Influence of sodium hexa metaphosphate antiscalant on the corrosion of carbon steel in industrial cooling water system

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1 Indian Journal of Chemical Technology Vol. 17, May, pp Influence of sodium hexa metaphosphate antiscalant on the corrosion of carbon steel in industrial cooling water system Harish Kumar 1 * & R S Chaudhary 1 Department of Chemistry, Ch. Devi Lal University, Sirsa 1 0, India Department of Chemistry, M. D. University, Rohtak , India harimoudgil1@yahoo.co.in; rschaudhary1@yahoo.co.in Received 13 August 09; revised January In order to have clear understanding about the contribution of antiscalant and zinc ions separately towards the corrosion control of carbon steel in six different synthetic cooling waters, a single antiscalant, sodium hexa meta phosphate (SHMP), was tried with and without zinc ions, which may control scaling as well as corrosion problem in cooling water system (CWS). Weight loss, electrochemical polarization and metallurgical research microscopy technique were carried out at 30, and 0 C using different concentrations of the inhibitor and zinc ions. Mechanism of inhibition is explained on the basis of molecular structure of the inhibitor. The inhibitor provides good inhibition efficiency and acts as anodic inhibitor when present alone and mixed inhibitor with zinc ions. SHMP due to its cyclic structure, forms cagecomplex with Fe + ions from any two adjacent negatively charged phosphoryl oxygen anions. Moreover, addition of zinc ions to cooling water along with SHMP, do not increase inhibition efficiency to an appreciable extent. Keywords : Corrosion inhibitor, Antiscalant, Cooling water system, Carbon steel, SHMP The cooling water chemistry has changed steadily over the past decade. Use of highperformance antiscalants, corrosion inhibitors, polymer for better dispersion, total prevention of microbial deposits by using powerful biocides and biodispersants, ph boosters, and advanced hardness stabilizers is the current practice. But addition of so many different chemicals to the cooling water system makes the cooling water chemistry so complex and complicated that it is very difficult to control it by the operator. Many problems such as high cost of operation, compatibility with other chemicals, difficulty in understanding the cooling water chemistry, mechanism of inhibition, dosage of inhibitor required, etc. arise in cooling water system (CWS). A clear understanding of the contribution of antiscalant and zinc ions separately towards the corrosion control in a cooling water system is required. Further, the question whether it is possible to use one chemical only, which may control scaling as well as corrosion, is also to be answered. If it is possible to find an antiscalant, which can also provide protection from corrosion in the cooling water system, it will not only make cooling water chemistry simple but also reduce the operational cost. It is common to use antiscalants like phosphates 13, polyphosphates 46, organophosphonates 7 etc. as antiscalants in industrial cooling water systems. Some of these antiscalants may also influence the corrosion in cooling water systems. However, a systematic and detailed investigation has not been reported in the literature. Further, zinc ions are also added to the cooling water system to control the corrosion Derivatives of azoles 14,1, thiourea 1619, amines 3 and quaternary ammonium compounds 4 have been reported as corrosion inhibitors for carbon steel. But addition of these organic corrosion inhibitors to CWS creates fouling and microbial growth problems in cooling water systems 69. A new technique to determine antiscalant s efficiency for industrial cooling water systems has been reported earlier 30. In the present work, the influence of an antiscalant, sodium hexa metaphosphate (SHMP), has been carried out to understand its contribution towards corrosion control for carbon steel in synthetic cooling waters of various qualities in presence and absence of zinc ions. The purpose of taking six different compositions of cooling water was the wide range of hardness available in cooling waters in industry depending upon the source of water. Consequently, six different compositions of cooling water varying

