Electropolishing of Stainless Steel

G JEYASHREE, A SUBRAMANIAN, T VASUDEVAN, S MOHAN· AND R VENKATACHALAM·

Department of Industrial Chemistry, Alagappa University, Karaikudi 630 003. INDIA * Central Electrochemical Research Institute, Karaikudi 630 006. INDIA

[Received: 24 July 1998 Accepted: 07 May 1999]

Three diITerent electropolishing baths were formulated in H3P04 -H2S04 medium with mono, di and triethanolamines as additives for polishing of stainless steel panels. For each bath, the polishing current densities were fixed through Hull Cell experiments. The eITect of varying current density, temperature and electroplating duration were studied for these baths. The specular reflectance measurements were made to assess the extent of polishing under different experimental conditions. A detailed analysis of the experimental results is presented.

Keywords: Electropolishing and stainless steel

INTRODUCTION caustic) at 323 K for 2 minutes at 7-8 A.dm-;!. The specimens were wJshed in running water and rinsed in dei~)l1ised water and then dried. Table I presents the Electropolishing is an electrochemical process wherein composition of various baths used. preferential dissolution takes place from macro and micro peaks that appear on the surface. Elimination of the large Hull cell studies scale irregularities (> I !! m) causes smoothening of the The Hull cell of volume 267 ml was used. The polishing surface while the removal of the smaller irregularities solution was poured into the cell upto the brim. Hull cell « 0.0 I !! m) results in brightening. patterns were obtained using lead as a cathode and Innumerable reports on the electropolishing compOSitIOns stainless steel as anode. The cell currents of I, 2 and based on phosphoric acids and non-phosphoric acid 3 A.dm-;! were used for 5 minutes. Experiments were solutions containing organic substances are available [1-5]. carried out at 333, 343 and 353 K. These baths contain higher amount of H P0 and 3 4 TABLE I: Composition of hazardous chemicals. The present paper deals with the various electropolishing baths - development of H3P04 H2S04 electropolishing baths containing ethanol amines for AISI 304 stainless steel. Bath Constituent Concentration The extent of polishing was assessed by specular (mVIitre) reflectivity measurements. A ~P~ 500 H S0 360 EXPERIMENTAL 2 4 Monoethanolamine 20

The stainless steel specimens (0.8% max. carbon, 18-20% B H3P04 500 chromium, 8-11 % nickel and remaining iron) of size H2S04 360 2.5 x 2.5 x 0.1 cm were cut and the sides of the Diethanolamine 20 specimens were ground to remove the burrs. They were C H3P04 500 degreascd with benzene or trichloroethylene and H2S04 360 electrocleaned cathodically in an alkaline solution (10% Triethanolamine 20

