Most of the industries are adding biocide and inhibitors at the same point in cooling water systems. Inhibitors are added continuously and biocides are added weekJy once or once in fifteen days. It is not known as to wh.etber intel-ference effect between biocides and inhibitors will lead to any adverse effect. The present study bas been undertaken to know the influence of biocides on corrosion. Besides, to avoid the interference between biocides and inhibitors, some experiments have been carried out. Formaldehyde, Ethylene diamine tetra acetic acid (EDTA) and Cetryl trimethyl ammonium bromide (CTAB) have been evaluated for their biocidal efficiency. Further, the role of biocides on corrosion in the presence of inhibitors (morpholine phosphate + Zinc) has been im'estigated. From the corrosion point of view, morpholine phosphate with zinc acts as "very good mixed inhibitor". MOI-pholine pho phatc when used alone acts as a good biocide, but when combined with zinc, the killing efficiency was nil. Results suggest that bacteria should be killed first and inhibitors should be added later for getting higher efficiency and for avoiding the interference between biocides and inhibitors

Indiral, K.R.

Maruthamuthu, S.

Ponmariappan, S.

Rengaswamy, N.S.

Subramanian, A.

IR@CECRI

Hlll/elill otElel'lrochelllisll'Y 16 (5) Ma 2000, pp 209-213 0256-1654120001 3-'\0 © 200 ECRI INTERFERENCE BETWEEN BIOCIDES AND INHIBITORS IN COOLING WATER SYSTEMS S MARUTHAM THU, S _PO MARIAPPA . KR I DIR L, A SUBRAMANIAN" A D N S REI GA WAMY Centr I Electrochemical Research Institute, Karai udi 630 006. IND1A * Al gappa University, Karaikudi 630 003. INDIA [Recei cd: 30 November 1998 Accepted: 18 November 19 9J Most of the industries are adding biocide and inhibitors at the same point in cooling water systems. Inhibitors are added continuously and biocides are added weekJy once or once in fifteen days. It is not known as to wh.etber intel-ference effect between biocides and inhibitors will lead to any adverse effect. The present study bas been undertaken to know the influence of biocides on corrosion. Besides, to avoid the interference between biocides and inhibitors, some experiments have been carried out. Formaldehyde, Ethylene diamine tetra acetic acid (EDTA) and Cetryl trimethyl ammonium bromide (CTAB) have been evaluated for their biocidal efficiency. Further, the role of biocides on corrosion in the presence of inhibitors (morpholine phosphate + Zinc) has been im'estigated. From the corrosion point of ,'iew, morpholine phosphate with zinc acts as "very good mixed inhibitor". MOI-pholine pho phatc when used alone acts as a good biocide, but when combined with zinc, the killing efficiency was nil. Results suggest that bacteria should be killed first and inhibitors should be added later for getting higher efficiency and for avoiding the interference between biocides and inhibitors. Keywords: Biocidcs, inhibitor, biocidal activity, mixed inhibitor, interference. INTRODUCTION	 hydroxyethylidene diphosphonic acid (H DP), hyJroxy phosphonoacetic acid (HPA) etc. as co rosion inhibitor have been rep rted [1-41. the effect morpholine phosphatc with Fouling and corrosion are two important operational zinc a inl ibitor in the cooling water system pipcline~ ha' prohlem~ in heat xchangers and associated cooling water not been studied yet. sy ·tem pipelines. pipelines are known to be particularly In the present study morph line phosphate was used as su 'ceptible to [ uling induced corro ion due lo their basic inhibit r while biocidcs viz.. formaldehyde. ethylene dlal11il1c design characteristics and recirculati n of water. Though tetra aceli acid (EDTA) and cetyltrimethyl ammonium hiocides and inhibitors are used. problems have been noticed bromide (CTAB) were used to study their Inter[crCIlC' dlecl in various cooling water systcms. The prabl ms included on corrosion prucess_ Formaldehyde