It is important to monitor and locate internal corrosion along natural gas pipelines to prevent methane leaks and catastrophic failures. Corrosion proxy materials enable optical fiber sensors to provide insight into corrosive environments where the pipeline materials tend to corrode. A distributed optical fiber corrosion sensor was demonstrated using a metallic film-coated, single-mode optical fiber interrogated with an optical backscatter reflectometer (OBR), based on strain changes during metallic film dissolution. Electroless plating leads to inherent internal stress in the metallic coating and therefore induces strains on optical fibers. As the metallic film gets dissolved or corroded, the internal stress will be released and cause opposite changes in strain, which can be used for corrosion monitoring. The microstrains induced solely by electroless plating or metal dissolution were measured in situ and in real time using the OBR, and interferences from temperature changes and water-induced swelling were compensated through comparison between the treated section (sensitized and activated) and an untreated control section. High-pH Ni plating had a faster deposition rate with branching on the film and induced negative microstrains, whereas low-pH Ni plating had a slower deposition rate with smooth coating and induced negligible (near-zero) strains. Eletroless Fe plating with high pH didn’t cause significant microstrains. When exposed to corrosive HCl solutions, dissolution of high-pH Ni plated films induced positive microstrains, opposite to the Ni plating. The OBR allows for distributed monitoring of strain changes due to Ni dissolution, demonstrating the capability of identifying corrosion locations along the optical fiber.
The presence of water can provide aqueous electrolytes for corrosion to occur inside the pipelines. The capability of monitoring water vapor condensation enables in-situ monitoring of internal corrosion in natural gas transmission pipelines. Previously, a fully distributed optical fiber sensor for water and humidity monitoring has been demonstrated, consisting of an unmodified off-the-shelf single mode (SM) optical fiber connected to an optical backscatter reflectometer. The intrinsic polymer jacket of the SM fiber is hygroscopic and can serve as a water sensing layer due to expansion/swelling from water absorption. In this work, strain changes were measured and calibrated in jacketed and unjacketed sections at different relative humidity levels (RH, 0% to 100%) and different temperatures (T, 21 to 50°C). In the jacketed section, the sensitivity to humidity decreased from 1.2 to 0.6 με/%RH and then diminished as T increased from 21 to 50 °C, which could be due to the intrinsic absorption property of polymer at higher T or the wet gas flow at room temperature being absorbed in the polymer jacket. The unjacketed section demonstrated a minimal sensitivity to humidity (<0.2 με/%RH) at 21-50 °C and a relatively consistent sensitivity to temperature.
Internal corrosion can occur when aqueous electrolytes are present inside natural gas transmission pipelines. Despite upstream gas dehydration treatments, liquid water can form through condensation of water vapor or may be introduced from plant upsets. With dissolved salts and acidic gases such as CO2 and H2S, aqueous electrolytes become very corrosive with increased conductivity and lower pH. Since water provides the electrolytes that initiate and sustain corrosion, detection of water can locate the spots for potential internal corrosion inside the pipelines. In this work, a simple optical fiber-based sensor for fully distributed water monitoring has been demonstrated and studied. The system consists of an unmodified off-the-shelf single mode (SM) optical fiber and an optical backscatter reflectometer (OBR) capable of measuring the spatial profile of strain changes along the fiber. The polymer jacket coating of the SM fiber is hydroscopic and serves as the water sensing layer due to expansion/swelling from water absorption. The swelling induced strain change is interrogated with the OBR to enable fully distributed water monitoring. This strain-based H2O sensor is sensitive to H2O molecules regardless of the phase (liquid or vapor) or the surrounding media. Strain changes were measured at different relative humidity levels from 0% to 100% to demonstrate reversibility and linear correlation between humidity and strain. This sensor has the advantages of fully distributed sensing, low cost, simple preparation, easy operation, and good sensitivity.
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