388 JEYASHREE et al. - Electropolishing of stainless steel

RESULTS ~~~~I ~ 3A 00o DO 6 6M 6 6 I 4 o ~ 66 666 6 6 6 ~I 2A 333K Hull cell studies DO 666666 666 4 6666~1 lA 66 6 66 4 In Hull cell experiments, a specified current is passed for a duration of five minutes. The current density at a OO~I~ 0 0 0 0~1 6 6 6 6 6 6 61 3A 000 DO 0 0 0 0 6 6 6 6 6.l:>. 6 4 specified point on the specimen is ~I~O DO ~ 16 66 6 66~1 2A 343K bo DO DOD 66666 ts6 4 lA ~I 4 A = I (5.102-5.24 log L) (1) 000~10 ~ 16 666 66 6 6 1 3A 0000 0 0 6 6 6 6 6 6 6 6 4 ~ 4 2A 353K where A is the current density (A.dm-;l), I is the current I~ 4 lA passed through the cell and L is the length of the polishing region from the nearest end of the anode. Various cell currents (1-3 A.dm-;l) were passed at Fig. J: Hull cell pattern for different cell currents and different bath temperatures. temperatures 333, 343 and 353 K for bath A (1) Very bright; (2) Bright; (3) Dull bright; (4) Dull Monoethanolamine At all temperatures mostly dull region dominated followed by a dull bright zone when 1 and 2 Amp currents were Reflectivity measurements passed. At 353 Kthe passage of 3 Amp has resulted in The stainless steel specimens were electropolished at a very bright region to a considerable extent (Fig. 1). 333 K using 15.5, 31 and 46.5 A.dm-;l at different time Diethanolamine intervals. They were washed in running water and dried. ., At 333 K very bright polishing regions were seen when The reflectivity of the specimens were measured using 1- 3 Amp. currents were passed. At 343 K and 3 Amp. ML 4B gloss/reflectance meter. current, very bright polishing was obtained at the nearest Weight loss measurements end of the anode. Again at 353 K, only at higher currents The specimens were kept at different anodic polishing 'very bright polishing could be obtained (Fig. 2). current densities for specified time intervals at desired Triethanolamine temperatures in the polishing bath using lead cathode. The At 333 and 343 K bright polishing was obtained at 2 and changes in weights were recorded. 3 Amp. Very bright polishing was seen at the nearest end of the anode at 353 K when 2 or 3 Amp currents were passed (Fig. 3). OOOO~I DO 000 ~ 1666 6 ~ 1 3A 00000 0 0 0 0 0 0 666 6 6 4 0~1 0 0 0 16 6 6 6 66 6 61 333 K 00 DO 0 66666666 4 2A 000010000,I6A6 6 6 6 661 A 3A 000~100 0~166 661 lA 0000000066666666 00000000 6666 4 0000~166666666~1 DO DO 0 66l\ 6 6 6 66A 4 2A 333K 0~1.0 DODD 00000 0 166~1 4 3A 00 0 0 0 0 0 DO DO 0 0 666 66~ lA 666 4 0~16 666666661 2A 343K DO 666666666 4 ~"8f000 0000~1666 666614 0000000006666666 3A 6 66~1 lA 666 6 4 00~1~ 00 0,166666 6 ~I 000 0 0 0 6 6 6 6 6 6 6 A 2A 343K ~I DODD 0 ~166 6 6 66.~ 3A ~1666 66 6 6~1 000 000 DOC 666666 4 066666 66l> 4 lA ~O 0 0 0~16 6 666~1 2A 353K DO 0 DO 666666 4 O~O ODD OJ66 6 66666 1 00 DO ODD 66666666 4 3A 66~1 lA 1D.66 1 ~OO 0001166 6 6 66~1 2A 353K 00000 6666666 4 0-1 0-2. Ll-3 666~1 6666 ~- lA

Fig 2: Hull cell pattern for different cell currents and Fig. 3: Hull cell pattern for different cell currents and temperatures 333, 343 and 353 K for bath B tempcraturcs 333, 343 and 353 K for bath C (l) Very bright; (2) Bright; (3) Dull bright; (4) Dull (1) Vel:v bright; (2) Bright; (3) Dull bright; (4) Dull

389

- I lEYASHREE et al. - Electropolishing of stainless steel

Bath B At 333 K maximum reflectance was seen only at 46.5 Adm-2 (Fig. 5). Both at 343 and 353 K maximum reflectance was observed at lower current density of 31 Adm-z. a Bath C

b At 333 K maximum of 99% reflectance occurred at the end of 12 minutes when 15.5 A.dm-z was passed (Fig. 6). c At 343 and 353 K, the maximum reflectance occurred at the same current density but at lower polishing time of 9 minutes. 84 To conclude, at 333 K both the baths A and C were found to be better than bath B from the current 80 requirement points of view. Among the two, bath A is 76J.--~Ir--_j.-----k------hl"---·---i-- superior to achieve maximum reflectance in 9 minutes.

