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Corrosivity of paper mill effluent and corrosion performance of stainless steel a
b
Chhotu Ram , Chhaya Sharma & A.K. Singh
a
a
Department of Applied Science & Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India b
Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India Accepted author version posted online: 04 Sep 2014.Published online: 25 Sep 2014.
Click for updates To cite this article: Chhotu Ram, Chhaya Sharma & A.K. Singh (2015) Corrosivity of paper mill effluent and corrosion performance of stainless steel, Environmental Technology, 36:6, 742-749, DOI: 10.1080/09593330.2014.960477 To link to this article: http://dx.doi.org/10.1080/09593330.2014.960477
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Environmental Technology, 2015 Vol. 36, No. 6, 742–749, http://dx.doi.org/10.1080/09593330.2014.960477
Corrosivity of paper mill effluent and corrosion performance of stainless steel Chhotu Rama∗ , Chhaya Sharmab and A.K. Singha a Department of Applied Science & Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India; b Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India
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(Received 30 July 2013; final version received 27 August 2014 ) Present study relates to the corrosivity of paper mill effluent and corrosion performance of stainless steel (SS) as a construction material for the effluent treatment plant (ETP). Accordingly, immersion test and electrochemical polarization tests were performed on SS 304 L, 316 L and duplex 2205 in paper mill effluent and synthetic effluent. This paper presents electrochemical polarization measurements, performed for the first time to the best of the authors’ information, to see the influence of chlorophenols on the corrosivity of effluents. The corrosivity of the effluent was observed to increase with the decrease in pH and increase in Cl− content while the addition of SO− 4 tends to inhibit corrosion. Mill effluent was found to be more corrosive as compared to synthetic effluent and has been attributed to the presence of various chlorophenols. Corrosion performance of SS was observed to govern by the presence of Cr, Mo and N contents. Keywords: stainless steel; paper mill effluent; chlorophenols; electrochemical tests; immersion test
1. Introduction Corrosion of metal occurs due to electrochemical reactions between metal and its surrounding environment. It affects the useful life of plant machinery in many industries including equipments used in effluent treatment plants (ETPs). Various wastewater handling equipments affected by corrosion are piping, sludge scrapping equipments, activated sludge piping, pure oxygen reactor, pumps, valves, etc. as they are manufactured of metals. Concentration and nature of chemicals, present in the wastewater and those added due to the treatment process, affect the type and rate of corrosion on mild steel and stainless steel (SS)fabricated plant machinery meant for treating urban and industrial wastewater. In one of the earlier studies, mild steel and SS 304 in an anaerobic wastewater treatment plant is observed to show general corrosion on the former and localized corrosion on SS.[1] In another study, uniform corrosion was found to increase due to action of Thiobacillus ferrooxidans in an aerobic treatment system, whereas localized corrosion was observed on AISI 304 during the anaerobic treatment.[2] In one field test involving exposure of SS in five wastewater treatment plants for one year, AISI 304 was found to suffer from pitting in three plants, whereas AISI 316 was attacked in one plant. No corrosion attack was found on duplex 2205.[3] Galvanic corrosion of steel coupled with copper in refinery wastewater showed increase in corrosion rate (CR) with an increase in MgCl2 concentration, the cathode and anode area ratio and operating temperature.[4] Dave et al.,[5] reported microbial corrosion on steels in activated sludge effluent
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
treatment systems and correlated with sulphate-reducing bacteria (SRB) and sulphur-oxidizing bacteria. Corrosion behaviour of steels, studied electrochemically in synthetic wastewater, shows no localized corrosion on SS while mild steel exhibited higher CR, than by the stainless steel alloys.[6] In dairy effluent, microbial-induced corrosion (MIC) was observed to initiate pitting on mild steel.[7] Influence of steel element corrosion was investigated in dairy wastewater treatment and a strong correlation was observed between anaerobic corrosion and efficiency of phosphorus removal.[8] In another study, CR of mild steel was found high enough to require continuous maintenance and early replacement of machinery in a municipal wastewater treatment plant. Accordingly, mild steel was replaced by SS (types 304/304 L, 316/316 L) for bar screens, grit removers, weirs, bolting, slide gates and aeration basin, digester and sludge piping in these plants.[9] Another study reports mild steel corrosion in cassava mill effluent and correlated with such microbes presence as Pseudomonas sp., Streptococcus sp., Micrococcus sp., Bacillus sp., Neisseria sp. and Lactobacillus sp. [10]. Thus it is observed that corrosion of metals in ETPs can be both chemical and microbial-influenced corrosion. Paper industry is among the industries using high amount of water (up to ∼ 60,000 gallons per tonne of product), which results in large amount of effluent generation,[11] due to its contamination with chemicals discharged from digester house, recovery, bleaching and paper-making stages.[12] Thus, corrosivity of paper mill effluent is expected to be high due to its pH,
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Environmental Technology chloride content, dissolved carbon dioxide, corrosioncausing microbes, etc. The corrosive conditions are further enhanced due to agitation of the effluent during the process of its aeration. Effluent streams from pulping and bleaching processes normally are more corrosive than streams from paper-making processes. Corrosion problems observed in the ETP include oxygen pitting of carbon steel, attack by condensed acid, MIC of carbon and SS and abrasionenhanced corrosion of grit chamber. Chloride concentration above 200 ppm, which is possible in bleach plant effluents, cause localized corrosion of SS. Sulphate ion inhibits corrosion of carbon steel and SS but it provides nutrient for SRB. These SRBs thrive in localized anaerobic conditions under slime and deposits and reduce sulphate to produce H2 S, which, in turn, is metabolized by sulphideoxidizing bacteria (Thiobacillus sp.) to produce sulphuric acid. This biogenically formed acid, common in septic municipal sewers, corrode steels under biofilm deposits. The pH in these media could be < 2, causing aggressive corrosion of carbon steel.[13] SRBs have also been observed to cause under-deposit pitting of carbon steel and SS in return activated sludge system in paper mill ETP particularly in piping systems and in clear effluent piping.[5] Corrosion performance of mild steel in primary and secondary treated paper mill effluent was investigated and CR observed was higher in primary effluent due to more chemicals than treated effluent.[14] Thus ETPs in paper mill are found to be severely affected by corrosion. However, not much effort has been put in systematically investigating corrosion through evaluation and characterization of the corrosivity and suitability of materials for constructing various equipments in the case of paper mill ETPs. It was, therefore, planned to undertake corrosion investigations, considering both chemical and microbial aspects, in effluents obtained from various stages of ETP of a nearby paper mill. To begin with, studies were initiated on chemical corrosion on commonly used steels in effluent obtained from the primary treatment stage and in simulated synthetic effluent. The present paper is a report on these investigations.
