Effect of Ascorbic Acid on Hydrogen Peroxide ... - Springer Link

ISSN 20702051, Protection of Metals and Physical Chemistry of Surfaces, 2014, Vol. 50, No. 3, pp. 384–390. © Pleiades Publishing, Ltd., 2014.

NEW SUBSTANCES, MATERIALS, AND COATINGS

Effect of Ascorbic Acid on Hydrogen Peroxide Decomposition into an Environmentally Friendly Mixture for Pickling of 316L Stainless Steel1 L. Narváez Hernández*, J. M. Miranda Vidales, and E. E. MartínezFlores a

Facultad del Hábitat, UASLP, Niño Artillero 150, 78290, San Luis Potosí, México de Metalurgia, UASLP, Sierra Leona 550, 78210, San Luis Potosí, México c Facultad de Ingeniería, UASLP, Dr. Manuel Nava 8, 78290, San Luis Potosí, México email: *[email protected] bInstituto

Received April 16, 2013

Abstract—One of main disadvantages of using environmentally friendly chemical mixtures for pickling, is stabilization of hydrogen peroxide (H2O2), due to its decomposition is affected by the presence of metal ions. In previous studies, ptoluenesulfonic acid (C7H10O4S) was used as a hydrogen peroxide stabilizer; however, benzene rings in its structure have a certain level of toxicity. In this work, ascorbic acid (C6H8O6) was tested as a stabilizer agent for H2O2 in a pickling mixture composed by sulfuric acid (H2SO4) and hydrofluoric acid (HF)–H2O2, this mixture was tested on 316L stainless steel (SS). Decomposition of H2O2 in pickling solu tion was evaluated for different ferric ions concentration between 0 to 40 g/L and at different temperatures from 25° to 60°C. Pickling rates at 25°C for 316L SS were 26873 mg/dm2 and 27799 mg/dm2 using ascorbic acid and ptoluenesulfonic acid, respectively. Ascorbic acid has a positive influence on stabilization processes of H2O2. DOI: 10.1134/S2070205114030058 1

INTRODUCTION

Oxide layers forming on steel surface are comprised of iron, chromium, nickel and other associated metal oxides, these layers must be removed by a technique known as pickling. Stainless steel pickling includes mechanical, electrochemical and chemical successive operations. Usually, stainless steel is pickled using a combination of nitric and hydrofluoric acids, this reaction results in the emission of nitrous gases, NOx (nitric oxide and nitrogen dioxide) [1]. These gases cause air and water pollution, and also are toxic to operators [2].

acts as an oxidizing agent in pickling solutions forming water. However, during pickling processes metals ions are formed, they pass into the acidic solution and then catalyze decomposition of hydrogen peroxide which reduces its efficacy and stability. Decomposition mechanism of H2O2 in presence of iron ions is shown in Eqs. (1) to (5), are generated dur ing these processes hydroxyl radical (OH•), perhy • – droxyl radical (H O 2 ), superoxide radical anion ( O 2•), –

and hydroperoxide anion (H O 2 ) [6, 7].

Indeed, environmental concern is an important issue in industrial operations along with quality and cost. Therefore, there is an increasing interest in replacing traditional blends with nitric acidfree pickling mixtures which are environmentally friendly [3–5]. Such mixtures contain at least one acid and one oxi dizing agent.

H2O2 + Fe2+ → OH• + OH– + Fe3+,

Ferric ions balanced by hydrogen peroxide to obtain a proper Fe3+/Fe2+ ratio (or redox potential), is a suitable oxidizing agent to dissolve the chromium depleted layer [3].

H O 2• + Fe2+ → H O 2 + Fe3+.

Hydrogen peroxide is completely nonpolluting, water miscible, weakly acidic in aqueous solutions and 1 The article is published in the original.

