Corrosion Behavior of LaFe10.85Co0.65Si1.5C0.2 Alloys in Neutral and Acidic Solutions
NaZn13-type(1∶13)La(Fe,Si)13 compounds are considered to be a promising candidate of magnetic refrigerant because of the large magnetic entropy change,adjustable Curie temperature(T),low raw material price and non-toxic raw materials.However,instability performance arising from the poor corrosion is still a main challenge for practical application.Nowadays,a relatively small number of studies concern the study of corrosion behavior in different solutions.The corrosion behavior of annealed LaFe10.85Co0.65Si1.5C0.2 magnetic refrigeration materials in deionized water,NaCl,HNO3 and H2SO4 solutions was studied by potentiody-namic polarization technique and electrochemical impedance spesctroscopy in this study.Scanning electron microscopy(SEM)and en-ergy dispersive spectrometer(EDS)were used to examine the surface morphology and composition of LaFe10.85Co0.65Si1.5C0.2 alloys before and after corrosion.The results showed that pitting corrosion occurred on the surface of LaFe10.85Co0.65Si1.5C0.2 alloys after soaking in de-ionized water and 0.01 mol·L1 NaCl solution for 30 min.The corrosion spots on the surface of LaFe10.85Co0.65Si1.5C0.2 alloys were distribut-ed randomly.The main phase and La-rich phase were corroded,and part of La-rich phase was exfoliated within the corroded area.Dur-ing the same corrosion time,the number of corrosion spots in NaCl solution was significantly more than that in deionized water.The po-larization curves exhibited anodic dissolution of LaFe10.85Co0.65Si1.5C0.2 alloys in deionized water.However,passivation zone appeared on the anode branch of the polarization curves in NaCl solution.The width of passivation region for the compounds in 0.0025,0.01 and 0.025 mol·L-1 NaCl solutions were 0.17,0.14 and 0.08 V,respectively.The passication region became narrow with the increase of Na-Cl concentration.Then the corrosion current density increased rapidly with the increasing electrode potential after electrode potential exceeded the critical value of passivation zone.The passivation film could not protect effectively.It could be seen from the fitting re-sults that the corrosion potential shifted negatively,the compounds surface state became more active,and the corrosion tendency be-came larger with the increase of NaCl concentration.In addition,the self-corrosion current density increased with the increase of NaCl concentration.Self-corrosion current density of compounds in NaCl was two orders of magnitude larger than that in deionized water.Combined with electrochemical impedance graph and component parameters,it could be seen that the impedance of passivated film(Rtl)decreased and the damage of Cl-increased with the increase of Cl-concentration.Furthermore,with the increase of Cl-concentra-tion,the resistance to charge transfer(Rt2)decreased,the pitting resistance decreased,and the corrosion rate increased.LaFe10.85Co0.65Si1.5C0.2 alloys were immersed in 0.01 mol·L-1 HNO3 solution and 0.01 mol·L-1 H2SO4 solution for 30 min,respectively.The re-sults showed that α-Fe phase was retained as the cathode after immersed in HNO3 solution for 30 min,while 1∶13 phase and La-rich phase were uniformly corroded.Comparative analysis shows that 1∶13 phase and La-rich phase were also corroded after soaking in H2SO4 solution.In addition,most of La-rich phase fell off to from holes or pits,which also suggested more serious corrosion.According to the calculation of polarization curve fitting parameters,the corrosion current density increased from 4.68×10-4to 2.14×10-3 A·cm-2 for the concentration of HNO3 solution increased from 0.01 to 0.1 mol·L-1,respectively.Compared with the same concentration of HNO3 solution and H2SO4 solution,the corrosion current density of the compounds in H2SO4 solution was one order of magnitude higher than that in HNO3 solution,and the linear polarization resistance was much lower than that in HNO3 solution.Corrosion rate of com-pounds in H2SO4 solution was faster than that in HNO3 solution.The electrical impedance diagram of LaFe10.85Co0.65Si1.5C0.2 alloys in acid-ic solution showed high frequency capacity reactance arc,medium frequency capacity reactance arc and low frequency inductive arc.In the high frequency region,the higher the concentration of HNO3 solution,the faster dissolution rate of oxide film on LaFe10.85Co0.65Si1.5C0.2 alloys surface,and the smaller film impedance(Rt1).In the intermediate frequency region,the charge transfer re-sistance of LaFe10.85Co0.65Si1.5C0.2 alloys in 0.01 mol·L-1 HNO3,0.025 mol·L-1 HNO3 and 0.1 mol·L-1 HNO3 solutions were 55,26 and 6.5 Ω·cm2,respectively.The charge transfer resistance decreased with the increase of H+concentration.Compared with 0.025 mol·L-1 HNO3 solution and 0.025 mol·L-1 H2SO4 solution,the charge transfer resistance of the LaFe10.85Co0.65Si1.5C0.2 alloys in H2SO4 solution was lower and the corrosion rate was faster than that in HNO3 solution.By comparing the corrosion behavior of LaFe10.85Co0.65Si1.5C0.2 alloys in neutral and acidic solutions,it could be seen that LaFe10.85Co0.65Si1.5C0.2 alloys was passivated in NaCl solution,while it was active dis-solved in deionized water,HNO3 and H2SO4 solutions.The corrosion current densities of LaFe10.85Co0.65Si1.5C0.2 alloys in deionized water,0.025 mol·L-1 NaCl,0.025 mol·L-1 HNO3 and 0.025 mol·L-1 H2SO4 were 6.24×10-7,3.555×10-5,4.92×10-4 and 1.33×10-3 A·cm-2,re-spectively.Compared with neutral solution,the corrosion rate of LaFe10.85Co0.65Si1.5C0.2 alloys in strong acidic solution was faster,the corro-sion degree was deeper,and the corrosion was more serious.
LaFe10.85Co0.65Si1.5C0.2 magnetic refrigeration materialsneutral solutionacid solutioncorrosion behavior
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