Research On Wear And Electrochemical Corrosion Behavior Of Copper Strip During Cold Rolling Lubrication

As an important material in the fields of electronics,aerospace,heat conduction and dissipation,copper(Cu)is developing towards the direction of high dimensional accuracy,high surface quality and high conductivity.It also puts forward new requirements for technical lubrication.Rolling oils are commonly used for finish rolling stage,researches show that the addition of extreme-pressure(EP)additives to rolling oils can effectively reduce friction and wear.However,it is lack of theoretical study on the adsorption characteristics,anti-wear mechanism and synergistic effect of composite EP additives on the Cu surface.Oil-in-water(O/W)emulsions used for rough and medium rolling stage generally possess lubricating,cooling and cleaning effects.But it is easy to produce genetic surface corrosion such as incremental porosity and color spot expansion.Therefore,in this paper,the anti-wear mechanism of EP additives,the preparation of O/W emulsions,as well as the corrosion and inhibition of emulsions on the Cu surface were studied.EP additives were effectively selected for rolling oils preparation based on their molecular structures and physicochemical properties.When the modified phosphate DDPE was interacted on the Cu surface,S-O and P=S bonds were broken,O and S contents were adsorbed on the surface to form Cu-O and Cu-S chemical bonds.This can generate a stable chemisorption film on the Cu surface and thus ensure the extreme pressure and anti-wear performance of rolling oils.Adhesive wear,plowing wear and oxidative wear were the three main wear forms during Cu/Fe friction process.The relative wear rate of the oxidative wear was as low as 22.5%.A chemisorption layer that composed of Cu2O,CuFeS2 and FePO4,and a physisorption layer composed of free B2O3,Fe and Cu substance were formed on the steel surface,which also achieved synergistic anti-wear effects.The multifunctional O/W emulsions used for Cu cold rolling were prepared.Characterized by SEM and TEM,the size of emulsion droplets was between 10?m and 50 ?m,with a shape of "large oil droplets" wrapped by "small water droplets".The tribological properties of the emulsions were found to be improved by using a variety of phosphorus and sulfur EP additives.Comparing to rolling oils,the rolling force and finish rolling thickness of Cu strip reduced by 2.7%and 4.7%,respectively.However,emulsion spots were easy to adsorb on the Cu surface and then generated oxidizes and corrosion that affected its surface quality.The static corrosion measurements were carried out to investigate the corrosion behavior of Cu strip in emulsions.The dynamic relationship of Cu corrosion at room temperature could be explained as lg(D)=1.6841g(t)-4.338.After immersion tests,the corrosion products on the Cu surface were successively proved by Cu2O,CuO,Cu(OH)2,Cu(OOCR)2 and Cu2(OH)2CuCO3,which caused the oxidative discoloration.In addition,the effect of different factors on the corrosion of Cu strip in emulsion were further studied via electrochemical methods.When the annealing temperature was higher than 600?,the average grain size of Cu strip grew rapidly and its corrosion current density decreased to 7.84×10-8 A·cm.2.This significantly improved the anti-corrosion property of Cu in emulsions.Affected by the combination of stress corrosion and pitting corrosion,Cl,S,and C ions were likely to adsorb on the surface defects such as microcracks,metastable pits and dislocations to form pits nucleation,which accelerated the pits expansion.Therefore,the corrosive tendency of Cu was increased with the accumulation of rolling deformation.The current density of Cu strip in emulsion prepared by hard water was greater than that prepared by soft water,due to the higher concentration ions such as Ca2+,Mg2+,Cl-,SO42-.The efficiency of inhibitor was also reduced.In the alkaline emulsion,multilayer passive films that composed of Cu2O and CuO/Cu(OH)2 had stronger protective effect on the Cu surface than CuCl and CuCl2-formed in the acid condition.Therefore,weak alkaline emulsions were usually used for Cu cold rolling.Two modified corrosion inhibitors,NBTAH and BTDA were added to O/W emulsions and their inhibition behavior was studied.When 5 mM NBTAH and 8 mM BTDA were combined used,the inhibition efficiency on Cu can be achieved by 96.4%.A synergistic mechanism of adsorption and shielding of composite inhibitors on the Cu surface was proposed.The adsorption effect is reflected by charge transfer of polar atoms of benzene ring,azoles ring and thiazole ring on the Cu surface to form multilayer passive films that composed of Cu-NBTAH and Cu-BTDA chelates.While the shielding effect is shown in the inhibition effect of composite inhibitor films on the diffusion of H2O,Cl-,CO2,SO42-in the O/W emulsions.Under different cold rolling temperature,the diffusion behavior of Cl-in composite inhibitor films followed the thermodynamic relationship of D=37144.6exp(-5616.7/T),and its diffusion activation energy reached 4.670×104 J·mol-1.This also illustrated the composite inhibitor films possessed the most obvious inhibition effect on pitting corrosion of Cu strips.