Effects Of Sands Contaminated By NaCl Or KCl On Electrochemical Corrosion Behaviors Of X70 Steel

In this thesis,the electrochemical corrosion behaviors and corrosion mechanism of X70 steel in a sand contaminated by Na Cl or KCl are studied by use of electrochemical impedance spectroscopy(EIS),polarization curves(PC)and scanning electronic microscopy(SEM)in door tests.Based on the macro and micro pictures of surfaces of X70 steel,it can be seen that the corrosion type of X70 steel is a localized corrosion and the corrosion products of X70 steel are mainly Fe2O3 and Fe3O4 in the end of corrosive process for both NaCl contaminated sand and KCl contaminated sand.When the concentration of contaminant is smaller(3% and 9%),the surface corrosion of X70 steel in NaCl contaminated sand system is more serious than that in KCl contaminated sand system.However,when the concentration of contaminants is greater than 9%,the surface corrosion of X70 steel in KCl contaminated sand system is more serious than that in NaCl contaminated sand system.The analysis of EIS results for X70 steel shows that the patterns of graph ofX70 steel bured in contaminated sand either NaCl or KCl vary with buried time and contaminate concentration.When buried time is less or equal to 14 d,the Nyquist graph of X70 steel mainly consists of two parts,i.e.,a small segment of incomplete capacitive arcs in high frequency section and a segment of capacitive arcs in middle and low frequency sections,the pattern of Nyquist graph is not related with the type and concentration of contaminants.But,when buried time is longer than 14 d,the pattern of Nyquist graph is not only related with the type and concentration of contaminant,but also related with the buried time.The charge transfer resistance(Rct)is an important index to represent effect of system on corrosion of X70 steel.It is shown that Rct increases first and decreases later with the increment of contaminate concentration for both systems in this study.From the results of polarzation curve(PC)of X70 steel,it is shown that the passivation does not appear either in NaCl contaminated sand or in KCl contaminated sand.Ingeneral,for NaCl contaminated sand system,the corrosive speed of X70 steel increases first,then decreases with the concentration of NaCl increases,the maxium speed is reached at the NaCl concentration of 9%;for KCl contaminated sand system,the samilar tendency is seen too,but the maxium speed is reached at the KCl concentration of 15%.The analytical results of EIS and PC state that when buried time for X70 steel is less or equal to 28 d,the corrosivity of NaCl contaminated sand system to X70 steel is more serious than that of KCl contaminated sand system at concentration of contaminant ?9%;on the contrary,at concentration ofcontaminant greater than 9%,the corrosivity of KCl system to X70 steel is more serious than that of NaCl system.However,when the buried time is 60 d,the corrosivity of KCl system to X70 steel is more serious than that of NaCl system at any concentration of contaminant.