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Corrosion of pipes and how to limit it
K.S. Mohamed
Abstract (Water distribution systems were used over centuries. Pipes of steel and iron is the basic elements of these systems.
Because these pipes are always exposed to atmosphere, oxygen and aqueous conditions, corrosion takes place. Corrosion is the
degradation of a metal due to exposure to environmental effects around it. Water we drink came from these pipelines which can
be corroded. Pieces of iron mix with water that turn it to red water. The corroded pipes are also good place for microbes and
increase the accumulation of arsenic and radium. Corrosion first takes place at the pipe walls and increase towards center.
Because of water around pipe surfaces, the corrosion results in rising the amount of soluble Fe (||).This paper will discuss how we
can solve such a problem using cathodic protection using Mg as a sacrificial anode. This will limit the corrosion of water pipes
and lead to deliver water safely.
INTRODUCTION1
Cast iron pipes corrosion has been one the most serious
problems for just as long. A complex process as pipes
corrosion can appear in many various and different ways.
These appearances can be pipe degradation and it can be
measured by weight loss, oxygen consumption, or corrosion
current. It can also be scale formation and it’s measured by
head loss or scale deposition. The last form of corrosion is
by-product release and this is measured by iron concentration, color, staining, turbidity, or number of customer
complaints. The main parameter that effects the corrosion
are several water quality parameters. These qualities can be
PH, alkalinity, and buffer intensity. Increasing the PH
makes both weight loss and degree of tuberculation
increase, in the PH range from 7 to 9. In contrast, increasing
the Alkalinity of water leads to decreasing weight loss and
corrosion rate. Increasing the alkalinity makes buffer
intensity increase, so, they almost have the same behavior
on the rate of corrosion and weight loss. Considering a pipe
is immersed in soil where there’s water and exposed
somehow to the atmosphere, anodic reaction is the reason
why corrosion occurs, as following,
Fe —–> Fe+2 + 2e ,
One of the main reasons of corrosion is the dissolved
oxygen (DO), it will make the following cathodic reaction,
O2 + 2H2O + 4e —–> 4OH–
DO makes iron an electron acceptor, the overall reaction
can be written as,
Fe (metal) + 0.5O2+ H2O↔Fe+2+ 2OH–
The oxidation of ferrous iron (Fe+2) or iron scales happens
because of dissolved oxygen, for example:
Fe+2+ 0.25O2+ 0.5H2O + 2OH–↔Fe (OH) 3(s)
3FeCO3(s) + 0.5O2↔Fe3O4(s) + 3CO2
4Fe3O4(s) + O2↔6Fe2O3(s)
So, the rate of corrosion can be increased by increasing the
dissolved oxygen. An important factor that must be
considered is the polarization. There are two types of
polarization, activation polarization and concentration
polarization. It happens when the hydrogen ions are
attached to the surface of the metal. Due to this attachment,
reduction will occur to the metal and this will result of
electron transfer. Then, hydrogen molecules will be formed
due to the combination the hydrogen ions with the
transferred electrons. Finally, Hydrogen gas will be formed
due to the accumulation of hydrogen molecules. The second
type is concentration polarization, it’s an electrochemical
reaction that happens due to the diffusion in the electrolyte
where the hydrogen ions in the soluble solution is diffused
to the metal surface. [1]
METHODS
The first method, electrochemical method, in this way we
are going to show the degree of PH after the long contact of
water with calcite. The difference in pH rapid-test is given
from the relationship (the difference of pH rapid-experiment
= PH (before -after)), where pH before is the value before
adding calcite and pH after the pH value using the marble
test. If the difference in pH rapid-experiment is greater than
zero, water is corrosive and is going to dissolve CaCO3. If
the difference of pH rapid-experiment is greater than 0,
water is supersaturated and is going to precipitate a layer of
CaCO3. If the Δ pH rapid-experiment is less than 0.1, then
water is saturated (in equilibrium) with CaCO3. The second
method, Physical test determining the sampling point, two
sampling bottles were in equilibrium in terms of
temperature with water from the sampling point in a water
bath. One of the two bottles contains sample water and
CaCo3 powder while the other one contains only sample
water. After 2 h under stirring in the laboratory, the total
alkalinity, pH and temperature was determined for both
samples. The water pH, Ca2+ concentration, DIC and total
alkalinity increase when the water is under saturated and
decrease if the water is supersaturated [2]. After knowing
the degree of corrosion from samples, we are going to use a
protection way to limit this corrosion. We will use an active
metal “more active than the material of the pipe” so as to
expose it to corrosion not the metal of the pipe. The metal
we will use is Mg. the sacrificial anode will be connected to
the protected sample through wielding them together using
wires [3].
RESULTS AND DISCUSSION
The redox reaction between the protected metal in the
cathode and the more reactive metal, sacrificial metal, in the
anode takes place spontaneously and the current flows from
the anode to the cathode forming a circuit. There’s higher
electromotive potential of the sacrificial anode than the
protected metal. An oxidation reaction occurs at the anode,
which means that the erosion will occur to the sacrificial
metal. On the other hand, the cathodic side will be reduced,
protecting the metal from erosion [4]. One advantage of this
system that it works spontaneously, so it requires no power
input. Another advantage is that it’s inexpensive and its
initial cost is low. Furthermore, it’s comparatively simple
installation and additional anodes can easily be added when
the initial ones prove to be inadequate and once installed,
testing the system components is relatively simple. On the
other hand, the current capacity is limited by anode mass
and therefore, protected metal consumption at low current
density. The driving voltage is low, so in high resistive
environment the anodes may not work [5].
CONCLUSION
Since these pipeline systems work for much time,
controlling the corrosion is very important to maintain
quality of water and the pipe in order not to leak. This paper
demonstrates how corrosion happens, methods to solve this
problem and results obtained. Mg was selected as it is more
active than the materials of the pipes, and it is acting as the
sacrificial anode so as to protect the pipe from corrosion.
The results was that Mg anode corroded instead of the pipe
“protected the pipe”
REFERENCES
[1] S. MCNEILL AND M. EDWARDS, “IRON PIPE
CORROSION IN DISTRIBUTION SYSTEMS,” AMERICAN
WATER WORKS ASSOCIATION, 01-JUL-2001. [ONLINE].
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02/J.1551-8833.2001.TB09246.X. [ACCESSED: 01-NOV2019].
[2] P. MELIDIS, M. SANOZIDOU, A. MANDUSA, AND K.
OUZOUNIS, “CORROSION CONTROL BY USING INDIRECT
METHODS,” DESALINATION, 05-JUL-2007. [ONLINE].
AVAILABLE:
HTTPS://WWW.SCIENCEDIRECT.COM/SCIENCE/ARTICLE/ABS/PI
I/S0011916407003165. [ACCESSED: 01-NOV-2019].
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ACRIFICIAL_ANODE. [ACCESSED: 01-NOV-2019].
[4] “SACRIFICIAL ANODE,” SACRIFICIAL ANODE - AN
OVERVIEW | SCIENCEDIRECT TOPICS. [ONLINE]. AVAILABLE:
HTTPS://WWW.SCIENCEDIRECT.COM/TOPICS/ENGINEERING/SA
CRIFICIAL-ANODE. [ACCESSED: 01-NOV-2019].
[5] “US5714045A - JACKETED SACRIFICIAL ANODE
CATHODIC PROTECTION SYSTEM,” GOOGLE PATENTS.
[ONLINE]. AVAILABLE:
HTTPS://PATENTS.GOOGLE.COM/PATENT/US5714045A/EN.
[ACCESSED: 01-NOV-2019].