Sheffield AB, Sweden. Mild steel coupons ofvw dimension 3.6 × 3.6 × 0.54 cm and SS coupons of dimension 5.0 × 1.8 × 0.23 cm were used for the immersion test. These coupons were polished progressively from coarse to fine (up to 1000 grit) emery paper and then subjected to 4/0 polishing for the final finishing as per the ASTM A279 standard.[15] Prior to exposing to the test, the polished coupons were ultrasonically degreased in acetone. For the electrochemical studies, metal specimens of 1 cm2 were embedded in a mould of epoxy resin and an electrical connection was established via a copper wire. The test material was polished and degreased as described above. 2.2. Characterization of effluent Primary treated effluent was collected from ETP of a nearby paper mill and was immediately stored at 4°C to minimize any compositional change likely to occur during storage. Effluent was characterized using AR grade chemicals as per the standard method.[16] Thus, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of effluent were analysed by the five-day BOD bottle method and titration methods, respectively. Chloride and sulphate were estimated by the argentometric titration and gravimetric methods, respectively. Total dissolved solid (TDS) and total suspended solid (TSS) were estimated by the gravimetric method. Colour was measured using the Pt–Co unit method. Synthetic effluent simulating the composition of paper mill effluent, as far as possible, was also prepared to conduct the electrochemical test. In this solution, potassium hydrogen phthalate and glucose–glutamic acid were added to achieve the target values (observed in mill effluent) of COD and BOD, respectively.[16] Table 2 presents the composition of paper mill effluent used to perform electrochemical tests. 2.3. Corrosion test To investigate corrosion behaviour of various steels, the immersion test and electrochemical polarization tests were conducted. For the immersion test, duplicate coupons of steels were kept immersed in effluent for six months. To keep a check on the variation in composition of effluent, it was monitored every 10th day and the effluent was replaced by a fresh sample. The representative composition (Table 3) of the effluent was obtained by averaging these values for the total duration of test. After exposure,
2. Materials and methods 2.1. Materials Corrosion tests were carried out using mild steel and austenitic SS 304 L, 316 L and duplex 2205. SS coupons (composition in Table 1) were supplied by Avesta Table 1. Sample 304 L 316 L 2205
Composition of steel samples (wt%). C
Si
Mn
P
S
Cr
Ni
Mo
N
Cu
0.025 0.029 0.020
0.344 0.298 0.52
1.619 1.652 1.450
0.027 0.028 0.020
0.001 0.001 0.002
18.214 16.919 22.25
8.258 10.332 5.48
0.294 2.174 3.08
0.074 0.044 0.150
0.269 0.373 –
Balance % – Fe.
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Table 2. Composition of primary treatment stage effluent and synthetic effluent.
Table 4. CR of steels in paper mill effluent by the immersion test.
S. no.
Parameter
Mill effluent
S. no.
Sample
1. 2. 3. 4. 5. 6. 7. 8. 9.
pH COD BOD Chloride Sulphate TDS TSS Ec Colour
7.78 692 128 500 172 2380 112 2.5 1107
1. 2. 3. 4.
ms 304 L 316 L 2205
± ± ± ± ± ± ± ± ±
0.02 11 16 25 18 9 16 0.1 14
Synthetic effluent 7.78 692 128 500 172 2004 30 2.2
± ± ± ± ± ± ± ± –
0.03 8 14 20 15 5 8 0.1
CR (mpy)
Maximum pit depth (µm)
± ± ± ±
– 9 8 4
0.890 0.015 0.013 0.009
0.005 0.015 0.001 0.001
4. All experiments were carried out at room temperature. Each electrochemical test was repeated to check the reproducibility of the results.
All values in mg/L except pH, Ec (mmho), colour (Pt–Co units).
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Table 3. Composition of paper mill effluent during immersion test. S. no. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Parameter pH COD BOD Chloride Sulphate TDS TSS Ec Colour Temperature
Values 8.01 851 208 401 212 1873 223 2.95 1950 27
± ± ± ± ± ± ± ± ± ±
0.54 329 117 159 150 532 120 1.2 873 5
All values in mg/L except pH, Ec (mmho), colour (Pt–Co units), temperature (°C).
the corroded coupons were cleaned as per the ASTM G172 [15] for estimating the extent of uniform and localized corrosion. CR was calculated by measuring weight loss, experienced by the coupons after the immersion test, using the formula: CR (in mils per year) = 534 × w/(d × A × T), where w is the weight loss (mg), d is the density of the material (gm/cm3 ), A is the exposed area of coupon (inch2 ) and T is the duration of exposure (hours). The extent of pitting was estimated by measuring the maximum pit depth on the surface of cleaned corroded coupons using a stereo microscope (Olympus) and an optical microscope (Leica Q500MC). Scanning electron microscope (SEM-Quanta 200 FCG, The Netherlands) was used for examination of coupons after immersion and polarization tests. Electrochemical polarization tests were carried out in effluents (Table 2) using Radiometer ‘Voltalab’ Electrochemical Laboratory Model PGZ301. The set-up uses a saturated calomel electrode as the reference electrode, graphite rods as the auxiliary and the test specimen was the working electrode. The tests conducted were open circuit potential (OCP), Tafel plot, cyclic polarization curve and potentiostatic test. Electrochemical corrosion parameters were obtained with the help of software VoltaMaster
3. Results 3.1. Immersion test CR, indicating the extent of uniform corrosion, maximum pit depth and number of pits per unit area, indicating the extent of localized corrosion, experienced by various steel samples is given in Table 4. One observes that (i) CR is very low in the SS as compared to mild steel, (ii) SS can be put in following order of decreasing CR: 304 L > 316 L > 2205 and (iii) pitting was observed on all three varieties of SS with maximum attack on 304 L and minimum on 2205. SEM photograph (Figure 1) shows pitting on SS exposed to mill effluent after the immersion test. 3.2. Electrochemical test Table 5 presents corrosion related parameters derived from electrochemical polarization measurements (some of the representative polarization curves are shown in Figure 2) performed on mill effluent. One observes that OCP of 304 L is the lowest followed by 316 L and 2205. CR is observed to be the maximum in case of 304 L and minimum for 2205, whereas resistance (Rp ) is the highest in case of 2205 and lowest for 304 L. These three parameters show 304 L to exhibit the lowest and 2205 highest resistance against uniform corrosion. Pitting potential (from both potentiodynamic polarization and potentiostatic tests) is observed to be the lowest for 304 L followed by 316 L and 2205. Pitting potentials observed, in potentiostatic tests, are slightly lower than the values for the respective alloy, as observed from potentiodynamic polarization tests. This is so because the sample remains at a given polarization potential for a longer duration in potentiostatic measurement than in the case of anodic polarization measurement. Evidence of pitting is shown by the SEM photograph (Figure 3) on SS 304 L and 316 L after having undergone cyclic polarization test. Passivation range, showing margin of safety against pitting attack in case of a metal, is observed to be the maximum for 2205 and minimum for 304 L (Table 5). The passivation current density (ip ), representing the CR while under passivation and hence the protection characteristic of the passive film, is observed
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B
C
D
Figure 1. SEM images of SS before and after the immersion test: (A) 304 L before test (100 × ), (B) 304 L (100 × ) after test, (C) 316 L (200 × ) before test and (D) 316 L (200 × ) after test.
