Study On Pitting Mechanism And Influencing Factors Of Copper Tubes In Heat Exchanger

Locomotive heat exchanger is the key equipment for normal operation of diesel locomotives.Corrosion and fouling of locomotive heat exchangers can severely reduce the efficiency of heat transfer,resulting in failure or even shutdown of the heat exchanger tubes and in increased operating costs.The material constituting the locomotive heat exchanger is mainly copper and its alloys.The corrosion of copper and its alloys can be attributed to uniform corrosion,pitting,erosion and atmospheric corrosion.Corrosion inhibitors for copper and its alloys are mainly formulated with borax,sodium nitrite,sodium silicate,and benzoyl triazole.However,during actual use,the brass tubes in the locomotive heat exchangers are prone to appear pitting.This paper is focused on brass flat tubes,brass galvanized flat tubes,T2 copper and T2 copper tubes as the research object.The pitting corrosion mechanism and influencing factors of copper tubes in locomotive heat exchangers are investigated by weight-increasing,weightlessness,SEM and EDS.The results are as follows:The degree of corrosion affecting on the T2 copper coupons shows a decline trend with the order followed temperature > borax> sodium silicate > benzotriazole > sodium silicate.However,for T2 copper pipes,the order of the effect is temperature > borax > sodium silicate > sodium nitrite > benzotriazole.Additionally,the degree of corrosion influencing on the brass pipe from large to small is temperature > sodium nitrite > sodium silicate >benzotriazole > borax,while for brass galvanized flat tube the influence decreases with the order of temperature > borax > sodium silicate > sodium nitrite > benzotriazole.The SEM results show that the main cause of pitting corrosion of the T2 copper tube and the brass flat tube is the partial rupture of the cuprous oxide protective film on the copper surface.Causes copper at the bottom of the pit to act as an anode and the cuprous oxide film on the surface acts as a cathode to form a galvanic couple,which causes pitting of the copper dissolved in the pit,and breakage of the oxide film and excessive carbon residue and deposition of dirt(foreign matter)on the surface.,caused by mechanical attacks,and the material’s own defects.In the corrosion test for coupons of the same length,T2 copper has almost no corrosion.The corrosion of T2 copper pipe is at the initial stage of pitting corrosion,while the corrosion of brass flat pipe is at the transition stage of pitting to uniform corrosion.The brass tinned flat tube For the uniform corrosion phase.EDS results show that the elements on the unetched copper surface are oxygen and copper(ie,cuprous oxide),and the pitting elements on the surface are oxygen,carbon,copper and silicon(a mixture of basic copper carbonate and silicon dioxide);The elements on the corroded brass surface are oxygen,zinc,and copper(ie,cuprous oxide and zinc metal).The elements on the pitting surface are oxygen,carbon,copper,iron,zinc,and silicon(basic copper carbonate,and dioxide.The carbon content of the corroded test piece was reduced in the mixture of silicon and iron oxide.Based on the experimental results,the corrosion inhibitors(borax,sodium nitrite,sodiumsilicate and benzotriazole)for brass flat tubes,brass galvanized flat tubes,T2 copper sheets and T2 copper tubes are summarized.The best ratio for the practical application provides some guidance in operation.