Hydrogen Flux Method For Online Corrosion Monitoring On Oil And Gas Pipelines

As a non-intrusive corrosion monitoring technology, electrochemical hydrogenflux method can measure the inner corrosion rate of pipes without perforation on thepipe wall. However, many current electrochemical hydrogen flux methods can onlyevaluate the concentrations of hydrogen atoms in metals other than the corrosion ratesof metals. Particularly, the correlation of the hydrogen permeation current densities (ihp)with the corrosion rates of metals is kept unclear up to now. Thanks to the detailstudies on the correlation between them, this thesis evaluated the feasibility of thecorrosion rate measurement via hydrogen flux monitoring, aiming at facilitating theelectrochemical hydrogen flux method to online non-intrusive corrosion monitoring ofoil pipelines.Via the experimental pipe simulating oil pipeline for acidic medium transportation,three main factors effecting on the correlations of the ihpwith the corrosion rate ofQ235A carbon steel were studied by potentiostatic anodic polarization and weight lossmethod. In addition, the PVA/CMC/NaOH gel electrolyte was synthesized and itsphysical properties such as proton conductivity and water loss rate were alsoinvestigated. The gel electrolyte was then applied for the preparation of a self-designedelectrochemical hydrogen sensor. The average hydrogen permeation current density(iAp) in the wall of an experimental pipe was measured by step potentiostaticpolarization via the hydrogen sensor. Meanwhile, the inner corrosion rate of theexperimental pipe was tested by weight loss and Ac impedance method in parallel, soas to prove the correlations between the iApand the corrosion rate of the pipe on thesame conditions. The results obtained are shown as following:(1) It was found that good linear dependence between the ihpand the corrosionrate of the Q235A carbon steel irrespective of the various acidity and temperature.When the concentrations of H2S vary from5to200mg/L, a quadratic correlation canbe found between them. However, no simple function between them in the aqueoussolution with different salinity.(2) Although the changes of the free water loss, conductivity and viscosity of PVA/CMC/NaOH gel electrolyte with temperatures and service time, they all still ingood condition. When the0.2mol/L NaOH electrolyte in the hydrogen flux sensor wasreplaced by the gel electrolyte, some key characteristics such as background currentflow, reproducibility and the proton diffusion rate were almost constant.(3) Good linear correlation between the iApand the corrosion rate of theexperimental pipe was found, which is very similar to the measurement results byDevanathan-Stachurski electrolytic cell. Thus, the electrochemical hydrogenpermeation method was proven to be suitable for non-intrusive online corrosionmonitoring on oil and gas pipelines, especially for the direct corrosion ratemeasurement.