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Table 5. Corrosion parameters of SS by electrochemical techniques. Metals
Mill effluent
Open circuit potential (V) 304 L − 0.190 ± 0.003 316 L − 0.172 ± 0.004 2205 − 0.159 ± 0.005 CR (mpy) 304 L 316 L 2205
0.036 ± 0.002 0.023 ± 0.001 0.013 ± 0.002
Surface resistance (Kohm.m2 ) 304 L 0.024 ± 0.001 316 L 0.044 ± 0.012 2205 0.090 ± 0.023
Synthetic effluent − 0.177 ± 0.004 − 0.139 ± 0.006 − 0.131 ± 0.002 0.019 ± 0.003 0.016 ± 0.004 0.010 ± 0.002 0.052 ± 0.004 0.074 ± 0.006 0.120 ± 0.005
Pitting potential (cyclic polarization test) (V) 304 L 0.420 ± 0.020 0.490 ± 0.022 316 L 0.542 ± 0.018 0.631 ± 0.020 2205 1.045 ± 0.008 – Pitting potential (potentiostatic test) (V) 304 L 0.300–0.350 0.500–0.550 316 L 0.400–0.450 0.600–0.650 2205 1.000–1.050 – Repassivation potential (V) 304 L 0.058 ± 0.008 316 L 0.195 ± 0.020 2205 1.010 ± 0.015
0.150 ± 0.015 0.315 ± 0.015 –
Passivation range (V) 304 L 0.757 ± 0.014 316 L 0.880 ± 0.020 2205 1.370 ± 0.025
0.780 ± 0.010 0.950 ± 0.020 –
to be the maximum for 304 L followed by 316 L and minimum for 2205 (Figure 2(C)). Passivation current density has also been suggested as inversely proportional to pitting corrosion resistance.[17] Repassivation potential, showing resistance of metal against crevice corrosion, is observed to be the maximum for 2205 followed by 316 L and minimum for 304 L. Thus, from all parameters, SS 2205 is observed to show the maximum resistance against localized corrosion
while 304 L have the minimum resistance. Electrochemical polarization tests were also conducted in synthetic effluent (Figure 4). The comparison of different electrochemical parameters (Table 5) proves that mill effluent is more corrosive than synthetic effluent. Duplex SS 2205 shows resistance against pitting in synthetic effluent and it was further confirmed by potentiostatic tests. Increase in current, in cyclic polarization curve of 2205, around ∼ 1100 mV potential in synthetic effluent which could be misunderstood as due to onset of pitting can be assigned to the Cl− /OCl− reaction system.
4. Discussion 4.1. Composition of effluents To check the dependence of corrosivity of mill effluent on its pH and constituents, anodic polarization curves of SS 304 L (Figure 5) were recorded in synthetic solutions with varying pH, Cl− , SO− 4 and chemicals for imparting BOD and COD values as observed in mill effluent. Curve-a may be considered as a reference curve which corresponds to the solution having pH 7.78, Cl− ∼ 500 ppm and SO− 4 ∼ 172 ppm, as observed in mill effluent. However, curve-b (solution with pH ∼ 6) shows lower pitting potential as compared to curve-a (solution pH ∼ 7.78). Thus, decrease in pH will result in increasing corrosivity of the solution. Therefore, changes in the process which might lead to decrease in pH of mill effluent (here from 7.78 to 6) will make it more corrosive. It has been observed earlier also that effluents having pH below 4.5 corrode steels while more alkaline liquors favour the formation of the protective oxide layer.[13] The effect of chloride ions on SS shows reduction in the localized corrosion resistance (Figure 5). Thus, curve-e (Cl− ∼ 1500 ppm) shows lower pitting potential and higher current density, in comparison to curve-d (Cl− ∼ 500 ppm). Addition of Cl− ions has been suggested to substantially decrease protective characteristics of the passive film. The small size and negative charge of chloride ion helps it to penetrate through the passive layer to the positively charged metal cation.[18,19]
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A
B
C
D
Figure 2. (A) E vs. time curve, (B) Tafel plot, (C) cyclic polarization curve and (D) current vs. time curve (potentiostatic curve of 304 L) of SS in paper mill effluent. A
B
C
Figure 3. SEM images (200 × ) of SS: (A) 304 L before test, (B) 304 L after cyclic polarization test and (C) 316 L after cyclic polarization test.
SS sustain severe damage due to pitting in wastewater solutions containing chlorides.[4] Sulphate addition enhances pitting potential of SS in simulated effluent, as observed by the polarization test (Figure 5). Thus, anodic polarization curve of 304 L measured in solution with the polarization test (Figure 5). Thus, anodic polarization curve of 304 L measured in solution with SO− 4 ∼
172 ppm (curve-a) shows higher pitting potential as compared to curve-d (solution without SO− 4 ). Sulphate has been reported to inhibit initiation of pits on 304 L SS [20] and another report suggests that addition of sulphate in chloride solution decreases pit initiation site as well as current density.[21] Thus, more acidic effluents having higher Cl− but lower SO− 4 are expected to be more
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C
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B
D
Figure 4. (A) E vs. time curve, (B) Tafel plot, (C) cyclic polarization curve and (D) current vs. time curve (potentiostatic curve of 304 L) of stainless steels in synthetic effluent.
Figure 5. Anodic polarization curves of SS 304 L in synthetic solutions by varying pH, Cl− , SO− 4 , COD and BOD (wherever not indicated, pH, Cl− and SO− 4 correspond to 7.78, 500 and 172 ppm, respectively).
corrosive. Addition of potassium hydrogen phthalate and glucose–glutamic acid in solution provides target BOD and COD values does not show any significant change to the
localized corrosion behaviour of SS in the solution (curvesa and-c in Figure 5). However, the mill effluent is observed to be more corrosive as compared to synthetic effluent having similar pH, Cl− , SO− 4 , BOD and COD. It seems that nature of chemicals (even though they are contributing to same BOD and COD values) is more important in determining the corrosive nature of the mill effluent. According to earlier studies,[22] bleaching process using chlorine, ClO2 , or other chlorine compounds in paper mill leads to generation of chlorinated organic compounds. These have been identified as chlorate, chloroform, chlorophenolics viz. chlorophenols, chlorocatechols, chloroguaiacols, etc. [23]. These chemicals in mill effluent can be collectively analysed as adsorbable organic halides, which has been estimated ∼ 15–20 ppm in paper mill effluents and majority of them are chlorophenols.[24,25] Thus increased corrosivity of mill effluent was suspected to be due to chlorophenols and was tested accordingly. With a view to check the influence of chlorophenols, anodic polarization measurements were made on SS 304 L exposed in a solution having pH and Cl− similar to that in mill effluent and additionally 1 ppm of 2,
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C. Ram et al. by the decrease in phenolic compound concentration which have large over potential for oxygen evolution.[25] However, more investigations involving estimation of electrochemical potential of chlorophenols in the solution and analysis of corroding solution after the test are required to further corroborate the influence of chlorophenols on corrosivity of paper mill effluents. 4.2. Corrosion resistance of stainless steel Results of the immersion test and electrochemical tests show resistance of tested SS, against uniform and localized corrosion, to increase in the following order:
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304L < 316L < 2205. Figure 6. Anodic polarization curves of SS 304 L in chlorophenols (1 ppm ∼ 2, 4-DCP, 2, 5-DCP and 2, 4, 5-TCP) with and without chloride ( ∼ 500 ppm) at pH ( ∼ 7.78).
