study on prevention of corrosion for stainless steel …€¦ · hydrogen sulfide is a common...

*Author to whom all correspondence should be addressed: Email: [email protected]

STUDY ON PREVENTION OF CORROSION FOR STAINLESS STEEL PIPE IN

WASTEWATER TRANSPORTATION

Tauseef Khaliq*, Meisheng Liang, Walzli Yousaf, Muhammad Moeen, Zawar Hussain, Mudassir

Habib

College of Environmental Science and Engineering, Taiyuan University of Technology, Taiyuan 030024,

China.

Abstract 1

A wastewater transportation system is an essential system in an urban society that conveys the wastewater 2

streams from the commercial or residential areas towards the wastewater treatment plants. Wastewater 3

transportation system includes pipelines and other necessary installations to transport the water from one 4

place to another. So, the pipeline materials need to be able to withstand a variety of corrosive conditions that 5

can be encountered while delivering wastewater. The most common type of corrosion found in the stainless-6

steel pipes installed in wastewater is MIC (Microbiologically influenced corrosion). This study is focused 7

on studying the corrosion behaviour of the stainless-steel pipes installed in sewage water. The behaviour of 8

Stainless-steel type AISI 304 in wastewater has been studied. Different field tests were performed to study 9

the behavior in detail. Experimentation included measuring corrosion potentials, cathodic response, and 10

CPT (Critical pitting temperatures) for the AISI 304 type steel. This study provides an alternative material 11

AISI 316L steel and suggests this material to be used in place of AISI 3014 steel. In addition to the solution 12

provided by an alternative material (AISI 316L) in place of AISI 304 stainless steel, this study offers 13

another solution, for controlling the corrosion of pipes in wastewater streams, i.e., Cathodic protection. 14

Cathodic protection was applied on stainless steel type AISI 304, and significant improvement in protection 15

against the corrosion was observed. 16

17

Keywords—AISI 316L, Cathodic Protection, Cathodic Response, Corrosion potential, Manganese 18

Oxidizing Bacteria, MIC, Protection Potential, Stainless steel AISI 304. 19

20

21

International Journal of Advancements in Research & Technology, Volume 8, Issue 10, October-2019 ISSN 2278-7763 1

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1. Introduction 22

Stainless steels have been applied widely for transporting wastewater throughout the world (Rodrigo et 23

al., 2010). They are applied in a variety of diverse applications like delivery pipes, treatment equipment, 24

valves, etc. due to their durability and corrosion resistant properties (Ryan et al., 2002). Stainless steels have 25

corrosion resistance properties which are provided by the oxide film of Fe-Cr alloy. But even these steels can 26

be susceptible to different forms of corrosion including crevice corrosion, intergranular corrosion, and pitting 27

corrosion, etc. (Hu et al., 2011). In general, AISI 304 steel, AISI 316L, chromium nickel, chromium nickel 28

molybdenum, and duplex steels are commonly used materials for pipes in wastewater. 29

Major modes of corrosion in wastewater include microbiologically influenced corrosion (MIC), and 30

abrasive corrosion. MIC is usually caused by the presence of different microbiological agents within the 31

water which alters the local chemistry in the pipeline by generating corrosive chemicals. Wastewater 32

contains many biological and organic materials which are significantly active in the wastewater streams. The 33

bacteria which metabolizes the sulfur compounds are the most important from the corrosion‟s point of view 34

because this process may produce some acidic chemicals which cause corrosion in steel pipes. Some bacteria 35

can also oxidize the ferrous ions into ferric ions that result in even more corrosive environment. High 36

corrosion rates reduce the life of the pipes significantly (Maluckov, 2012). 37

Abrasive corrosion is another common type of corrosion during the transportation of the wastewater. It 38

includes the gradual degrading of a pipe surface due to mechanical wear. Mechanical wear can be caused by 39

suspended matter or entrained air bubbles (Chernov et al., 2002). 40

Organic components present in raw sewage include greases, fats, pesticides, surfactants, oils, and many 41

other aggressive compounds. Some of these compounds react with one another and create new substances. 42

