The Cathodic Protection Of316l Stainless Steel In Simulated Oilfield Produced Water

In oil industry, most of the oilfields entered the stage with high water content.Oilfield produced water become the key research object of the oilfield anticorrosionwork. This environment had strong and complex corrosive to the equipments andpipings due to its characters: complex composition, high salinity, high temperatureand usually low pH. Therefore,316L stainless steel with excellent corrosionresistance was chosen for this kind of environment. While, a large amount of Cl-inthe oilfield water would destroy the passive film, lead to pitting corrosion orperforation. So, protections were needed. Cathodic protection had been widely used insea water as a typical anticorrosion measure. But the research of cathodic protectionin oilfield produced water is less, and the research with316L stainless steel as theobject in this environment is little.In this paper, cyclic voltammetry curves were used to choose different cathodicpotentials to study in60℃simulated oilfield produced water with saturated CO2.Protection current density detections, weight loss testings, morphological observationswere used to study the protective effects of the different potentials for316L stainlesssteel. And then, methods such as electrochemical impedance spectroscopies,microcosmic appearance observations, composition analysis were used to study thereduction of passive film and the formation of deposits with different cathodicpotentials. At last, the influences of different potentials for316L stainless steelhydrogen embrittlement were studied, using slow strain rate tensile experiments, thefracture morphology observations and micro-hardness testings. Combining all theexperimental results, suitable cathodic protection potential range was chosen for theprotection of316L stainless steel in simulated oilfield produced water.The results indicated: In60℃simulated oilfield produced water with saturatedCO2,316L stainless steel showed a good resistance to general corrosion, but appeared pitting cession after30days immersion.-300mV showed no protective effect to316Lstainless steel,-400~-700mV showed significantly inhibit to general corrosion.-400mV did not inhibit pitting corrosion completely, while-600~-700mV did; In therange of-400mV~-700mV, the reduction rates of passive films increased with thedecreased potentials. With polarizations of7days, the chemical compositions on thesurfaces of316L stainless steel were very different at-600mV and-700mV fromother potentials, because of the high reduction rates of passive films, and there wereenrichments of Cr and Mo on the surfaces. As a result of acidic simulated oilfieldproduced water and low cathodic current densities, the formation rates of calcareousdeposits were slow. Just some small crystal nucleuses were observed on the surfaceswith polarizations of7days. When the polarization time increased to30days, a littleof CaCO3crystals were observed at-500mV, and a large amount of crystals wereobserved at-700mV.316L stainless steel itself had strong plasticity, but as thecathodic polarization potential become negative, the plasticity was reduced. The316Lstainless steel fractures had obvious characteristics of ductile fractures at the corrosionpotential. With polarizations lower than-600mV, the tendencies of hydrogenembrittlements and brittle fractures of316L stainless steel increased. Withpolarizations of-700mV the tendencies increased. With-800mV the brittle fractureswere evident; Suitable cathodic protection potential scope of316L stainless steel isonly in-500~-700mV in simulated oilfield produced water.