2 18 INDIAN J. CHEM. TECHNOL., MAY from soft to hard water have been used for carrying out the experiments. Experimental Procedure Six synthetic cooling waters with different calcium hardness and chloride ions concentration were used for the experiments. The compositions of synthetic cooling waters are given in Table 1. Calcium ion concentration in synthetic cooling waters varied from 7 to 300 ppm. Concentration of the SHMP used as corrosion inhibitor was changed from to 0 ppm. The experiments were performed at 30, and 0 C as the temperature of cooling water varies from C at cooling tower to 0 C near operating machinery and equipment. All the chemicals used were of analytical reagent (AR) grade and solutions were prepared using double distilled water. The ph of all the cooling waters was maintained at 6. by adding sodium hydroxide or glacial acetic acid solution as per requirement. Duplicate or in some cases triplicate experiments were performed to verify the reproducibility and consistency of the experimental data. Carbon steel used for the investigations was in the form of sheet (0. mm thick) and had the following composition: C, 0.18; Si, 0.03; Mn, 0.1; S, 0.0; P, 0.06; Ni, 0.01; Cu, 0.01, and Fe, balance (w/w). For weight loss measurements, carbon steel specimens of 3 1. cm size were cut from the carbon steel sheet whereas for electrochemical polarization investigations specimens of 1. cm size were used. All the specimens were mechanically polished, in order to have mirror like finish, successively with the help of emery papers of grades 0, 0, 300, 0, and 0 micron, and thoroughly washed with double distilled water and then with acetone. The specimens were dried and stored in a desiccator over silica gel. After recording the initial weights of carbon steel specimens, they were immersed in tilted position in 0 ml beaker having 0 ml of synthetic cooling waters as corroding medium with or without the Table 1 Composition of synthetic cooling waters. inhibitor. Experiments were carried out in an electronically controlled air thermostat at 30,, and 0ºC with in an accuracy of ±0.1 C. After exposing the specimens for 30 days, the specimens were taken out from the beaker and washed initially under running tap water. Loosely adhering corrosion products were removed with the help of rubber cork and the specimen was again washed thoroughly with distilled water and dried and then weighed again. Corrosion rate in mils per year (mpy) and percentage inhibition efficiency were calculated. Influence of addition of zinc ion was also investigated on corrosion rate of carbon steel. For the surface study of the carbon steel samples before and after exposing to different synthetic cooling waters for 30 days, inverted trinocular metallurgical research microscope from Getner, Japan with measurement software attached with PC and printer was used. This technique was used to study the morphology of the corroded specimens of the carbon steel exposed to the synthetic cooling water maintained at ph 6. (by adding acetic acid or NaOH solution) for 30 days at 30ºC with and without SHMP. The comparison of micrographs of the corroded specimens with and without SHMP clearly gave an idea about the effectiveness of SHMP antiscalant as corrosion inhibitor. Electrochemical polarization studies were carried out in 00 ml glass cell with threeelectrodes system assembly. Potentiostatic polarization of the working electrode was carried out by using a Potentiostat/Galvanostat PGS 1 T (Tacussel, France). The working electrode was carbon steel under study; platinum electrode was used as counter electrode or an auxiliary electrode. All potentials were measured against a penciltype saturated calomel electrode as reference electrode. A Luggin capillary filled with test solution was used to connect the reference electrode with the cell. The tip of the Luggin capillary was kept very close to the working area of the electrode but without touching it in order to minimize the IR drop and this distance was kept Conc. (ppm) C. W. I C. W. II C. W. III C. W. IV C. W. V C. W. VI Ca Mg HCO CO Cl SO Na

