Corrosion Behavior Of 316L Stainless Steel Under Stress And Corrosive Media Synergy

In recent years, 316 L stainless steel with its excellent mechanical properties and corrosion resistance are more widely used in thermonuclear industry, petrochemical industry and other fields. Particularly in the oil industry, 316 L stainless steel have a relatively high corrosion resistance in carbon dioxide and other corrosive media, commonly used oilfield pipes and tanks. But, 316 L stainless steel will be vulnerable to pit in the high Cl- concentration of the environment so that these devices exist the potential risks. Currently, there are not enough data to show that 316 L stainless steel pitting and stress corrosion cracking if will be promoted under the synergistic effect of higher force and High Cl- concentration, which brings a high risk to the application and promotion of the material.In response to this phenomenon, In this paper, we mainly study 316 L stainless steel corrosion behavior and mechanical behavior in different pH and Clconcentration of the solution corrosion, adopting to build constant load and constant deformation conditions in the corrosive medium. Analyzing 316 L stainless steel corrosion behavior, stress corrosion cracking and corrosion creep problems under conditions of constant stress by scanning electron microscopy(SEM), electrochemical analysis systems and other equipment. The main conclusions are as follows:(1) 316 L stainless steel in synergy stress and corrosive media, its corrosion potential to move forward and the corrosion current density to reduce gradually with the increase of the solution pH value and lowering the concentration of Cl-, The fracture time of the sample increased from 2.8 days to 9.5 days, and the plastic loss decreased from 86% to 68%, indicating 316 L stainless steel stress corrosion sensitivity decreased; With the increase concentration of Cl- in solution, the fracture time of the sample decreased from 37 days to 12 days, and the plastic loss decreased from 40% to 4%, indicating the stress corrosion sensitivity of 316 L stainless steel was enhanced. Through the fracture morphology observed: with the decrease of the etching solution pH value, microscopic fracture morphology dimple becomes large, the number of the tough nest is reduced, and the fracture mode from ductile fracture into brittle fracture. With corrosion solution of Cl- concentration increased, microscopic fracture morphology dimple becomes large and the number of the tough nest is reduced, appearing to the shape of the river pattern. With corrosion solution of Cl- concentration increased or pH decreased, the fracture mode of 316 L stainless steel ductile fracture to brittle fracture transition. In addition, 316 L stainless steel corrosion creep phenomenon will be occured in synergies stress and corrosive media. corrosion creep rate are experiencing decelerating creep stage, stage of steady creep and accelerated creep stage. With increase of the pH value of the solution, the steady creep rate from 1.2×10-6s-1 reduced to 5×10-7s-1, steady-state creep stage increased. With the decrease of the Cl- concentration, the steady creep rate from 4×10-7s-1 reduced to 1.8×10-7s-1, and steady-state creep stage also increases.(2) The microstructure of 316 L stainless steel weld center is ferrite austenite, and ferrite is distributed in the austenitic matrix. The microstructure of the weld near the fusion line is divided into two parts: A two-phase microstructure consisting of austenite and ferrite near the side of the weld is formed by a single austenite near the side of the heat affected zone. The microstructure of the heat affected zone is similar to that of the base metal, but the grain size of the substrate is obviously bigger than that of the substrate. The microstructure of the parent material is mainly single solid solution austenite, and the grain size distribution is relatively uniform. The samples were soaked in NaCl solution with different pH and Cl- concentration, After a period of time, the samples have undergone serious corrosion, With corrosion solution of Clconcentration increased or pH decreased, The corrosion degree of the sample is increased, and the corrosion of the weld is more serious than that of the matrix. observing the microstructure of the samples, 316 L weld fusion line occurs stress corrosion cracking under acidic condition and the characteristic of stress corrosion cracking was not presented under the condition of acid condition.