The description of localized, smooth, wavy thinning on the bottom of the pipe is characteristic of erosion-corrosion. In lean amine service with trace acid gases, the combination of fluid flow (potentially with entrained droplets or turbulence) and the corrosive environment leads to accelerated metal loss in specific areas, distinct from uniform sulfidic or acidic corrosion.

For REAC service in hydroprocessing units where ammonium bisulfide corrosion and chloride stress corrosion cracking (CISCC) are concerns, carbon steel is the standard and most practical material. It is immune to CISCC. While alloys like Monel resist CISCC, carbon steel with proper corrosion allowance, wash water injection, and velocity control is the industry-standard, cost-effective solution for this specific environment.

The combination of cyclic service (temperature cycling), the location of the crack (stress concentration zone like a weld HAZ), and the branched, oxidized nature of the crack are classic indicators of thermal fatigue. This mechanism is driven by repeated thermal stresses from startup/shutdown or process cycles, not by steady-state high temperature or specific contaminants.

Naphthenic acid corrosion is most severe in high-velocity areas at temperatures between 450°F and 750°F. The vacuum column transfer line (and associated equipment) in an FCC unit sees some of the hottest temperatures and highest velocities of streams containing naphthenic acids, making it a prime location for this form of corrosion-erosion.

The formation of iron sulfide scale on low-alloy steel in high-temperature hydrogen service is a key indicator of High-Temperature Hydrogen Attack (HTHA). The hydrogen diffuses into the steel, reacting with carbides to form methane, which leads to decarburization and fissuring. The iron is left to react with sulfur, forming the observed scale. This is a serious integrity concern as it reduces strength.

Automated Ultrasonic Testing (AUT) with shear wave (angle beam) techniques is the most effective non-intrusive method for detecting internal planar flaws like hydrogen-induced cracking (HIC), stepwise cracking (SWC), and stress-oriented hydrogen-induced cracking (SOHIC). It provides detailed imaging of damage within the wall, unlike straight-beam UT (for thickness) or AE (which is more global).

The conditions (sour water with H2S, pH < 7, temperature < 150°F) are classic for Wet H2S damage mechanisms, including hydrogen blistering and HIC/SOHIC. The primary risk is not uniform thinning but the formation of internal cracks from atomic hydrogen penetration. Inspection should therefore focus on crack detection using shear wave ultrasonic methods, not just thickness monitoring.

Naphthenic acid corrosion severity is highly dependent on temperature (peaking between 450°F-750°F) and the sulfur content of the crude. Sulfur compounds can form protective iron sulfide scales that mitigate acid attack. A high TAN crude with low sulfur (‘sweet crude’) is much more corrosive than a high TAN crude with high sulfur. Velocity is a factor, but temperature and sulfur content are the primary governing parameters.

A white, water-soluble, neutral pH deposit in a light ends fractionator is characteristic of ammonium chloride (NH4Cl). It forms from reaction of ammonia (NH3) and hydrogen chloride (HCl). This salt is hygroscopic and can absorb water, leading to highly localized, severe under-deposit corrosion and pitting of carbon steel. Its presence mandates a thorough inspection for pitting underneath deposit areas.

The description of localized, smooth, wavy thinning on the bottom of the pipe is characteristic of erosion-corrosion. In lean amine service with trace acid gases, the combination of fluid flow (potentially with entrained droplets or turbulence) and the corrosive environment leads to accelerated metal loss in specific areas, distinct from uniform sulfidic or acidic corrosion.0