H2O2 +

Fe3+

→ H O 2• +

Fe2+

+

H+,

H2O2 + OH• → H O 2• + H2O, –

H O 2• ↔ O 2• + H+, –

(1) (2) (3) (4) (5)

In order to retard peroxide decomposition in acidic solution and in presence of ferric ions, it has been pro posed a lot of inorganic and organic substances, which can often be alternatively described as chelating agents or free radical inhibitors. Studies have shown that addition of stabilizers such as alcohols [8], carboxylic

384

EFFECT OF ASCORBIC ACID ON HYDROGEN PEROXIDE DECOMPOSITION

acids [9], organic acids [9], or phosphonic acid [10] could prevent decomposition of hydrogen peroxide. Previous studies have demonstrated that ptolue nesulfonic acid (pTSA) is an efficient stabilizer for hydrogen peroxide in H2O2–H2SO4–HF pickling mixtures used for 316L SS [11]. However, the pTSA presents ecological disadvantages due to the presence of benzene rings in its structure; this could be hazard ous in case of skin or eye contact (irritant), of inges tion, of inhalation (lung irritant). It could also have potential chronic health effects because it is carcino genic [12]. The aim of this work was to investigate influence of ascorbic acid on decomposition of H2O2 in presence of ferric ions in pickling mixtures based on H2SO4–HF– H2O2. Ascorbic acid (AA) belongs to a class of non toxic compounds, water soluble and stable in air. In solution, it is rapidly oxidized by air and gradually decomposed into dehydroascorbic acid (DHA), which is more reactive. In addition, ferric ions concentration and temperature effects were studied to obtain optimum con ditions in an environmental friendly chemical pickling treatment for 316L SS using H2SO4–HF–H2O2. MATERIALS AND METHODS Stainless Steel Specimens 316L SS alloy used in all experiments was supplied by Thyseen Krupp Corporation with a chemical com position of: 16.67 Cr wt %, 10.07 Ni, 2.06 Mo, 1.22 Mn, 0.26 S and Fe for balancing. Specimens 25 mm × 50 mm (area ≈ 2500 mm2) were cut from a 0.7 mm thick sheet and then cleaned in an ultrasonic bath using acetone. The stainless steel was annealed at 1110°C for about 3 hours and it was possible to obtain an oxide layer with a thickness of 13 microns on the sample surface and with a weight gain of approxi mately 1.7 mg/cm2. After heat treatment, samples were exposed to an electrochemical pickling process; for removing chromium and manganese oxides from depleted external surfaces [13]. Pickling Mixture The pickling bath solution was prepared as follows: 80 g/L H2SO4, 25 g/L HF, 20 g/L of ferric ions as fer ric sulphate [Fe2(SO4)3] and 30 g/L H2O2. All the chemicals were reagent grade. 3hydroxibenzoic acid was used as an inhibitor agent in all mixtures. The mix tures tested were mixed with a magnetic stirrer; all experiments were performed in triplicate at pH 2.0. Stabilization of H2O2 was made according to the following procedure: ascorbic acid was added into aqueous solution of H2O2 previously dissolved in eth anol, weight ratio was 5 : 100. In order to establish optimal mixing conditions and avoid excessive decomposition of H2O2, following parameters were evaluated:

385

(a) Effect of ascorbic acid. The decomposition of H2O2 was evaluated as a function of AA concentra tions using 2.5, 5.0 and 7.5% relative to the content of H2O2. Assays were performed at 25°C in absence of stainless steel specimen. (b) Effect of temperature. Decomposition of H2O2 was evaluated in pickling mixture at temperatures of 25, 40, 50 and 60°C. The tests were conducted in the absence of SS sample. (c) Effect of iron ions. Decomposition of hydrogen peroxide as a function of ferric ions concentration was studied covering a wide range: 0, 5, 10, 15, 20, 30 and 40 g/L. These tests were conducted by immersing a 316L SS specimen in 250 mL of each pickling mixture at 25°C. After one hour of immersion, samples were removed, dried and weighed using an analytical bal ance with a precision of ±0.1 × 10–3 g. Decomposition of H2O2 in pickling mixture was evaluated by chemical titration using potassium per manganate (KMnO4) 0.1 N [14]. The stabilizing effect of ascorbic acid was compared with that of pTSA used in other works [15]. Pickling Treatment The 316L SS specimens were immersed in pickling solution at 25°C for 3 min, to ensure complete removal of remaining oxides layer (depleted chromium). Then samples were removed, dried and weighed using an analytical balance with a precision of ±0.1 × 10–3 g. Measurements were taken at the beginning of the experiments and at the end, the weight lost was deter mined according to ASTM G10294 [16]. All exper iments were performed in triplicate. The reproducibil ity of the gravimetric results was higher than 95%. These processes were compared with a traditional mixture based HNO3HF [17]. Electrochemical Techniques Polarization measurements were performed with a Gamry 3000 potentiostat at a potential scan rate of 0.16 mV/s, a saturated calomel electrode (SCE) was used as reference, titanium electrode as a counter electrode and 316L SS specimen with a surface area of 2 cm2, as working electrode. Micrographic Observations JEOL JSM6610LV scanning electron microscopy was used for morphological observations, with a sec ondary electron (SE) detector at 20 kV. The working distance was maintained at 10 mm.

PROTECTION OF METALS AND PHYSICAL CHEMISTRY OF SURFACES Vol. 50 No. 3 2014

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NARVÁEZ HERNÁNDEZ et al. 100

80

H2O2 concentration, %


100

2.5% AA 5.0% AA 7.5% AA

60 40 20

90

80

70 pTSA AA 60 10

0

20

40 60 Time, min

80

100

0

100

200 Time, min

300

400

Fig. 1. H2O2 decomposition in the pickling solution with 2.5, 5.0 and 7.5 wt % of AA concentration.

Fig. 2. H2O2 decomposition in the pickling solution in presence of pTSA and AA at 25°C.

RESULTS AND DISCUSSION

The variation of H2O2 concentration in presence of either pTSA or AA stabilizers was evaluated as func tion of time, it was used an aqueous solution with 30 g/L H2O2 at 25°C, results are shown in Fig. 2. It is observed that with pTSA after 300 minutes of experi mentation hydrogen peroxide loss was about 18%, while using AA it decomposes only 10% after the same time. These results may suggest that it is adequate to use AA as stabilizer of hydrogen peroxide into a pick ling mixture, should also be considered that this com pound does not produce environmental pollution. Common pickling processes for stainless steel using HNO3HF operate at temperatures of about 50°C. For ecological mixture proposed in this paper was neces sary to determine the optimum operating tempera

Figure 1 shows decomposition of hydrogen perox ide in pickling mixture for different AA concentrations at 25°C. Results are given for additions of 2.5, 5.0 and 7.5 wt % of ascorbic acid. An improvement in H2O2 stability can be observed with 5 wt % of ascorbic acid was used. After 90 minutes H2O2 content in the mix ture was about 90%. In contrast with mixture having 2.5 wt % of AA, that produce hydrogen peroxide half life decreased after 40 minutes. For a composition of 7.5 wt % of AA since first minutes of the test, values were close to halflife. In pickling processes, it must have an optimum quantity of stabilizer which is able to react with ions of Fe3+ in the solution. Below this optimum concentra tion, there is an excess of Fe3+ ions that can catalyze the decomposition reaction of hydrogen peroxide in a shorter time, as shown in Fig. 1 for 2.5 wt % of AA. Above this optimal concentration, AA is in excess which is probably cause of decomposition reaction of hydrogen peroxide to about 45% in the first few min utes (Fig. 1, 7.5 wt % AA). According to the results, 5 wt % of AA acts as a suitable concentration for pick ling bath, so it is possible to obtain a stabilization of hydrogen peroxide for over 90 min [18].

