic stainless-steel SS 304 12” insulated pipe.
Stress Corrosion Cracking and Non-destructive Testing
Failure analysis of stress corrosion cracking in carbon steels, stainless steels, and aluminum alloys dates to the adoption of corrosion resistant metallic alloys for a wide range of chemical process [1-2]. While SCC tends to be application specific due to the alloy used, operational environment, and applied stress the process from initiation through failure is described by the Parkins’ stress corrosion spectrum adapted in Figure 1 [3,4,5]. The three-stage model describes the important primary stage electrochemically, and later stage, mechanically driven deterioration methods. Stage I and II SCC initiation is largely driven by electrochemical forces. The corrosion protective oxide film is broken down on the surface of the component and pitting corrosion begins. As the pits deepen and stress concentrations increase SCC initiates. Towards the end of Stage II, mechanical forces drive the SCC propagation until failure occurs in Stage III.
Figure 2: Three stage SCC progression model [3,4,5].
Early-stage passive layer breakdown, stainless steel pitting initiation, and SCC initiation are long term processes with very small physical features that are challenging for advanced NDT techniques like eddy-current testing and phased array ultrasonic testing to detect. In-situ acoustic emission has been identified as an advanced NDT method for detection and characterization of early-stage SCC electrochemically driven deterioration processes [6,7].
Acoustic Emission Testing for Stress Corrosion Cracking
An excellent summary on how acoustic emission testing may be used to detect and identify stress corrosion cracking in stainless steel is provided by Calabrese . The main SCC corrosion specific acoustic emission sources were identified as: SCC crack initiation and propagation, hydrogen bubble evolution, and corrosion protective surface oxide layers fracture. More traditional stress corrosion crack tip acoustic emission sources include slip, twinning and fracture of precipitates and nonmetallic inclusions.