THE INVESTIGATION OF CORROSION BEHAVIOR OF 316 Cb AUSTENITIC STAINLESS STEEL IN ACIDIC SOLUTIONS

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1 316 Cb OSTENİTİK PASLANMAZ ÇELİĞİN ASİDİK ÇÖZELTİLERDEKİ KOROZİF DAVRANIŞININ İNCELENMESİ M. SONCU*, S. ZOR**, L.ÇAPAN* *Kocaeli Üniversitesi Makine Mühendisliği Bölümü, 41380, Kocaeli **Kocaeli Üniversitesi Fen Edebiyat Fakültesi Kimya Bölümü,41380, Kocaeli ÖZET: 316 Cb ostenitik paslanmaz çeliğin (0.1M, 1M, 2M, 3M) HCl, H 2 SO 4 ve H 2 SO 4 + HCl (1:1) asidik çözeltilerindeki korozyon davranışları potansiyodinamik polarizasyon ve kütle kaybı yöntemiyle incelenmiştir. Ayrıca, belli sürelerde asidik çözeltilerde bırakılan ostenitik paslanmaz çeliğin korozyon dayanımı sertlik ölçümleri ile belirlenmiştir. Asidik çözeltilere uzun süre daldırılan 316 Cb ostenitik paslanmaz çeliğin elemental analiz yöntemiyle krom miktarı ölçülmüştür. Anahtar Kelimeler: ostenitik paslanmaz çelik, korozyon, asidik çözeltiler. THE INVESTIGATION OF CORROSION BEHAVIOR OF 316 Cb AUSTENITIC STAINLESS STEEL IN ACIDIC SOLUTIONS ABSTRACT: The corrosion behaviors of 316 Cb austenitic stainless steel in (0.1M, 1M, 2M, 3M) HCl, H 2 SO 4 and H 2 SO 4 + HCl(1:1) the acidic solutions have been investigated by potentiodynamic polarization method and weight loss method. Furthermore, the corrosion strength of austenitic stainless steel which is exposed to acidic solutions for specific time was determined by hardness measurements. The measurement of amount of chrome content of 316 Cb is austenitic stainless steel which is exposed to long time in acidic solutions was made by elemental analyse method. Keywords: austenitic stainless steel, corrosion, acidic solutions. 1. INTRODUCTION: Stainless steels which can achieve non-corrossive character by performing invisible, adherent and whealty with regard to chrome oxide layer through chrome and oxygen in media are iron base alloys. The stability of passive film and corrosion resistance of steel is increased with increasing chrome content in structure. Although higher costs of the stainless steel which are not different from the other kind of steels about mechanical characteristics, the main reason of common usage is having corrosion resistance. The stainless steels have large range of application. Over the one-third of the produced stainless steels are used in structural applications for chemistry & power engineering industries. These applications include application areas such as, nuclear reactor canals, heat converters, the tubes that are used in oil industries, chemistry applications & paper industries' components, the pieces of boilers & furnaces are used in nuclear central (1,2). There are so many studies about corrosion behaviors of 316 austenitic stainless steel. A. Pardo at all. investigated Cu and Sn effect on pitting corrosion resistance of 304 and 316 stainless steels in media with chloride. At the end of the measurements of cyclic polarization which are potential static and electro-chemistry in 3.5% NaCl solution, following results was found: When copper was added to the structure, it was helping to nucleation and was stopping these pits growing, in opposition to copper, tin was making nucleation difficult and it was helping the pits, were formed, for growing (3).