100 H2O2 concentration, %

Effect of Ascorbic Acid

80 60 40 40°C pTSA 50°C pTSA 60°C pTSA 40°C AA 50°C AA 60°C AA

20

Stabilizers Comparison As a Function of Temperature (pTSA vs. AA) Hydrogen peroxide is thermodynamically unstable so that mechanisms controlling its decomposition can be complicated by presence of some metal ions released during pickling process, these ions can cata lyze decomposition. For this reason it is necessary to use a compound that prevents decomposition of H2O2.

0

50

100

150 200 Time, min

250

300

Fig. 3. H2O2 concentration versus time as function of the temperature in the presence of pTSA and AA as stabilizers.



387

(a) pTSA AA

1200 100 Corrosion rate, mdd


1000 80 60 0 g/L 5 g/L 10 g/L 15 g/L 20 g/L 30 g/L 40 g/L

40 20

800 600 400 200 0

0

10

20

30

40 50 60 Time, min (b)

70

80

90


80 60 0 g/L 5 g/L 10 g/L 15 g/L 20 g/L 30 g/L 40 g/L

20

0

10

20

30

40 50 60 Time, min

40

Fig. 5. Corrosion rate (mdd) versus ferric ion concentra tion for 316L SS in a pickling solution with AA and pTSA.

100

40

10 20 30 Fe3+ ions concentration, g/L

70

80

90

Fig. 4. H2O2 concentration versus time as a function of fer ric ion concentration using (a) pTSA and (b) AA as stabi lizers at 25°C in presence of 316L SS.

ture, taking into consideration H2O2 stability [19]. In addition to tests at 25°C (Fig. 2), assays at different temperatures were performed: 40°, 50° and 60°C for both pTSA and AA, these results are shown in Fig. 3. The best behavior is obtained in test at 25°C (Fig. 2) because it achieves a minimal loss in H2O2 concentra tion for a longer time. A decomposition of about 90% of hydrogen peroxide was obtained after 135 min in test performed at 60°C for both stabilizers. The loss of hydrogen peroxide to ascorbic acid at 50°C was approximately 50% after 300 minutes. However for test at 40°C decomposed only 20% H2O2 after 300 min. On the other hand, 75% of H2O2 loss was obtained using pTSA at 50°C after 250 min. Based on results, it is possible to propose that 25°C is a suitable temperature for pickling. It can be seen that hydrogen peroxide decomposition is strongly dependent on temperature. For temperatures higher than 25°C, a negative effect on H2O2 stability was observed.

Effect of Presence of Ferric Ions In a pickling solution based on hydrogen peroxide, it is important to add ferric sulphate as an oxidizing agent, since at least 55% of total iron ions in the bath must be in the form of ferric ion [20]. Adding hydrof luoric acid to this solution provides a good pickling finish for a variety of SS products. Hydrofluoric acid is widely used to increase dissolution kinetics; it acts as a source of H+ ions, and is an effective complexing agent for both Fe3+ and Cr3+ ions. The fluoride ion can react with any of Fe3+ and Cr3+ ions to produce complexing ions [21, 22]: Fe3+ + F– → FeF2+, +

Fe3+ + 2F– → Fe F 2 ,

(6)

→ FeF3. Effect of presence of different ferric ion concentra tions on hydrogen peroxide decomposition versus time was evaluated. Results that were obtained when pTSA was used as stabilizer are shown in Fig. 4a and those obtained with AA stabilizer as shown in Fig. 4b. For both stabilizers H2O2 concentration decreases when ferric ion concentrations is greater than 20 g/L, for values less or equal than this ferric ion concentra tions, amount of H2O2 decreases slowly, requiring about 90 min for a loss of approximately 18%, while for ferric ion concentration of 30 and 40 g/L a loss of about 80% was obtained in the same period. A thresh old can be defined for a Fe3+ ions composition of 20 g/L. Above this composition, loss of H2O2 increases dramatically. This increase in decomposition of hydrogen peroxide can be attributed to catalytic decomposition promoted by the presence of ferric ions dissolved in the medium, according to Eq. (2). Hydrogen peroxide is a stable compound when kept at room temperature without presence of ions Fe3+ +

