How to detect whether nitrate stress corrosion cracking has occurred in equipment
Thread Content
Given your previous interest in the carbon steel equipment used in the regeneration systems of refinery catalytic cracking units, multiple approaches can be employed to determine whether nitrate stress corrosion cracking has occurred in such equipment: a preliminary visual inspection is carried out, focusing on areas where stress concentrations occur such as welds and heat-affected zones, to check for brittle cracks that do not show signs of uniform corrosion; these cracks typically have a branched pattern and exhibit no signs of plastic deformation. Precise determination in the laboratory is achieved by using metallographic and fracture scanning electron microscopy to directly observe the grain-boundary/through-grain branching characteristics of cracks; this is the most reliable method for identifying such types of cracking, allowing for an accurate determination of their nature. Non-destructive testing on-site: First, data on the device’s operating conditions are collected to assess the risk level of nitridation cracking; thereafter, ultrasonic testing (UT) is carried out to determine the location and depth of internal cracks, thereby meeting the testing requirements specific to catalytic cracking regeneration systems. Auxiliary verification methods involve using EDS composition analysis to detect the characteristic nitrate elements of corrosion products within cracks, and taking into account whether there is a condensing environment with temperatures below the dew point under operating conditions, in order to further support the test conclusions.
The testing approach shared by the original poster is quite professional; in particular, it combines preliminary macroscopic inspections, metallographic microscopy, and on-site ultrasonics, covering essentially all the key steps from qualitative to quantitative analysis. I would like to add two details that are easily overlooked in practical applications: First, when performing on-site ultrasonic testing, attention should be paid to the choice of coupling agent; some oily coupling agents may interfere with the detection of crack signals, so it is recommended to use water-soluble or specialized high-viscosity coupling agents ； Secondly, for carbon steel equipment in regeneration systems, if the operating temperature fluctuates frequently or wet conditions prevail, it is recommended to shorten the inspection interval, as the propagation rate of nitridic cracking in such environments may be faster than expected. Furthermore, if the testing conditions permit, combining magnetic particle inspection (MT) for surface crack verification can reduce the risk of missed detections.