Research On Critical Pitting Temperature Of 316L Stainless Steel

Critical pitting temperature(CPT)is deemed as the parameter for screening pitting susceptibility of stainless steels and can be used for ranking pitting resistance.This thesis focused on the basic research on CPT of stainless steels from the aspects of environmental factors,pitting nucleation,corrosion control strategies and the correlation between laboratory level results with CPT results in practical engineering in order to provide necessary technical reference and theoretical direction.A model was established by response surface methodology to correlate the typical environmental factors such as NaCl concentration,pH and pressure in complex and coupled environment with CPT for the purpose of analysising the effects of environmental factors on CPT.The results showed that with the increase of NaCl concentration,CPT decreased.CPT was slightly higher in neutral environment than that in acid or alkaline condition.With the increase of pressure,CPT decreased slightly.Interaction between Na Cl concentration and pressure existed on CPT,but the interaction was small enough to be ignored.In summary,the main influencing factor for CPT was NaCl concentration,followed by pH and pressure.The developed model could be used for simplifying the corrosion research problems in complex coupled environment with multivariates.The mechanisum of pitting nucleation on CPT was studied systematically through nitric acid passivation treatment.The CPT of 316 L SS was improved from 12.5 ? to 32.5 ? significantly after nitric acid passivation treatment for 24 h.After nitric acid passivation treatment,some of the nonmetallic inclusions on the surface of 316 L SS dissolved and the content of Cr2O3 in passive film improved,both of which led to the restrain of pitting nucleation.The improvement of CPT was mainly attributed to the inhibition of pitting nucleation by passivation treatment,but had noting to do with pitting growth process.Former researches mainly focused on the effects of pitting growth progress on CPT.However,this thesis elucidated the mechanisum of pitting nucleation on CPT,which was a supplement to the existing theoretical model of CPT.The effects of several representative organic functional groups on CPT were researched.The results showed that the polar group of heterocyclic nitrogenous organic inhibitor imidazole had strong chemical affinity,which could reduce the dissolution rate of metals significantly and improve CPT effectively.However,the conformation of nonpolar group of alkynyl organic inhibitor was extremely complicated,which retarded the diffusion of ion remarkably.As a result,CPT decreased.The addition of heterocyclic nitrogenous organic inhibitor could be used as an effective corrosion control stategy for the improvement of pitting resistance of stainless steels.The effects of several organic functional groups on CPT were illuminated by this research.For the selection of organic inhibitors in practical engineering,it was a better choice to choose the strong adsorption of the polar group and weak block of the nonpolar group as the inhibitor for the improvement of CPT of stainless steels.The relationship between laboratory level results and CPT results in practical engineering was established using Gumbel function.By extrapolating to large specimens from the results obtained by laboratorial small specimens,the CPT could be predicted in practical engineering.It turned out that CPT was a function of the double logarithm of specimen area.As the increase of specimen area,CPT decreased dramatically and then approached to a limiting value,which could be used as a theoretical ly predicted value for the large specimen in practical engineering.That the corrosion problems in practical engineering were severer than laboratory level was not only due to the variability in practical engineering,but also could be attributed to the specimen area.