4-dichlorophenol (DCP), 2, 4, 5-trichlorophenol (TCP) and 2, 5-dichlorophenol. It is observed in all the three cases (Figure 6) that pitting potential decreases down to 400–430 mV which is somewhat lower than that observed in mill effluent (Table 5). A review of literature does not show any previous work relating to corrosion due to these chlorophenols. However, one study found that white water is responsible for corrosion in paper-making section where these organic compounds may be responsible for corrosion.[26] In another work, it was suggested that electrochemical treatment of chlorophenols initially leads to anodic and/or cathodic release of non-chlorinated phenolic and quinonic compounds along with chloride ions.[27,28] These chloride ions released from chlorophenolic compounds may be responsible for the decrease in pitting potential of SS 304 L in an aqueous solution [19] and thereby making it less passive. The electrochemical oxidation reactions involved in anodic oxidation of chlorophenols [28] are as follows: H2 O → OH∗ + e− + H+ . These reactions might occur due to the SS electrode being polarized to anodic potentials during the anodic polarization measurements in the present work. Another study suggests that passivation of electrode may be reduced OH
OH
The relative resistance of the studied SS against corrosion may be correlated with their composition through determination of pitting resistance equivalent number (PREN) [29,30] as given below: PREN = %Cr + 3.3x %Mo + 16x %N. Accordingly, the PREN of 2205 is the maximum (34.8), that of 316 L is 25.2 and 304 L is the minimum (19.9) showing 2205 to be most resistant while 304 L least resistant against corrosion. These conclusions are in accordance with those derived on the basis of the experimental results. Thus the presence of Mo and N alongside Cr is crucial for providing corrosion resistance to the SS. Authors have investigated the effect of nitrogen addition in austenitic SS (316 L) by polarization techniques (pitting/crevice corrosion and repassivation) and found that an increase in the nitrogen content retarded the crevice CR.[31] 5. Conclusions The present paper reports on corrosivity of paper mill effluent. Different corrosion tests were performed on SS 304 L, 316 L and 2205 for this purpose. The tests show an increase in corrosivity of effluent, with a decrease in pH and an increase in Cl− content. Presence of SO− 4 helps in reducing the effect of corrosion. A comparison of mill and synthetically prepared effluents, with simulating composition, shows the former to be more corrosive. The increased O
Cl
Cl
OH
OH
Cl
COOH COOH +
+ COOH
COOH OH ads Chlorophenols or dichlorophenol
O ads Chlorobenzoquinone
Carboxylic acid
Cl-
Environmental Technology corrosivity is suggested to be due to the presence of various chlorophenols which release Cl− as a result of electrochemical reactions. The comparative corrosion resistance of various SS is observed to depend upon their Cr, Mo and N contents.
Acknowledgement Mr Chhotu Ram is highly thankful to the Ministry of Human Resource Development (MHRD), New Delhi, India for the financial research grant for pursuing Ph.D. work.
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References [1] Englert GE, Muller IL. The corrosion behaviour of mild steel and type 304 stainless steel in media from an anaerobic biodigestor. Int J Biodeter Biodegr. 1996;37: 173–180. [2] Perego P, Fabiano B, Pastorino R, Randi G. Microbiological corrosion in aerobic and anaerobic waste purification plants: safety and efficiency problems. Bioprocess Eng. 1997;17:103–109. [3] Iversen A. Microbially influenced corrosion on stainless steels in wastewater treatment plants: part 1. Br Corros J. 2001;36:277–283. [4] Nosier SA. The effects of petroleum refinery wastewater on the rate of corrosion of steel equipment. Anti-Corros Methods Mater. 2003;50:217–222. [5] Dave C, Craig R, Bacon D. Corrosion in secondary effluent treatment systems. NACE International Corrosion Conference; 2004; New Orleans La. [6] Jabalera RS, Campo EA, Chacón-Nava JG, Malo-Tamayo JM, Mora-Mendoza JL, Martínez-Villafañe A. Corrosion behavior of engineering alloys in synthetic wastewater. J Mater Eng Perform. 2006;15:53–58. [7] Babu BR, Maruthamuthu S, Rajasekar A, Muthukumar N, Palaniswamy N. Microbiologically influenced corrosion in dairy effluent. Environ Sci Technol. 2006;3:159–166. [8] J˛edrzejewska-Cici´nska M, Krzemieniewski M. Effect of corrosion of steel elements on the treatment of dairy wastewater in a UASB reactor. Environ Technol. 2010;31: 585–589. [9] Tuthill AH, Lamb S. Stainless steel in municipal wastewater treatment plant, Nickel Development Institute, NiDI Technical Series 10076. Available from: www.nidi. org/index.cfm/ci_id/10627.htm [10] Rim-Rukeh A. Microbiologically influenced corrosion of S45C mild steel in cassava mill effluent. Res J Eng Appl Sci. 2012;1:284–290. [11] Nemerow NL, Dasgupta A. Industrial and hazardous waste management. New York: Van Nostrand Reinhold; 1991. [12] Pokhrel D, Viraraghavan T. Treatment of pulp and paper mill wastewater – a review. Sci Total Environ. 2004;333: 37–58. [13] Nixon RA, Bennett DC. Corrosion in waste-water treatment plants at pulp and paper mills – overview, Pulping and Environmental Conference Proceedings, TAPPI Engineering; 2006; Canada.
