Electrodissolution of AISI 304 stainless steel in concentrated acids leading to electropolishing

Transcription

1 Indian Journal of Chemical Technology Vol.2,July 1995,pp Electrodissolution of AISI 304 stainless steel in concentrated acids leading to electropolishing V B Singh & Udai Arvind Department of Chemistry, Banaras Hindu University, Varanasi , India Received 16 August 1994; accepted 2 January 1995 The electrochemical polarization behaviour of austenitic stainless steel 304 has been studied with a special reference to electropolishing in concentrated phosphoric and sulphuric acid and in various compositions of their mixtures containing acetic acid at 30 ± 1 C in deaerated condition. The specimen ofss 304 invariably showed active, passive and transpassive behaviour in different solution mixtures. The conditions which cause electropolishing of the specimen have been worked out. Highly reflecting and smooth electropolished surface of the SS 304 was obtained only in a few compositions ofthe electrolytes. The electropolishing was observed in the transpassive potential region at or beyond a limiting current plateau. Phosphoric acid based solutions with acetic acid appeared to be more suitable for electropolishing of SS 304 in comparison to phosphoric acid-sulphuric acid mixtures and the presence of acetic acid in overall reaction mixtureappeared to be beneficial. Electropolishing of steels and stainless steels is widely practiced in industries. Phosphoric acid based electrolytes are commonly employed for the stainless steep. The use of acetic acid as a component in different electropolishing solutions is also reported2. A phosphoric acid solution containing sulphuric acid gave a better surface finish than pure phosphoric acid3. Acetic acid has been found beneficial in electropolishing for different materials and phosphoric-sulphuric acid mixtures have also been found suitable for electropolishing of stainless steep. Therefore, it was of interest to search electropolishing electrolytes containing concentrated H3P04, H2S04 and or CH3COOH and favourable experimental condition for electropolishing of stainless steels. Moreover, scarcely any information is available related to the anodic behaviour of austenitic stainless steels in concentrated acid solutions under electro- polishing condition. Therefore, in an attempt to examine the electropolishing conditions the anodic polarization behaviour of stainless steels in concentrated phosphoric, sui ph uric and acetic acid and their mixtures has been studied over a wide composition range at 30 C. Experimental Procedure The experimental set up, working procedures and preparation of the specimen are the same as described earlier4 s. The polarization curves have been determined potentiostatically (POS 73) for austenitic stainless steel 304 (18 Cr - 10 Ni) of about 2 cm2 area held vertically in the electrolyte in conjugation with a platinum counter electrode and a SCE reference electrode. The experiments were performed at 30 ± 1 C in deaerated solutions of concentrated phosphoric and sulphuric acid and their mixtures containing acetic acid under unstirred conditions. The solution mixtures were prepared by taking required volumes of each acids (H2S04, H3P04 and CH3COOH) from their concentrated stock solution and percentage has been reported in terms of volume per cent (V%). Results and Discussion Open circuit potential (OCP) of the austenitic stainless steel 304 in individual acids and in their mixtures always possesses a negative value of potential ( m V). The OCP was found to attain a stable value within half an hour in each case. The polarization behaviour of the stainless steel in individual concentrated acids, phosphoric acid based solution mixtures containing sulphuric and acetic acids and in sulphuric acid based solutions, containing other two acids, in different volumes, has been shown in Figs 1-3, respectively. Examination of these curves reveals that invariably active, passive and transpassive behaviour are obtained for the stainless steel with a little bit variation in current densities depend~ng upon the composition of the solution. The cathodic behaviour up to a certain

