In a piping system there are a number of
factors working alone or in combination that can effect
the rate of corrosion occurring in pipe. Depending on the
degree of these factors a new piping system can show signs
of corrosive wear in as little as two years after installation.
The corrosive effect the source water can have on the piping
system does not mean poor water quality is being delivered.
In fact in the Pacific Northwest region of the United States
and Canada, the source water quality has been rated as some
of the best drinking water in the world. Yet, that water
has a very corrosive effect on piping systems.
The rate of corrosion on a piping system is related to
basically these factors:
the pH of the water
the amount of oxygen in the water
the chemical make up of the water
the amount of galvanic corrosion from the use of dissimilar
metals contained in or in contact with the piping system
the temperature of the water
the velocity/pressure of the water in the pipe
THE pH OF THE WATER
Acidity of a water sample is measured on a pH scale. The
pH scale ranges from zero (maximum acidity) to 14 (maximum
alkalinity); the middle of the scale, 7, represents the
neutral point. Acidity increases from neutral toward 0.
The pH scale is based on logarithmic progression like the
commonly used “Richter” scale for earthquake
measurement. A difference of one pH unit represents a tenfold
change in acidity. Normal rain has a pH of 5.6 – slightly
acidic because of the carbon dioxide picked up in the earth’s
atmosphere by the rain. Acid rain with a pH of 3.6 has 100
times the acidity of normal rain with a pH of 5.6!
In copper pipe systems when the pH is more than 8, a copper
oxide film is usually formed on the pipe walls. This film
acts as a barrier that slows the effects of corrosion. However,
when the pH in the water supply is lower than 8 in the water
supply the copper oxide film (barrier) is dissolved leaving
no protective barrier and the pipe is subject to the corrosive
action of the water.
THE AMOUNT OF OXYGEN IN THE WATER
A domestic water system is an “open” system
where the water in use is always being replenished with
fresh oxygenated water. Dissolved air in water consists
of about 30% oxygen and the rest is mostly nitrogen, which
is non-corrosive. Oxygen degrades metals through an electro-chemical
process of internal oxidation. The result is that metal
gradually gets converted to an oxide (rust), becoming thinner
and weaker in the process. As the pipe corrodes the impurities
are deposited in the water lines. Encrusted build up is
the direct result of the oxidation process.
While oxygen content decreases under higher water temperature
and higher pressure it is these higher temperatures and
higher pressure conditions that speed up the oxidation process.
Experience shows that corrosion is more pronounced in hot
THE CHEMICAL MAKE UP OF WATER
Dissolved minerals in the water and the basic chemical
composition of the water may have differing effects on the
corrosive forces at play. For example a moderate to high
level of calcium would help form a protective coating on
the pipe which could slow down the corrosive effects. However,
high levels of calcium may cause a calcium build up in the
DISSIMILAR METALS - GALVANIC CORROSION
Galvanic corrosion, also known as electrolysis, occurs
when different metals come into contact with each other.
When conflicting metals are touching each other one of the
metals has a tendency to give up electrons. Basically at
the point of contact the metal giving up the electrons dissolves
over time. The most frequent cases of this happening are
when galvanized pipe and copper are connected; copper pipes
touch steel studs, or steel pipe hangers. Contrary to common
belief, the effects of galvanic corrosion are limited to
the immediate area of contact. The use of dielectric fittings
helps stop the problem but does not repair the resulting
thin walled and damaged pipe.
The higher the water temperature the faster the rate of
oxidation. Experience shows that corrosion is more pronounced
in hot water lines.
Water velocity problems are usually associated with a “closed”
loop piping system where the need to pump or circulate the
water is required.
Erosion corrosion occurs at locations where water turbulence
develops. Turbulence can be caused by excessive velocity,
sudden changes in direction (sharp turns, elbows) and through
“flow” obstacles such as burrs and solder excess.
In the case of copper pipe, a copper oxide layer is established
under the right pH conditions on the inside surface of the
pipe and this acts as a protective barrier. However, when
the water velocities are above 4 ft/sec the copper oxide
layer is destroyed or cannot form in the first place. Without
this oxide layer the metal will tend to deteriorate at a
more rapid rate.
The major contributing factors to this type of erosion
water velocities exceed 4 ft/sec
oversized circulation pumps
installation of undersized distribution lines
multiple or abrupt changes in the direction of the pipe
burrs on the inside of the pipe
improper soldered joints
improper balanced system
The internal corrosion of piping systems raises health concerns.
These health concerns include the negative health effects
associated with the leaching of lead, copper and other harmful
metals from water pipes into the drinking water supply.