| The underground metallic facilities have long been placed in corrosive soil environment,which not only suffer from the corroding of the soil medium,but also the stray current corrosion.In recent years,this type of corrosion damage becomes more and more serious due to the increase of voltage grade for transmission lines and numbers of electric railway.The corrosion mechanism has not yet been determined due to the difference of the voltage,current and frequency of stray current and the soil composition is complex.As a result,the lack of theoretical guidance for further corrosion protection of metals is occur.In this work,Q235 steel and its weld were adopted as research object,and the simulated acidic soil solution as corrosion medium.The corrosion behavior and electrochemical process with DC and.AC stray current were studied,respectively.The polarization curve and AC impedance,etc.were carried out and analyzed based on electrochemical principle.The corrosion morphology and corrosion products were analyzed.Depending on these,the corrosion initiation and development of the metals with stray current were compared,and the corresponding corrosion mechanism was discussed.Moreover,a type of magnesium sacrificial anode with gadolinium was optimized to study the corrosion behavior and protection effect with interference of stray current.The research results shows:With the influence of stray current,the anode surface evolves from activation area to passivation area,and then to secondary activation area and final to hydrogen evolution process over one period,dominating by electrochemical process;the corrosion of metal was accelerated by the stray current in positive half cycle.Hydrogen corrosion occurred in both the base metal and the weld in negative half cycle,and the hydrogen evolution on the base metal is earlier than the weld;the corrosion rate of Q235 steel with alternating current density confirms to exponential motion.The corrosion rate with variation of AC stray current frequency matches negative exponent relation.The higher the frequency,the slower the corrosion rate.Potential fluctuations of Q235 steel and its weld under low frequency AC stray current are similar to the impressed alternating current wave,which confirms to the formula of E(t)=(ECP+?E)+A sin(2?·f·t+?)excepting a discrepancy of phase angle ??,The ? of base metal is larger than that of the weld.When the frequency is larger than 50 Hz,the potential-time curve of Q235 steel obtained through conventional electrochemical method displays a nearly straight line due to the existence of inductance.This is able to be calculated by applied DC stray current approximately;the corrosion potential decreases with the increase of frequency.Current oscillation on the electrode surface is observed.There is the presence of three electrical parameters including inductance,capacitance and resistance.The larger the stray current,the higher the capacitance value,and the maximum value of interface resistance appears.With the interference of DC voltage,the corrosion rate of Q235 steel and its weld with DC voltage satisfy the index laws:the base metal confirms to Vcor=-215.39eUDC/-16 93+216.45 and the weld confirms to Vcorr=-132.3793eUDC/-7 0452303+133.95 5 79,respectively;The corrosion rates of the both materials firstly keep in slow growth with the increase of voltage and then rise sharply.The polarization rate of the base metal is slower than the weld.It means that the base metal is located in weak polarization region even in the linear polarization region,while the weld reaches the strong polarization region.Low voltage interference can destroy the local passivation membrane on the Q235 steel surface.Local corrosion is observed although low DC voltage.Then the corrosion pits are expanded and spread continuously under high DC voltage,and finally connected to form homogeneous corrosion.Besides increasing reaction speed of the metal interface,DC stray current also affects the change of double electric layer on the metal surface.Under the interference of external stray current,the surface impedance decreases initially,and then increases and finally decreases with the increase of external voltage.The total galvanic corrosion resistance namely the sum of solution resistance and double electric layer surface resistance drops depending on index law with the increase of external voltage.Although the impedance of the weld varies disorderly,mainly attributing to coarse structure and sever pitting corrosion,the general rule of the overall surface impedance obeys the same law as the base metal,and the total resistance of double electric layer decreases faster than base metal.The phase composition of the corrosion product of base material and weld metal is consistent under the influence of AC/DC stray current,and does not change with the type of stray current.The mainly corrosion products are y-FeOOH.With lwt.%addition of Gd into AZ63 magnesium alloy,the current efficiency of sacrificial anode can be increased by about 10%.With the addition of the rare earth element Gd,the annular? phase of magnesium alloy are damaged and the grain is more refined.As a result,the current efficiency of sacrificial anode is improved effectively.While the as-prepared AZ63-Gd1.0 sacrificial anode is used in DC stray current protection,the voltage of stray current between 0?2 V,the protection effect of alloy for the underground facilities decreases with the increase of impressed voltage,when the impressed induction electric field voltage of stray current is higher than 2 V or voltage of leak stray current is greater than 4 V,the sacrificial anode output current is smaller and can't meet the requirement of cathode protection and merely performs as a good leakage device for stray current.When the AC stray current is less than 20 A·m-2,the magnesium alloy sacrificial anode has a good protective effect.When the AC stray current is higher than 75 A·m-2,the sacrificial anode potential increases beyond the self-corrosion potential of Q235 steel. |
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