3 KUMAR & CHAUDHARY: INFLUENCE OF SHMP ANTISCALANT ON CORROSION OF CARBON STEEL IN CWS 183 constant during the whole study. Only 1 cm area of specimen was used as working area and the rest of the area of the specimen was covered with a lacquer except the other tip of specimen which was used for making electrical connection. The specimen was left in the test solution until a constant opencircuit potential (OCP) was attained. Linear polarization resistance (LPR) measurements were carried out potentiostatically by scanning through a potential range of 14 mv above and below the OCP value in steps of mv. Experiments were carried out in absence and presence of SHMP at 0,, 0,,,, and ppm concentrations at 30,, and 0ºC. The resulting current was plotted against the potential and slope of the line was measured. The corrosion current, I corr is related to the slope of the line by SternGeary equation. E βa β = c i.303 I ( β + β ) corr a c (1) E where, is the slope which is linear polarization i resistance (R p ), β a and β c are anodic and cathodic Tafel slopes respectively and I corr is the corrosion current density in µa/cm. The anodic and cathodic Tafel slopes were measured after recording anodic and cathodic polarization curves of the specimen up to a maximum shift of ± 1 mv from OCP value in steps of mv. Experiments were carried out in absence and presence of the inhibitor at their 0,, 0,,,, and ppm concentrations at 30, and 0 C. Rearranging the above equation: I corr βaβc 1 = x.303( β + β ) R a c p () The corrosion current density I corr, is related to the corrosion rate by the equation, Corrosion rate (C.R.) (mpy) ICorr Eq. Wt. = (3) Density Results and Discussion Conductance, ph, total dissolved solids (TDS), P and M alkalinity, total hardness, calcium hardness, Langelier saturation index (LSI), G 31 and saturation index (SI) 3 were measured for each synthetic cooling water and are shown in Table. Percentage corrosion inhibition efficiency (PCIE) values of SHMP as corrosion inhibitor at various concentrations (,,,,, 0, and 0 ppm) in all the six synthetic cooling waters measured by weight loss method at ph 6. and temperature 30, and 0ºC have been shown in Table 3. The PCIE values for SHMP and zinc ion system at ph 6. and 30 C are given in Table 4. Cathodic and anodic polarization curves of carbon steel in synthetic cooling water III in absence and presence of,,,,, 0, and 0 ppm SHMP at 30ºC are plotted in Fig. 1. Similar polarization curves were obtained at 0 and ºC and have not been included in the paper. Cathodic and anodic polarization curves of carbon steel in synthetic cooling water III in absence and presence of,,,,, 0, and 0 ppm SHMP along with 0 ppm zinc ions at 30ºC are plotted in Fig.. PCIE versus concentration of SHMP and with 0 ppm zinc ions at 30ºC in synthetic cooling water VI are plotted in Fig. 3. It was observed that PCIE increases with increase of concentration of SHMP from to 0 ppm, decreases with the increase of chloride ion concentration in synthetic cooling waters I to VI (Fig. 4) and decreases with increase of Table Various parameters of six different synthetic cooling waters Parameters C. W. I C. W. II C. W. III C. W. IV C. W. V C. W. VI ph Conductance T.D.S palkalinity MAlkalinity CaHardness Total hardness L.S.I. from activity coeff G value S.I. value

4 184 INDIAN J. CHEM. TECHNOL., MAY Table 3 Percentage corrosion inhibition efficiency of SHMP in all six cooling waters by weight loss method at ph 6. and 30, and 0 o C. Water type CW I CW II CW III CW IV CW V CW VI SHMP Temperature (ºC) conc. (ppm) Table 4 Percentage corrosion inhibition efficiency of SHMP with zinc ions in all the six cooling waters by weight loss method at ph 6. and 30 o C. Water type C W I CW II C W III C W IV C W V C W VI SHMP (ppm) Zn + (ppm) temperature from 30 to 0ºC (Fig. ). SHMP was found to provide almost complete protection from corrosion at 0 ppm concentration to carbon steel in synthetic cooling waters I to III. It is evident from the Table 3 that PCIE decreases with increase in temperature from 30 to 0ºC. At 0ºC in CW I, SHMP gives 99.9% inhibition efficiency at 0 ppm concentration. At 30ºC, 0 percent efficiency is provided by SHMP at ppm

