Research On The Corrosion Behavior Of Corrosion Resistant Steel Q315NS Arc Welded Joint

As the main corrosion form of coal-or heavy oil-fired boilers,dew point corrosion occurs when corrosive gases are co oled and converted to condensed acids.The condensed acids are much corrosive to steel,causing corrosion damage to plant materials.The service temperature is designed lower and lower to improve energy efficiency recently,which makes dew point corrosion more and more serious.In recent years,the government begin to implement more stringent emission standards for air pollutant due to the gradual strengthening of domestic environmental awareness.Though air pollution reduced through the process of desulfurization and denitrification,it also reduces the emission temperature of the flue gas and makes the dew point corrosion more serious,which brings great pressure on service life and security to the related equipment in the power and metallurgical industrie s.Q315 NS corrosion-resistant steel,which is developed for dew point corrosion,has excellent properties such as low cost,good mechanical properties and good dew point corrosion resistance.It has broad application prospects for the production of flue ga s system components in power and metallurgical industries.As an efficient and low-cost process,welding is an essential process in the utilization of Q315 NS.The corrosion mechanism of the heat affected zone is much complex due to the presence of microstructure gradients,which is largely determined by the welding thermal cycle.The microstructure and performance of Q315 NS corrosion-resistant steel will change significantly after welding process.However,the mechanism of passivation process,active dissolution and pitting corrosion tendency of Q315 NS welded joint are still unclear.Therefore,it is necessary to carry out the research on corrosion behavior of Q315 NS welded joint in different service conditions.In this thesis,the microstructure of Q315 NS corrosion resistant steel and its welded joint were studied firstly.The process of passivation,the activity dissolution process and the pitting of welded joint of the Q315 NS corrosion resistant steel were systematically studied.The corrosion behavior differences of Q315 NS corrosionresistant steel welded joints in different regions were analyzed and discussed in detail.The microstructure and propertie of Q315 NS welded joint was analyzed.The microstructure of the welded metal of Q315 NS corrosion resistant steel welded joint was mainly composed of a small number of pre eutectoid ferrite,a large number of acicular ferrite and granular bainite.The microstructure of the coarse grain HAZ（CGHAZ）was mainly composed of large bainite phase.The microstructure of the fine grain HAZ（FGHAZ）and incomplete recrystallization HAZ（ICHAZ）were mainly composed of ferrite and a small amount of pearlite.All of them appear as equiaxial crystal.The post weld heat treatment process the microstructure of the weld metal and CGHAZ were translated into the irregular massive ferrite with a small amount of pearlite and bainite.Besides the post weld heat treatment process effectively reduced the residual stress of the welded joint.The difference on corrosion product morphology between Q315 NS corrosion resistant steel and CGHAZ in strong oxidizing acid solution（50% H 2SO4 solution and H₂SO₄/H₃PO₄ solution in 70 ℃）was analyzed,the corresponding physical model was established and the corresponding corrosion kinetics analysis was carried out.A porous-structured corrosion product was formed on base metal（BM）of Q315 NS while a rod-shaped corrosion product was formed on the surface of CGHAZ in 50 wt.% H₂SO₄ solution due to the difference on microstructure between BM and CGHAZ.There was no difference on components between CGHAZ and BM,and the main components were mainly iron sulfate,phosphate and with a small amount of Cu O and Sb₂O₃.The rod-shaped corrosion product caused a smaller corrosion film thickness and larger diffusion coefficient,which reduced the inhibition effect of corrosion product film on corrosion.In order to study the corrosion behavior of Q315 NS welded joints in nonoxidizing acid solution,HCl solution with mass fraction of 3.65% and H₃PO₄ solution with mass fraction of 3.27% were used as corrosion experimental solution at 60 ℃ respectively.In the active dissolution process,the microstructure of coarse grain HAZ had the smallest corrosion current density because of the maximum charge transfer resistance,while the corrosion current density in fine grain HAZ and incomplete recrystallization HAZ were roughly equivalent to that of the BM.The inhomogeneity between the grains caused the microcouple effect and increased the active dissolution rate.The current density of the microcouple effect was proportional to the reciprocal of the cubic of grain diameter.The mechanism of Cl^-on pitting of Q315 NS welded joint in strong oxidizing acid solution containing Cl^-was studied by semiconductor electrochemical method.The effect of Cl^-doping in corrosion products on pitting inoculation rate was analyzed based on shot noise model.The Cl^-doping in the corrosion products of Q315 NS welded joint brought about the higher donor carrier concentration,which mean the higher pitting strength.The diffusion rate of cations at the interface of metal/passivation film was mainly related to the thickness of corrosion product film and the concentration of cation vacancy in the corrosion product film.That is,the higher the concentration of the donor carrier in the corrosion product film and the smaller the thickness of the space layer caused the greater the probability of pitting.Besides,the higher the surface activity was,the higher the pitting strength was.The post-weld heat treatment of Q315 NS corrosion-resistant steel at 800 ℃ for 1 h did not affect the type of corrosion products semiconductor and majority carrier densitie formed in the H₂SO₄/HCl mixed corrosion solution at 70 ℃.But the post-weld heat treatment made the thickness of space charge layer and corrosion products film increased,which had a positive effect on the improvement of corrosion resistance.