The Passivity And Pitting Of Al-Zn-Mg Alloy Based On PDM

Aluminum alloys are widely used in aviation,ships,high-speed rail and many other fields due to their excellent properties?low density,corrosion resistance and good weldability?.As the speed and service life of marine ships and high-speed rails increase,the safety and reliability of aluminum alloy welded structures have put forward stricter requirements.High-speed rail will be affected by the corrosive environment during long-term operation,which increases the possibility of corrosion failure of aluminum alloy welded structures,especially in the coastal environment containing chlorine.In addition to high humidity,there is also a salt spray that causes a large amount of salt in the air?including NaCl,Na₂SO₄,MgCl₂?.The closer to the sea,the higher the salt content in the air.These salts are deliquescent and absorb moisture,and easily form electrolyte films,which accelerates the corrosion of aluminum alloys and other metals.In particular,sodium chloride is ionized to form free chloride in the liquid electrolyte membrane.Due to the dry-wet cycle,the surface activity of the chloride is very high,and the protective barrier layer on the aluminum alloy can be locally damaged,resulting in deep local corrosion,such as pitting,stress corrosion cracking and corrosion fatigue.Cracks often nucleate at"supercritical"pits.Therefore,studying the corrosion behavior of aluminum alloys in chloride environments has important practical engineering value.Research indicates:1.Dynamic potential polarization research shows that the passive state of Al-Zn-Mg alloy is significantly different under different pH conditions.From the perspective of passive current,the passivation of Al-Zn-Mg alloy under weak alkaline conditions the film has better protection.Under constant potential polarization conditions,the logarithmic form of the passive current of Al-Zn-Mg alloy in borate buffer solution and the voltage is linearly proportional to the voltage.According to PDM prediction,the passive film of Al-Zn-Mg alloy in borate buffer solution is a P-type semiconductor,which is consistent with the results of Mott-Schottky analysis.2.Al-Zn-Mg alloy will cause pitting in a solution containing chloride ions.An increase in chloride ion concentration or a decrease in the pH of the solution will reduce the pitting potential of the Al-Zn-Mg alloy,making pitting easier occur.When the pH of the solution is10±0.2,the chloride ion concentration is 0.01 mol/L,0.02 mol/L,0.05 mol/L and 1 mol/L.The corresponding average fracture potentials of the aluminum alloy are 91.86 mV,-645.8mV,respectively.-812.2 mV and-929.6 mV.When the chloride ion concentration in the solution is 0.05 mol/L,the average values of the critical pitting potentials of the aluminum alloy in the solutions of pH 9.18,10.19 and 11.50 are-847 mV,-812.2 mV and-781.7 mV,respectively.3.According to the relationship predicted by PDM,the critical metal cation vacancy concentration?area density?required to cause the Al-Zn-Mg alloy passive film can be estimated to be about 8.85x10¹⁴ No./cm².This result is related to aluminum and oxidation.The lattice area density of aluminum atoms in the aluminum crystal(1.220x10¹⁵ No./cm²,8.74x10¹⁴ No./cm²)agrees.The deposition of metal cation vacancies may occur on the aluminum substrate or the alumina passive film lattice.4.Under different chloride ion concentration and solution pH conditions,the cumulative probability distribution of the rupture potential of the aluminum alloy passive film obtained by the experiment is approximately a normal distribution curve.After optimizing the model parameters,the cumulative probability distribution of the rupture potential of the aluminum alloy passive film calculated by PDM is in good agreement with the experimental values.The feasibility of the pitting behavior of Al-Zn-Mg alloy calculated by using PDM in chlorine-containing environment was proved.According to the cumulative probability distribution curve,when the electrode potential decreases,the probability of the passive film rupture will be infinitely close to zero,but it will never be zero.This indicates that the passive film may crack even at a very low potential,which is due to the abnormally high cation vacancy diffusion coefficients that may occur in some locations,especially considering the actual Al-Zn-Mg alloy in coastal areas very long service time.