is	 ::l protein soluhle now blockage of pipes, pipe punctures and unacceptable hi cide, EDTA is a very go xl chelator and CTAS is a gcn r~ll corrosion ratcs of the system components, cationic biocide [5] fOI controlling fouling and corrosion, inhibitors are added 'onlinuulIsly and biocide are added weekly once or on 'e in aterials and method tiltc;en days. Hence. it is quite essential to study the Mild steel specimens (compositi n of sulphur 0.02 -O_lo/f. Il1lerfen;nce hetween biocides and inhibit rs for cooling ph phorous 003 -0.08%, manganese 0.4 -0.5'71·, carbon water systcm_ It was decided to evaluate three biocides along 2.0% and iron the r 5t) of dimensions J	 x. 4 x 0_2 cm and with one II1hibitor s) stem, 1 cm" were cut mechanically and polished to n arer fini h Though several studies all the usc of phosphonie acid Viz. me hanicall using emery finc grade paper and decreased lill11lnOlrin elhlcne phophonic acid (ATMP), with lri hloroethylne. Side f 4 x 0.2 cm 'pecimens used 3	 209 MAR THAMUTHU et (Jl. - Interference between biocides and inhibitors in cooling waler system for weight loss studies and I cm 2 specimen used for r Ilari/.ation tudies. Inhibitors and Biocides used h.rpholinc phosphate with zinc was used as inhibitor "y~lem. Formaldehyde (HCHO), Ethylene diamine !...etra acetic acid (EDTA) and Cetyl trimelhyl ammonium bromide (CTAB) W'fe used as biocidcs. Corrosion studies - Weight 10 s Data Initial weight of the mild tecl sp cimen before corrosion te l was n ted. Mild steel pecimen in tripli at were immer. cd in the test solution for a period of seven days. ftcr . even days, p cimens were pickled and washed in water and air rit.: . The los in weight wa detennined and cffi.,;iencie ass sscd for the following four 'ystems. y tern I T) a 'ess the influence f iocides in corro i n inhibiti n process, f nnaldchyd (125 ppm), EDTA (150 ppm) and C AB (25 P m) were added in pond water and weight loss data w r obtained after seven days. Sy tern n To assess th inhibit r efficiency, of morpholine phosphate (200 ppm) with zinc (25 ppm) was used as orrosion inhihl Ilr. tern ill To assess the influence of biocidcs (formaldehyde, EDTA and CTAB) and inhibitor (morpholine phosphate + zinc) and the interference effect between biocides and inhibitors on .:orrOSlon pr es, this system has been tudied. Biocides and inhi itors were added at the same time and corrosion inhibition effici ncies were calculated. System IV Bio.:ides were added in pond water to kill bacteria. After 24 hours, inhibitor was added in the same pond water and subsequently coupons were immersed for seven days to find out the weight Iss. ElectrochemicaJ Measurements A thrc electrode ell assembly was used. I cm2 mild steel specimen was used as th working electrode, large platinum foil count r electrode and saturated calomel electrode (SC ) a reference electrode respectively. Polaris tion studies were carried out u ing EG&G PAR model 173 potentiostat in combinati n with model 175 uni ersal programmer and X-Y recorder. Potential (6) Vs log current (i) plots were recorded at a swe p rate of I mV/sec. Corr 'ion pot ntial (E ), tafel slopes ba and b werecorrdet rmined from the plots. For electrochemical study, syst ms m and IV were used. TABLE I: Physical and chemical characteristic of pond water Parameter Value pH 8.2 Conductivity (11 ohms/em) 980 Alkalinity rnAJK 250 otal hardness (ppm) 216 Ca-hardness (ppm) 130 Mg-h, rdness (ppm) 86 Chloride (ppm) 160 , ulphate (ppm) 114 Total solid (mgfl) 736 Dis 01 ved olid' m"/l) 53 l Suspended lids (mg/l) 197 Chemical and Biological analysis of water Chemical 'hamct ristics of the pond water used in the investigation were nalysed by standard analytical method [6]. The pelagic bacterial population of pond water was enumerated using nutrient agar adopting pour plat technique. The chemical characteristics of pond w ler :lrc presented in Table 1. RESULTS