Corrosion Behavior Of Copper Alloy Protected By BTA In Atmospheric Environment

Copper alloy is a kind of Non-ferrous metal alloy closely related to modern society,which is widely used in power transmission and transformation engineering,electronic industry,transportation,marine industry and other fields.In addition,the consumption proportion can be divided according to the service performance:the electrical conductivity is 64%.the corrosion resistance is 23%,the structural strength is 12%,and the decoration is 1%.Copper-nickel alloys have been selected as coinage materials because of their good machinability and decorative properties,but it will corrode like other metals in the atmospheric environment.Although benzotriazole(BTA)is a kind of corrosion inhibitor for copper and its alloys,the corrosion mechanism of BTA for copper coins needs to be studied in combination with practical application environment.In the process of circulation,copper coins will not only remain sweat on its surface,but also contact with corrosive media in the air.All these factors will cause corrosion of copper coins.However,in the presence of BTA,there are few researches on copper coins influenced by circulation environment.Therefore,this paper designs the experiment according to the specific application background of copper coins.In the experiment,copper-nickel for coinage is used as the raw material,kinetic experiment as the main breakthrough point,and the blank group added as the contrast.The corrosion mechanism is investigated by electrochemical measurement,corrosion product morphology and composition analysis.According to the designed experimental environment,the research content is divided into three parts,which are simulated sweat environment,urban atmosphere environment and simulated industrial atmosphere environment.In the early stage of corrosion test in artificial sweat environment,the chemical conversion film formed by BTA treatment can provide limited protection for the copper-nickel coin.with the extension of the test time,the weight gain of the coins significantly increased,which is far greater than that of the blank group,and this chemical conversion film becomes the factor of accelerated corrosion.In the later stage of the test,the corrosion law of the two groups of samples tends to be the same.The main corrosion factors in artificial sweat(ISO 3160-2:2015))are Cl-and H+.The chemical bond formed between BTA and metal ions will be destroyed due to the participation of chloride ions in the acidic environment.The active metal ions will react with the corrosion medium quickly and form the corrosion products covering substrate surface,which is the reason for the difference in compared with the kinetic curves of the blank group.The components are Cu2(OH)3Cl、NiO/Ni(OH)2 and ZnCl2/ZnO,In the early stage of electrochemical test,BTA film has a larger impedance and shows fine corrosion resistance,but it also delays the formation of protective corrosion products.The corrosion rate in the later stage is faster than that in the blank group.BTA is not suitable for long-term protection of copper coins in circulation environmentThe chemical conversion film formed by BTA treatment in urban atmospheric environment plays a limited role in the protection of copper coins.Even if the local damage is caused by corrosion medium,the chemical conversion film layer formed by BTA treatment can still hinder the formation of corrosion micro cell with "large cathode,small anode" structure,and gradually form uniform corrosion due to its further damage indirectly hinders the further expansion of pitting corrosion.In the outdoor industrial atmospheric environment,chloride ion and SO2 are the main factors of copper coin’s surface corrosion.The presence of BTA did not change the composition of corrosion products.Except for the accumulation of solid dust particles,the main corrosion products on the copper coin surface were Cu2(OH)3Cl and Cu5(SO4)2(OH)6·5H2O,which were consistent with the blank group.Removing the impurities on the surface can prolong the corrosion inhibition time of BTA on copper coins.Under the simulated urban atmospheric environment,the chemical conversion film formed by BTA treatment has a certain protective effect on the corrosion of the copper-nickel matrix.The corrosion of the blank group is always more serious than that of BTA group,and the corrosion law of the two groups tends to be the same in the later stage of the test.The chemical conversion film formed by BTA treatment can increase the contact angle between copper-nickel and deionized water drop from 53°to 69 ° and has a certain resistance to corrosive medium.Compared with the surface morphology of different corrosion cycles,the corrosion of BTA group copper-nickel will expand from the damaged part of the film,and the final corrosion products are porous structure,the main components are ZnSO4·6H2O and Cu4(SO4)(OH)6.In the parameters obtained from the polarization curve,the corrosion current density of BTA group decreases first and then increases.The reason is that in the process of corrosion,besides the chemical conversion film formed by BTA treatment,there are also protective oxide films on the surface of copper-nickel due to corrosion.The joint action of the two further delays the corrosion.With the extension of corrosion time,the damage range of the chemical conversion film formed by BTA treatment expands,and the overall corrosion current density also increases.In conclusion,a chemical conversion film can be formed on the surface of copper-nickel coins by BTA treatment.This film can play a limited role in protecting coins under some environmental conditions.According to the corrosion status of copper-nickel in different environments,the effect of corrosion factors on chemical conversion film formed by BTA treatment is analyzed.It can be found that the corrosion effect of Cl-is stronger than that of SO2.In the simulated sweat,the corrosion of copper-nickel containing BTA is faster.The reason is that Cl’ can promote the decomposition of chemical conversion film in acid environment,and its smaller ion radius makes Cl-adsorb on the damaged part of the protective film,and even react with the substrate directly.BTA is suitable for the protection of copper coins that do not contact with sweat frequently,such as commemorative coins,while it does not have enough protection for copper coins that undertake circulation mission.