5 KUMAR & CHAUDHARY: INFLUENCE OF SHMP ANTISCALANT ON CORROSION OF CARBON STEEL IN CWS 18 Fig. 3 Percentage corrosion inhibition efficiency of SHMP and with 0 ppm Zn ions at different concentrations at 30 C in synthetic cooling water VI. Fig. 1 Polarization curve of carbon steel in C. W. III in absence and presence of different concentrations of SHMP at 30 C and 6. ph. Fig. Polarization curve of carbon steel in C. W. III in absence and presence of different concentrations of SHMP along with 0 ppm Zn + ions at 30 C and 6. ph. concentration. A rise of ºC in temperature of cooling water nearly doubles the concentration of SHMP for achieving the same protection from corrosion. At 0ºC the PCIE of SHMP in CW VI was Fig. 4 Percentage corrosion inhibition efficiency of SHMP at 0 ppm concentration and with 0 ppm Zn ions at C as a function of chloride ion concentration in all six synthetic cooling waters. very low (63.7) at 0 ppm concentration. To increase this efficiency upto.3 the concentration of SHMP had to be increased to 0 ppm. Various corrosion parameters like open circuit potential (OCP), β a (anodic Tafel slope), β c (cathodic Tafel slope), R p (Polarization resistance), i corr (critical current density), corrosion rate and percentage corrosion inhibition efficiency were calculated from

6 186 INDIAN J. CHEM. TECHNOL., MAY electrochemical polarization experiments for carbon steel in presence of different concentrations of SHMP at 30, and 0ºC in CW III and VI and have been shown in Tables and 6 respectively. In a previous study 30, percentage antiscaling efficiency of SHMP antiscalant at 30ºC has been reported (Table 7). It is clear from the Table 7 that SHMP is a good antiscalant and shows good antiscaling efficiency even in hard synthetic cooling water VI at a very low concentration i.e. from to ppm. PCIE of SHMP antiscalant measured in presence of different concentration of zinc ions from 0 to 0 ppm in all six cooling waters by weight loss method at ph 6. and 30ºC have been shown in Table 4. PCIE of SHMP at 0 ppm concentration versus zinc ion concentration at 30ºC in cooling water VI are plotted in Fig. 6. It is clear from the plot that PCIE increases slightly with increase in concentration of zinc ions in cooling water. PCIE of SHMP ( ppm) in cooling water I with out zinc ions was 81.0 but with 0 ppm Fig. Percentage corrosion inhibition efficiency of SHMP at 0ppm concentration and at 30, and 0 C in synthetic cooling water VI. Fig. 6 Percentage corrosion inhibition efficiency of SHMP (0 ppm) with 0, 0, 0 and 0 ppm of zinc ion in synthetic cooling water VI. Table Various corrosion parameters calculated from electrochemical experiments for carbon steel in C. W. III in presence of different concentrations of SHMP at 30, and 0 C. Conc. of inhibitor (ppm) Blank Temp. ( C) OCP (mv) β a (mv/decade) β c (mv/decade) R p (Ohmx 3 ) I corr (µa/cm ) Corrosion rate (mpy) % Inhibition efficiency by LPR % Inhibition efficiency by weight loss

7 KUMAR & CHAUDHARY: INFLUENCE OF SHMP ANTISCALANT ON CORROSION OF CARBON STEEL IN CWS 187 Table 6 Various corrosion parameters calculated from electrochemical experiments for carbon steel in C. W. VI in presence of different concentrations of SHMP at 30, and 0 C. Conc. of inhibitor (ppm) Temp. ( C) OCP (mv) β a β c (mv/decade) (mv/decade) R p (Ohmx 3 ) I corr (µa/cm ) Corrosion rate % Inhibition (mpy) efficiency by LPR % Inhibition efficiency by weight loss Blank Table 7 Percentage antiscaling efficiency of SHMP antiscalant at five concentrations in six types of synthetic cooling waters at 30 C. Water type CW I CW II CW III Conc. of SHMP (ppm) % antiscaling Efficiency zinc ions its efficiency increased to PCIE of SHMP (0 ppm) in cooling water VI without zinc ions was 78.4 but with 0 ppm zinc ions its efficiency increased to It is very much clear that addition of 0 ppm zinc ions to the cooling water do not increase the efficiency of SHMP appreciably. Clear pits are visible due to pitting type of corrosion in micrographs of the corroded carbon steel samples when exposed to CW VI for 30 days at 30ºC Water type CW IV CW V CW VI Conc. of SHMP (ppm) % antiscaling Efficiency (Fig. 7). The pitting corrosion in CW VI was due to presence of 69 ppm of chloride ions. No such pits are visible in micrographs of the carbon steel samples when exposed to CW III for 30 days. There is uniform type of corrosion in CW III in absence of inhibitor. There is no corrosion on carbon steel samples and surface is very smooth and clear as observed in micrographs in presence of 0 ppm SHMP + 0 ppm zinc ions in cooling water III.