Weight loss data The c rrosion rate of mild steel in pond water in pI' sen .e of biocide and inhibitor are presented in Tables II-V. Biocides on corrosion (System n Table IT brings out the influence of biocidcs on con·usion. The tested biocides did not contribute to corrosion but showed corrosion inhibition in the range between 3391 and 39% TABLE U: Corrosion rate of mild steel in pond water in presence and absence of biocide for 7 days (system I) Concn of Corrosion rate Efficiency Biocide biocide (mmpy) (%) (ppm) Control 1.14990 x 10-4 HCHO 125 7.64380 x 10-<; 34 EDTA 150 7.51850 X IO-s 35 CTAB 25 7.05115xlO-s 39 210 MARUTHAMUTHU et at. -Interference between biociJ~ and inhibitors in cooling water sy tell15 TABLE III: Corrosion rate of mild steel in pond water in presence of inhibitor for 7 days (system II) Inhibitor Concn of inhibitor (ppm) Corrosion rate (mmpy) Efficiency (%) Morpholine phosphate Morpholine pho'phate + /.lnc 200 200 + 25 7.9067 x 4.4226 x \ 0-5 10-6 31 96 Inhibitor on corrosion ( ystem II) Influcnce of inhibitor on corrosion is presented in Table III. Mar holine phosphat al '0 slightly inhibited corrosion and Lhe COlT sion inhibition efficiency was about 3\ %. While addi ng Zn, m rpholine phosphate drastically decreased (4.4226 x 1O--{;) thc corrosion rate and it showed a high inhibition efficiency of 96%. Biocides and inhibitor on corro ion (Systems ill and IV) In system ill, biocides and inhibito (Morph line pho phate+Zn) were added at the same time and corr sion efficiency was calculated. The inhibition ef Iciency is given in T ble II. In Lhe presence of form Jdehyde with inhibit r, the ef lcieney was bout 340/£. In the presence 0 ED 'A and CTAB with inhibitor, thc cfficiency wa about 350/£ and 31 % respectively. In sy t m IV, after kiIlin"of bacteria by biocide:, in presence of fom1ald hyde the corrosion inhibition was about 920/(.. While aUdin EDTA and CTAB the inhibition efficiencie~ ere about 85% and 94~ resp ctiv 1•• Electrochemical study The poLentiostaLic polarisali n d La of mild steel immer ed in p nd water r r ySLems III and IV are given in Table VI. T ABLE IV: Corrosion rate of mild steel in pond water in presence of inhibitor + biocides for 7 days (system III) Concn of Corrosion Inhibitor Biocide inhibitor & rate Efficiency biocide (mmpy) (%) (ppm) MPO,,+Zn HCRO 200+25+125 7.6438x I 0-" 34 MPO,,+Zn EDTA 2 0+25+150 7.5185x.1O-5 35 MPO~+Zn CTAB 200+25+25 7.9313xl0-5 31 TABLE V: Efficiency of inhibitor after kilhng bacteria by addition of biocides for 7 days Is)!>t m 1\'\ Biocide Inhibitor addition addition after 2nd da ( o/c l II (, l)::'HCRO MPO" + Zn g.8453 .< 10-<;EDT MP04 + Zn 1.6953 x. X5 CTAB MP04 + Zn 1.":\711 x 10 t. 94 In y'tcrn III, the inhibition efficiency with lorm;.\ldchYlk: was bOllt 75%, the ocr was about -6g5 ilIV. In pre ·CII(I.' of EDTA the inhibition efficiency as ahout RO';'; <lncl lit..: OCP was n positi v side y abouL 100 11 V wilen cumparcu to control. CTAB witll inhibit r go 'C COlT sinn efflcienc) o!' ab ut 78% but OCP was more or less same as with COnLn)1 In SYSl III IV, the C01Tosion inhibitiv_ percentage was hIgher wh n compared to syslem III. In presence (If l'orlllJkkhydc. the flicicncy wr at> ut 9 ,9% and OCP Wa.'; ,( 7( 111 V Inhibitor with . TAhiflcJ lh OCP Lo n gall e ~id(' 10 about -72 lV when comp,u'cd to cont]' 1 an ~)~It:1ll III The inhibition effiel 'ncies L,r ED A & CTAB were alhlUI 99,95 0 and 99.8771 r ~pL:cli\'cly in the'c systems TABLE VI: orro ion rate of mild st·c1 in pond water in presence ~md absence of biocides by polal"isation method OCP lror , Corrosion Effici· System (mV"s (~A/clln rate SCE) (mmpyl Control -653 -670 4.000 9().~702() Ill. Mixing of biocide + inhibilor ( .~/ tem III) x -685 -7 5 1.000 22.71760 7'i(1l1 y -560 -610 I.J( 0 24. LJHLJ 311 KUfHI Z -652 -692 0900 2.445!)