There are many inorganic components in raw sewage including nitrogen, heavy metals, Sulphur, 43

phosphorous, strong alkalis, and various acids. 44

Some gases like methane, ammonia, hydrogen sulfide, nitrogen, and carbon dioxide are also commonly 45

found in wastewater (Dai and Victoria, 2013). Organic materials including nitrogen and Sulphur are 46

decomposed anaerobically which produces odorous compounds like volatile fatty acids, hydrogen sulfides, 47

and amines. Ozone disinfecting agents and chlorine add further threats of corrosion near the treatment plants 48

(Suffet et al., 2004). In addition to these corrosive compounds, many flocculent, coagulants, emulsifiers, 49

neutralizers, odor control agents, and anti-foaming agents are added in the streams of the wastewater at 50



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various points. So, the stream of wastewater carries enough corrosive compounds which can corrode stainless 51

steel pipes. 52

The interaction of the primary compounds produces secondary gases and chemicals with even more 53

corrosive properties. Microorganisms present at different stages of transportation produce gaseous and 54

chemical by-products. Hydrogen sulfide is a common by-product of bacteria related to MIC. SRB or Sulphur 55

reducing bacteria can reduce sulfates to sulfites in the presence of the anaerobic environment and produce 56

H2S (Hydrogen sulfide gas). Other aerobes including a strain of Thiobacillus may oxidize Sulphur to sulfuric 57

acid and lower the PH values up to 1.0 and attack the metallic pipes (Huber et al., 2016). Ferro/Manganese 58

oxidizing bacteria can also be responsible for the corrosion process. 59

Crevice and pitting corrosion can result when stainless steel encounters the wastewater which contains 60

microorganisms. MIC is not itself a form of corrosion, instead it is corrosion aggravated and initiated by the 61

presence of the microorganisms. 62

The average dimensions of microorganisms are usually in micrometers which allow them to settle in 63

inaccessible areas like interiors of pits and crevices where they can easily avoid the shear of the velocity of 64

the fluid. The small size of these microorganisms facilitates the dispersion and growth of microbial cells. 65

The problematic situation happens when plenty of these organisms combine to form a slime biofilm or 66

matrix. Structure of biofilm can easily be observed with the help of electron microscopes. 67

Various techniques of corrosion monitoring, and detection are reported in the literature. These techniques 68

include fractionation, mineralogy, PH test, and other qualitative analyses to find the presence of sulfides and 69

carbonates. The surface of pipes and corrosion scales are analyzed using Scanning Electron Microscope, 70

and elemental composition is usually determined by coupled EDS (Cui et al., 2016). The elemental 71

composition can reveal the root causes of the corrosion by identifying the composition of the corrosion 72

product (Little et al., 2011). Optical micrographs have also been used for observing the electrochemically 73

etched samples to study microstructures in detail. 74

As far as the protection techniques against corrosion are concerned, many approaches are being currently 75

used. One of the methods is to reduce the number of microorganisms in the wastewater by using many 76

oxidizing and non-oxidizing biocides. These additives kill the organisms which enter the wastewater stream 77

and decrease their growth rate in the biofilm (Liu et al., 2016). 78

Use of Biofilms has also been considered as a method to control MIC. Making use of biofilms is 79

reported in the following ways: 1) Using biofilms as a diffusion barrier to products of corrosion to suppress 80

the metal dissolution; 2) Respiring microorganisms in the biofilms are made to consume oxygen which 81

decreases the amount of reactant at the surface of the metal; 3) Some metabolic products produced by these 82



