Effect of dietary spices on the pitting behavior of ...

Materials Letters 65 (2011) 2241–2244

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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t

Effect of dietary spices on the pitting behavior of stainless steel orthodontic bands Neelima Mahato a,⁎, Manu R. Sharma b, T.P. Chaturvedi b, M.M. Singh a a b

Department of Applied Chemistry, Institute of Technology, BHU Varanasi-5, India Division of Orthodontics, Faculty of Dental Sciences, Institute of Medical Sciences, BHU Varanasi-5, India

a r t i c l e

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Article history: Received 23 February 2011 Accepted 7 April 2011 Available online 14 April 2011 Keywords: Corrosion Metals and alloys Surfaces

a b s t r a c t A comparative study of the effect of commonly used dietary spices on the pitting behavior of orthodontic bands made of AISI 304L was carried out using electrochemical experiments. Corrosion parameters were measured after immersing the steel for 24 h in artificial saliva containing spice extract. Cinnamon, cumin, red chili and turmeric caused pitting even in the absence of chloride ions. On adding NaCl, red chili, cinnamon and fenugreek caused severe pitting and coriander was found to inhibit pitting. Micrographs of the specimens show formation of hemispherical pits covered with remnants of passive film. © 2011 Elsevier B.V. All rights reserved.

1. Introduction Austenitic stainless steels belonging to 300 series are commonly used as metallic parts of orthodontic appliances due to their biocompatibility, corrosion resistance, favorable mechanical properties and ease of workability [1,2]. Appliances adapted around the tooth continuously bathe in saliva which is a complex mixture of dissolved electrolytes and its salinity is equivalent to sea water [3]. Moreover, a diet rich in NaCl and acidic spices provides a regular supply of corrosive agents. A spice is a dried seed, fruit, root, bark, leaf, or vegetative substance used in significant quantities as food additive for the purposes of enhancing flavor and color, as a preservative or as a medicine. For our investigation, we selected ten most commonly used spices. These were turmeric (Curcuma longa), black pepper (Piper nigrum), red chili (Capsicum annuum), clove (Syzygium aromaticum), coriander (Coriandrum sativum), cumin (Cuminum cyminum), fenugreek (Trigonella foenumgraecum), fennel (Foeniculum vulgaris), cinnamon bark (Cinnamomum verum Presl.) and bishop's weed (Trachyspermum ammi). The objective is to perform a detailed investigation of the effects of spices (corrosive or inhibitive) on the steel surface using electrochemical and surface analytical techniques.

2. Experimental 2.1. Preparation of test solution All the chemicals were procured from Merck, India Ltd. Artificial saliva (AS) was prepared in double distilled water with the following composition in g/L: K2HPO4(0.340), NaH2PO4.2H2O (0.445), KHCO3 ⁎ Corresponding author. Tel.: + 91 9984760112; fax: + 91 5422368428. E-mail address: [email protected] (N. Mahato). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.04.034

(1.500), NaCl (0.585), MgCl2.6H2O (0.031), citric acid (0.032) and CaCl2 (0.012). Spices were crushed separately and extract of each spice was prepared by refluxing a quantity of 50 g in 250 ml distilled water for an hour. The contents were filtered by Celite 545 on silica gel 20–120 mesh using a vacuum pump to get a clear extract. Test solutions containing 20% extract by volume in AS were prepared. An additional set of test solutions containing 1% NaCl was also prepared. 2.2. Materials and methods Commonly used orthodontic band made of AISI 304L with composition Cr (18.34%), Ni (10.48%), Mn (0.88%), Si (0.84%), C (0.02%) and Fe (67.19%) was taken for the study. The band has a shiny and smooth front surface and a rough rear surface. For the adaptation (during clinical processes), the front side is exposed while the rear side is cemented along the tooth. The rear side being rough holds the cement strongly. Accordingly, the electrodes were prepared. Specimens of dimensions (3× 0.44 × 0.012 cm3) with 1 cm exposed length on the front side were used as working electrodes. The cell consisted of a platinum counter electrode and Ag/AgCl (Orion, Beverly, MA, USA) as the reference electrode. The working electrode was immersed in 25 ml test solution for 24 h and the electrochemical experiments were carried out at 37 °C using an Electrochemical Analyzer (CHI 604 C). Polarization studies were carried out in the range (−0.4 to 1.0 V) at a scan rate of 1 mV s−1. All the experiments were performed for four times and the mean values of the obtained results are recorded in Table 1. 3. Results and discussion 3.1. Electrochemical tests The individual effects of spice extracts on the polarization behavior of stainless steel in AS are shown in Fig. 1a–d. The curves show small