(l Ii'UHl IV. Inhibitor addition after killing baclerial by addition of biocide (sy tem IV) x -676 -720 OJl04 (W)OH7 99 <.iO y -729 -635 O. 02 0.04544 IN )) Z -652 -693 0.005 () I 136lJ l)l) Xi x = HCRO + MP04 + Zn; Y = EDTA + MPO~ Zn Z = CTAB + MPO4 + ZI1 211 MARUTHAMUTHU et al. - Interference between biocides and inhibitors in cooling water systems Bacterial killing efficiency The bacterial killing efficiency of biocides along with inhihitor is presented in Table VII. A bacterial activity of about 101 was observed in 125 ppm of formaldehyde. In presence of EDTA the bacterial count was about 102, and in presence of CTAB the bacterial count was nil. It is indicated that CTAB acts as a very good biocide even at 25 ppm. The killing efficiency of morpholine phosphate was 100% but a large number of unshaped dead cells effici~ncy of morpholine phosphate was 100% but a large number of unshaped dead cells were observed in petridishes. When morpholine phosphate was combined with zinc, the bacterial growth was about 3.2 x 105. While the biocides formaldehyde and EDTA in combination with the inhibitor system. the bacterial count was very high, in presence of CTAB with inhibitor, the bacterial count was about 102. The variation in the pH value of pond water in various systems is shown in Table VIII. DISCUSSION Historically phosphates and chromates have been used as scale and corrosion inhibitors in cooling water systems [7-11]. Since phosphates act as good nutrients [12] for bacteria and inhibitor like chromate is toxic to environment, development of chemicals replacing phosphates and chromates are needed. Polyphosphates have easily hydrolyzable P-O bonds resulting in the formation of ortho phosphates. The ortho phosphates are not good corrosion inhibitors but good feed for bacteria and alagae resulting in biofouling effects, phosphates as such are not good corrosion inhibitors but in presence of zinc Ions they function as good corrosion inhibitors. TABLE VII: Bacterial density (Pelagic bacteria)of pond water in various systems for 24 hours Concentration Bacterial System (ppm) count (CFU/ml) Control (PW) 5.7 X 106 HCHO 125 1.3 x 10 EDTA 150 5.2 X 102 CTAB 25 Nil MP04 MP04 + Zn MPO.j + Zn + HCHO 200 + 25 200 200 + 25 + 125 Nil 3.2 x 105 5.2 X 106 MPO.j + Zn + EDTA MP04 + Zn + CTAB 200 + 25 + 150 200 + 25 + 25 5.4 X 106 5.4 X 102 CFU = Colony forming unit Corrosion It is observed -from the results that the combination of morpholine l'hosphate with zinc acts as a good inhibitor (96%). It is also found that the biocides reduce the inhibition efficiency while combining with inhibitor. Besides. in system IV, the inhibitor gives higher efficiencies and indicates that intt(rference between biocides and inhibitor is very low. The inhibition may be due to the formation or zinc phosphate or zinc hydroxide on metal surface [II]. But in system III. the biocides formaldehyde, EDTA and CTAB complex may disturb the absorption of zinc phosphate and zinc hydroxide which may interfere with the action of inhibitor. But in system IV, within 24 hours biocides killed the hacteria and interference between biocides and inhibitor was least. While adding morpholine to pond water, pH was about 9.62. and 7 days the pH decreased to 8.46. But the pH of the morpholine phosphate was about 7.86 on 1st day and 7th day, when morpholine phosphate combined with zinc. the pH was about 5.76 on 1st day and 6.15 on 7th day. However. the biocides did not influence the pH of the morpholine phosphate along with zinc. It indicates that pH influenced the activity of biocides and inhibitor. Adsorption of zinc phosphate or zinc hydroxide may be disturbed by biocides. In system IV, after addition of formaldehyde and CTAB. the pH was in the neutral range. But in prcsence of EDTA the pH was about 4.83 on first day and while adding inhibitor the pH was about 7.23 on 7th day. It can be infcn'ed that the inhibitor was working well in neutral pH and gave higher inhibition efficiency (95%). The