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microorganisms are also used as the corrosion inhibitors, for example, siderophores; 4) Some antibiotics 83

produced by microorganisms can also avoid the proliferation of the organisms like SRB which cause 84

corrosion (Bhola et al., 2010). Another method used for protecting steel pipes in wastewater transportation 85

is the cathodic protection which applies a negative potential to the materials and makes use of suitable 86

anodes to protect the steel pipes. This study focuses on the protection of stainless-steel pipes in the 87

wastewater by having a better combination of materials and applying cathodic protection method. 88

89

2. Materials and methods 90

A. Materials 91

Samples of stainless-steel pipe coupons and corrosion scales were collected from a local wastewater 92

transportation channel. Samples were taken from two pipes installed at different locations. One of the pipes 93

was made of AISI 304 type stainless steel, whereas the second pipe was made of AISI 316L steel. The 94

detailed composition of both grades is shown in table 1. These pipes had a diameter of about 250 mm. 95

96

97

Type EN C% Cr% Ni% Mn%

AISI

304

EN

1.4301

0.08 18.3 8.7 1.20

AISI

316L

EN

1.4435

0.03 17 11 2

98

The deposits and scales were removed from the pipes after tests by tapping them with a rubber hammer, 99

and these samples were stored in plastic bags for investigation. Digital photos of the samples were taken by 100

using HS digital camera (SX600). Then corrosion scales were converted into fine powders by grinding them 101

once they were pretreated (vacuum freeze-dried) (Yang et al., 2012). Pipe samples were obtained in the 102

form of ingots by cutting the pieces. The dimensions of these samples were 2mm x 50mm x 90mm. 103

Corrosion product was removed from these samples by using an iron brush, and then they were cleaned for 104

15 mins in acetone by using an ultrasonic washer. These samples were then air dried at normal room 105

temperature before using them for different analyses (Yang et al., 2014). 106

Table1: Chemical composition of AISI 304 and AISI 316L stainless steels.



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B. Field Test 107

The field tests were conducted on the pipe coupons of the dimensions 2mm x 50mm x 90mm. The 108

system made of PVC cell was designed for the system pulling water from the two sewage pipelines placed 109

near the treatment plant. These tests were conducted by using coupons of stainless steel. Two different types 110

of steels were used for a different source of water. The schematic diagram of the system is shown in figure 111

1. Corrosion potential for each coupon was measured during exposure of the samples by using two 112

reference electrodes and the data logger system. The temperature and conductivity of the water were also 113

measured. The flow rate through the cell was kept 5 liters/min that corresponded a flow rate of 0.0051 m/s 114

over the surface of the coupon. 115

116

117

118

119

120

121

122

123

Field experiments also included collecting water samples from the sewage streamline. These samples 124

were analyzed using potentiometric methods like ICP-OES to find out the conductivity and pH of the water 125

source (Mathiesen et al., 2003). 126

Polarization tests were conducted by using CorrOcean potentiostat (MKII Portable), and MMO coated 127

titanium was used as a counter electrode. The polarization was created from the corrosion potential in the 128

cathodic direction. Scan rate in the test was 1mV/s. Deposits obtained from the samples were analyzed with 129

ESEM, and the coupled EDS was used to determine the elemental composition. 130

Figure 1: Schematic diagram of field tests for

stainless steel samples



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Microbiological analysis of biofilms on the tested samples and water samples was done after the 131

exposure of at least 120 days. This analysis involved a wide range of evaluation methods which included 132

incubation of nitrate, iron, manganese, and SRB related bacteria. 133

CPT or the “critical pitting temperature” was determined with the help of polarization at the fixed 134

temperature in the lab. Flushed port cell like the standard ASTM G150 was used to avoid crevice corrosion. 135

Polarization in anodic direction was made from the corrosion potential, and the scan rate of 5.9 mV/min was 136

used. The solution for the test was prepared by adding different amounts of NaCl specifically (100, 500, and 137

1000mg/L Cl-) in de-mineralized water. The solution was unbuffered, and critical pitting potentials were 138

observed at 10A/cm2. Then these potentials were converted into iso-potential curves of CPT (Arnvig and 139