3F–


388

NARVÁEZ HERNÁNDEZ et al. 2200 316L inmersed in HF + pTSA mixture 316L inmersed in HF + AA mixture

2000 1800 E (mV)sce

1600 1400 1200 1000 800 600 400 200 1

10

(а)

20 µm

(b)

20 µm

(c)

20 µm

1000 1000 10000 100000 logi [µA/cm2]

Fig. 6. Anodic potentiostatic polarization curves of 316L SS in pickling mixture using AA and pTSA respectively.

which catalyze its decomposition as shown in Figs. 4a and 4b, when ferric ion composition was 0 g/L. Cata lytic decomposition of hydrogen peroxide has been studied previously [23]. In the pickling process is very important presence of Fe3+, because these ions act as an oxidizing agent in the environmentally friendly pickling solution; this is the role of HNO3 in conven tional HNO3–HF pickling mixture and the high acti vating power of the F– ion confers to the medium a strong depassivating character [24]. However, due to immediate reaction between H2O2 and metallic ions (Eqs. (1) and (2)), it is necessary to use stabilizers. The role of AA used as a stabilizer in this work can be explained in different ways; some studies suggest that the critical performance of these compounds depends on their capacity to stabilize the higher valence state of iron, and therefore not only to stimulate the oxidation •– of Fe2+ by O 2 and H2O2, but also to prevent the effec •–

•

tive reduction of Fe3+ by O 2 , H O 2 and H2O2 [25]. On the other hand, some researches propose that delay mechanism is the trapping of hydroxyl radicals by metallic ions. So, the additional decomposition of hydrogen peroxide is delayed [23]. Corrosion Rate of Stainless Steel Substrate Immersed in a Pickling Mixture The corrosion rate expressed as mg/dm2 day (mdd) for a 316L SS specimen was evaluated using a mixture of H2SO4–HF–H2O2 for both AA and pTSA stabi lizer and as function of ferric ion concentration, results after one hour of immersion are shown in Fig. 5. In both cases it can be observed that 316L SS pick ling rate decreases when Fe3+ ions concentration increase in environmental friendly pickling solution. According to Figure 5, ascorbic acid is more aggressive

Fig. 7. Surface morphology of the 316L SS after it has been pickled in the mixtures: (a) mixture based in H2O2 stabi lized with AA, (b) mixture based in H2O2 stabilized with p TSA and (c) mixture based in HNO3HF as reference.

than pTSA at concentrations below 13 g/L of Fe3+ ions approximately. However, for a concentration of 20 g/L of Fe3+ ions, the behavior of both stabilizers was very similar. The ability of the mixtures to pickle 316L SS spec imen depends strongly on the presence of ferric ions. These results are of practical importance due to the ferric ion concentration can be controlled to obtain a