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[14] Ram C, Sharma C, Singh AK. Corrosion performance of mild steel in paper mill effluent. Adv Mater Res. 2012;585: 522–527. [15] American Society Testing and Material Standard, Wear and Erosion, Metal Corrosion 3, Vol. 2, 1991. [16] Standard methods for the examination of water and wastewater. 17th ed. Washington, DC: American Public Health Association; 1989. [17] Werner H. The passivation current density as a parameter for a non-destructive test on plants of the pitting corrosion resistance of welded NiCrMo alloys. J Solid State Electrochem. 2006;10:753–757. [18] Ibrahim MAM, Abd El Rehim SS, Hamza MM. Corrosion behaviour of some austenitic stainless steels in chloride environments. Mater Chem Phys. 2009;115:80–85. [19] Roberge PR. Hand book of corrosion engineering. New York, USA: McGraw-Hill; 2000. [20] Wang Y, Singh PM. Effect of white water chemistry on passivation behaviour of 304 L stainless steel, peers conference; 2012; 463–476. [21] Pistorius PC, Brustein GT. Growth of corrosion pit on stainless steel in chloride containing dilute sulphate. Corros Sci. 1992;33:1885–1897. [22] Sharma C, Mahanty S, Kumar S, Rao NJ. Gas chromatographic determination of pollutants in the chlorination and caustic extraction stage effluent from the bleaching of a bamboo pulp. Talanta. 1997;44:1911–1918. [23] Freire CSR, Silvestre AJD, Neto CP. Carbohydrate derived chlorinated compounds in ECF bleaching of hardwood pulps: formation, degradation and contribution to AOX in a bleached kraft pulp mill. Environ Sci Technol. 2003;37:811–814. [24] Choudhary AK, Kumar S, Sharma C. Removal of chlorophenolics from pulp and paper mill wastewater through constructed wetland. Water Environ Res. 2013;85: 54–62. [25] Ezerskis Z, Jusys Z. Oxidation of chlorophenols on Pt electrode in alkaline solution studied by cyclic voltammetry, galvanostatic electrolysis, and gas chromatography mass spectrometry. Pure Appl Chem. 2001;73:1929–1940. [26] Lacorte S, Latorre A, Barcelo D, Rigol A, Malmqvist A, Welander T. Organic compounds in paper-mill process waters and effluents. Trends Anal Chem. 2003;22:725–737. [27] Canizares P, Garcia-Gomez J, Saez C, Rodrigo MA. Electrochemical oxidation of several chlorophenols on diamond electrodes Part I. Reaction mechanism. J Appl Electrochem. 2003;33:917–927. [28] Polcaro AM, Palmas S. Electrochemical oxidation of chlorophenols. Ind Eng Chem Res. 1997;36:1791–1798. [29] Muñoz AI, Anton JG, Nuevalos SL, Guinon JL, Herranz VP. Corrosion studies of austenitic and duplex stainless steels in aqueous lithium bromide solution at different temperatures. Corros Sci. 2004;46:2955–2974. [30] Merello R, Botana b FJ, Botella J, Matres MV, Marcos M. Influence of chemical composition on the pitting corrosion resistance of non-standard low-Ni high-Mn–N duplex stainless steels. Corros Sci. 2003;45:909–921. [31] Baba H, Kodama T, Katada Y. Role of nitrogen on the corrosion behaviour of austenitic stainless steels. Corros Sci. 2002;44:2393–2407.
Environmental Technology Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/tent20
Corrosivity of paper mill effluent and corrosion performance of stainless steel a
b
Chhotu Ram , Chhaya Sharma & A.K. Singh
a
a
Department of Applied Science & Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India b
Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India Accepted author version posted online: 04 Sep 2014.Published online: 25 Sep 2014.
Click for updates To cite this article: Chhotu Ram, Chhaya Sharma & A.K. Singh (2015) Corrosivity of paper mill effluent and corrosion performance of stainless steel, Environmental Technology, 36:6, 742-749, DOI: 10.1080/09593330.2014.960477 To link to this article: http://dx.doi.org/10.1080/09593330.2014.960477
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Environmental Technology, 2015 Vol. 36, No. 6, 742–749, http://dx.doi.org/10.1080/09593330.2014.960477
Corrosivity of paper mill effluent and corrosion performance of stainless steel Chhotu Rama∗ , Chhaya Sharmab and A.K. Singha a Department of Applied Science & Engineering, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India; b Department of Paper Technology, Indian Institute of Technology Roorkee, Saharanpur Campus, Saharanpur 247001, India
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(Received 30 July 2013; final version received 27 August 2014 ) Present study relates to the corrosivity of paper mill effluent and corrosion performance of stainless steel (SS) as a construction material for the effluent treatment plant (ETP). Accordingly, immersion test and electrochemical polarization tests were performed on SS 304 L, 316 L and duplex 2205 in paper mill effluent and synthetic effluent. This paper presents electrochemical polarization measurements, performed for the first time to the best of the authors’ information, to see the influence of chlorophenols on the corrosivity of effluents. The corrosivity of the effluent was observed to increase with the decrease in pH and increase in Cl− content while the addition of SO− 4 tends to inhibit corrosion. Mill effluent was found to be more corrosive as compared to synthetic effluent and has been attributed to the presence of various chlorophenols. Corrosion performance of SS was observed to govern by the presence of Cr, Mo and N contents. Keywords: stainless steel; paper mill effluent; chlorophenols; electrochemical tests; immersion test
1. Introduction Corrosion of metal occurs due to electrochemical reactions between metal and its surrounding environment. It affects the useful life of plant machinery in many industries including equipments used in effluent treatment plants (ETPs). Various wastewater handling equipments affected by corrosion are piping, sludge scrapping equipments, activated sludge piping, pure oxygen reactor, pumps, valves, etc. as they are manufactured of metals. Concentration and nature of chemicals, present in the wastewater and those added due to the treatment process, affect the type and rate of corrosion on mild steel and stainless steel (SS)fabricated plant machinery meant for treating urban and industrial wastewater. In one of the earlier studies, mild steel and SS 304 in an anaerobic wastewater treatment plant is observed to show general corrosion on the former and localized corrosion on SS.[1] In another study, uniform corrosion was found to increase due to action of Thiobacillus ferrooxidans in an aerobic treatment system, whereas localized corrosion was observed on AISI 304 during the anaerobic treatment.[2] In one field test involving exposure of SS in five wastewater treatment plants for one year, AISI 304 was found to suffer from pitting in three plants, whereas AISI 316 was attacked in one plant. No corrosion attack was found on duplex 2205.[3] Galvanic corrosion of steel coupled with copper in refinery wastewater showed increase in corrosion rate (CR) with an increase in MgCl2 concentration, the cathode and anode area ratio and operating temperature.[4] Dave et al.,[5] reported microbial corrosion on steels in activated sludge effluent
*Corresponding author. Email: [email protected] © 2014 Taylor & Francis
