Microstructure Evolution And Corrosion Resistance Of Fe-Based Amorphous Alloy During Low Temperature Heat Treatment

Fe-based amorphous alloy is widely used in the field of electricity,electronic,machinery,chemical and so on because of its excellent magnetic properties.But normal corrosion is unavoidable in many applications where the Fe-based amorphous alloys are used.So,they also need to have excellent corrosion resistance.In this paper,Fe78Si9B13 and Fe73.5Si13.5B9Cu1Nb3(at.%)amorphous alloys are taken as the research object and annealed at different low temperatures and time.The phase structure,thermal stability,corrosion behavior and corrosion morphology of Fe-based amorphous alloy were studied.This study is focused on the evolution of microstructure and the corrosion behavior of Fe-based amorphous alloy during heat treatment and the mechanism of corrosion resistance was explained through the microstructure analysis.Firstly,the phase structure,thermal stability and corrosion behavior of Fe78Si9B13 and Fe73.5Si13.5B9Cu1Nb3 amorphous alloys in 3.5%NaCl solution,5%H2SO4 solution and 10%NaOH solution were analyzed.The results show that Fe78Si9B13 ?Fe73 5Si13.5B9Cu1Nb3 alloys are of single amorphous structure.Compared with Fe78Si9B13,Fe73.5Si13.5B9Cu1Nb3 showed better thermal stability.Besides,Fe73.5Si13.5B9Cu1Nb3 amorphous alloy presented better corrosion resistance in three corrosive media.The structural analysis of the Fe78Si9B13 and Fe73.5Si13.5B9Cu1Nb3 amorphous alloys during the low temperature heat treatment process shows a general trend that the size of the atomic clusters decreases first and then increases.Atomic diffusion will occur during the heat treatment process which makes the structure and composition more uniform when the annealing temperature is lower.But it will promote the further growth of atomic clusters with the increasing of annealing temperature.The corrosion resistance of two kinds of amorphous in 3.5%NaCl solution after annealed at low temperature is depended on its microstructure.The corrosion rate is almost positively correlated with the size of atomic clusters,that is,the smaller the size of atomic clusters are,the smaller the corrosion rate and the better the corrosion resistance is.This can be attributed to changes of the number of corrosion microcells which caused by the changes of microstructure.The smaller the size of the atomic clusters of the amorphous alloy is,the more uniform of composition and structure is.Thus,less corrosion microcells can be formed and the corrosion resistance becomes better.For the corrosion resistance of two kinds of amorphous alloy in 5%H2SO4 solution after heat treatment,the size of atomic clusters is not the only factor,but also may be related to stress corrosion in acidic environments.The internal stress generated during the preparing process can be released during the low temperature heat treatment which makes the free energy of the amorphous system reduced.Thus the corrosion rate decreases and the corrosion resistance is enhanced.But when the heat treatment temperature is higher,the further increase in the size of the atomic clusters leads to a decrease in the uniformity of the structure and composition.On the other hand,the diffusion coefficient of Si increases and a large number of Si spread to the surface resulting in the surface composition uniformity decreasing.As a result,some areas corrode preferentially and the corrosion rate increases.Therefore,in order to improve the corrosion resistance,it is necessary to make the internal stress released by heat treatment,but also to prevent the rapid growth of atomic clusters.The corrosion resistance of the two amorphous alloys in 10%NaOH solution after annealed at different temperature and time is related to the formation of the passive film.The composition and structure of the amorphous alloy becomes more uniform after annealed at the lower temperature(200?,250?)which is conducive to the formation of a more continuous and dense passive film that can provide better protection for the metal matrix.Thus,the corrosion rate decreases and the corrosion resistance is improved.After annealed at 400?,the higher temperature can promote Si to migrate to the surface to promote the formation of the passivation film.On the other hand,although some nanoscale crystalline phases precipitated in the amorphous matrix,the corrosion mechanism in alkaline solution is not to depend on galvanic corrosion,the precipitation of these nanocrystals does not accelerate corrosion.Therefore,the corrosion resistance of the sample after heat treatment is higher than that of the untreated.