Study On The Corrosion And Protection Of LA51 And LA141 Mg-Li Alloys

Mg-Li alloys are promising engineering materials in the transportation and electronic industries because of their high specific strength,excellent elastic modulus,good damping capacity,and acceptable mechanical properties.However,in practice,the applications of the Mg-Li alloys are limited mainly because of their poor corrosion resistance,which is attributed to the presence of Li,an element with a higher electrochemical and chemical activity than that of Mg.It accelerates the corrosion kinetics of Mg-Li alloys to a great extent.Therefore,it is very necessary to improve the corrosion resistance of Mg-Li alloys though effective measurements.Based on characterisation of the microstructure,electrochemical measurements and immersion tests,the corrosion and protection of Mg-Li alloys in NaCl solution was systematically investigated by four methods(i.e.different rolling reductions,corrosion inhibitors,alloying,and B4 C coating).The contents of this dissertation are listed as follows:The microstructure and corrosion resistance of two single phase Mg-Li alloys(i.e.LA51 and LA141)have been investigated.The results showed that LA51 cast alloy was composed of ?-Mg phase;LA141 cast alloy was composed of ?-Li phase and Mg LiAl2 particles.The microstructure of two cast alloys was remarkably refined after the extrusion,and the corrosion resistance of the two cast alloys was improved by the extrusion.Furthermore,the corrosion resistance of the two alloys was decreased with increasing the contents of Li.The effect of rolling-induced microstructural changes of LA51 alloy sheet on its corrosion behaviour in 3.5wt.% NaCl solution was systematically investigated.The results reveal that the corrosion performance of LA51 alloy sheet improved as a function of rolling reduction.The notable increase in corrosion resistance of the rolled LA51 alloy sheet was attributed to its characteristic texture,texture-intensity,and the formation of twins and protective oxide surface films.The inhibition effect of methionine and tryptophane on the corrosion of LA51 alloy sheets in 3.5 wt.% NaCI solution was investigated.The results showed that the both inhibitors had inhibition for LA51 alloy sheets;the inhibition efficiency increases firstly and then decreases with the increase of inhibitor concentration.However,tryptophane had better protection than methionine at the same concentration.The adsorption of methionine and tryptophane on the surface of LA51 alloy sheets in 3.5 wt.% NaCl solution obeyed the Langmuir adsorption and its inhibition efficiency decreases with the temperature increasing.The corrosion behaviour of LA141 alloy sheets containing 0.3 wt.% RE(La or Y)in 3.5 wt.% NaCl solution was studied.The results showed that the alloy sheets containing 0.3 wt.% RE had better corrosion resistance.And the LA141-0.3Y alloy sheet had best corrosion resistance.The main cause of better corrosion resistance was due to a lower amount of micro-galvanic corrosion because of a decreased volume fraction of the LiMg Al2 phase,and the formation of a new phase(Al2La or Al2Y),and a more-compact,more-protective corrosion product film.The corrosion behaviour of LA141 alloy sheets containing 0.3 wt.% Sr or 0.3 wt.% Y in 3.5 wt.% NaCl solution was studied.The results showed that the LA141-0.3Sr alloy sheet had worst corrosion resistance.And the LA141-0.3Sr-0.3Y alloy sheet had best corrosion resistance.The main cause of better corrosion resistance was due to a lower amount of micro-galvanic corrosion,and a more-protective corrosion product film.The inhibition effect of tryptophane on the corrosion of LA141 alloy sheets in 3.5 wt.% NaCI solution was investigated.The results showed that the tryptophane had inhibition for LA141 alloy sheets;the inhibition efficiency increases firstly and then decreases with the increase of tryptophane concentration.The adsorption of tryptophane on the surface of LA141 alloy sheets in 3.5 wt.% NaCl solution obeyed the Langmuir adsorption and its inhibition efficiency decreases with the temperature increasing.Moreover,the inhibition efficiency of tryptophane on LA141 alloy sheets had worse than that on LA51 alloy sheets.The corrosion behaviour of LA141 alloy sheets with and without B4 C coatings in 3.5 wt.% NaCl solution was studied.The results showed that EPD was successfully used to obtain B4 C coatings on LA141 alloy sheets.The corrosion resistance of the LA141 alloy sheets could be improved by the B4 C coatings.Furthermore,the corrosion resistance of LA141 alloy sheets was improved with increasing the deposition weight of B4 C coating.