EXPERIMENTAL APPROACH TO VALIDATE A REAL-TIME CORROSION MONITORING SYSTEM

Abstract

Monitoring corrosion is a vital step to prevent catastrophic structural failure. With accurate corrosion rate data, one can mitigate the risk of failure by performing routine maintenance on components that have become compromised. This monitoring process becomes more complicated when the corroding side of the material is not accessible; such as the inside of pipes or the backside of in-ground structures. Non-destructive evaluation (NDE) techniques must be used to determine the condition of the structure to maintain its integrity. We investigated the use of Ultrasonic Thickness Sensors (UTS) for corrosion monitoring. A system was designed to utilize Electrochemical Machining (ECM) to simulate a material corroding at various rates in a localized region (simulating a pit of 1 cm in diameter) while its thickness was monitored by the UTS. Finite Element Analysis (FEA) was used to better understand the current distribution on a steel substrate induced by a planar circular counter electrode for various electrolyte conductivities and current densities. The FEA analyses revealed that the current distribution is more highly localized near the counter electrode for low electrolyte conductivity, and hence, ECM would be a viable technique. Multiple ECM experiments were conducted in electrolytes consisting of 1, 10, and 100 g/L of NaCl. Occasionally, variations in pit formation were observed and likely caused by the orientation of the counter electrode to the steel surface. The smallest and most consistent surface roughness was obtained using the 10 g/L concentration. Larger variations in surface roughness were observed at 1 and 100 g/L NaCl concentrations. Hence, the 10 g/L NaCl electrolyte was selected for the experiments with lower current densities. The ultrasonic sensors have a resolution of a limit of 1 mil (25.4μm), and the drift was measured to be within this limit over 7 days in a temperature-controlled environment. The ECM system allowed for reproducible machining rates that produced results that showed the UTS were capable of monitoring even the slowest corrosion rates at 1.3 mils (33.02 μm) per day. Discrepancies of a few mils were measured between the UTS and 3D profilometer values and UTS and depth-gage values. This is not unexpected as the UTS gives a single depth value over its 1.59 mm (1/16”) diameter active area; whereas, the 3D profilometer reports the average and maximum values over any selected area, and the values of the depth-gauge report using a 1.52 mm (0.06”) radius carbide-tipped ball point. This is a reminder that the ultrasonic sensor provides a single depth over a very small active area even though there may be gradients and variations, which should be taken into consideration when using and interpreting UTS data for field use. From the data gathered the UTS are capable of monitoring a fixed corrosion rate that was set by the ECM system that produced a controlled corrosion rate. Further testing is needed to validate the sensor at the slowest machining rate of 0.13 mils (3.302 μm) per day.