2 212 INDIAN 1. CHEM. TECHNOL., JULY / o 85'10 H3PO" o 95 'I. H2so" A 50'1. ~50" + 30'1. H3PO" + 20'1. H20 lij U III 1400 > 1000 > E a :;500 c:.: o Do 200 o ~oO Current density, }!A/em2 Fig. I-Polarization behaviour of AISI 304 stainless steel in sulphuric and phosphoric acids > uii> ~0 > c E C>- oil w 0 o 50'I,H3P04 + 3Q'4H2S04+ 20'I,CH3COOH o 60 'I, H3P '4H2S 'I,CH~ -"00' ~- -~cp 70 'I, H3P '4 H2SO4 + 20'1, CH3COOH 75'1, H3P04+ O'4H2S0,,+25 I.CH3COOH 103 Current density, foja/cm2 Fig. 2-Polarization behaviour of AISI 304 stainless steel in solution mixture of phosphoric acid based electrolyte containing sulphuric and acetic acids I II'l "" "1'1~1111"'11 1 "'"I, "'! '11' 11"11 III" 1 1

3 SINGH & ARVIND: ELECTRODISSOLUTION OF STAINLESS STEEL 213 Z800 o 50 'I. HZS04 + ZO'4 H3P '4CH3COOH o 60 ',. HZS04 + ZO'l. H3P '4 CH~OOH Z300~ 70'10 HZS '1. H3PO" +10'4CH3COOH A 75'10 HZS04 + 0'4 H3P04 + 2S'4CH3COOH 1800 w UIII :e c: 1) Q. 300 o -ZOO -700~ Cur-r-.nt density. }JA/cmZ Fig. 3--Polarization behaviour of AISI 304 stainless steel in solution mixtures of sulphuric acid based electrolyte containing phosphoric and acetic acids potential region is almost similar in all the cases. However, some difference was observed in case of solution containing higher concentrations of sulphuric acid (Fig. 3) particularly at more negative potentials. The cathodic reaction in each case is hydrogen evolution. It is seen from Fig. 2 that the passivity current density is found to decrease regularly with increasing concentration ofh3p04. The nature of the curves in general in the passive region is quite similar to each other, however, in the concentration range 50-60% H3P04 these are very close and some times overlap. A stable passive region (at least up to a minimum potential of 800 mv) for SS 304 is obtained in each solution composition of phosphoric acid based electrolytes. A rapid increase in the current is observed with successive increase of the anodic potential beyond the passive region in each solution mixture. The potential at which such increase was observed was found to depend on the composition of solution mixture. Such rapid increase in the current was found comparatively at lower' potential in concentrated phosphoric acid solution mixtures (70-75% H3P04). The large increase in the current due to a small increase in the anodic potential beyond the passive region indicates transpassive dissolution of the metal (SS 304). In the transpassive region another current peak and a current plateau (beyond 1.2 V), are invariably obtained in this class of electrolytes. At potentials higher than 1.4 V the anodic current again increased with increase in the potential. Gas evolution was observed under these conditions. Potentiostatic experiments were performed to determine the influence of applied anodic potential on the surface morphology of the AISI SS 304. These experiments were carried out at different carefully selected anodic potentials corresponding to currents below, at and above the transpassive limiting current plateau. The surface of the specimen was examined by an optical microscope below the second current peak and in the limiting current region in each case. In phosphoric acid rich electrolytes the surface of the specimen did not show any sign of grain boundary or localized attack on it below the second current peak and limiting current region. Therefore, the sharp increase of current in the transpassive potential region is due to uniform dissolution of metal/passive film. Tnphosphoric acid hased solutions wi,th two other constituents H2S04 and CH3COOH it is observed that as the concentration of H3P04 increases in the solution mixture the smoothness as well as brightness of the specimen significa!ltly improve. A very fine,