treatment systems and correlated with sulphate-reducing bacteria (SRB) and sulphur-oxidizing bacteria. Corrosion behaviour of steels, studied electrochemically in synthetic wastewater, shows no localized corrosion on SS while mild steel exhibited higher CR, than by the stainless steel alloys.[6] In dairy effluent, microbial-induced corrosion (MIC) was observed to initiate pitting on mild steel.[7] Influence of steel element corrosion was investigated in dairy wastewater treatment and a strong correlation was observed between anaerobic corrosion and efficiency of phosphorus removal.[8] In another study, CR of mild steel was found high enough to require continuous maintenance and early replacement of machinery in a municipal wastewater treatment plant. Accordingly, mild steel was replaced by SS (types 304/304 L, 316/316 L) for bar screens, grit removers, weirs, bolting, slide gates and aeration basin, digester and sludge piping in these plants.[9] Another study reports mild steel corrosion in cassava mill effluent and correlated with such microbes presence as Pseudomonas sp., Streptococcus sp., Micrococcus sp., Bacillus sp., Neisseria sp. and Lactobacillus sp. [10]. Thus it is observed that corrosion of metals in ETPs can be both chemical and microbial-influenced corrosion. Paper industry is among the industries using high amount of water (up to ∼ 60,000 gallons per tonne of product), which results in large amount of effluent generation,[11] due to its contamination with chemicals discharged from digester house, recovery, bleaching and paper-making stages.[12] Thus, corrosivity of paper mill effluent is expected to be high due to its pH,
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Environmental Technology chloride content, dissolved carbon dioxide, corrosioncausing microbes, etc. The corrosive conditions are further enhanced due to agitation of the effluent during the process of its aeration. Effluent streams from pulping and bleaching processes normally are more corrosive than streams from paper-making processes. Corrosion problems observed in the ETP include oxygen pitting of carbon steel, attack by condensed acid, MIC of carbon and SS and abrasionenhanced corrosion of grit chamber. Chloride concentration above 200 ppm, which is possible in bleach plant effluents, cause localized corrosion of SS. Sulphate ion inhibits corrosion of carbon steel and SS but it provides nutrient for SRB. These SRBs thrive in localized anaerobic conditions under slime and deposits and reduce sulphate to produce H2 S, which, in turn, is metabolized by sulphideoxidizing bacteria (Thiobacillus sp.) to produce sulphuric acid. This biogenically formed acid, common in septic municipal sewers, corrode steels under biofilm deposits. The pH in these media could be < 2, causing aggressive corrosion of carbon steel.[13] SRBs have also been observed to cause under-deposit pitting of carbon steel and SS in return activated sludge system in paper mill ETP particularly in piping systems and in clear effluent piping.[5] Corrosion performance of mild steel in primary and secondary treated paper mill effluent was investigated and CR observed was higher in primary effluent due to more chemicals than treated effluent.[14] Thus ETPs in paper mill are found to be severely affected by corrosion. However, not much effort has been put in systematically investigating corrosion through evaluation and characterization of the corrosivity and suitability of materials for constructing various equipments in the case of paper mill ETPs. It was, therefore, planned to undertake corrosion investigations, considering both chemical and microbial aspects, in effluents obtained from various stages of ETP of a nearby paper mill. To begin with, studies were initiated on chemical corrosion on commonly used steels in effluent obtained from the primary treatment stage and in simulated synthetic effluent. The present paper is a report on these investigations.
Sheffield AB, Sweden. Mild steel coupons ofvw dimension 3.6 × 3.6 × 0.54 cm and SS coupons of dimension 5.0 × 1.8 × 0.23 cm were used for the immersion test. These coupons were polished progressively from coarse to fine (up to 1000 grit) emery paper and then subjected to 4/0 polishing for the final finishing as per the ASTM A279 standard.[15] Prior to exposing to the test, the polished coupons were ultrasonically degreased in acetone. For the electrochemical studies, metal specimens of 1 cm2 were embedded in a mould of epoxy resin and an electrical connection was established via a copper wire. The test material was polished and degreased as described above. 2.2. Characterization of effluent Primary treated effluent was collected from ETP of a nearby paper mill and was immediately stored at 4°C to minimize any compositional change likely to occur during storage. Effluent was characterized using AR grade chemicals as per the standard method.[16] Thus, biochemical oxygen demand (BOD) and chemical oxygen demand (COD) of effluent were analysed by the five-day BOD bottle method and titration methods, respectively. Chloride and sulphate were estimated by the argentometric titration and gravimetric methods, respectively. Total dissolved solid (TDS) and total suspended solid (TSS) were estimated by the gravimetric method. Colour was measured using the Pt–Co unit method. Synthetic effluent simulating the composition of paper mill effluent, as far as possible, was also prepared to conduct the electrochemical test. In this solution, potassium hydrogen phthalate and glucose–glutamic acid were added to achieve the target values (observed in mill effluent) of COD and BOD, respectively.[16] Table 2 presents the composition of paper mill effluent used to perform electrochemical tests. 2.3. Corrosion test To investigate corrosion behaviour of various steels, the immersion test and electrochemical polarization tests were conducted. For the immersion test, duplicate coupons of steels were kept immersed in effluent for six months. To keep a check on the variation in composition of effluent, it was monitored every 10th day and the effluent was replaced by a fresh sample. The representative composition (Table 3) of the effluent was obtained by averaging these values for the total duration of test. After exposure,
2. Materials and methods 2.1. Materials Corrosion tests were carried out using mild steel and austenitic SS 304 L, 316 L and duplex 2205. SS coupons (composition in Table 1) were supplied by Avesta Table 1. Sample 304 L 316 L 2205
Composition of steel samples (wt%). C
Si
Mn
P
S
Cr
Ni
Mo
N
Cu
0.025 0.029 0.020
0.344 0.298 0.52
1.619 1.652 1.450
0.027 0.028 0.020
0.001 0.001 0.002
18.214 16.919 22.25
8.258 10.332 5.48
0.294 2.174 3.08
0.074 0.044 0.150
0.269 0.373 –
Balance % – Fe.
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Table 2. Composition of primary treatment stage effluent and synthetic effluent.
Table 4. CR of steels in paper mill effluent by the immersion test.
S. no.
Parameter
Mill effluent
S. no.
Sample
1. 2. 3. 4. 5. 6. 7. 8. 9.
pH COD BOD Chloride Sulphate TDS TSS Ec Colour
7.78 692 128 500 172 2380 112 2.5 1107
1. 2. 3. 4.
ms 304 L 316 L 2205
± ± ± ± ± ± ± ± ±
0.02 11 16 25 18 9 16 0.1 14
Synthetic effluent 7.78 692 128 500 172 2004 30 2.2
± ± ± ± ± ± ± ± –
0.03 8 14 20 15 5 8 0.1
CR (mpy)
Maximum pit depth (µm)
± ± ± ±
– 9 8 4
0.890 0.015 0.013 0.009
0.005 0.015 0.001 0.001
4. All experiments were carried out at room temperature. Each electrochemical test was repeated to check the reproducibility of the results.
All values in mg/L except pH, Ec (mmho), colour (Pt–Co units).
