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Electrochimica Acta 54 (2008) 74–79

Contents lists available at ScienceDirect

Electrochimica Acta

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iocorrosion of stainless steel grade 304L (SS304L) in sugar cane juice

. Durmooa,b, C. Richarda,∗, G. Berangera, Y. Moutiab

Université de Technologie de Compiègne (UTC), Laboratoire Roberval, UMR CNRS 6253, BP 20529, 60205 Compiègne cedex, FranceMauritius Sugar Industry Research Institute (MSIRI) and University of Mauritius (UoM), Le Reduit, Mauritius

r t i c l e i n f o

rticle history:eceived 7 November 2007eceived in revised form 6 May 2008ccepted 10 June 2008vailable online 24 June 2008

eywords:ane sugar juiceustenitic stainless steelittingiocorrosioniofilm

a b s t r a c t

The main objective of this research study is to assess the behaviour of stainless steel AISI 304L in contactwith sugar cane juice. Stainless steel grade 304L is largely used in the sugar cane industry but the acidicnature of sugar cane juice pose a serious challenge in maintaining the life span of vital components.

Sugar cane juice is acid having a pH value of around 5.6 at extraction. This acidic property is accountedfor by the presence of a variety of acids namely: aconitic, citric, malic, oxalic, glycolic, mesaonic, tartic,succinic, fumaric and syringic in sugar cane juice.

In addition to these acids there are approximately 50 different kinds of microorganisms present in thegreen cane and which are very active. These microorganisms will act as a contributor to a rather quickdrop in pH (pH∼3.1) of the sugar cane juice once extracted.

The more so, several minerals like water, salt, sulphate and silica are also present throughout the processline and are other contributor in the surface degradation wear mechanism.

Faced with all these adverse elements, it is therefore fundamental to investigate thoroughly in the wearcorrosion mechanism and biocorrosion on stainless steel grade 304L.

To evaluate the mass loss, several corrosion experimentations were carried with the help of a poten-tiostat both in a sterilized juice and none sterilized juice. From these experimentations, it has been notedthat corrosion was present on the surface of the disc (SS304L) in contact with none sterilized juice in theform of pitting while no corrosion wear was observed in the case of sterilized juice.

The presence of biofilm was also observed on the sample disc surface. Biofilm formed on the surface ofthe sample disc was transferred to a potato dextrose agar (PDA) plate by stamping the disc in a circularsequence on the plate in sterilized condition in view to measure the density and resistance of the film.It was noted through this stamping protocol that the biofilm was very resistant due to the fact that after12 stamping time we still observe the trace of the biofilm on the PDA plate. Microorganisms present inthe biofilm have growth on the PDA plate and isolation of each colony was carried out in view of their

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. Introduction

Stainless steels of austenitic type are well known in its capacityo preserve the structure at ordinary temperature and are usu-lly called 18-8 or 18-10 according to the percentage of chromiumnd nickel. These stainless steels grades are heavily used in theugar industry and agro industry. Their relative facility in fabrica-ion and their aptitude of deformation and also their good corrosionesistance are so many reasons of using them in various fields,

articularly in the sugar industry [1].

Indeed, the sugar industry installations can be particularly sub-ect to corrosion phenomena. At every step along the process linefrom crushing to crystallisation of sugar) the composition of the

∗ Corresponding author. Tel.: +33 3 44 23 46 17; fax: +33 3 44 23 49 84.

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013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2008.06.028

© 2008 Elsevier Ltd. All rights reserved.

ane juice tends to favour corrosion to many types of equipment.he types of corrosion recorded are of wide range (with no prefer-nce order): bacterial corrosion, stress corrosion, corrosion undereposit, intergranular corrosion, fatigue corrosion and pitting cor-osion. All of these corrosion phenomena are also present in theeet sugar industry [2].

The first work mentioning corrosion phenomenon in the sugarndustry (beet sugar industry) in France goes back to the years950, more particularly at the stages process of the diffusionnd evaporation, critical step in the manufacture of sugar (beetugar).

Among the most widespread forms of corrosion, the localiseditting corrosion is certainly the most frequent.