Bisgard, 1996). 140

The results of cathodic protection were investigated with the help of a full-scale system within a sand 141

filter. The impressed current was supplied to provide protection by using titanium anodes. The area covered 142

by each anode was 2.1 m2 in 250 mm pipes. The corrosion potential at 1.45 m from anodes was monitored. 143

The current applied in each system was also checked in the designed experiment. 144

3. RESULTS 145

C. Field Test 146

The PH of the water was found to be 7.12, and its temperature was between 20 and 25° C. Contents of 147

Manganese and chloride present in the water source are given below in table 2. Corrosion potentials were 148

measured for both the materials which are shown in the table. The behavior of potential was not very steady 149

initially but reached a high level above400mV SCE approximately within 40 to 50 days of exposure which 150

represented the formation of a biofilm. After this, the potential remained almost stable between the range of 151

420 and 450 mV SCE. It was observed that the stable potential level of the coupons was within a band of 20 152

mV approximately. 153

154

155

156

157



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158

159

160

161

162

163

164

165

D. Polarization Measurements 166

Polarization experiments were conducted along with the potential measurement to investigate the 167

cathodic behavior of the exposed steel samples. In general, both the materials showed a similar trend. Three 168

samples of each grade were tested, and the results agreed well with the potential measurements. Figure 2 169

shows the cathode curves for both the grades along with a fresh reference sample of AISI 304 (pickled) 170

which did not have any biofilm or surface deposits. 171

It was observed that cathodic response for AISI 316L type was lower as compared to AISI 304 type. 172

AISI 316 L has shown potential behavior close to the fresh sample. Curves for AISI 316L and the fresh 173

sample represented the limited current of about 10A/cm2 between 500 to 600 mV SCE. This current can be 174

associated with oxygen‟s limited transport to the surface. The current peak of AISI 316 L may reflect the 175

deposit which needs longer polarization time to be consumed completely. Once it gets consumed, the 176

cathodic rate may lower to the oxygen-limiting current. 177

178

Material Mn

mg/L

Cl-

mg/L

Ecorr (mV

SCE) max value

Deposit

AISI 304 0.15 261 450 Mn, Fe

AISI 316L 0.18 258 430 Mn, Fe

Table 2: Mn and Cl- contents in wastewater sources for two types of steels, maximum noted

corrosion potentials in the given situation, and deposits discovered in field tests



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179

180

E. Critical Pitting Temperature 181

Systematic CPT experiments were conducted to understand the risk level of pitting of stainless-steel 182

samples at relevant conditions of sewage streams by using a solution which was prepared for providing 183

different chloride contents. Measurements for steel types including EN 1.4435 (316L), and AISI 304 were 184

recorded at chloride contents between 100 and 1000mg/L. Highly oxidizing conditions and moderate 185

temperatures were used. The iso-potential curves for CPT were obtained with respect to the chloride 186

concentration and various temperatures. The potential of pitting was noted at the spot where the corrosion 187

current came to be more than 10 uA/cm2. In some combinations of temperature and chloride, the stainless 188

steel could not show pitting phenomenon regardless of the potential applied. In such a situation, the 189

temperature was below PICPT (potential independent critical pitting temperature) of the steel type. From 190

the data obtained from the provided chloride concentrations, iso-potential CPT and PICPT curves were 191

estimated for 300, 450, and 600 mVSCE. 192

The sample of AISI 304 steel was tested. It can be seen in figure 3 that for a particular value of corrosion 193

potential, the CPT decreases with an increase in chloride content which indicates the decrease in pitting 194

resistance. The curve at 450 mV is higher than the curve at 600 mV which shows that at 450 mV, the same 195

material is more resistant to pitting corrosion than at 600 mV. AISI 304 curve at 600Mv shows that material 196

has CPT value above 20° C at chloride concentration lesser than about 400mg/L whereas the curve at 197

450Mv shows that CPT is above 20° C for the chloride contents up to about 700mg/L. The material has 198

further improved CPT above 30° C at 450Mv for the concentration of chloride lesser than 500mg/L. 199

Figure 2: Cathode polarization curves of stainless-steel samples

after 100 days to exposure to wastewater.