2 G.O. Ilevbare at all. and A. Pardo at all. investigated Mo, Mn and chromate ions effect on pitting corrosion resistance of 304 and 316 stainless steels. According to this study, it has been seen that ions obstruct growing up of pitting and make the steel more strength (4,5). In this study, the corrosion resistance of 316 Cb austenitic stainless steel which was not made many studies about it, was investigated by potentiodynamic polarization curves and weight loss method in acidic solutions. Furthermore, hardness measurements and elemental analyse measurements of 316 Cb steel which is exposed to long time corrosive media were made. 2. MATERIAL AND METHOD: 2.1. Material: Austenitic stainless steel which standart composition was given in Table 1, it was sembolized with G-X CrNiMoNb and its standart number is , AISI number was 316 Cb,is used in all experiments. 316 Cb Austenitic stainless steel which uses as experiment specimen, was performed quenching treatment with two steps. At the first step, it was heated to a temperature interval C and was cooled in oil. Afterwards, at the second step steel was heated to a temperature interval C and again was cooled in the oil. Later on, experiment specimen was worked at suitable dimensions for experiment. AISI No C Table 1: Standart composition of experiment specimen (6). Cr Ni Mo Mn Nb Si P S 316 Cb xC Weight Loss Method: 316 Cb austenitic stainless steel plates at 5x2x0.3 cm dimensions were used to determine corrosion rate with weight loss method. The surfaces of the plates were cleaned by chemical process. Plates which are prepared for the experiments were cleaned with 3HNO 3 :1H 2 SO 4 :1H 2 O mixed solution and then respectively cleaned with water, pure water and CCl 4. Specimens which were cleaned were hold in furnace to fixed mass. Masses of fixed mass specimens were determined and recorded. This specimens were immersed in 3M H 2 SO 4, 3M HCl ve 3M H 2 SO 4 + 3M HCl(1:1) mixed solutions. Two specimens were taken from every solutions in specificied time (10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120 days). The stainless steel specimens were taken from solutions were hold in %5 citric acid (ph 4.5) for 2 hours and after that they were rinsed with pure water. Later, Plates were dried again in 110 C in furnace and weighted. In this way weight losses are determined and corrosion rates were calculated by formula 2.1 (7). weight loss corrosion rate = (2.1) (7) surface area of of metal time

3 2.3. Electrochemical Method: Electrochemical measurements were made by PAR 263 A potentiostate/galvanostate. In electrochemical measurements the platinum was used as against electrode, saturated calomel electrode (SCE) was used as a reference electrode and 316 Cb was used as a working electrode. The Surfaces of using working electrode were covered with polyester except the parts which have connections with the solutions. Before measuring that after from surface of working electrode was polished like mirror shinning with polishing device (METKON GRIPO 2V) which used sandpaper 800 and 1200 mesh, polarization curves were obtained between 1.00 V and V in 1 mv/s scan rate. During the measurement the solutions were mixed by magnetic mixer Hardness Test: As it told before at table (2.2 and 2.3) cleaned plates were immersed in 3M H 2 SO 4, 3M HCl ve 3M H 2 SO 4 + 3M HCl(1:1) solutions. Plates which is taking specified times (0, 30, 60, 90, 120 days) were cleaned and then prepared for hardness test. The hardnesses of prepared plates for measurement were determined by BROOKS Model MAT 10/250 machine with Rockwell B hardness method. 3. DISCUSSION AND RESULT: 3.1.Weight Loss Results: The Varying of corrosion rate according to time for 316 Cb austenitic stainless steel in 3M H 2 SO 4, 3M HCl ve 3M H 2 SO 4 + 3M HCl(1:1) solutions is given at Figure M HCl 3 M H 2 SO 4 3 M Mixing Corrosion velocity (m/dd) Time (Day) Fig. 1. Varying of corrosion rate according to time for 316 Cb austenitic stainless steel in 3M H 2 SO 4, 3M HCl ve 3M H 2 SO 4 + 3M HCl(1:1) solutions.

