Flow accelerated corrosion (FAC), also termed flow assisted corrosion, is a phenomenon wherein carbon steel components with a protective film oxide degrade over time. FAC occurs only when the fluid is moving (not static), when it contains water, and is unsaturated in iron.
It is imperative to understand FAC since it is different from simple erosion — the mechanisms of these two processes are different. In short, FAC does not require particle impingement, cavitation or bubbling, which produce crater-like wear. The dissolution of the protective oxide layer that is typically poorly soluble is achieved through various means like the combination of mass transfer, water chemistry and electrochemical actions.
Flow accelerated corrosion (FAC) is mostly observed in steam lines in power plants, components made up of carbon steel, tubing and vessels that are exposed to flowing water (single-phase) or wet steam (two-phase). The most vulnerable areas of FAC include bends/elbows, tees, orifices, and any location where turbulent flow exists. In fact, FAC can be modeled as a turbulent mass transfer process .
In carbon steels, FAC occurs under well-known and specific parameters. The primary parameters include pH value, oxygen content, temperature and water velocity. Two major methods for avoiding or at least significantly mitigating this issue in operating plants are available. Often, the easiest method is to change the water chemistry by raising its pH and creating an oxidizing environment. If this is not sufficient, or if other factors e.g., presence of copper-alloy components prevent this, it may be inevitable to replace carbon steel tube sections facing wear with a more resistant material, such as low-alloy steel containing some chrome .
The major non-destructive testing challenges in inspecting for flow accelerated corrosion are:
· Steam-lines operate at elevated temperatures like 450 to 500, making in-service nondestructive testing (NDT) difficult
· Considerable downtime due to removal of insulation and re-insulation post-inspection required for nondestructive testing (NDT)
· FAC is an internal wall loss phenomenon that requires ultrasonic thickness testing (UTT), pulsed eddy current testing (PECT), and/or remote visual inspection (RVI) for detection
· Shutdown is mandatory for nondestructive evaluation (NDE)
FAC inspection using PECT and UTT
The Pulsed Eddy Current Testing (PECT) technology was originally developed and patented by Shell Oil & Gas in the 1990s. The PECT method measures the differences between the conductivity and permeability of different metals, and the quantity of those metals in comparative readings. Typically, a test involves inspecting the insulated component, identifying a consistent area of thicker metal, and place the reference point (RP) in the middle of that area. The client has to make this area accessible for inspection by removing the insulation so that an Ultrasonic Thickness Testing (UTT) can be performed and used as a RP for normalization. As a result, some limited insulation removal is often required to quantify metal loss with PECT. Subsequently, the software normalizes the dataset, and compares all data to the RP thickness, converting them all from a percentage to an average wall thickness. Figure 1 shows an example PECT grid marked out on a steam line elbow. Figure 2 shows and example UTT grid marked out on a steam line elbow.
Figure 1: Example pulsed eddy-current grid marked up on a 10” steam line.