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The sample of AISI 316L (EN 1.4435) type of stainless steel was also tested for 300, 450, and 600 200

mV SCE. Figure 4 shows the CPT curves for AISI 316L. The overall behavior of the curves is the same as 201

observed in the case of AISI 304. The CPT decreases for a particular value of corrosion potential when 202

chloride content increases. This trend indicates a decrease in pitting resistance. For 600 mV SCE, stainless 203

steel could not show pitting phenomenon regardless of the potential applied. In this case, the temperature is 204

below PICPT. The curves at 200 and 300 are higher than the curve at 450mv which shows that at 200 and 205

300 mV, the same material is more resistant to pitting corrosion than at 450 mV. 206

The conditions of corrosion potential between 300 and 500 mV, and the chloride content of 200 and 600 207

mg/L are essential to consider because mostly, wastewater streams have conditions within this range. It can 208

be noted that 300Mv and 450 mV curves for AISI 316 L sample (Fig 4) have CPT values above 30°C for 209

the concentrations of chloride up to 600mg/L. For the chloride concentrations higher than 600mg/L, CPT 210

values start falling but still above 20°C. On the other hand, 450 mV curve for AISI 304 has CPT values 211

above 30°C up to chloride concentration of 500 mg/L after which they start falling below 30°C. The 300Mv 212

curve for AISI 304 shows almost the same behavior where both the materials are having CPT above 40°C 213

up to the chloride concentration of 500mg/L. 214

215

216

217 Figure 3: CPT (Critical pitting temperature) of SS

304. Potentials were relative to SCE.



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218

219

220

221

222

223

224

225

226

227

228

229

230

But with respect to the conditions of the wastewater under current study which has chloride content 231

ranging between 250 and 270 mg/L, and corrosion potentials near 450 mV within the band of 20 mv, 232

encircled points are shown in figure 3 and figure 4 which represent the CPT value for respective types of 233

steel. 234

It is clear from the figure 4 that AISI 316 L has shown higher resistance to pitting in these circumstances, 235

and it has also shown higher chloride tolerance for overall results as compared to AISI 304 in figure 3. AISI 236

304 type of steel has shown lesser resistance in the presence of manganese bacteria unless strict conditions 237

are applied to achieve the optimum level of corrosion resistance for this type. 238

239

F. Surface Analysis of Deposits 240

SEM analysis was done to investigate the surface deposits of the pipe coupons after 100 days of 241

exposure. The major compounds found in the surface deposits were known by SEM/EDS analysis as shown 242

in table 3. Iron and manganese were found to be the major constituents of the surface deposits. 243

Figure 4: CPT (Critical pitting temperature)

of SS 316L. Potentials were relative to SCE.



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244

245

Element Weight % in deposits

on stainless steels

AISI 304 AISI 316L

Fe 37 31

Mn 0.2 0.08

C 0.05 0.04

246

Exposed coupons were found to have more, or less deposited dark brown deposit as shown in figure 5. 247

That deposit was confirmed as manganese dioxide by EDS analysis. Figure 5a shows the moderate deposit 248

on AISI 316L type steel whereas, Figure 5b represents the extreme deposit on AISI 304 type steel. 249

After removing the deposits by the process mentioned earlier in section II, the corrosion affected area of 250

the stainless steels was visible which is shown in figure 5c and 5d. Figure 5c represents the corrosion found 251

on the surface of the AISI steel which was exposed for over 250 days. Figure 5d shows the surface of the 252

AISI 316L after removing the deposited film, and no corrosion was found until that time. 253

254

255

256

257

258

259

260

261

262

263 Figure 5: Exposed coupons of stainless steel for over 250 days showing a) moderate deposit (AISI 316L). b)

Extreme deposit (AISI 304). c) Surface of AISI 304 after removing film. d) Surface of AISI 316L after

removing film

Table 3: Elemental analysis of biofilm obtained from the surface of exposed samples



