THE EFFECT OF PH ADJUSTMENT ON THE INTERNAL CORROSION RATE OF CAST IRON AND ...

191 Pages·2010·11.47 MB·English

THE EFFECT OF PH ADJUSTMENT ON THE INTERNAL CORROSION RATE OF
CAST IRON AND COPPER WATER DISTRIBUTION PIPES
by
LOUISE MILLETTE
A THESIS SUBMITTED IN PARTIAL FULFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
FACULTY OF GRADUATE STUDIES
Department of Civil Engineering
We accept this thesis as conforming
to the required standard
UNIVERSITY OF BRITISH COLUMBIA
December 1985
Louise Millette, 1985
In presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, I agree that the Library shall make it
freely available for reference and study. I further agree that permission for extensive
copying of this thesis for scholarly purposes may be granted by the Head of my
Department or by his or her representatives. It is understood that copying or
publication of this thesis for financial gain shall not be allowed without my written
permission.
Department of Civil Engineering
University of British Columbia
2075 Wesbrook Place
Vancouver, Canada
V6T 1W5
Date: December 1985
ABSTRACT
Two experimental sessions were conducted to examine to effects of pH
adjustment on internal corrosion of cast iron and copper water distribution pipes. The
Greater Vancouver Regional District uses surface lakes as its potable water source. As
confirmed by chemical water characteristics monitoring, the supplied tap water has
several of the attributes of an aggressive water: low pH (4.9 to 5.7), low alkalinity
(10 to 15 mg/L as CaC0), low hardness (6 to 7 mg/L as CaC0), and dissolved
3 3
oxygen consistently near saturation.
Because of this aggressive nature, the tap water has a tendency to dissolve
water distribution pipes, and in particular domestic copper pipes (municipal cast iron
mains are cement lined for corrosion protection). This accelerated corrosion of copper
pipes not only increases the maintenance costs for home owners, it also encourages
high levels of copper in their tap water. This last finding was confirmed by a
preliminary tap water metal concentration survey wherein, after a month of sampling of
six dwellings, it was found that the recommended maximum copper level of 1.0 mg/L
was exceeded in 67% of the cold water morning first flush samples.
The investigated corrosion control measure consisted of pH adjustment to target
values of 6, 7 and 8, through the addition of hydrated lime (Ca(OH)). Cast iron and
2
copper pipe samples were exposed to pH adjusted water for durations ranging from
one to twelve weeks. For the most part, the adjusted pH experimental flow-through
units were gravity-fed; however, to examine the effects of the much higher normal
distribution system pressure, another set of control units were maintained at system
pressure.
Although it was found that the absolute winter corrosion rates were higher
than the summer rates, analysis of the relative coupon weight loss variations, with
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reference to the gravity control unit, lead to two major findings. The corrosion rates
of cast iron were ten times those of copper and the increased pH acted to enhance
these cast iron corrosion rates by approximately 15%. However, pH adjustment
successfully decreased copper corrosion by 68%. The effects of the increased pressure
on corrosion were different for both metals. The corrosion rates of cast iron coupons
in the control pressurized cells were twice the rates of the coupons in the gravity
control units. The effect of the increased pressure on copper was not as marked, but
the rates were found to be slightly lower than expected from the pressurized water
lower pH.