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Table 3. Composition of paper mill effluent during immersion test. S. no. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
Parameter pH COD BOD Chloride Sulphate TDS TSS Ec Colour Temperature
Values 8.01 851 208 401 212 1873 223 2.95 1950 27
± ± ± ± ± ± ± ± ± ±
0.54 329 117 159 150 532 120 1.2 873 5
All values in mg/L except pH, Ec (mmho), colour (Pt–Co units), temperature (°C).
the corroded coupons were cleaned as per the ASTM G172 [15] for estimating the extent of uniform and localized corrosion. CR was calculated by measuring weight loss, experienced by the coupons after the immersion test, using the formula: CR (in mils per year) = 534 × w/(d × A × T), where w is the weight loss (mg), d is the density of the material (gm/cm3 ), A is the exposed area of coupon (inch2 ) and T is the duration of exposure (hours). The extent of pitting was estimated by measuring the maximum pit depth on the surface of cleaned corroded coupons using a stereo microscope (Olympus) and an optical microscope (Leica Q500MC). Scanning electron microscope (SEM-Quanta 200 FCG, The Netherlands) was used for examination of coupons after immersion and polarization tests. Electrochemical polarization tests were carried out in effluents (Table 2) using Radiometer ‘Voltalab’ Electrochemical Laboratory Model PGZ301. The set-up uses a saturated calomel electrode as the reference electrode, graphite rods as the auxiliary and the test specimen was the working electrode. The tests conducted were open circuit potential (OCP), Tafel plot, cyclic polarization curve and potentiostatic test. Electrochemical corrosion parameters were obtained with the help of software VoltaMaster
3. Results 3.1. Immersion test CR, indicating the extent of uniform corrosion, maximum pit depth and number of pits per unit area, indicating the extent of localized corrosion, experienced by various steel samples is given in Table 4. One observes that (i) CR is very low in the SS as compared to mild steel, (ii) SS can be put in following order of decreasing CR: 304 L > 316 L > 2205 and (iii) pitting was observed on all three varieties of SS with maximum attack on 304 L and minimum on 2205. SEM photograph (Figure 1) shows pitting on SS exposed to mill effluent after the immersion test. 3.2. Electrochemical test Table 5 presents corrosion related parameters derived from electrochemical polarization measurements (some of the representative polarization curves are shown in Figure 2) performed on mill effluent. One observes that OCP of 304 L is the lowest followed by 316 L and 2205. CR is observed to be the maximum in case of 304 L and minimum for 2205, whereas resistance (Rp ) is the highest in case of 2205 and lowest for 304 L. These three parameters show 304 L to exhibit the lowest and 2205 highest resistance against uniform corrosion. Pitting potential (from both potentiodynamic polarization and potentiostatic tests) is observed to be the lowest for 304 L followed by 316 L and 2205. Pitting potentials observed, in potentiostatic tests, are slightly lower than the values for the respective alloy, as observed from potentiodynamic polarization tests. This is so because the sample remains at a given polarization potential for a longer duration in potentiostatic measurement than in the case of anodic polarization measurement. Evidence of pitting is shown by the SEM photograph (Figure 3) on SS 304 L and 316 L after having undergone cyclic polarization test. Passivation range, showing margin of safety against pitting attack in case of a metal, is observed to be the maximum for 2205 and minimum for 304 L (Table 5). The passivation current density (ip ), representing the CR while under passivation and hence the protection characteristic of the passive film, is observed
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B
C
D
Figure 1. SEM images of SS before and after the immersion test: (A) 304 L before test (100 × ), (B) 304 L (100 × ) after test, (C) 316 L (200 × ) before test and (D) 316 L (200 × ) after test.
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Table 5. Corrosion parameters of SS by electrochemical techniques. Metals
Mill effluent
Open circuit potential (V) 304 L − 0.190 ± 0.003 316 L − 0.172 ± 0.004 2205 − 0.159 ± 0.005 CR (mpy) 304 L 316 L 2205
0.036 ± 0.002 0.023 ± 0.001 0.013 ± 0.002
Surface resistance (Kohm.m2 ) 304 L 0.024 ± 0.001 316 L 0.044 ± 0.012 2205 0.090 ± 0.023
Synthetic effluent − 0.177 ± 0.004 − 0.139 ± 0.006 − 0.131 ± 0.002 0.019 ± 0.003 0.016 ± 0.004 0.010 ± 0.002 0.052 ± 0.004 0.074 ± 0.006 0.120 ± 0.005
Pitting potential (cyclic polarization test) (V) 304 L 0.420 ± 0.020 0.490 ± 0.022 316 L 0.542 ± 0.018 0.631 ± 0.020 2205 1.045 ± 0.008 – Pitting potential (potentiostatic test) (V) 304 L 0.300–0.350 0.500–0.550 316 L 0.400–0.450 0.600–0.650 2205 1.000–1.050 – Repassivation potential (V) 304 L 0.058 ± 0.008 316 L 0.195 ± 0.020 2205 1.010 ± 0.015
0.150 ± 0.015 0.315 ± 0.015 –
Passivation range (V) 304 L 0.757 ± 0.014 316 L 0.880 ± 0.020 2205 1.370 ± 0.025
0.780 ± 0.010 0.950 ± 0.020 –
to be the maximum for 304 L followed by 316 L and minimum for 2205 (Figure 2(C)). Passivation current density has also been suggested as inversely proportional to pitting corrosion resistance.[17] Repassivation potential, showing resistance of metal against crevice corrosion, is observed to be the maximum for 2205 followed by 316 L and minimum for 304 L. Thus, from all parameters, SS 2205 is observed to show the maximum resistance against localized corrosion
while 304 L have the minimum resistance. Electrochemical polarization tests were also conducted in synthetic effluent (Figure 4). The comparison of different electrochemical parameters (Table 5) proves that mill effluent is more corrosive than synthetic effluent. Duplex SS 2205 shows resistance against pitting in synthetic effluent and it was further confirmed by potentiostatic tests. Increase in current, in cyclic polarization curve of 2205, around ∼ 1100 mV potential in synthetic effluent which could be misunderstood as due to onset of pitting can be assigned to the Cl− /OCl− reaction system.
4. Discussion 4.1. Composition of effluents To check the dependence of corrosivity of mill effluent on its pH and constituents, anodic polarization curves of SS 304 L (Figure 5) were recorded in synthetic solutions with varying pH, Cl− , SO− 4 and chemicals for imparting BOD and COD values as observed in mill effluent. Curve-a may be considered as a reference curve which corresponds to the solution having pH 7.78, Cl− ∼ 500 ppm and SO− 4 ∼ 172 ppm, as observed in mill effluent. However, curve-b (solution with pH ∼ 6) shows lower pitting potential as compared to curve-a (solution pH ∼ 7.78). Thus, decrease in pH will result in increasing corrosivity of the solution. Therefore, changes in the process which might lead to decrease in pH of mill effluent (here from 7.78 to 6) will make it more corrosive. It has been observed earlier also that effluents having pH below 4.5 corrode steels while more alkaline liquors favour the formation of the protective oxide layer.[13] The effect of chloride ions on SS shows reduction in the localized corrosion resistance (Figure 5). Thus, curve-e (Cl− ∼ 1500 ppm) shows lower pitting potential and higher current density, in comparison to curve-d (Cl− ∼ 500 ppm). Addition of Cl− ions has been suggested to substantially decrease protective characteristics of the passive film. The small size and negative charge of chloride ion helps it to penetrate through the passive layer to the positively charged metal cation.[18,19]
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A
B
C
D
Figure 2. (A) E vs. time curve, (B) Tafel plot, (C) cyclic polarization curve and (D) current vs. time curve (potentiostatic curve of 304 L) of SS in paper mill effluent. A
B
C
Figure 3. SEM images (200 × ) of SS: (A) 304 L before test, (B) 304 L after cyclic polarization test and (C) 316 L after cyclic polarization test.