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264

The deposits which depicted potential ennoblement, due to increased cathodic response, were also 265

studied in SEM. The samples were examined at low vacuum and wet conditions to preserve bacteria. SEM 266

image of one of the samples is shown in figure 6 which confirms the formation of MnO2 deposits. 267

268

269

270

271

272

273

274

275

The microbiological analysis of the samples confirmed the formation of biofilm which contained bacteria 276

products and cells. Traces of bacteria mainly included denitrifying, and manganese oxidizing bacteria. But 277

these bacteria were not identified in all of the samples observed for two types of steels under study. On the 278

other hand, Sulfur-oxidizing bacteria was only found in the wastewater sample. 279

280

G. Cathodic Protection 281

Experiments were conducted on newly installed cathodic protection of stainless-steel pipe AISI 304 at 282

the same location of sewage streamline. The impressed current and potential were given at three different 283

positions of the pipeline. Titanium anodes were used for the cathodic protection of the pipes by applying 284

constant power supply. 285

Three reference electrodes and four anodes were arranged alternatively and at equal distance of about 7% 286

of the diameter of the pipe. Figure 7 shows the schematic diagram of the setup. To calculate the range of 287

every anode, the current was supplied to anodes in stages for the first 30 days as shown in figure 9. The 288

protection of the anode “A” reached the nearest reference electrode “X” placed at the distance of about 7% 289

of the pipe dia. The next anode “D” which was connected after 18 days primarily affected its nearest 290

reference electrode „Z.” 291

Figure 6: SEM image of deposits found on the surface of the exposed AISI 304 steel showing

MnO2 deposits.



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292

293

294

295

296

After 30 days all anodes were connected, each supplying 10mA(0.5µA/cm2). From this point onwards 297

potential stayed at a low level for the next two to three weeks as shown in figure 8. Then this protection 298

potential started increasing slowly and reached between 140 and 180 mVSCE and remained stable for the 299

next 500 days. This protection potential was still having a lower value than the corrosion potential of pipe 300

having no protection. As we observed in CPT diagrams that the curves at 200mVSCE and 300 mVSCE 301

were higher than the curve at 450mv which showed that at 200 and 300 mV, the same material was more 302

resistant to pitting corrosion than at 450 mV. So, corrosion protection resulted in reduced potential, as 303

mentioned earlier in this section, that can have lesser risk of corrosion as evident from the CPT data. The 304

impressed current was then increased to 30 mA. The potential decreased to about 70 mA SCE. It was 305

observed that polarization data (figure 2) was in fair agreement with the current demand for CP. 306

An important observation during the tests was that the polarization as a result of cathodic protection was 307

extended to a distance equal to the diameter of the several pipes. When all the anodes were connected, the 308

potential started rising again after 3 or 4 weeks that was probably due to the alterations in the biofilm. At 309

this point, a further rise in current had the limiting effect. Cathodic protection was found sufficient even 310

after 600 days that was confirmed by removing the current for some time. 311

312

313

314

315

316

317

318

Figure 7: Cathode protection system and electrode arrangement

Figure 8: Potential development of the pipe and Impressed cathodic current for the cathodic

protection setup.



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319

4. Discussion 320

Corrosion observed in sewage pipelines is believed to be associated with the biological oxidation of iron 321

or manganese (Linhardt, 2010). In the case of manganese, it is biologically oxidized into manganese dioxide 322

which behaves as an active oxidizing agent when it is precipitated on the surface of the steel (Lutey and 323

Richardson, 2002). Failures due to such deposits have been recorded for many applications including 324

sewage streams, power plants, cooling water systems, etc. Another associated method has also been 325

reported which includes the oxidation of manganese because of strong oxidizers like hypochlorite or 326

chlorine present in water streams containing manganese content (R.E et al., 1996). Peroxide has also been 327

reported by some studies as a mechanism for oxidation of manganese (K.C.W). Although peroxide is 328

created by metabolism of a particular bacteria, these oxidizers are not very common in sewage streamlines. 329