4 According to graph, while corrosion rate of 316 Cb austenitic stainless steels in 3M HCl are shown increasing, in other solutions are not shown clear increasing through protective oxide layer cause passivity. Increasing corrosion rate of specimen in 3M HCl approximately 11 multiple reached 2060 mdd at the end of 90 days. While increasing corrosion rate of 316 Cb plates in 3M HCl solution first 20 days, that changing like oscillation to 90. day due to making protective oxide layer on metal surface then again breaking down its. After 90 days, increasing corrosion rate of specimen due to breakdown protective oxide layer reached 1379 mdd at the end of 120 days. Corrosion rate values of 316 Cb austenitic stainless steel in acid mixing solution changed in constant value range to end of 70 days due to perform protective oxide layer. Corrosion rate increases to 120. day due to at the end of 70 day breaking down protective oxide layer on metal surface. Corrosion rate of 316 Cb austenitic stainless steel in 3M HCl is the highest value because of effecting Cl - ions, that following respectively 3M H 2 SO 4 and 3M H 2 SO 4 + 3M HCl(1:1) acidic solutions (Fig.1) 3.1. Electrochemical Results: Potantiodynamic polarization curves of 316 Cb austenitic stainless steel in HCl, H 2 SO 4 and H 2 SO 4 + HCl(1:1) acidic solutions which have different concentration (0.1M, 1M, 2M, 3M) are given respectively at Figure ,8 0,6 0,4 0,2 0,0 E, V (SCE) -0,2-0,4-0,6-0,8-1,0-1,2-1,4 0.1M HCl 1M HCl 2M HCl 3M HCl -7,0-6,5-6,0-5,5-5,0-4,5-4,0-3,5-3,0-2,5-2,0-1,5-1,0-0,5 0,0 Log i, (A/cm 2 ) Fig. 2. Potentiodynamic polarization curves of 316 Cb austenitic stainless steel in HCl solutions at different concentration (0.1M, 1M, 2M, 3M). Polarization curves in Fig.2 increasing acid concentration of 316 Cb austenitic stainless steel in HCl, corrosion current density rises from µa/cm 2 (0.1M HCl) to 4505 µa/cm 2 (3M HCl). In this, Cl - ions penetrate into metal surface and increasing solution of metal. Also this effect increasing with chloride concentration rises. Furthermore, passivity

5 was not seen at HCl solutions that was seen at solution H 2 SO 4 ve HCl + H 2 SO 4 (1:1) for Cl - ions obstruct to existence oxide at metal surface (from compared Fıg.2 With Fig. 3 and 4.) potential of corrosion increasing with HCl concentration that convert to more negative values. Polarization curves of 316 Cb austenitic stainless steel in 0.1M, 1M, 2M, 3M H 2 SO 4 solutions is given with Fig.3. Passivity was seen due to oxide occurrence on metal surface at all curves. Cathodic reaction at the acidic solutions is hydrogen reduction. Full passivity potential region is V and 0.55 V at 0.1M ve 1M H 2 SO 4 that (-0.25 V)-(0.52 V) at 2M H 2 SO 4 and also (-0.26 V)-(0.5 V) at 3M H 2 SO 4. Full passivity current density is 4.90µA/cm 2 at 0.1M H 2 SO 4 that µa/cm 2 at 1M H 2 SO 4, µa/cm 2 at 2M H 2 SO 4 and also µa/cm 2 at 3M H 2 SO 4. Full passivity potential region and potential of breaking passivity is decreased with Increasing H 2 SO 4 concentration. Full passivity current density is increased with increasing acid concentration. 0,8 0,6 0,4 0,2 0,0 E, V (SCE) -0,2-0,4-0,6-0,8-1,0-1,2-1,4 0.1M H 2 SO 4 1M H 2 SO 4 2M H 2 SO 4 3M H 2 SO 4-7,0-6,5-6,0-5,5-5,0-4,5-4,0-3,5-3,0-2,5-2,0-1,5-1,0-0,5 0,0 Log i, (A/cm 2 ) Fig. 3. Potentiodynamic polarization curves of 316 Cb austenitic stainless steel in H 2 SO 4 solutions which have different concentration (0.1M, 1M, 2M, 3M).