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Table of Contents
ABSTRACT ii
LIST OF TABLES viii
LIST OF FIGURES ix
ACKNOWLEDGMENTS xi
1. INTRODUCTION 1
1.1 Background Information 1
1.2 Statement of Purpose 4
1.3 Audience 4
1.4 Information sources 5
1.5 Working definition 5
1.6 Limitations of the Report 6
1.7 Scope of coverage 6
2. BACKGROUND AND LITERATURE REVIEW 8
2.1 Corrosion Phenomenon 8
2.1.1 Types of Corrosion 9
2.1.2 Mechanisms of Action 11
2.1.2.1 Electrochemistry 12
2.1.2.2 Thermodynamics 14
2.1.2.3 Kinetics 17
2.1.3 Monitoring and Evaluation 20
2.1.4 Control 22
2.2 Corrosion in Water Distribution Systems 26
2.2.1 Types of Corrosion 26
2.2.2 Influence of Pipes 28
2.2.3 Influence of the Water Quality 28
2.2.3.1 Water Stability Indexes 29
2.2.3.2 Calcium Carbonate 31
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2.2.3.3 Ions, Dissolved Gases and pH 33
2.2.3.4 Temperature and Flow Velocity 35
2.2.3.5 Bacterial Growth 37
2.2.4 External Influences 38
2.2.4.1 Stray Current 39
2.2.4.2 Users 40
2.2.4.3 Acid Precipitation 40
2.2.5 Control Measures 41
2.3 Copper Levels in Drinking Water 43
3. METHODOLOGY 46
3.1 Introduction 46
3.2 Laboratory Set-Up 46
3.2.1 Gravity Cells 46
3.2.2 Pressurized Cells 48
3.3 Water Quality Monitoring 49
3.3.1 Sampling Procedure 49
3.3.2 Analytical Procedures 50
3.4 Metal samples manipulations 54
3.4.1 Cutting and identification 54
3.4.2 Chemical preparation 57
3.4.3 Sampling procedures and schedules 59
3.4.4 Corrosion rate calculations 61
3.5 Private dwelling tap water survey 62
3.5.1 Sampling sites and methodology 62
3.5.2 Analytical procedures 63
4. RESULTS AND DISCUSSION ....65
4.1 Laboratory Water Characteristics 65
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4.1.1 pH, Hardness, Alkalinity, Acidity and Calcium 66
4.1.1.1 First Experimental Sequence (1984) 67
4.1.1.2 Second Experimental Sequence (1985) 74
4.1.1.3 Comparison and Discussion 80
4.1.2 Dissolved Oxygen, Chlorides, Nitrates+Nitrites and Sulfates 84
4.1.2.1 First Experimental Sequence (1984) 84
4.1.2.2 Second Experimental Sequence (1985) 89
4.1.2.3 Comparison and Discussion 94
4.2 Weight Loss and Corrosion Rate 98
4.2.1 First Experimental Sequence (1984) 98
4.2.2 Second Experimental Sequence (1985) 105
4.2.3 Comparison and Discussion 110
4.2.4 Laboratory Water Metal Content 116
4.3 Tap Water Survey Program 120
4.3.1 Average Concentration 121
4.3.2 Dwelling Comparison 124
4.3.3 Temperature and Residence Time Comparisons 129
4.3.4 Discussion 133
5. CONCLUSIONS AND RECOMMENDATIONS 135
5.1 Summary of Findings 135
5.2 Major Conclusions 137
5.3 Recommendations for Future Studies 138
6. REFERENCES 140
APPENDICES 149
" A. SURFACE AREA AND CORROSION RATE CALCULATIONS
EXAMPLE 150
B. WATER SAMPLING METHODOLOGY AND RESIDENT
QUESTIONNAIRE 153
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C. LABORATORY WATER AVERAGE METAL CONCENTRATION 156
D. FORTRAN PROGRAMS AND WEIGHT LOSS SUMMARY DATA ...158
,> E. AVERAGE METAL CONCENTRATION IN PRIVATE HOUSES 174
.v. F. MAGNESIUM, POTASSIUM, SODIUM AND ZINC LABORATORY
WATER CONCENTRATIONS VERSUS TIME 177
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UST OF TABLES
Table Page
2.1.1 Oxidation potentials 15
3.4.1 Content of Washing Baths 58
3.5.1 Private Dwellings Sampled for Tap Water 63
4.1.1 Summary of pH Related Data - First Experiment 67
4.1.2 Degree of Hardness 72
4.1.3 Variations in Calcium and Hardness - First Experiment 74
4.1.4 Summary of pH Related Data - Second Experiment 75
4.1.5 Variations in Calcium and Hardness - Second Experiment 80
4.1.6 Student t variations, Acid., AIL, Hard., Calcium 83
4.1.7 Summary of DO, CL', NO; + NO" , and SO;2 - First 85
2