Sugar cane juice is a complex medium due to the continuousermentation process occurring once extracted and as well as theresence of others mechanisms in the juice: such as chemical,

S. Durmoo et al. / Electrochimica Acta 54 (2008) 74–79 75

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wjvextraction (2 bars) of cane chips using a French press model.Throughout the duration of these tests, corrosion potential wasmeasured against time. The polarisation curves were also recordedin the range of ±1500 mV/SSE for a sweep rate of 2.5 mV s−1. The

ig. 1. Schematic diagram of the jar device used to verify the growth of biofilm onur specimen at the laboratory of MSIRI.

icroorganism (bacteria, yeast and fungi), abrasion due to theresence of suspended solids along the process line, etc.

To assess each of these mechanisms on the behaviour of stainlessteel grade 304L, different kind of measurements and techniquesere required: electrochemical measurements for the understand-

ng of the corrosion phenomena, the transfer of biofilm formed onhe samples to identify bacterias and yeast, the use of tribocorro-ion device to simulate erosion corrosion and abrasion corrosionhenomenon.

Throughout this research work, substantial amount of resultsave been compiled on the behaviour of SS304L surface in contactith the sugar cane juice for the different degradation mechanisms

uniform corrosion, localised pitting corrosion, biocorrosion, androsion–abrasion corrosion). However, in this article only the bio-orrosion and more particularly pitting results are presented. Someesults for uniform corrosion are also given.

. Methodology

In this research work, while studying the corrosion behaviourf specimens (SS304L) in contact with the sugar cane juice, theresence of biofilm was observed on the exposed surface and morearticularly in the region where pitting corrosion was occurred. Itas therefore essential to transfer the biofilm to a medium thatromoted the growth of these microorganisms in order to iden-ify (and observe) their morphology and aid in their identification.he potato dextrose agar (PDA) medium was chosen as it is a richedium in terms of nutrient composition and has the potential of

romoting the rapid growth of microorganisms. It is nonselectivend will therefore allow a large array of microorganisms to grow.his investigation of the biofilm and the identification of microor-anisms were carried out at the Mauritius Sugar Industry Researchnstitute (MSIRI).

Fresh and sterilized sugar cane juice was poured separately in00 ml sterile flask and sealed with a cotton plug. The specimenSS304L) of polished surface of grain 800 �m having a diameterf 10 mm was first cleaned with 70% alcohol followed by severalashes in distilled water prior to sterilization at 119 ◦C for 35 min

n order to eliminate the presence of foreign matter, bacteria andther microorganisms on the surface. It was then placed in a verticallane and immersed in the juice solution at a specific depth (15 cm)o as to standardise experimental conditions across every experi-

ent. The purpose of placing the specimen in a vertical plane in the

olution was to ascertain that adhesion of microorganisms was dueo their ability to form a biofilm against the natural downward forcef attraction due to gravity. Fig. 1 illustrates the principle describedbove and the photo in Fig. 2 shows the specimen before and after

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ig. 2. Photo showing the specimen in the flask, before and after pouring fresh juice.

etting in contact (i.e. flask and specimen) with fresh sugar caneuice.

The flasks were then placed on an arbitral shaker (Gallenghampodel) set at 100 rpm for 2 days at a constant temperature of 25 ◦C

o allow a continuous mixing of the juice in view to keep the solu-ion homogeneous (Fig. 3).

After a period of incubation of 2 days, the specimen was removedrom the flask under the laminar flow cabinet to prevent exter-al contamination. It was then rinsed in sterile distilled water andreated in an ultrasonic bath (Branson model) for 5 min. This wasone in order to ensure that only a strongly adhered biofilm wouldemain on the specimen.

The technique of transferring the biofilm from the specimen tohe culture medium was carried out according to a predefined pro-ocol which consisted of imprinting the specimen on the PDA platen a clockwise direction under sterilize atmosphere as illustrated inig. 4. This imprinting procedure was repeated four times on a PDAetri dish. In order to estimate the extent and tenacity with whichhe biofilm was adhering to the specimen, a total of 12 prints were

ade with a single specimen on a total of three PDA Petri dishes.To evaluate the impact of corrosion rate, electrochemical tests

ere performed on fresh, aged and sterilized sugar cane juiceointly at the University of Mauritius and at the laboratory Rober-al (UTC). The juice was obtained from the MSIRI by high-pressure

ig. 3. Fresh juice in sterilised jar and placed on an arbitral shaker at constantemperature of 25 ◦C.

76 S. Durmoo et al. / Electrochimica Acta 54 (2008) 74–79

Fig. 4. Schematic diagram showing the technique of transferring biofilm from spec-imen of SS304L to the PDA medium in sterilised condition.