On the other hand, there are many facts which point out the presence of Ferro/manganese bacteria in the 330

sewage streamlines that makes stainless steel corrode (Lewandowski and Hamilton, 2002). The presence of 331

manganese-oxidizing bacteria in most of the samples of both the steel types as confirmed by 332

microbiological analysis confirms the activity of these bacteria. Schematic diagram of bacterial activity is 333

shown in figure 9 which demonstrates the formation of pits as a result of MIC. 334

335

336

The manganese dioxide which was observed on the tested samples is found to be a cathodic material. 337

The presence of this cathodic material on the surface of the exposed samples explains the increased 338

potential in combination with an increased cathodic response as shown in figure 2. 339

Figure 9: Schematic diagram of Manganese oxidizing bacteria in the steel pipe applied

in the wastewater stream



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The concentration of manganese was measured in the wastewater as shown in table 2. This 340

concentration varied from 0.15 mg/L, for the wastewater in which AISI 304 was tested, to 0.18 for the 341

wastewater in which AISI 316L was present. No clear relation was found between the concentration of 342

manganese in sewage and the deposition of manganese dioxide film on the surface of the exposed samples. 343

ASI 304 was tested in the water having manganese concentration of 0.15mg/L, but the deposit on this 344

sample contained 0.2% of Mn. On the other hand, AISI 316L was tested in the wastewater having 345

manganese content of 0.18 mg/L, but the Mn content found in its deposit was 0.08 as shown in table 3. This 346

behavior shows a highly selective mechanism of manganese deposition. This behavior, in the absence of 347

any strong oxidizer, can only be referred to as the activity of the bacteria (Dickinson and Pick, 2002). 348

It took about 20 to 30 days for the potential to reach its maximum value. This behavior can be linked to 349

the time-dependent nature of the biofilm buildup on the surface of the exposed steel (Wiatr, 2013). The 350

same level of corrosion could not be reproduced on the tested samples as it is usually seen on the damaged 351

pipe systems. It was due to this reason that initiation of this type of corrosion takes considerable time and 352

the area of the coupons was also too small to have required cathodic area to initiate the active attack. But, 353

the behavior of the results obtained from the tests and corrosion potentials of the material well depicted the 354

behavior of the material in the given circumstances which are obvious from the obtained analysis of the 355

deposition films. 356

While selecting the stainless materials, it was a bit difficult to predict the exact conditions of the 357

discussed sewage streamlines. So, the general potential criteria of 300 mV SCE was used to determine the 358

CPT (Critical pitting temperature) in the sewage water. It was expected that this value would provide a safe 359

margin for AISI 304 steel type. But, the results of the tests have shown that there is a need to reconsider the 360

design criterion while selecting grades of stainless steels for the sewage water streamlines. CPT diagrams 361

were achieved for the potentials and chloride contents of the relevant range. 362

With respect to the conditions of the wastewater under current study which has chloride content ranging 363

between 250 and 270 mg/L, and corrosion potentials near 450 mV within the band of 20 mv, it is clear that 364

AISI 316 L has shown higher resistance to pitting in these circumstances, and it has also shown higher 365

chloride tolerance for overall results (figure 4) as compared to AISI 304 in figure 3. 366

After the careful selection of the materials, another approach to prevent the process of corrosion can be 367

cathodic protection. This method can be applied to the systems which are already in use or suffer from 368

corrosion. By providing the appropriate cathodic protection, these systems can be protected from the 369