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N. Mahato et al. / Materials Letters 65 (2011) 2241–2244

Table 1 Corrosion parameters of AISI 304L in AS containing spice and NaCl (polarization studies). Medium

βa (V/d)

βc (V/d)

Ecorr (V)

Icorr (μA/cm2)

Epit (V)

pH (before)

pH (after)

AS AS + salt Bishop's weed Bishop's weed+salt Black pepper Black pepper+salt Cinnamon Cinnamon + salt Cloves Cloves + salt Coriander Coriander + salt Cumin Cumin + salt Fennel Fennel + salt Fenugreek Fenugreek + salt Red Chili Red Chili + salt Turmeric Turmeric + salt

0.28 0.58 0.16 0.34 0.23 0.28 0.22 0.47 0.33 0.30 0.20 0.22 0.28 0.19 0.41 0.18 0.28 0.28 0.23 0.27 0.18 0.22

0.07 0.08 0.11 0.08 0.12 0.08 0.08 0.06 0.07 0.08 0.13 0.12 0.11 0.13 0.07 0.11 0.12 0.11 0.08 0.07 0.08 0.11

−0.21 −0.22 −0.28 −0.31 −0.30 −0.26 −0.33 −0.29 −0.34 −0.25 −0.16 −0.21 −0.26 −0.08 −0.28 −0.44 −0.30 −0.28 −0.26 −0.24 −0.31 −0.21

0.537 0.993 0.106 0.114 0.598 0.486 0.377 0.099 0.029 0.238 0.088 0.045 0.267 1.064 0.968 0.143 0.826 0.719 0.360 0.377 0.026 0.286

Absent 0.59 Absent 0.51 Absent 0.59 0.97 0.35 Absent 0.74 Absent Absent 0.75 0.80 Absent 0.50 Absent 0.46 1.05 0.39 0.93 0.74

7.4 7.41 6.91 6.93 6.66 6.68 6.02 6.01 5.85 5.84 5.11 5.13 5.41 5.43 6.43 6.45 6.44 6.43 5.93 5.96 6.12 6.14

7.67 7.53 6.99 7.25 7.09 6.98 8.84 6.87 5.95 6.18 5.62 5.28 5.74 6.08 7.79 6.82 7.51 6.65 6.96 6.62 6.27 6.36

active region followed by a well defined passive region for most of the spices. In cumin, bishop's weed and cloves, the plots exhibit a hump in the passive region attributed to repassivation process. The Ecorr values shifted towards more negative potential except coriander indicating an early onset of anodic processes in the presence of spices. Also, the passive current density was found higher compared to that in AS. The current density in the passive region of the steel in electrolyte is related to the charge transfer resistance through the passive film. A lower current density represents higher charge transfer resistance through the passive film and a better protection to the surface. Addition of spice to the AS solution decreases this resistance and increases the current