Table 1Definition of electrochemical parameters used to study corrosion, erosion corrosionand abrasion corrosion of stainless steel 304L in contact with sugar cane juice

Item Description Parameters

WE Working electrode Area 0.73 cm2 forcorrosion and 0.58 cm2

for erosioncorrosion/abrasioncorrosion

RE Reference electrode Hg/Hg2SO4

(SSE = +408 mV/SCE at25 ◦C)

AE Auxiliary electrode Platinum sheetMedium Sugar cane juice pH range 5.6–3.4 for

corrosion and pH 5 forerosioncorrosion/abrasion

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Fig. 5. Evolution of the extracted sugar cane juice pH (cane category M695/69) withtime at ambient temperature.

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corrosionApplied potential ±1500 mVSweep rate 2.5 mV/s

eference electrode (RE) used for all electrochemical tests (studyf corrosion, erosion corrosion and abrasion corrosion) was ang/Hg2SO4 reference electrode (SSE) intended for the applicationshere chloride could interfere with the measurement. It is noted

hat the reference potential at 25 ◦C for an electrode SSE filled withaturated K2SO4 solution is approximately +408 mV with respect tosaturated calomel electrode (SSE = +408 mV/SCE). The test speci-en, of cylindrical shape, was used as the working electrode WE.platinum sheet was used as our auxiliary electrode AE. All these

ata are summarised in Table 1. Potentiostat used in this researchork was a Volta Lab PGP 201.

It is also important to have in mind that the acid content ofugar cane juice depends on its geological place and its variety. Inhis study, the commercial variety M695/69 was used for the wholetudy and the extracted juice was supplied by the MSIRI.

. Results and discussion

Knowing that the sugar cane juice was a complex and an evolu-ionary medium, it was essential to characterize its variation with

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able 2ummary of thickness loss of results of SS304L in contact with sugar cane juice of pH ran

ase Type of test pH Ecorrosion

Corrosion: fresh sugar cane juice 5 −445Corrosion: aged sugar cane juice 3.6 −575Corrosion: sterilized juice 5.6 −500

ig. 6. Semi-logarithmic scale representing overlapped polarization curves (−1500o +1000 mV) with a sweep rate 2.5 mV/s for some pH values of sugar cane juiceolution (fresh and aged) and a control sterilised juice at ambient temperature.

ime by measuring its pH evolution. Fig. 5 shows a typical pH evo-ution of the sugar cane juice extracted from variety M695/69 withime. Fresh sugar cane juice has a pH value of around 5.4; this valuend this curve may vary depending on the variety of sugar cane, itsrowth stage and also on the geographical location and environ-ental conditions. Step by step, the pH value dropped to about 3.1.

acteria, yeast and fungi are among the three main elements in thisomplex microorganism system, responsible for this drop in pH aslready stated in some published papers [3–6].

To assess the corrosion impact on the surface of SS304L, elec-rochemical experimentation was carried out with sugar cane juicef variety M 695/69 of different pH value at ambient temperaturend also with sterilized juice to identify the effect of bacteria. Theesults are presented in Fig. 6. It is clearly demonstrated from thevolution of the corrosion potential (Ecorrosion) with pH that when

gainst corrosion. With sterilized juice the pH value remains stable;his is due to the fact that all microorganisms present in the solu-ion have been eliminated and in terms of corrosion the specimenS304L exhibits very small surface degradation.

ging from 5.6 to 3.6 (corrosion scenario)

(mV/SSE) Icorrosion (�A/cm2) Thickness loss �e (mm/year)

8.38 0.09812.40 0.1450.50 0.006

S. Durmoo et al. / Electrochimica Acta 54 (2008) 74–79 77

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ig. 7. Observation (ESEM) pitting corrosion on SS304L surface in contact with freshf 2.5 mV s−1 at ambient temperature.

Taking into account the evolution of the electrochemical curvest can be emphasized that a modification of uniform corrosion canccur. From curves of Fig. 6, the uniform corrosion rate in term ofhickness loss was estimated for corrosion scenario of stainless steelS304L in contact with sugar cane juice of pH ranging from 5.6 to 3.6t ambient temperature (Table 2). The weight loss measurementsere not significant.

From Table 2, it is clearly shown that while pH drop the rate oforrosion increases in agreement with different results obtainedn various media [3]. Let us note that insignificant metal loss isbserved in a sterilized medium (case C).