6 0,8 0,6 0,4 0,2 0,0 E, V (SCE) -0,2-0,4-0,6-0,8-1,0-1,2-1,4 0.1M Mixing 1M Mixing 2M Mixing 3M Mixing -7,0-6,5-6,0-5,5-5,0-4,5-4,0-3,5-3,0-2,5-2,0-1,5-1,0-0,5 0,0 Log i, (A/cm 2 ) Fig. 4. Potentiodynamic polarization curves of 316 Cb austenitic stainless steel in HCl+ H 2 SO 4 (1:1) mixing acid solutions at different concentration (0.1M, 1M, 2M, 3M). Polarization curves of 316 Cb austenitic stainless steel in HCl+ H 2 SO 4 (1:1) solutions is given Fig.4. Full passiviation is performed in 1M, 2M and 3M HCl+H 2 SO 4 (1:1) mixing solutions. Flade potential at all concentration is similar values, while acid concentration increases full passivity potential region decreases. While acid concentration increases full passivity current density is increased Hardness Test: In specified time hardness measurements of 316 Cb austenitic stainless steel plates which hold to HCl, H 2 SO 4 and H 2 SO 4 + HCl(1:1) acidic solutions is measured with Rockwell B hardness method. Hardness measuring from five different point for every surfaces are made and average of values which obtained by Rockwell B hardness method was taken. Varying of specimen hardness values according to holding time in solutions is shown at Figure 5. When hardness value of specimen at the beginning is 91 R B, this value decreased according to exposing time in corrosive media. The lowest values for 3M HCl solution and the highest values for 3M H 2 SO 4 + 3M HCl (1:1) solution were obtained. In this manner, hardness and also according to this mechanical strength decreases while corrosion effect increases.

7 Fig. 5.Varying of 316 Cb austenitic stainless steel hardness values(r B ) according to holding time in different acidic solutions Elemental Analyse Elemental analyse of 316 Cb austenitic stainless steel which exposed to 120 days in acidic solutions was made. According to analyse results, when chrome content of specimen was determined as at the beginning, this content was determined as for 3M HCl, was determined as for 3M H 2 SO 4, is determined as for 3M H 2 SO 4 + 3M HCl(1:1) at the end of 120 days. This decrease of chrome content shows that pitting corrosion occur as preferential on the grain boundary. 4.CONCLUSIONS Corrosion Rate of 316 Cb austenitic stainless steel is increased according to weight loss method. The most increase is determined in 3M HCl solution. Full passivity in HCl solutions was not observed at the potentiodynamic polarization measurements. According to hardness tests, hardness of 316 Cb austenitic stainless steel decreases while corrosion effect increases. Decrease in chrome content was recorded from elemental analyse results.

8 REFERENCES 1. Metals Handbook, Ninth Edition, Vol. 3, Properties and Selection: Stainless Steels, Tool Materials and Special Purpose Metals, ASM, 3-104, Metals Handbook, Second Edition, J.R. Davis, Davis& Associates, ASM, , A. Pardo, M.C. Merino, M. Carboneras, A.E. Coy, R. Arrabal, Pitting corrosion behaviour of austenitic stainless steels with Cu and Sn additions, Corrosion Science, Volume 49, Issue 2, , February G. O. Ilevbare, G. T. Burstein, The inhibition of pitting corrosion of stainless steels by chromate and molybdate ions, Corrosion Science, Volume 45, Issue 7, , July A. Pardo, M.C. Merino, A.E. Coy, F. Viejo, R. Arrabal and E. Matykina, Pitting corrosion behaviour of austenitic stainless steels-combining effects of Mn and Mo additions, Corrosion Science, Volume 50, Issue 6, , June Wegst, C.W., Stahlschlüssel, Werlag Stahlschlüssel Wegst GMBH, Üneri, S., Korozyon ve Önlenmesi, Korozyon Derneği-ANKARA, 1998.