Corrosion
of Water Pipes
Albert C. Holler
A paper presented at the North Central Sect, meeting on Sep.
30, 1971, by Albert C. Holler (Active Member, AWWA), vice
pres.-chem.. Twin City Testing and Engrg. Lab., Inc., St Paul,
Minn.
Many factors contribute to water-line corrosion. The
author briefly reviews the more common types, with
emphasis on the problems confronting copper services.
Although copper lines have performed well in many in
stallations, in some, the results have been disasterous. Cop
per, it was found, could be as subject to corrosion as iron or
steel.
Forms of Corrosion
Uniform attack. In uniform attack the water reacts with the
metal to give a uniform depth of penetration of the metal over
the entire surface. Acid solutions normally give rise to this
type of attack. Water with a high content of dissolved salts
and high electrical conductivity also give a uniform corrosion
attack.
A surface covered with a uniform deposit probably will un
dergo a reasonably uniform type of attack. However, a deposit
like calcium carbonate, which occurs from hard water will
reduce the rate of attack to almost zero.
Grooving. Grooving attack is the severe thinning of the pipe
section in certain areas. The grooves are free from corros;ion
products. Grooving is characteristic of acid-condensate at
tack; that is, condensed steam that contains dissolved carbon
dioxide (carbonic acid). Steel and copper condenser tubes are
corroded by this grooving when large amounts of carbonates
are present in the boiler-feed water. Severe thinning of the
lower half of the horizontal returns is caused by the flow of
acid condensate through the partially filled pipes. Grooving
can also occur at a bimetal junction and will be largely con
centrated on the anodic metal close to the line of contact
Pitting. Pitting corrosion is the most serious form of
localized attack because depth of penetration usually approxi
mates the diameter of the corroded area.
Pitting often results in the perforation of the pipe even
though the amount of metal corrosion is very small. The size
of pits may vary from shallow-hemispherical to pin-hole size
in which the depth of penetration is much greater than the
diameter.
Corrosion by pitting is usually expressed in terms of the
deepest pit, which makes more sense than expressing il in
terms of distribution or of an average number of the deepest
pits.
The pitting phenomenon is not clearly understood. It starts
with a small anodic area and a large cathodic area. The rate; of
attack at this small anode is a function of the conductivity of
the water and the cathodic and anodic polarization.
Tuberculation. Mounds of corrosion product can form on the
surface of metal from corrosion processes occurring on the
surface; the mounds appear as tubercules, thus the name
tuberculation.
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MANAGEMENT
Mounds of corrosion product are associated with pitting
and overlie anodic areas where localized attack is taking place.
These mounds are often so large and numerous that frictional
resistance to flow is increased, and the carrying capacity of the
pipe is greatly reduced.
These tubercules seen in steel or cast-iron water mains
have a hard outer crust of brown hydrated ferric oxide and an
inner layer of black magnetic or green rust. The crust
physically separates the anodic area within the pit from the
area outside where the oxygen reduction takes place.
Sulfates and chlorides concentrate within the pits, resulting
in a solution that is slightly acid, irrespective of the pH of the
water outside. The oxygen concentration in the tubercule is
about zero. The acidity tends to accelerate the rate of attack
by providing a secondary cathodic reaction with the liberation
of hydrogen.
It should be noted that tuberculation in copper and its
alloys is rare.
Exfoliation. This peculiar form of corrosion can occur with
80/20 or 70/30 cupro-nickel tubes in high-pressure feedwater heaters. The corrosion is severe, with an exfoliating
scale consisting of a mixture of copper and nickel oxides in
the same proportion as the metals present in the alloy. The
scale grows and flakes off.
Waterline Attack. Localized corrosive attack can take place at
a three-phase boundary metal-water-air as would occur
at a waterline or at the edge of a droplet of water on a metal
surface.
Corrosion arises from two sources the ready access of ox
ygen to the metal surface through the thin meniscus region
and the difficult access of solution to the surface owing to the
crevice formed between the air-liquid surface and the
metal-liquid surface. The greater access of oxygen to this
region creates a concentration cell in which the area of the
meniscus is cathodic with respect to the rest of the surface.
Crevice attack. A crevice is an ideal spot for localized corro
sion because of the limited access of electrolyte to the area in
the crevice. This results in a corrosion concentration cell
because of differences in salt, hydrogen ions, or oxygen con
centrations.
Crevices can be formed when a metal makes contact with
another piece of the same metal, other metals, or any nonmetallic materials.
Crevice attack can start in inaccessible corners produced by
bad design or beneath foreign matter that settles on the sur
face.
Carbonaceous matter can act as a cathode; more commonly
known cathodes, however, are mud, sand, stones, cinders,
iron scale, shells, and textile fibers. Intersection of wires,
tubes rolled into tubes, threaded junctions, screwed joints, or
inverted seams can form good crevices and places for attack.
The shape of the crevice is important, and the intensity of
attack within the crevice is dependent on the ratio of the area
between the opening and the exposed cathodic area.