The observation of the metal surface (under environmentalcanning electron microscope (ESEM)) revealed the occurrence ofitting phenomena on the sample surface while in contact withhe fresh juice of pH 5.6. Sugar cane juice contains a high level ofhloride (approximately 1300 ppm) [7,8]. Chlorides tend to becomeoncentrated with sugar processing [9]. Nevertheless, no pitting

as observed when same experimentation was performed with

terilized juice pH 5.6. Fig. 7 shows pitting corrosion and presence ofiofilm around the pitting cavities after a polarisation test of −1500o +1500 mV, at a sweep rate of 2.5 mV s−1. Under same experimen-

ig. 8. Absence of corrosion is noted on the surface of SS304L after a polarisationest.

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r cane juice of pH 5 under polarisation test (−1500 to +1000 mV) with a sweep rate

al condition but with a sterilized medium (Fig. 8) no presence ofiofilm on the surface or pitting corrosion was observed. The pres-nce of some crystal sugar which is due to the concentration of theuice solution was only detected at a high magnification (8000×).

The fact that the presence of biofilm was observed on the sur-ace especially around the pitting perimeter and even in the pittingole then it is essential to investigate the presence of the biofilmn the surface. Through the printing protocol as explained earliern Section 2, the biofilm was transferred from the SS304L speci-

en surface to the PDA plate and was maintained at 30 ◦C in anncubator. The experiments have been carried out with two sep-rate samples: one in contact with fresh juice and the other withterilized juice both during a period of 1 day. There is clear dis-inct growth of microorganisms after 1 day of incubation periods shown in Fig. 9(a) which confirms the presence of adhesion ofiofilm on the SS304L specimen surface in contact with fresh juiceedium.Contrary to Fig. 9(a) there is no growth of microorganisms even

fter 3 days of incubation period as shown in Fig. 9(b) where theane juice used was sterilised and therefore confirms the predictionarlier formulated.

To verify the adhesion of the biofilm adhesion growing on the

urface of the specimen when fresh juice was used, the same pro-edure was repeated 12 times in total on three separate PDA platess shown in Fig. 10. It is clearly observed that even after 12 imprintshere is still residue of biofilm present on the surface of the sample.

ig. 9. (a) Observation of microorganism growth on PDA medium for SS304L samplen contact with fresh juice after an incubation period of 1 day at 30 ◦C and (b) no

icroorganism growth on PDA medium for SS304L in contact with sterilized juicefter an incubation period of 3 days at 30 ◦C.

78 S. Durmoo et al. / Electrochimica Acta 54 (2008) 74–79

F the sample was imprinted four times on the first PDA medium under sterilized condition,( ndition and (c) four other more imprinting was made with the same sample in sterilizedc

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ig. 10. Verification of the adhesion capacity of the biofilm on the sample SS304L: (a)b) four more imprinting was made from the same sample again under sterilised coondition, and all three PDA plating was kept in an incubator for 1 day at 30 ◦C.

From the imprints on PDA plates several types of colonies wererowing and the idea was to separate each colony without contam-nation to obtain pure colonies and this technique is called isolationf colonies. After repeating this exercise two to three times, a pureolony is obtained as shown in Fig. 11. Once each colony is isolated,he next step is to proceed to its identification and lyophilisation.owever, with experience yeast and bacteria can be easily distin-uished by observing their shape and morphology with the aid ofmedium to high power lens of a light microscope.

During the phase of purification of colony (having well sepa-ated colonies), secretion of pigmentation was observed in certainases on PDA plate after 3 days of incubation as shown in Fig. 12. Ateast two types of colonies can be observed from this figure, secret-ng each a specific acid content which can be distinguished by theolour of the pigmentation on the plate.

Finally to put results beyond doubt concerning the adhesion ofhe biofilm on the surface of SS304L, is to confirm visually andmpirically the same specimen with the help of an ESEM.

The same precautionary procedures as outlaid earlier in theransfer of the biofilm to the PDA plate were applied on the sam-le in contact with the sugar cane juice of variety M 695/69. In facthe specimen was removed after 1 day of contact with the juice,ashed with sterilized de-mineralised water and then observednder ESEM.

Fig. 13 under a magnification factor of 1000 times; reveal the

resence of microorganism adhering to the SS304L surface.

Different clustering colonies grouped together can be observednd among which it is easy to distinguish yeast species whichave the particular characteristics of their bright and round sur-

aces.