pH data .suggest that mixing of inhibitor and biocides at the same time reduces the pH of the electrolyte which in turn may reduce the inhibition efficiency. On the other hand by adding biocides first and inhibitor subscquently the influence of pH on corrosion was found to be the least. It may be due to the higher pH on the 7th day. Potentiostatic polarisation results also indicate that the corrosion efficiency was low in presence of both inhibitor and biocides. While adding biocides first and inhibitor suhsequ~ntly. the efficiency was improved. It also indicates that the interference between biocides and inhibitor was the least in system IV. The nature of curves also explain that morpholinc phosphate + zinc along with biocides act as "mixed inhibitor" . Bacterial killing efficiency The killing efficiency of biocides (formaldehyde. EDTA and CTAB) with and without inhibitor are presentcd in Table VII. The bacterial density of about IOJ, 102 and nil were observed in presence of above biocides respectively. Besides, the inhibitor morpholine phosphate also acts as a 212 MARUTHAMUTHU et al. - Interference between biocides and inhibitors in cooling water systems -TABLE VIII: pH of the pond water in various systems for 7 days System Initial pH Final pH (1st day) (7th day) Control (PW) 7.90 7.50 HCHO 6.6] 8.00 EDTA 3.74 6.20 CTAB 5.41 8.49 Morpholine 9.62 8.46 MP04 7.86 7.87 MP04 + Zn 5.76 6.15 MP04 + Zn + HCHO 5.76 6.52 MP04 + Zn + EDTA 5.60 6.46 MP04 + Zn + CTAB 5.75 6.00 Inhibitor addition after killing bacteria by addition of biocide MP04 + Zn + HCHO 6.67 7.80 MP04 + Zn + EDTA 4.83 7.23 MP04 + Zn + CTAB 7.09 8.19 very good biocide. Formaldehyde kills the bacteria by solubilization of protein in phospholipid molecules. EDTA enhances the killing efficiency by chelating the trace metals present inside the bacteria. Eventhough, bacteria effectively utilize morpholine, CTAB acts as good biocide by disturbance of the arrangements of phospholipid molecules in the cell of the bacteria as the energy source and able to grow in high concentration of morpholine, Accumulation of ammonia inhibit the growth of bacteria [4]. Conversion ratio of morpholine to ammonia was reported as I :0.82 by Pseudononas sp. Morpholine phosphate when combined with line chloride gave low killing efficiency, This reduction in killing tendency may be due to the formation of morpholinium chloride which may not be easily degradable, Because, the process of degradation may be very slow, requiring 235 to 300 hrs for complete removal of morpholine from the electrolyte [4,] 3]. When biocides and inhibitors were simultaneously injected the bacterial killing efficiency of biocides was not found effective. Probably, biocides lost their activity against hacteria. Hence it is suggested that biocides and inhibitor should be added separately for getting higher efficiency. The present investigation has clearly shown that CTAB and inhibitor combination aets as good system for bacterial control, when compared to other biocidcs. The bacterial count was below the limiting level. CONCLUSION *	 From the corrosion point of view, 11l0rpholine phosphate with zinc acts as a very good mixed inhibitor system. *	 Bacterial killing efficiency of biocide CTAB along with inhibitor appears to be better than another systems. *	 It is not advisable to inject biocide and inhibitor at the same time. Bacteria should be killed first and inhibitor should be added later for gelling higher efficiency. Acknowledgment: Authors are grateful to Dr N Palaniswamy and Dr V S Muralidharan for their helpful discussion during the study. Authors thank the Director, CECRI, Karaikudi for providing necessary facilities. REFERENCES I.	 J Mathiyarasu, R Natarajan. 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Interference between biocides and inhibitors in cooling water systems

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