4 214 INDIAN 1. CHEM. TECHNOL., JULY 1995 smooth and highly reflecting surface was obtained in a solution mixture containing 75% H3P % CH3COOH without sulphuric acid. This concentration of the solution thus offers the best composition for electropolishing ofss 304 in this set of electrolytic solution. It is important to mention that all four curves Fig. 2) obtained in the solution mixtrure containing R~P04, H2S04 and CH3COOH are very similar, however, the curves obtained in 50-70% H3P04 are close to each other in the current plateau region. The curve obtained in 75% H3P % CH3COOH without H2S04 is associated with low current in the plateau region. The plateau region is found to decrease with increasing HJP04 concentration in the solution mixture. This can be explained in terms of varying water content and viscosity of the solution mixtures. With increasing content of HJP04 both water content and viscosity increase. The increasing content of water will give rise to comparatively less prominent plateau which has actually been observed. Whereas more prominent plateau with low current is expected with increasing viscosity of the solution mixtures. Low current and less prominent plateau region observed in 75% HJP % CHJCOOH are possibly due to the dominent role of viscosity in this solution mixture. The potential at which gas evolution occurs decreases (less noble) with increasing concentration of HJP04 in the solution mixture. Thus, it is arrived from these observations that the current plateau followed by a second current peak seems to be a necessary condition for electropolishing. It is also worth mentioning that a bright yellow film formed on the specimen surface is a precurser of electropolishing. These observations are in accordance with the widely held view that electropolishing occurs in presence of both a diffusion-controlled dissolution in solution and in presence of a non-passive surface film2 The experiments were also performed in another type of electrolytic solutions based on H2S04 (keeping 20% HJP04 constant). The polarization curves (Fig. 3) show active, passive and transpassive behaviour for SS 304 in different solution mixtures containing sulphuric, phosphoric and acetic acid. The current densities in different characteristic regions v:aried to some extent depending upon the solution composition. In this case too, there is a sharp increase in the anodic current above I V beyond the passive region which showed very clear plateau region for the current at successive higher potentials. However, before the beginning of the limiting current region there is some indication of second current peak which is apparent in lower concentration of sulphuric acid (50-60%) in the solution mixture. The value of limiting current is observed to decrease with increasing concentration of H2S04 in the solutiofl. mixture. Such a decrease has also been reported earlier6-8. Since the water content can be assumed to vary only slightly in the mixtures except for 75% H2S % CHJCOOH without HJP04, therefore, the decrease in the value of limiting current can be attributed mainly to the increasing viscosity with increasing H2S04 concentration in the solution mixture. The current plateau region is quite prominent particularly in the satutions containing 70 and 75% sulphuric acid. The decreasing value of the limiting current with increasing H2S04 content does not alone permit to distinguish the species being transported because solubility of the corrosion product and concentration of free anions decrease whereas the water concentration vary slightly with increasing H2S04 concentration. The contribution of water towards observed limiting current and the plateau range can be identified in phosphoric acid free solution mixture (having least water content) when compared with rest of the mixtures of this category. This is also indicated from the observed limiting nature at higher negative potentials and most prominent plateau in the transpassive region in the solution mixture. The experiments have also been performed in third type of electrolytic solutions keeping constant concentration of sulphuric acid (40%) with a variation in acetic and phosphoric acid concentration. The polarization curves (Fig. 4) show active, passive and transpassive behaviour for AISI SS 304 in different compositions of the solution mixtures. The critical current and passivity current densities are found to have almost same order of values 111 lower acetic acid content (i.e. 10 and 20% CH3COOH) but are having greater values for concentrated acetic acid (i.e. 30 and 50% CH3COOH) in the solution mixture. In these electlolytic solutions too there is a sharp increase in the anodic current beyond the passive region (> IV) and invariably a clear current plateau region at higher potentials is obtained. In the transpassive region a current plateau is observed above 1.4 V in each case. It is also found that beyond 1.2 V the anodic current decreases significantly and the limiting current region becomes increasingly prominent with increasing concentration of acetic acid in the solution mixture. Such behaviour can be attributed to the decreasing content of water in the solution mixture as phosphoric acid is successively replaced by acetic acid. The successive replacement of more viscous solution (H3P04) by less viscous acetic acid in solution mixture is expected to result in higher Iii 1:1 ;'" "I'll' "!'I'I" "I "I II!I\"rlll I~ 1q I,.111;,I" II, 'II!' ; II IIII I I ~ I l; i I', I I