desirable pickling rate of 316L SS and avoiding corro sion of substrate. Results obtained by gravimetric measurements made on 316L stainless steel free oxide samples immersed in pickling solution at 25°C, using both AA and pTSA, showed weight loss of about 185 mdd, similar for both stabilizers. Conversely, when the pick ling mixture based on HNO3–HF was used, large dif ferences in weight loss was observed, with values of about 16500 mdd. These results confirm that proposed mixture based on H2O2 is much less aggressive on the substrate than traditional mixtures. Polarization Curves to Evaluate the Aggressiveness of the Pickling Mixture on the Substrate Figure 6 shows anodic potentiostatic polarization curves for 316L SS into a pickling mixture in presence of any of AA or pTSA stabilizer at 25°C. It can be seen from figure that the behavior of both compounds are very similar, showing active performance in according with the results obtained in Fig. 5. Removal of Depleted Chromium Oxide Layer Using Three Kinds of Pickling Mixtures Based on the above results, it was possible to estab lish working conditions for pickling mixture to remove chromium depleted oxides that remain on 316L stain less steel surface. The mixture has the following com position: 80 g/L H2SO4, 25 g/L HF, 20 g/L of ferric ions and 30 g/L stabilized with 5 wt % relative to the content of H2O2. Samples were immersed in a pickling bath at 25°C for 180 s. table shows results of pickling rate during the removal of oxide from 316L SS surface using any of both pickling mixtures proposed. These results were also compared with traditional pickling mixture based on HNO3–HF. Results in table show that pickling rates were very similar for any of three pickling mix tures that were studied. Microstructure Observations Figure 7 shows the scanning electron micrograph (SEM) of 316L SS samples after it has been pickled in any of mixtures proposed: (a) sample pickled with a mixture based in H2O2 stabilized with AA, (b) speci men pickled whit a mixture based in H2O2 stabilized with pTSA and (c) sample pickled with a mixture based in HNO3–HF which is used as reference. Speci mens were pickled at 25°C during 180 s (a and b) and at 50°C (c). To each of mixture studied it was observed that 316L SS surface after pickling was completely oxides free, intergranular corrosion is not appreciated and grain boundaries are well defined. In all cases the oxide layer depleted in chromium was totally removed indi cating an effective process.

389

Pickling rate of 316L SS immersed in three different pick ling mixtures at 25°C during 180 seconds Pickling mixture

Rpm, mdd

Base in HNO3HF

29652

Base in H2O2 + pTSA

27799

Base in H2O2 + AA

26873

CONCLUSIONS The results of this work allow establishing following conclusions: —Ascorbic acid has a positive influence on hydro gen peroxide stabilization in the pickling mixture, in order to maintain a high concentration of H2O2 for sufficient time for ending pickling process. —Optimal conditions were established for ecolog ical pickling mixture: temperature, Fe3+ ions concen tration, stabilizer concentration. It should be noticed that ascorbic acid is a nontoxic compound. —Ascorbic acid has a good performance as a stabi lizer for hydrogen peroxide making it a good alterna tive to replace pTSA in pickling mixtures; AA is not toxic compound so it is environmentally friendly. —Ascorbic acid showed the best performance for pickling at lower temperature (25°C) in contrast to traditional pickling mixture which performs at 50°C regularly. —Pickling mixture based on H2O2 stabilized with ascorbic acid was capable of removing the oxide layer from 316L stainless steel sample surface obtaining a good final surface finish. REFERENCES 1. AlMayouf, A.M., AlAmeery, A.K., and AlSuhy bani, A.A., British Corrosion J., 2001, vol. 36, p. 127. 2. Madi, V.N., Leeker, J.W., and Van Scoy, C.A., Patent 2002/0174880 USA, 2002. 3. Li LianFu., Caenen, P., Daerden, M., et al., Corrosion Sci., 2005, vol. 47, p. 1307. 4. Narvaez, L., Cano, E., and Bastidas, J.M., Corrosion, 2005, vol. 61, p. 219. 5. Sinyavskii, V.S., Usova, V.V., and Eskin, G.I., Protec tion of Metals, 2004, vol. 40, p. 303. 6. Walling, C., Accounts of Chemical Research, 1975, vol. 8, p. 125. 7. De Laat, J. and Gallard, H., Environmental Science and Technology, 1999, vol. 33, p. 2726. 8. Ernst, R.E., Patent 3 869 401 USA, 1975. 9. Watts, R.J., Finn, D.D., Cutler, L.M., et al., J. of Con taminant Hydrology, 2007, vol. 91, p. 312. 10. Baciocchi, R., Boni, M.R., and Aprile, L., J. Hazard Materials B, 2004, vol. 107, p. 97.


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