Research On The Quantitative Relationship Between Salt Spray Corrosion And Electrochemical Corrosion Of The Multicomponent Permeation Layer

In this paper,low temperature gas multicomponent permeation technique which was developed by Southwest Jiaotong University has been used in the heat treatment of Q235 steel.The microstructure of the permeation layer was observed by OM and SEM,and XRD was used to determine the composition of the modified layer.Furthermore,the corrosion resistance of the samples were evaluated by Neutral Salt Spray(NSS)test,linear polarization test and OCP test.On the basis of these experiments,the quantitative relationship between the electrochemical parameters and the time of the NSS test was established.The results showed that:The Q235 steel treated by multicomponent permeation acquired a modified layer which was mainly consist of Fe-N compound.Fe3N and Fe3O4 were detected in the modified layer of the sample without high-temperature gas multicomponent permeation pretreatment,while the sample with the pretreatment found Fe4N and Fe2O3 in addition,it cause the complex microstructure of the layer.The corrosion time of the sample without high-temperature gas multicomponent permeation pretreatment was 96h when the corrosion degree in the NSS test was 8,while the sample with the pretreatment can reach 624h corresponded to the same degree.The corrosion damage mechanism of the treated samples was pitting corrosion.,but the corrosion behavior of the samples treated with different processes was not the same.The pitting was first appeared in the surface of the sample without high-temperature gas multicomponent permeation pretreatment,then expanded towards the matrix.However,the pitting in the sample with high-temperature gas multicomponent permeation pretreatment was first found in the subsurface,then expanded towards the surface.Moreover,the electrochemical parameters of the treated samples were not as stable as the untreated sample in the corrosion process,they showed the periodic mutation performance.Due to the complex microstructure of the permeation layer,the parameters of the sample with high-temperature gas multicomponent permeation pretreatment had many times of periodic mutation,while the sample without the pretreatment only had one,and the mutation time was corresponding to the time when the corrosion degree of the NSS test was 8.The ideal polarization curves and measured polarization curves of the Q235 steel were detected,and the number of electron transfer in the anode reaction of the permeation layer was confirmed as 6 by experiment,which was based on the relationship between the ideal polarization curves and the measured polarization curves.Thus the anodic reaction equation of Fe3N was inferred as Fe3N→3Fe2+ +6e + N(g).The density of Fe3N is 7.173g/cm3,which was calculated according to its crystal structure.On the basis of these experiments,the calculation model of the NSS test time when the corrosion degree was 8,was established for the Q235 steel treated by multicomponent permeation without high-temperature gas multicomponent permeation pretreatment,and the calculation formula,was deduced as:(?)...