| Duplex stainless steel is composed of two phases of ferrite and austenite which has two-phase characteristics.It is a material with excellent mechanical properties and strong corrosion resistance.It is widely used in marine chemical,petroleum and exploration fields.304 stainless steel is austenitic stainless steel with great corrosion resistance and low price.It is usually used in chemical and food processing industries.In the above application fields,some environments will contain organic acids such as formic acid or acetic acid,which are more corrosive,and a certain amount of chloride ions that can easily cause pitting corrosion of materials.There are few studies on the corrosion behavior of chlorinated organic acid solutions.This article uses high,medium and low grades of duplex stainless steels(2750,2205 and 2101)as the main research materials,and focuses on the pitting corrosion behavior of three duplex steels in chlorinated formic acid and acetic acid solution and the influence of organic acid concentration,temperature,solution pH value and other factors on the different stages of pitting corrosion and the comparison with the corrosion behavior of 304 austenitic stainless steel,possessing more important theoretical significance and practical guidance.In this article,the methods of slow potential polarization,potentiostatic polarization,X-ray electron spectroscopy(XPS),scanning electron microscopy and energy spectroscopy(SEM/EDS)were used to study the characteristics of the three duplex stainless steels 2750,2205 and 2101.Uniform corrosion behavior and pitting corrosion behavior in 0.6 mol/L NaCl solution containing different concentrations(0 mol/L,0.8 mol/L,1.6 mol/L and 3.2 mol/L)of formic acid and acetic acid,as well as material re-passivation ability after pitting corrosion;and studied the influence of different temperatures(25?,35? and 45?)and pH values(2,4 and 7)on the above corrosion behavior;in addition,the influence of 304 stainless steel in different concentrations(0 mol/L,0.8 mol/L,1.6 mol/L and 3.2 mol/L)uniform corrosion and pitting corrosion behavior in 0.6 mol/L NaCl solution of acetic acid.The main research results obtained are as follows:(1)Adding formic acid and acetic acid to the 0.6 mol/L NaCl solution will promote the uniform corrosion of the three duplex stainless steels.As the concentration of formic acid increases(0.8 mol/L,1.6 mol/L to 3.2 mol/L),the three corrosion rate of three duplex stainless steel increases;with acetic acid concentration increaseing from 0.8 mol/L to 1.6 mol/L,the corrosion rate of the three duplex stainless steels increases,while 3.2 mol/L acetic acid has a passivation effect on the three steels.Its uniform corrosion rate was reduced.(2)The results of the slow dynamic potential polarization curve show that the 2101 duplex stainless steel will have obvious metastable pitting corrosion before stable pits in the neutral 0.6 mol/L NaCl solution,and the addition of different concentrations(0.8 mol/L,1.6 mol/L and 3.2 mol/L)formic acid,the characteristic peaks of metastable pits of 2101 duplex steel disappeared,indicating that a certain concentration of formic acid has an inhibitory effect on the occurrence of metastable pits in 2101 steel;while adding acetic acid of 0.8 mol/L and 1.6 mol/L will also inhibit the occurrence of metastable pits in 2101 steel,but when the concentration of acetic acid increases to 3.2 mol/L,the characteristic peaks of metastable pits in 2101 duplex stainless steel reappear,indicating that a small amount acetic acid has an inhibitory effect on the occurrence of metastable pits in 2101 steel,but acetic acid at three concentrations can promote the growth of metastable pits in 2101 duplex steel and increase the life of metastable pits.(3)The temperature of the solution promotes the uniform corrosion of the three duplex stainless steels.At the three solution temperatures,the pitting corrosion potential Eb of 2750 and 2205 dual-phase steels is not much different,and is significantly higher than the Eb value of 2101 dual-phase steel.The difference(Eb-Ep)between the pitting potential Eb and the protection potential Ep of 2101 dual-phase steel at the three solution temperatures is very large.When the solution temperature of 2205 dual-phase steel increases from 25? to 35?,Eb-Ep value began to increase significantly.When the solution temperature of 2750 duplex stainless steel increased from 35? to 45?,the Eb-Ep value began to increase significantly.The increase in the Eb-Ep value indicated the repassivationability after pitting corrosion occurring becomes weak.The effect of temperature on the early stage(metastable pitting corrosion process)of 2101 dual-phase steel's pitting corrosion is that the increase in temperature(25? to 35? and 45?)increases the number of nucleation and growth rate of metastable pits.But the life of small pits is reduced,and metastable pits within a suitable growth rate range are easily transformed into stable pits.(4)When the pH value of the solution decreases from 7 to 4 and 2,the uniform corrosion rates of the three duplex stainless steels all increase.Under the three pH conditions,the pitting corrosion potential Eb of 2750 and 2205 dual-phase steel is significantly higher than that of 2101 dual-phase steel,indicating that 2750 and 2205 dual-phase steel has better pitting corrosion resistance than 2101 dual-phase steel;As the pH value decreased from 4 to 2,the passivation interval of 2101 dual-phase steel was significantly wider and the pitting potential Eb was significantly increased.When the pH value of the solution decreases,the Eb-Ep value of 2750 and 2205 dual-phase steel has no obvious change,while the Eb-Ep value of 2101 dual-phase steel has increased to varying degrees.It shows that the decrease of the pH value of the solution(?)little' effect on the repassivation ability of 2750 and 2205 dual-phase steels after pitting corrosion,and the repassivation ability of 2101 dual-phase steels is weakened.(5)Adding acetic acid to 0.6 mol/L NaCl solution will increase the uniform corrosion rate of 304 stainless steel.With the increase of acetic acid concentration,both the metastable pitting potential Em and Eb shifted negatively,the number of nucleation of metastable pits decreased significantly,and the size of metastable pits increased.After adding acetic acid to the solution,the pH value decreases,resulting in a decrease in the ratio of Cr2O3/Cr(OH)3 and Fe3+/Fe2+in the surface film of 304 stainless steel,and the film becomes thinner.Therefore,it is easy to produce pitting corrosion under the action of chloride ions.Acetate ion(Ac-)has little effect on the uniform corrosion of 304 stainless steel.The effect on pitting corrosion is mainly to inhibit the nucleation process of metastable pits,but it promotes the growth of nucleated pits which are easier to transform into stable pits.The role of acetic acid is not only related to its lowering of the pH value of the solution,but also related to the role of acetate. |
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