8 188 INDIAN J. CHEM. TECHNOL., MAY Fig. 7 Metallurgical micrographs of carbon steel samples when exposed to C.W. III & VI for 30 days under 0X magnification. The electrochemical data and results obtained with SHMP as corrosion inhibitor in presence of various concentrations of zinc ions at 30ºC in CW III have been given in Table. OCP value increases in presence of SHMP and this change increases with increase in concentration of SHMP. There is increase in OCP value with increase in temperature of the cooling water in presence of SHMP. Anodic and

9 KUMAR & CHAUDHARY: INFLUENCE OF SHMP ANTISCALANT ON CORROSION OF CARBON STEEL IN CWS 189 cathodic Tafel slopes decreases in presence of SHMP, however, the decrease in anodic Tafel slope is quite large. Tafel slopes value changes slightly with increase in concentration of SHMP at all the investigated temperatures. This slight change in Tafel slope values with concentration of SHMP is not of any significance considering the mechanism of inhibition. Linear polarization resistance (R p ) increases appreciably when the concentration of SHMP is sufficient to provide high corrosion inhibition efficiency. Anodic Tafel slope, cathodic Tafel slope and R p decrease with the change in quality of cooling water from CW III to CW VI. OCP and corrosion rate increased with increase in chloride ion concentration from CW III to CW VI. On adding zinc ions to CW III already having SHMP, the anodic and cathodic Tafel slope increased suggesting that zinc ions influence both anodic and cathodic reactions. The values of β a and β a remain almost constant with change in concentration of SHMP in presence of zinc ions as well with change of zinc ion concentration from 0 to 0 ppm (Table ). Considering this SHMP should be regarded as anodic inhibitor. From the anodic and cathodic polarization curve it is evident that SHMP suppresses anodic reactions in absence of zinc ions and hence acts as anodic inhibitor. The addition of zinc ions shifts the corrosion current density towards lower current density value and this trend increases with increase in concentration of zinc ions. It was observed that both anodes and cathodes are polarized to considerable extent in presence of SHMP and Zn + ions suggesting that SHMP + Zn + ions formulation provides a mixed type inhibition. In SHMP, pentavalent phosphorous is tetrahedrally surrounded by four phosphoryl oxygen. This polyphosphate is water soluble existing as anion in aqueous solution which has twelve equivalent negatively charged oxygen atoms after dissociation. In SHMP, due to resonance, negative charge is dispersed over two oxygen atoms of phosphate group and hence two oxygen on phosphate group in SHMP are equivalent. SHMP forms cagecomplex with Fe + ions from any two adjacent negatively charged phosphoryl oxygen anions. The cagecomplex is more stable than simple SHMPFe + complex. SHMP also forms complex with zinc ions. However, complexes formed with Fe + ions are more stable in comparison to that of Zn + ions. Consequently, Zn + ions will be free to form ZnOH + and/or Zn(OH) to afford cathodic protection. SHMP Fe + complexes are adsorbed on anodic sites and suppress the anodic reaction. Mixture of SHMP and Zn + ions, thus acts as a mixed inhibitor whereas SHMP alone is anodic inhibitor. Conclusions From the results of the study the following conclusion can be drawn: (i) Corrosion inhibition efficiency increases with increase in concentration from to 0 ppm. (ii) Corrosion inhibition efficiency decreases with increase in temperature