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process of corrosion. This system seems to be the only feasible system especially for the buried and 370

complex pipelines in sewage systems where environmental control or coatings may be difficult to apply. So, 371

the cathodic protection test was conducted on AISI 304 stainless steel pipe which have shown poor results 372

without any protection. To install new systems, material choice can be the option. But, for already installed 373

pipes and even for the new pipes cathodic protection can provide sufficient protection against the process of 374

corrosion. The protection potential obtained by applying cathodic protection was still having a lower value 375

than the corrosion potential of material which had no cathodic protection. So, based on these results, 376

cathodic protection can be considered as an appropriate technique to minimize the need for pipe 377

replacements at an extensive level (Liu and Frank, 2010). 378

It can also be observed that conventional analysis of water is usually an insufficient way to understand 379

the behavior of corrosion and the potential risk of MIC in case of the stainless steels in particular media. 380

The best approach to understand the behavior of corrosion can be the analysis of the CPT data along with 381

the study of careful potential measurements. 382

In addition to material selection and cathodic protection, another approach can also be used for reducing 383

the number of deposits on the surfaces of the pipes. Pipelines should have enough slope to have an 384

enhanced natural flow. Scour velocities of greater than 0.6m/s can minimize the sediment deposition 385

significantly and reduce the chances of corrosion. Suggested slopes for various pipe sizes are given below in 386

table 4 (North Carolina Department of Environmental and Natural Resources). These slopes can help 387

achieve the desired speed of 0.6m/s in the wastewater stream which can help reduce the corrosion. 388

389

390

391

392

393

394

395

396



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Table 4: Minimum suggested slopes for different pipe sizes. 397

398

399

400

401

402

403

404

405

406

407

408

409

5. Conclusion 410

The results of all the tests have shown that the primary form of corrosion found in stainless steel pipes 411

applied in the sewage streams is the microbiologically induced corrosion (MIC). The studied systems 412

revealed that the main reason behind this microbiologically induced corrosion was the presence of 413

manganese-oxidizing bacteria which oxidizes manganese to manganese dioxide which gets deposited on the 414

surface of the exposed stainless-steel pipes. Both AISI 304 and AISI 316L types of stainless-steel types 415

were attacked by the activity of the bacteria, but the type 316L showed more resistance in the given 416

conditions. 417

Results of field tests also confirmed the relationship of corrosion of stainless-steel pipes in wastewater 418

with the microbiological activity which resulted in the increase of corrosion potentials between the range of 419

410 and 450 mV SCE. 420

Pipe Diameter

(Inches)

Required slope

(Minimum)

(Feet/100 feet)

6 0.61

8 0.40

10 0.29

12 0.22

14 0.165

16 0.14

24 0.09

36 0.054



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The results obtained from the chemical analysis of the surface deposits of the exposed samples showed 421

the presence of manganese and iron content. The manganese content was higher even for the sample which 422

was exposed to wastewater having lower manganese content. This finding further confirms the results of 423

microbiological analyses which indicated the presence of oxidizing bacteria and predicted its presence to be 424

the main reason for the increase in corrosion potential and corrosion in stainless steel pipes. 425

So, this study suggests that corrosion of steel pipes applied in the wastewater streams can preferably be 426

avoided by selecting the proper grade of stainless steel carefully. In our case, it is suggested to prefer AISI 427

316L over AISI 304 because the results obtained from CPT data showed an increased risk of corrosion in 428

wastewater between 0°C and 20°C. 429

Alternatively, cathodic protection can be used to prevent the attack of corrosion in stainless steel pipes. 430

The protection potential after applying the cathodic protection was having a lower value than the corrosion 431

potential of the AISI 304, which shows that cathodic protection can prevent the occurrence of corrosion in 432

stainless steel pipes and thereby decrease the need of frequent replacements. 433

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6. FUTURE WORK 435

Future work includes measuring the corrosion behavior of stainless-steel pipes at increased velocities of 436

the wastewater. Wastewater flow will be enhanced up to the suggested flow speed in a test cell, and the 437

same tests will be conducted to study the detailed behavior of corrosion at enhanced speed under the same 438

conditions. 439

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ACKNOWLEDGEMENTS 448

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