Experiment
4.1.8 Percent Dissolved Oxygen Saturation - First Experiment 86
4.1.9 Summary of DO, CL", NO" + NO;, and SO;2 - Second 90
3
Experiment
4.1.10 Percent Dissolved Oxygen Saturation - Second Experiment 92
4.1.11 Student t variations, DO, Chlorides, Nitrates+Nitrites, Sulfates 95
4.2.1a) Comparison of Corrosion Rates - Cast Iron 111
4.2.1b) Comparison of Corrosion Rates - Copper 112
4.2.2a) Percent Changes in Corrosion Rate with pH - Cast Iron 113
4.2.2b) Percent Changes in Corrosion Rates with pH - Copper 114
4.2.3 Sept. 26, 1984 Turbid Tap Water Sample Metal Content 120
4.3.1 Maximum Acceptable Metals Concentration 121
4.3.2 Hot Water Temperature in Dwellings 132
A.1 Specific weights related data 152
Cl Metal Concentrations in mg/L - First Experiment 156
C.2 Metal Concentrations in mg/L - Second Experiment 157
E.1 Metal Concentrations in mg/L - HOUSE A 174
E.2 Metal Concentrations in mg/L - HOUSE B 174
E.3 Metal Concentrations in mg/L - HIGH-RISE A 175
E.4 Metal Concentrations in mg/L - HIGH-RISE B 175
E.5 Metal Concentrations in mg/L - LOW-RISE A 176
E6 Metal Concentrations in mg/L - LOW-RISE B 176
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L I ST O F F I G U R ES
Figure Page
1.1.1 GVRD Water Supply System 2
2.1.1a) Pourbaix diagram of copper in water . 16
2.1.1b) Pourbaix diagram of iron in water 16
2.1.2 General Polarization Diagram 18
2.1.3 Passivation and pit initiation 20
2.1.4 Cathodic protection by zinc addition 23
2.1.5 Anodic protection by impressed current 25
2.2.1 Effect of sodium chloride on the corrosion 34
3.2.1 Gravity Cells 47
3.2.2 Cell Dimensions 48
3.4.1 A Metallic Coupon 55
3.4.2 'Edge-clipping' Sites 57
3.4.3 One String of Coupons 59
3.4.4 Washing Motion 59
3.4.5a) First Experimental Schedule 61
3.4.5b) Second Experimental Schedule 61
3.5.1 Location of Sampled GVRD Private Households 64
4.1.1a) pH Values versus Time - First Experiment 68
4.1.1b) Frequency Distribution of pH - First Experiment 69
4.1.2 Acidity versus Time - First Experiment 70
4.1.3 Alkalinity versus Time - First Experiment 71
4.1.4 Hardness versus Time - First Experiment 72
4.1.5 Calcium versus Time - First Experiment 73
4.1.6a) pH Values versus Time - Second Experiment 76
4.1.6b) Frequency Distribution of pH - Second Experiment 77
4.1.7 Acidity versus Time - Second Experiment 77
4.1.8 Alkalinity versus Time - Second Experiment 78
4.1.9 Hardness versus Time - Second Experiment 79
4.1.10 Calcium versus Time - Second Experiment 80
4.1.11 Dissolved Oxygen Concentration with time - First Experiment 86
4.1.12 Chloride Concentration with Time - First Experiment 88
4.1.13 Nitrate+Nitrite Concentration with Time - First Experiment 88
4.1.14 Sulfate Concentration with Time - First Experiment 89
4.1.15 Dissolved Oxygen Concentration with time Second Experiment 91
4.1.16 Tap Water Temperature - Second Experiment 91
4.1.17 Chloride Concentration with Time - Second Experiment 92
4.1.18 Nitrate + Nitrite Concentration with Time - Second Experiment 93
4.1.19 Sulfate Concentration with Time - Second Experiment 94
4.2.1a) Specific Weight Losses of Cast Iron - First Experiment 100
4.2.1b) Specific Weight Losses of Copper - First Experiment 100
4.2.2a) Effect of High Pressure on Iron Noble Behavior 101
4.2.2b) Effect of System Pressure on Water Stability Region 101
4.2.3a) Corrosion Rates of Cast Iron - First Experiment 104
4.2.3b) Corrosion Rates of Copper - First Experiment 104
4.2.4a) Gravity Corrosion Rates of Cast Iron - First Experiment 105
4.2.4b) Gravity Corrosion Rates of Copper - First Experiment 105
4.2.5a) Specific Weight Losses of Cast Iron - Second Experiment 107
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