| Dew point corrosion originates from the top of the primary distillation tower/flash tower of a coal-fired power plant and a petroleum processing plant,the top of the atmospheric tower at the time of cold reflux at the top of the tower,and the oil and gas condensing cooler of the tower top temperature across the initial condensation zone and related pipelines Among them,the atmospheric and vacuum overhead cooling system is the most severely corroded,and the condensing and cooling system at the top of the atmospheric and vacuum distillation tower of the refinery often fails due to dew point corrosion of hydrochloric acid.Therefore,the study of dew point corrosion can better reduce and avoid the degree and occurrence of dew point corrosion,and provide a reliable basis for the selection of materials with severe dew point corrosion in the actual production process.Based on the self-designed Dew point corrosion(DPC)simulation device for strong acid environment dew point corrosion,the corrosion loss method,Tafel polarization method and electrochemical impedance method were used to study the HCl environment of 316L and HR-2 stainless steel at different concentrations and temperatures.Corrosion characteristics,focusing on passivation and electrochemical behavior of pitting of two materials,using metallographic microscopy,scanning electron microscopy(SEM),X-ray diffraction(XRD)and resistive probe combination to corrode samples or corrosion products for analysis.Through research and analysis,the following conclusions can be drawn:Through research and analysis,it shows that:(1)The corrosion rate of the two materials approaches a similar change in the corrosion rate of the two concentrations.Corrosion is accelerated for 24-48h,the maximum rate is reached at 48h;corrosion is gradually slowed down for 48-96h;since the corrosion product is less than 48h before corrosion,the corrosive medium can fully contact with the steel body,and the corrosion rate is accelerated;corrosion In the later period,the metal surface forms a stable and dense passivation film from 48h to 96h,which strengthens the protection of the steel body and hinders the further penetration of the corrosive medium,and the corrosion rate is slowed down.After 96h,the corrosion rate increases,and the passivation film formed by the surface steel body is intruded by a large amount of chloride ions in the liquid film,which destroys the passivation film and accelerates the corrosion rate.It can be seen from the overall corrosion situation that the overall corrosion loss of HR-2 is small and the corrosion rate is slow;and.the corrosion rate of the two steels in each stage of corrosion in 1mol/L hydrochloric acid solution is higher than 0.5mol/L hydrochloric acid solution;HR-2 steel has a high Cr content,Cr has anti-oxidation performance and can promote the formation of stable phase ?-FeOOH in the inner rust layer,so that the corrosion rust layer is more tightly bonded to the metal,and the steel body is more protective;therefore HR-2 has better corrosion resistance.(2)The two stainless steels can form a stable passivation zone in three different concentrations of hydrochloric acid,and the density of the blunt current Em is not much different.As the concentration of hydrochloric acid increases,the passivation interval generally increases,and when the polarization potential is higher than the overpotential,the corrosion current increases significantly,indicating that the passivation film is rapidly dissolved and the passivation film is unstable.Moreover,the pitting potential Eb of the electrode is continuously negatively shifted,indicating that the metal passivation film is more likely to rupture with the increase of the Cl-concentration,the sensitivity of the material pitting is enhanced,and the corrosion rate is accelerated.The overall HR-2 pitting potential is higher than 316L,and the passivation interval of HR-2 steel is also higher than 316L,indicating that HR-2 is more resistant to corrosion.(3)For the same stainless steel,Rp and Rs decrease with increasing temperature at the same concentration.316L stainless steel is smaller than Rp and Rs of HR-2 stainless steel,so HR-2 stainless steel is more resistant to corrosion than 316L stainless steel;at the same temperature and material,with the increase of hydrochloric acid concentration,Cl-concentration continues to increase,with the reaction Continuously,the corrosion product film on the metal surface is continuously thickened,hindering the mass transfer process of oxygen and Cl-,and then decelerating the corrosion process;in the later stage of corrosion,as the corrosion product increases,it is impossible to uniformly cover the metal surface,in the gap Cl-Enrichment occurs,which promotes the generation of pitting,which causes the corrosion rate to gradually increase.Therefore,the corrosion rate of the two stainless steels in the liquid membrane environment of hydrochloric acid is first decreased and then gradually increased.(4)By comparing the SEM images of the two materials for 24h and 144h,it can be seen that the corrosion of the 24h rust layer of the two materials is relatively flat,and the corrosion product of the surface 316L is more than that of HR-2;the 316L matrix can be clearly seen after 144h corrosion.There are a lot of pitting pits,there are obviously larger and deep pitting pits,and the surface of the steel body is seriously corroded,while the HR-2 and 316L are smaller than the corrosion pits.Compared with the same corrosive environment and time,HR-2 is more resistant to corrosion than 316L.EDS spectrum analysis shows that the Fe content of the corrosion products of the two materials is decreasing,and the corresponding O,C and Cl are on the rise.The change trend is consistent.At first,the matrix itself has no Cl-.After 144h,the Cl-content of 316L steel is obviously higher than that of HR-2 steel,which indicates that Cl-in the night film of steel body is involved in metal corrosion,and Cl'intrusion into 316L in corrosion system.More steel,pitting corrosion is more serious,that is,HR-2 steel is more resistant to corrosion than 316L steel.(5)Corrosion products of two kinds of stainless steel include Fe2O3,Fe3O4,FeOOH,Cr,and FeCl2·4H2O.For two kinds of stainless steels with different corrosion resistance,the corrosion product layer is basically the same,Cl-competitive adsorption causes localized oxygen deficiency on the stainless steel surface,hindering The oxidation of Cr on the surface of stainless steel hinders the formation of a new passivation film rich in Cr,resulting in dynamic equilibrium failure of the passivation film of stainless steel,and the thinning of the passivation film promotes the occurrence of pitting corrosion of stainless steel. |
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