from 30 to 0 C. (iii) SHMP provides good corrosion protection to carbon steel even in synthetic cooling water VI having 69 ppm of aggressive chloride ions. (iv) SHMP acts as anodic inhibitor when present alone and mixed inhibitor with zinc ions. (v) SHMP forms cagecomplex with iron ions of carbon steel from any adjacent negatively charged phosphoryl oxygen anions. (vi) Addition of zinc ions to cooling water along with inhibitor, do not increase inhibition efficiency of inhibitor to an appreciable extent. Hence, addition of zinc ions to cooling water may be avoided as zinc ions always creates problems in cooling water. (vii) Complexes formed by SHMP with Fe + ions of carbon steel are more stable in comparison to that of Zn + ions. Consequently Zn + ions will be free to form ZnOH + and/or Zn(OH) to afford cathodic protection to carbon steel. SHMPFe complexes are adsorbed on anodic sites and suppress the anodic reaction. Mixture of SHMP and Zn + ions, thus acts as a mixed inhibitor whereas SHMP alone is anodic inhibitor. (viii) In soft water i.e. CW III there is uniform type of corrosion and in hard water i.e. CW VI there is pitting type of corrosion. References 1 Rajan K S & Venkatnath K, Proc Int Conf on Corrosion CONCORN, Mumbai, 36 Dec, (1997) 67. Bahadur A, Mater Perform, 37(4) (1996). 3 Jefferies J & Bucher B, Mater Perform, Aug (1993) 0. 4 Beer W J & Ertel J F, Handbook of Industrial Water Conditions, Betz Laboratories Inc (1991). Delavan R J & Mullins M A, CORROSION/84, Paper No 317 (Houston), TX: NACE (1984). 6 Kuzenestsov Y I, Corrosion control by corrosion inhibitors (Plenum Press, New York), 01, 8. 7 Sekine I & Hirakawa Y, Corrosion, 4 (1986) 7. 8 Veres A, Reinhard G & Kalman E, Br Corros J, 7 (199) Galkin T, Kotenev V A, Arponen M, Forsen O & Ylasaari S, Proc 8th Euro Symp on Corrosion Inhibitors, Ferrara, Italy, 1 (199).

10 190 INDIAN J. CHEM. TECHNOL., MAY Hirozawa S T, Proc 8th Euro Symp on Corrosion Inhibitors, Ferrara, Italy, 1 (199). 11 Galkin T, Forsen O, Ylasaari S, Kotenev V A & Arponen M, EUROCORR'96 NACE, Paper No II OR (1996). 1 Rajendran S, Apparao B V & Palaniswamy N, Proc nd Arabian Corrosion Conference, Kuwait, (1996), Rajendran S, Apparao B V & Palaniswamy N, Proc 8th Eur Symp on Corrosion Inhibitors, Ferrara, Italy, 1 (199) Walker R, Corrosion, 30() (1976) Patel N K, Makawana S C & Patel M M, Corros Sci, 14(1) (1974) Patel M M, Patel N K & Vora J C, Indian J Technol, (1974) Desai M M & Shah V K, Trans SAEST, 6 (3) (197) Desai M M & Shah V K, Trans SAEST, 7 (4) (197) Desai M M & Punjani B K, Anti Corrosion Methods Mater, 18(11) (1971) 47. Desai M N & Shah Y C, Corros Sci, 1(9) (197) Desai M N & Gandhi G H, J Electrochem Soc India, 1(1) (197) 161. Desai M N & Gandhi G H, Trans SAEST, 7(1) (197) Desai M N, Shah Y C & Gandhi M H, Corros Sci, 9() (1969) 6. 4 ElNaggar M M, Corros Sci, 4() (00) 773. Karpagavalli R & Rajeshwari S, WorkshopcumSeminar on Electroanal Chem and Apllied Topics (ELAC00), 7 Nov 01 Dec, (00) Sastri V S, Corrosion Inhibitors (John Wiley and Sons, Chichester), 1998, 3. 7 Bradford S A, Corrosion Control (van Nostrand Reinhold, New York), 199, 3. 8 Fontana M G & Staehle R W (eds), Advances in Corrosion Science and Technology (Plenum Press, New York), 1 (1970) McCoy J W, The chemical treatment of cooling water (Chemical Publishing Co, New York), Chaudhary R S & Kumar H, Indian J Chem Technol, 11 (04) Snoeyink V L, Water Chemistry, 1 st edn (John Wiley & Sons, Inc.), 19, American water works Assoc Joint task group on calcium carbonate saturation, (1990) 71.