Research On Corrosion Behavior Of Aluminum Alloy Materials In Typical Parts Of Motor Vehicle Body

As a means of transport,the high-speed EMU train covers different climatic zones in China,with complex running environment.In order to reduce the weight of the train,a large number of aluminum alloy materials are used in the body of high-speed EMU.Different corrosion phenomena were found at the end door,side door area and other parts of the train when the EMU is overhauled in the factory at the fourth and fifth stage.In order to ensure the safe operation of the body of EMU So It is necessary to test and analyze the corrosion behavior of aluminium alloy in automobile body and evaluate it in running environment.In this paper,the corrosion status of the high-speed train body was preliminarily understood through field corrosion investigation.The corrosion of the areas of end doors,side doors and boxes was serious,with aluminum alloys in these parts covered by white corrosion products and a large number of corrosion pits on it.The corrosion specimens of aluminum alloys in the bottom frame of installation box in“A”end of car and top frame of gangway door in“B”end of car were selected,followed by analyze of macro-morphology,micro-morphology and component tests and EDS,XRD and electrochemical tests.Experiment selected commonly used aluminium alloy materials in automobile body,A5083-O and A7N01S-T5,to carry out corrosion test,including salt spray test,damp heat test,corrosion test of 3.5%sodium chloride medium and disinfectant medium,galvanic corrosion test and electrochemical test.The macro/micro corrosion morphology and corrosion kinetics of the tested aluminium alloy were observed and tested to analyse the corrosion behavior rule of the aluminium alloy in different corrosion environments.The results are as follows:?1?According to the corrosion investigation,the environment of the running train carriage was complex,including humid atmospheric environment,sewage medium?tap water,disinfectant,cleaning fluid?environment,dry and humid alternating environment.Aluminum alloy of the car body is connected with stainless steel material at the bottom of the carriage by a couple,which may cause corrosion in the humid environment by forming a couple battery.?2?A lot of white corrosion products were found on the surface of aluminium alloys of bottom frame of installation box in“A”end of car and top frame of gangway door in“B”end of car.The microstructure of surface corrosion products was obviously stratified,with many cracks on it.A large number of corrosion pits were found on the surface of the alloy while the corrosion products were removed.The corrosion products were mainly consist of Al?OH?₃,Al₂O₃·3H₂O,AlCl₃·3H₂O,Mg?OH?₂ and Zn?OH?₂.The chloride formed by Al?OH?₃ and chloride ions was soluble which can hardly protect the matrix.The electrochemical results showed that the corrosion current density in the anodic polarization region was too high to protect the aluminum alloy from corrosion products.?3?The surface of A5083-O and A7N01S-T5 in salt-spray and hygrothermal environments discolored,lost metallic luster,and covered with thin,pitting corrosion dominated white corrosion products.The corrosion rate of the two kinds of aluminium alloys gradually decreased with the time,and tended to be flat at the later stage of corrosion,which indicated that the corrosion products and oxygen on the surface reduced the corrosion rate by blocking the direct contact between most of the corrosive media and alloy.The polarization curve of A5083-O and A7N01S-T5 in the anodic region of salt spray environment was controlled by activation.The corrosion current density increased rapidly under anodic polarization,along with pitting pits increased.The corrosion potential of the two kinds of aluminium alloys shifted negatively first and then positively,and the corrosion tendency decreased during the later corrosion.The results of EIS showed that the protective ability of the oxide films and corrosion products of the aluminium alloys to matrix trended to decrease first and then increase.In hygrothermal environments,the two kinds of aluminum alloy anode polarization curve area were controlled by activation,in addition to passivation in A7N01S-T5 at 1000h.The Tafel slope in the anode area increased in the middle and later stages of corrosion,the resistance of anodic reaction increased,and the corrosion rate slowed down.According to EIS fitting results,the protective ability of the oxide films and corrosion products of the two aluminium alloys to matrix trended to decrease first and then increase.?4?The pitting corrosion of A5083-O and A7N01S-T5 was dominant in 3.5%NaCl corrosion.With the increase of immersion time,pitting pits on the surface of the aluminium alloys increased.The two kinds of aluminum alloy anode polarization curve area were controlled by activation,with oxygen reduction reaction occurred in cathodic,in addition to passivation in early stage?120h?of polarization curve in 3.5%NaCl in A5083–O.The results of the EIS showed that the protective ability of alloy oxide film and corrosion products to matrix trended to increase first and then decrease.The results of galvanic couple test showed that A5083-O and A7N01S-T5 have high potential difference with stainless steel in freshwater environment,with obvious galvanic corrosion occurred when they contacted.While the two kinds of aluminium alloys contacted with 304 stainless steel,a galvanic couple formed.Aluminum alloy acted as anode to provide cathodic protection for stainless steel and the corrosion of aluminium alloy was accelerated.The corrosion aggravated when the area ratio of aluminium alloy to 304 stainless steel was 1:2.The results of the electrochemical test showed that the disinfectant can accelerate the corrosion of A5083-O and A7N01S-T5,and the capacitive arc resistance radius decreased gradually with the increase of the effective chlorine content in the disinfectant.The cathodic polarization process of aluminium alloys in the disinfectant was mainly hydrogen evolution reaction,and the anodic polarization process showed a mode of activation-passivation-breakdown.The results of EIS showed that the surface corrosion degree of the two aluminium alloy was aggravated with the increase of available chlorine content,and pitting corrosion was the main corrosion mode of aluminium alloy in disinfection solution.With the increase of available chlorine content in disinfection solution,the corrosion point increased continuously,and the pitting corrosion changed from pitting corrosion to overall corrosion.