How to determine whether a device has suffered nitrate stress corrosion cracking
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Given your previous interest in the issue of nitridic cracking in carbon steel equipment used in the regeneration systems of catalytic cracking units, the following methods can be used to determine whether such equipment has suffered from nitrate stress corrosion cracking: Visual inspection – Over 90% of nitridic cracking occurrences are found at the ring welds; these cracks are usually transverse to the weld direction, without any signs of uniform corrosion, and appear as intermittent fissures. A small number of such cracks occur in the base metal area. Microscopic feature verification: The cracks extend inward from the surface in contact with the corrosive medium, appearing as dry tree branches with thin branches; the fracture surface exhibits a typical brittle morphology. In a nitrate environment, carbon steel fractures primarily in a grain-boundary mode. Supporting evidence from operating conditions: Verify the environment in which the equipment is located – whether there are conditions such as excessive levels of NOₓ in the flue gas (e.g., NO levels as high as 2610 μg/g, NO₂ levels as high as 196 μg/g), or whether the wall temperature is low enough to cause condensation of nitrate solutions. Non-destructive testing verification: First, device operation data is collected to assess the cracking risk level; thereafter, non-destructive testing methods such as UT (ultrasonic testing) are applied as appropriate. Based on the results of these tests, it is finally determined whether cracking has occurred.
The original poster has shared very professionally; by presenting these details on macro and micro-level assessments, it provides great value as a reference for troubleshooting equipment on-site. I would like to add two points that may need attention during actual inspections: Safety first – condensation is likely to occur when the wall temperature is low and the NOₓ concentration is high; before conducting tests, it is essential to ensure that the equipment has been cooled down properly and that all residual acidic substances have been removed, in order to avoid contact with them. Personal protection should also be worn when sampling or grinding. Supporting inspection methods: In addition to visual inspection and microscopic analysis, magnetic particle testing (MT) or penetrant testing (PT) can be used on-site to help detect minor surface cracks, especially in areas with circumferential welds; these methods enable more accurate identification of suspected defects. Additionally, if the original poster has actual cases in hand (such as a device that experienced similar cracks or deviations in operating parameters), it would be helpful to share the procedures taken at that time, so that everyone can understand better. Of course, if you want to confirm for sure whether it is brittle or not, it’s best to commission a professional institution to conduct fracture analysis and energy-dispersive spectroscopy for composition testing – safety assessments cannot be taken lightly.