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ig. 11. Purification process in view to separate each colony present in the biofilm withounder sterilized condition and spread on this PDA plating and kept growth in incubatorondition and spread again on a new PDA plate and kept growth in incubator for anothern incubator at 30 ◦C.

ig. 12. Secretion of pigmentation (red, blue and green colour) was observed on theDA plating after 3 days of incubation at 30 ◦C. (For interpretation of the referenceso colour in this figure legend, the reader is referred to the web version of the article.)

Fig. 14 which has been enlarged to higher magnification showshe texture and arrangement of species growing on the surface ofhe SS304L. It reveals in fact that the surface density of bacteriaovering a specimen surface is very high.

A single bacterium observed at higher magnification (Fig. 15)

eveals the presence of flagella which enable the bacterium to moverom one place to another within on the biofilm; this possibility of

igration and consequently the extension of the biofilm across thepecimen surface can explain the coating of the surface observedn Fig. 14.

t contamination: (a) a particular colony was collected from the imprinting PDA (a)for 1 day at 30 ◦C, (b) from plating (a) a small zone was collected under sterilisedday at 30 ◦C and (c) same procedure in (b) was repeated again and kept for 1 day in

S. Durmoo et al. / Electrochimic

Fig. 13. Bacteria and yeast colonies growing on the surface of SS304L observed underESEM.

Fig. 14. High surface density population growth of a colony on the surface of SS304Lobserved under ESEM.

Fig. 15. Observation of a bacterium with its flagella on the surface of SS304L underESEM.

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[[5] W.L. Owen, Microbiology of Sugar Manufacture and Refining, p. 385.[6] P.C. Irvine, Sugar Bull. 42 (23) (1964) 317.[7] B. Baroux, C. Lemaître, F. Dabosi, in: P. Lacombe, B. Baroux, G. Béranger (Eds.),

Les aciers inoxydables, Ed. Sci., Les Editions de Physique, 1990.[8] G.A. Cash, D.P. Schweinsberg, G.A. Hope, Corros. Sci. 30 (6, 7) (1990) 543.[9] G.A. Cash, G.A. Hope, D.P. Schweinsberg, Corros. Sci. 33 (5) (1992) 751.

a Acta 54 (2008) 74–79 79

It is important to note that pitting observed is of the diameterize of 20 �m in average, which can be considered as quite big. Thenhe possibility of corollas corrosion phenomena as described in theiterature review [2] may have been produced here.

. Conclusion

The findings of this study have shown that various forms of cor-osion were occurring when the SS304L specimens were in contactith sugar cane juices as

(i) Uniform corrosion. Nevertheless, the presence of this kind of cor-rosion was low and was depending on pH of the juices. The pHdropped with time.

ii) Pitting corrosion. Pitting phenomena generally occurred in freshor aged juice but not in sterilized juice. This corrosion grewunder the passive film due to a confinement and then gavesmall craters on the surface of the specimens. These phenom-ena arise from the complexity of the media with the presence ofchlorides and are also depending on time with a concomitantdecrease of pH. The question that can be raised: this pittingcorrosion is it chemical and electrochemical result or a bio-chemical result or a coupling of the two mechanisms? Indeed,bacteria can locally affect by their metabolism the passivefilm.

ther types of corrosion were observed on surface of SS304L usedn sugar cane industry particularly as tribocorrosion phenomenaabrasion–erosion corrosion). These results will be reported subse-uently.

From a technological point of view, the factor time is anmportant parameter and it is convenient to reduce the time ofontact with a given juice in order to prevent the effects of itsging.

It is worth mentioning that the results presented are not exhaus-ive but comprehensive enough to understand the behaviour and

echanism of the biofilm in the degradation of stainless steel grade04L.

cknowledgements

This work was supported by the AUF (Agence Universitaire de larancophonie) under grant of Dr. Seeva Durmoo. The work was alsossociated to the Biocorys programme and the authors are indebtedo Dr. Damien Féron from CEA Saclay (France), scientific supervisorf the Biocorys programme for his very helpful advice.

eferences

1] F. Dupoiron, M. Verneau, in: F. Dabosi, G. Béranger, B. Baroux (Eds.), CorrosionLocalisée, Ed. Sci., Les éditions de Physique, 1994, p. 605.

2] C. Poiré, Phénomènes de corrosion dans l‘industrie sucrière: comportement del‘acier inoxydable à 17% de chrome dans les évaporateurs, thèse UTC, 1986.

3] M.A. Clarke, Cane Hand Book, Sugar Processing Research INC., New OrleansLouisiana, p. 18.

4] W.L. Owen, Facts About Sugar 18 (1925) 566.