SS sustain severe damage due to pitting in wastewater solutions containing chlorides.[4] Sulphate addition enhances pitting potential of SS in simulated effluent, as observed by the polarization test (Figure 5). Thus, anodic polarization curve of 304 L measured in solution with the polarization test (Figure 5). Thus, anodic polarization curve of 304 L measured in solution with SO− 4 ∼
172 ppm (curve-a) shows higher pitting potential as compared to curve-d (solution without SO− 4 ). Sulphate has been reported to inhibit initiation of pits on 304 L SS [20] and another report suggests that addition of sulphate in chloride solution decreases pit initiation site as well as current density.[21] Thus, more acidic effluents having higher Cl− but lower SO− 4 are expected to be more
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C
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B
D
Figure 4. (A) E vs. time curve, (B) Tafel plot, (C) cyclic polarization curve and (D) current vs. time curve (potentiostatic curve of 304 L) of stainless steels in synthetic effluent.
Figure 5. Anodic polarization curves of SS 304 L in synthetic solutions by varying pH, Cl− , SO− 4 , COD and BOD (wherever not indicated, pH, Cl− and SO− 4 correspond to 7.78, 500 and 172 ppm, respectively).
corrosive. Addition of potassium hydrogen phthalate and glucose–glutamic acid in solution provides target BOD and COD values does not show any significant change to the
localized corrosion behaviour of SS in the solution (curvesa and-c in Figure 5). However, the mill effluent is observed to be more corrosive as compared to synthetic effluent having similar pH, Cl− , SO− 4 , BOD and COD. It seems that nature of chemicals (even though they are contributing to same BOD and COD values) is more important in determining the corrosive nature of the mill effluent. According to earlier studies,[22] bleaching process using chlorine, ClO2 , or other chlorine compounds in paper mill leads to generation of chlorinated organic compounds. These have been identified as chlorate, chloroform, chlorophenolics viz. chlorophenols, chlorocatechols, chloroguaiacols, etc. [23]. These chemicals in mill effluent can be collectively analysed as adsorbable organic halides, which has been estimated ∼ 15–20 ppm in paper mill effluents and majority of them are chlorophenols.[24,25] Thus increased corrosivity of mill effluent was suspected to be due to chlorophenols and was tested accordingly. With a view to check the influence of chlorophenols, anodic polarization measurements were made on SS 304 L exposed in a solution having pH and Cl− similar to that in mill effluent and additionally 1 ppm of 2,
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C. Ram et al. by the decrease in phenolic compound concentration which have large over potential for oxygen evolution.[25] However, more investigations involving estimation of electrochemical potential of chlorophenols in the solution and analysis of corroding solution after the test are required to further corroborate the influence of chlorophenols on corrosivity of paper mill effluents. 4.2. Corrosion resistance of stainless steel Results of the immersion test and electrochemical tests show resistance of tested SS, against uniform and localized corrosion, to increase in the following order:
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304L < 316L < 2205. Figure 6. Anodic polarization curves of SS 304 L in chlorophenols (1 ppm ∼ 2, 4-DCP, 2, 5-DCP and 2, 4, 5-TCP) with and without chloride ( ∼ 500 ppm) at pH ( ∼ 7.78).
4-dichlorophenol (DCP), 2, 4, 5-trichlorophenol (TCP) and 2, 5-dichlorophenol. It is observed in all the three cases (Figure 6) that pitting potential decreases down to 400–430 mV which is somewhat lower than that observed in mill effluent (Table 5). A review of literature does not show any previous work relating to corrosion due to these chlorophenols. However, one study found that white water is responsible for corrosion in paper-making section where these organic compounds may be responsible for corrosion.[26] In another work, it was suggested that electrochemical treatment of chlorophenols initially leads to anodic and/or cathodic release of non-chlorinated phenolic and quinonic compounds along with chloride ions.[27,28] These chloride ions released from chlorophenolic compounds may be responsible for the decrease in pitting potential of SS 304 L in an aqueous solution [19] and thereby making it less passive. The electrochemical oxidation reactions involved in anodic oxidation of chlorophenols [28] are as follows: H2 O → OH∗ + e− + H+ . These reactions might occur due to the SS electrode being polarized to anodic potentials during the anodic polarization measurements in the present work. Another study suggests that passivation of electrode may be reduced OH
OH
The relative resistance of the studied SS against corrosion may be correlated with their composition through determination of pitting resistance equivalent number (PREN) [29,30] as given below: PREN = %Cr + 3.3x %Mo + 16x %N. Accordingly, the PREN of 2205 is the maximum (34.8), that of 316 L is 25.2 and 304 L is the minimum (19.9) showing 2205 to be most resistant while 304 L least resistant against corrosion. These conclusions are in accordance with those derived on the basis of the experimental results. Thus the presence of Mo and N alongside Cr is crucial for providing corrosion resistance to the SS. Authors have investigated the effect of nitrogen addition in austenitic SS (316 L) by polarization techniques (pitting/crevice corrosion and repassivation) and found that an increase in the nitrogen content retarded the crevice CR.[31] 5. Conclusions The present paper reports on corrosivity of paper mill effluent. Different corrosion tests were performed on SS 304 L, 316 L and 2205 for this purpose. The tests show an increase in corrosivity of effluent, with a decrease in pH and an increase in Cl− content. Presence of SO− 4 helps in reducing the effect of corrosion. A comparison of mill and synthetically prepared effluents, with simulating composition, shows the former to be more corrosive. The increased O
Cl
Cl
OH
OH
Cl
COOH COOH +
+ COOH
COOH OH ads Chlorophenols or dichlorophenol
O ads Chlorobenzoquinone
Carboxylic acid
Cl-
Environmental Technology corrosivity is suggested to be due to the presence of various chlorophenols which release Cl− as a result of electrochemical reactions. The comparative corrosion resistance of various SS is observed to depend upon their Cr, Mo and N contents.
Acknowledgement Mr Chhotu Ram is highly thankful to the Ministry of Human Resource Development (MHRD), New Delhi, India for the financial research grant for pursuing Ph.D. work.
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