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.
Figure 3: Stress corrosion cracking source discrimination in acoustic emission duration / energy space.
Interestingly, there does appear to be some amplitude discrimination between the various stress corrosion cracking acoustic emission sources . At very low acoustic emission peak waveform amplitudes the source is commonly metal dissolution or passive film fracture. At low to moderate peak amplitude levels, hydrogen bubble evolution is observed as well as plastic deformation due to twinning. Moderate stress corrosion acoustic emission amplitude levels are observed from micro-cracking sources which may include intergranular cracking, transgranular cracking, and crack tip plastic deformation.
Eddy Current Testing for Stress Corrosion Cracking
Eddy current testing can be applied to stress corrosion cracking using standard eddy current probes, eddy current array (ECA) probes and remote field eddy current testing (RFET).ET and ECA  methods lend themselves to SCC detection in stainless steel components but probe, or array selection, is critical since cracks and lift-off have comparable phase. To detect cracks in stainless steel components and to avoid mischaracterizing lift-off variations with surface cracks it is recommended to use an ECA with cross-wound coils.Eddy-current array cross-wound coils consist of orthogonally wound coils. Cross-wound ECA probes exhibit a 180-degree lag between lift-off variations and surface cracks.
Figure 4: 180-lag between stainless steel surface crack and lift-off using a cross-wound coil .
ECAs are constructed using a multi-layer printed circuit board coil a multi-layer configuration dedicated to a cross-wound array on the lower layer and reflection coil probe on the upper layer. The eddy current reflection probe provides dynamic lift-off compensation for rough surfaces and weld reinforcements. In this configuration, eddy-current signal polarity can provide direction correlation to crack orientation relative to coil axis. The advantages of ET or ECA for SSC include inspection of coated components, less surface cleaning compared to visual or penetrant testing, and non-contact.
Ultrasonic Testing of Austenitic Stainless Steel for Cracks
Even though conventional (UT) and phased array ultrasonic testing (PAUT) methods have been routinely applied to crack detection in austenitic stainless steels for over 50 years , welded austenitic components are still more difficult to inspect compared to standard carbon steels. Specialized transducer configurations are required and include the transmit-receive longitudinal (TRL) dual element probe in the 2- 5 MHz range depending on the expected grain size of the targeted materials. This is one of the unique applications that uses higher angle longitudinal waves, with comparable longer wavelengths than same frequency shear waves, to minimize grain boundary scattering. The longer wavelength of this mode scatters less at the larger grain boundaries. The separation of the transducer transmitter and receiver elements provides better signal-to-noise ration and no dead zones due to the dual elements.
Figure 5. Scattering of smaller wavelength shear wave in coarse grain austenitic stainless steel welds.
Advanced non-destructive testing of stainless-steel pressurized components is still a challenge today with candidate NDT including acoustic emission testing, eddy current and eddy current array testing, and phased array ultrasonic testing. The optimal technique(s) depend on the SCC spectrum status, access to the affected area, desired accuracy/resolution.
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