5 . SINGH & ARVIND: ELECTRODISSOLUTION OF STAINLESS STEEL o 40.,. HZS04 + SO.,. H3P ,. CH3COOH D ~'Io H2S '" H3P 'IoCH3COOH 1800~ 40'" H2SO4 + 30'10 HJPOI 'I. CHaCOOH X.co.,. ~S ,.H3P O'l.CH3COOH l&l IJ III = o eoo Y)' 10' 103 Cu,.,..nt d.naity, JJA/crrl Fig. 4-Polarization behaviour of AISI 304 stainless steel in solution mixture of acetic, sulphuric and phosphoric acid's limiting current which is other wise observed here. This clearly indicates that limiting region is more dictated by the water content rather than the viscosity of the solution mixtures. It is further supported that when acetic acid is replaced by water in the solution mixture containing H3P04 and H2S04, the limiting current plateau becomes less pronounced (Fig. 1). An examination of the curves (Fig. 4) in the transpassive dissolution region indicates higher dissolution rate in the solution mixture containing lower concentration of acetic acid. The fact that better brightening is observed in the solution mixture containing 10% acetic acid suggests that water present in the solution plays some role in the electropolishing phenomenon. It emerges from the experimental results that good electropolishing is not achieved in presence of either high or low content of water in the solution mixtures, however, the water content appears to play some role in process of electropolishing. The role of viscosity is also inferred and it is found to be uncertain that the plateau region and.the current under these conditions are actually due to diffusion limitation of water only. Thus, the precise role of water in the polishing process certainly requires a further study9. Potentials higher than 1.8 V caused the anodic currents to increase as a result of gas evolution in each case but more in lower concentration of acetic acid in the solution. The surface of the specimen was examined in every composition of this class of electrolytic solution mixture after each experiment and it was found smooth throughout. The surface brightness, however, decreased as acetic acid concentration was increased in the solution mixture. The best surface finish was obtained in 10% CH3COOH +.40% H2S % H3P04 composition mixture among all the four sets (i.e % and 50% CH3COOH). Among the three mechanisms I 0 suggested for m!lss transport of the species in the solution during electropolishing, the water transport seems to be most probable which is diffusion controlled in the current plateau region. This is inferred because the water content is decreasing with increasing acetic acid and decreasing phosphoric acid concentration in the solution mixtures. Conclusions Ariodic polarization of AISI SS 304 in concentrated H3P04, H2S04, and their mixtures containing CH3COOH of different compositions reveals a typical active passive and transpassive behaviour. Its electropolishing is observed in the limiting current region where the secondary current peak is prominently observed. It emerges out of the results

6 216 INDIAN 1. CHEM. TECHNOL., JULY ]995 ihat for electropolishing of this stainless steel the use of concentrated phosphoric acid (> 50%) is necessary in the electrolytic solution with other two components (sulphuric and acetic acid) and the electropolishing seems to be better in the solution with low water content. Moreover, the electropolishing improves in terms of brightness and smoothness when H2S04 is replaced by CH3COOH (25%) and polishing commenced at relatively lower potentials in this solution mixture. Binary phosphoric acid rich solution with sulphuric or acetic acid is found to be more suitable in comparison to ternary solutions. Acknowledgement Financial assistance for the work from Council of Scientific and Industrial Research, New Delhi is gratefully acknowledged. References I Datta M & Vercruysse D, J Electrochem Soc, 137 (1990) 2 Gabe D R, Corros Sri, I3 (1973) Ponto L, Datta M & Landolt D, Surf Coatings Technol. 30 (1987) Rao N N & Singh V B, Corros Sci, 25 (1985) 47I. 5 Singh V K & Singh V B, Corros Sri, 28 (1988) Epelboin I & Keddam M, Cr A cad Sri Paris, 258 (1964) Daguenet M & Froment M, Cr Acad Sri Paris, 260 (1965) Alanis 1 L & Schilfrin D J, Elecrochim Acta, 27 (1979) MagainoS, MatloszM&LandoltD,J ElectrochemSoc,l40 (1993) Landolt D, l~croc"im Acto, 32 (1987) 1. I I "~I 'I' 'II' I'!'I"'II I'll" FII I 111"1" 11'111111'111 I II if 't'iiih 1111'1 ~ I "11.11II 11,1 II