TIME (SE'C) For 343 K, bath C is the best among the three and the maximum reflectance is achieved at the end of 9 minutes. For 353 K bath A is superior from current requirement and electropolishing time points of view. Fig. 4: Relationship between specular reflectivity and tillle of electropolishing at current densities Weight loss measurements ~ 15.5. 31.0 and 46.5 A.dm at 333 K for bath A At a constant polishing time of 5 minutes, the specimens were anodically kept at different current densities. Losses in weight of anodes were calculated. It was found that Reflectance studies losses in weight were linearly related to the applied Bath A current density and time of electropolishing (Fig. 7). A maximum reflectance of 98% was obtained at 333 and 353 K when a current density o'f 15.5 Adm-z was passed through the cell for 9 minutes (Fig. 4). However at 343 K 2 it was found that the current requirement is 31 A.dm- • 100 a

~80 ;:: ~~ ex

50 70 0 3 6 9 12 15 TIME (m) TIME (min)

Fig. 5: Relatio/lship between specular reflectivity and Fig. 6: Relationship between specular reflectivity and time of electropolishing at current densities time of electropolishing at current densities ~ ~ 15.5. 31,0 and 46.5 A.dm at 333 K for bath B 15.5. 31.0 and 46.5 A.dlll at 333 K for bath C

390 JEYASHREE et al. - Electropolishing of stainless steel

TABLE II: Reflectivity of electropolished specimens - Effect of bath ageing at 333 K a Time duration 1 hour

90 N Bath Current (A) Reflectivity (%) E u A 1 86 -~ 70 2 89 ~ 3 94 50 B 1 83 2 87 3 93 c 1 83 10 2 84 3 86

surface takes place. The adsorption on the surface of Fig. 7: Typical relationship between current density and crevices was hindered due to the existence of the thick weight loss at constant time of 5 lI1inutews for bath A electrolyte layer. The inhibition of metal dissolution from the peaks of the surface causes levelling. The passive Ageing of the bath films on the crevices are stable and the uniform rate of To study whether the electrolyte is stable during the dissolution from crevices and from the peaks causes the experimental conditions of electrolysis, electropolishing appearance of bright surface. experiments were carried out at different turrents of 1-3 for a longer duration of electrolysis namely one hour CONCLUSION against the normal requirements of 3-15 minutes of polishing time. The results are presented in Table II. Hull cell studies, reflectivity measurements and weight loss measurements revealed that electropolishing bath DISCUSSION containing monoethanolamine offered very bright surface When an anodic current passes through the solution, a at all temperatures compared to di and triethanol amines. liquid layer of anodic dissolution products is formed on This electropolishing bath offered maximum retlected the surface. This layer has a higher viscosity and a greater surface with minimum electrical power consumption. electrical resistivity than the bulk of the solution. The layer thickness is greater on crevices than on projections. REFERENCES Due to the non-uniform current density, metal from the projection dissolves more rapidly than from crevices, I. J Edwards. J Electrodep Tech Soc. 28 (1952) 133 which produces a surface levelling effect. The preferential 2. P A Jacquet, C R Acad Sci. 202 (1936) 402 dissolution may be due to [6-9] 3. P Michel. Met lnd, 26 (1949) 2175 4. P A Jacquet, Metal Finishing. 47 (1949) 6 J:l differences of the metal ion concentration in peaks and 5. H E Zen tIer-Gorden. J Electrodepositors Tech Soc, 26 crevices tor) (1950) 55 J:l differences in resistivities of peaks and crevices (or) 6. P A Jacquet. Metal Finishing. 47 (1949) 5-10; 48 (1950) J:l concentration of the electric field lines on peak in the 1-2 rough surface which leads to a change in the surface 7. W C Elmore, J Appl Physics. 10 (1939) 724; 11 (1940) 7970 tension of the films covering the peaks. 8. J Edwards. J Electrochell1 Soc, 100 (1953) 7; 189c (1953) When organic compounds viz., ethanolamines are added, 8; 223 c the adsorption of these compounds on the peaks of the 9. G S Vozdvizhanskii, Dall SSSR. 59 (1948) 1587

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