density. Based on the Icorr values, the order of aggressiveness of the spices can be given as: fennelN fenugreek N black pepper N ASN cinnamon N red chili N cumin N bishop's weed N coriander N cloves N turmeric. Although the Icorr values for fennel, fenugreek and black pepper are high, but the specimens collected after the experiment did not show any pitting. On the contrary, the specimens collected from cumin, turmeric, red chili and cinnamon test solutions, exhibited tiny pits. A sharp Epit is also apparent on the polarization curves. On adding 1% NaCl to AS, Icorr increases from 0.537 to 0.993 μA/cm2. This is due to an increase in the conductivity of the medium. But, this is not the case with every test solution containing NaCl. Also, we get a different order: cumin N AS N fenugreek N black pepper N red chili N turmeric N cloves N fennel bishop's weedN cinnamon N coriander. This is due to the complex composition of spices. Pitting occurred in all the cases where NaCl was added to the test solution except coriander. Chloride ion combines with metal ion to form metal chloride and facilitates dissolution of the latter. Once initiated, the pitting propagates in an autocatalytic manner. Since pitting is mainly responsible for the damage and failure of the material, Epit values give clearer picture. Based on the Epit values the order of aggressiveness is cinnamon N red chili N fenugreek N fennel N bishop's weed N black pepper ~ AS N cloves N cumin N turmeric N coriander. The polarization curve of steel in AS containing cinnamon test solution exhibits several spikes at lower potentials corresponding to the formation of metastable pits which repassivated soon after their formation. Cinnamon was found most detrimental to the surface. The change in pH values of the test solution before and after the completion of electrochemical experiments was found significant (Table 1). With cumin, Epit occurred at 0.75 V and 0.8 V in the presence and absence of NaCl respectively. Similarly, Epit in turmeric appeared at 0.93 V and 0.74 V. Cumin and turmeric on one hand aggravate pitting in the absence of salt but on the other hand, both retard pitting in its presence (i.e., Epit occurs at higher potential). Cathodic Tafel constant or βc values fall within two different ranges indicating either of the two cathodic reactions. βc values near 120 mV/d involving four electron transfer correspond to oxygen reduction and near 60 mV/d involving two electron transfer correspond to hydrogen

Fig. 1. Overlay of polarization curves of AISI 304L in AS containing (a, b) spice extract and (c, d) spice extract with 1% NaCl.


a

b

c

d

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Fig. 2. Micrographs of the front side (a) covered pits, (b) single pit with remnants of passive film, (c) severe pitting and (d) deformed edge line and material loss.

gas evolution. The βa values are scattered owing to variation in the nature of anodic reactions in different environments (Table 1). Unlike our findings, black pepper, cumin and fenugreek mucilage have been reported as corrosion inhibitors [4,5]. This might be due to the method of filtration we adopted to collect the extract which possibly removed the mucilage and similar protective constituents. The method was chosen in order to get a clear and transparent solution. Unless it was done, the test solutions would develop foul smell in 24 h due to the growth of microorganisms. This problem was eliminated when transparent extract was used. Results suggest that

coriander, turmeric and cumin may be explored further to develop eco friendly inhibitors like fenugreek and black pepper [6]. 3.2. Surface analysis The micrographs (JEOL SEM: JSM-5600) of the corroded surfaces exhibit pits on both sides, front as well as rear (Figs. 2 and 3) even though the latter was covered. Since the specimen is very thin, the electrolyte got percolated through the pits to reach the other side. Pitting pattern was found similar in all the experiments. Pits are

a

b

c

d

Fig. 3. Micrographs of the rear side (a) randomly scattered pits, (b) single pit, (c) and (d) severe pitting.

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covered by film like structures (Fig. 2a) and the magnification of the same (Fig. 2b) shows remnants of the adhered film. This indicates that the adherence of the passive film to the surface is very strong. Severe pitting and deformation of the edge line due to the material loss is shown in Figs. 2c and d. Figs. 3a–d show pitting pattern on the rear side. Pitting on the rear side was found more severe than the front side.

anodic reactions vary according to the medium. Pitting on the rear side is severe compared to the front side.

Acknowledgements Authors acknowledge Dr. DM Phase and Er VK Ahire, IUC-CSR, DAE, Indore, India for SEM facility and UGC, India for financial support.

4. Conclusions Pitting behavior of the steel is influenced by spices. Cinnamon, red chili, cumin and turmeric induce pitting even in the absence of NaCl. On adding NaCl, all experiments exhibit pitting except coriander. Hence, coriander acts as an efficient inhibitor. Cinnamon is rated most detrimental to the surface. Pitting process is dependent on pH of the test solution. Tafel constants indicate the reduction reaction to be either hydrogen gas evolution or oxygen reduction, whereas the

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