An example of crevice attack involving copper alloys would
be the combination of bronze and stainless steel. Under nor
mal conditions the two materials would be compatible but a
stainless steel axle running in a bronze bushing has proved
otherwise. This is because the lack of the oxygen supplied to
the stainless steel reduces its potential corrosion resistance to
iron, and a powerful galvanic cell is then formed between the
bronze and the active steel.
Dezincification. Brass can be selectively corroded by soft
water containing carbon dioxide. This results in the brass losJOURNAL AWWA
ing its zinc and being converted to a porous mass of soft, brit
tle copper. The same can occur with bronzes in which the tin
of the bronze is lost; this is destannification.
Corrosion erosion. When water flows at a sufficiently high
velocity so that appreciable turbulence is created, intense
localized corrosion may occur by the combined action of cor
rosion and mechanical abrasion. This is known as corro
sion-erosion and includes both impingement attack and
cavitation.
Impingement attack is caused by the local breakdown of a
protective layer by suspended solids in the water or gas bub
bles that impinge on the surface or by turbulence alone. Con
ditions for impingement attack can be found at the entrance
to pipes, sharp bends, near deposits, or where the cross sec
tion of the flow changes abruptly.
Large gas bubbles can strike the surface with enough force
to break up the smaller bubbles, releasing enough energy to
break any film or layer of corrosion product. Pits are formed
where these impacts keep occurring at the same surface spot.
These have a sharp-edged appearance, are free from corro
sion product, and usually show undercutting of the metal on
the downstream side. This gives rise to horse-shoe shape pits.
Copper is most susceptible to this type of corrosion.
Cavitation attack always occurs under conditions of tur
bulent flow and is a very serious form of severe and rapid
wastage of metal. It is caused by vibration and by the forma
tion and collapse of vapor-filled cavities at the water-metal
surface where sudden changes occur in pressure. This type of
corrosion is found on pump impellers and in valves; the main
damage to the metal is mechanical.
Cracking. Metals may fail as a result of cracks. In corrosion
cracking the amount of metal removed is small, but the
damage caused is great. The cracks formed may intergranulate the crack follows the grain boundaries; cracks may also
transgranulate the crack runs across the grains of metal.
When the metal pieces also are stressed or contain residual
stresses, the piece may fail by a process called stress-corrosion
cracking. Copper is not very susceptible, but brass is very
easily cracked by stress-corrosion, also termed season crack
ing. Vibration in brasses can also cause a corrosion-fatigue
situation.
Galvanic corrosion. The last form of corrosion to be con
sidered here is galvanic corrosion. A most common form of
galvanic corrosion used everyday but not usually considered a
corrosion process is found in a battery flashlight. Here, a
galvanic cell is set up between the zinc battery case and the
carbon electrode.
In water-distribution systems galvanic cells arise from the
use of dissimilar metals for piping. For example, the cast-iron
or steel water mains are connected to copper home-distribu
tor pipes. This results in a bimetallic couple, and corrosion oc
curs at the junction of the two metals.
Corrosion Characteristics
Pure copper has many technical uses despite its poor me
chanical strength because of its high chemical stability in a
number of corrowive solutions and its high-heat and highelectrical conductivity. It is limited in temperature range
because of the formation of relatively nonprotective oxides at
high temperatures.
The corrosion resistance of copper is due to its being a
relatively noble metal. The formation of thin adherent films
of corrosion products copper oxide and copper carbonateaccount for copper's satisfactory service in waters.
AUGUST 1974
However, in solutions that contain copper-complexing
agents such as cyanide or ammonia, and particularly in the
presence of oxygen from the air, copper is severely corroded
with the formation of complex ions.
Copper is attacked very slightly by distilled water. In this
case, copper ions are leached into solution.
Soft waters containing appreciable amounts of free carbon
dioxide have a great effect on copper. Such waters may be so
corrosive that green copperarbonate stains are found on
plumbing fixtures.
Copper carbonate forms a protective film on copper in hard
waters; thus, these waters are seldom corrosive to copper.
At 140F, waters with a high-temporary hardness, become
corrosive, since they are softened by heating. The sodium bi
carbonate present breaks down with the release of carbon
dioxide. Of the many copper-corrosion problems encountered
by the author, the majority have been of the corro
sion-erosion or erosion forms.
The first evidence of corrosion or trouble with the copper
tubing is that it leaks. Pinholes or leaking around the brazed
or sweated joints is the most common form of failure.
Solutions
The first approach to the solution is to examine the internal
surface microscopically.
Are there bright worn spots on the surface? Is it grooved?
Can the undercutting of the downstream side of the gouges be
seen? Are the insides of the grooves and pits free from corro
sion products? Does the surface of the grooves appear
smooth? Does the metal look like it is worn away? If the
answer is "yes" to these questions, then the copper tube has
failed by erosion. If the water analysis shows high carbon
dioxide and softening and the temperature is above 140F,
then corrosion erosion took place.
Example of Failure
People get into trouble because they have not taken into
consideration corrosion principles, and they lack knowledge
concerning corrosive resistance or nonresistance of materials.
A case in point involved an apartment complex in which
water pipes were leaking. This caused many problems for the
tenants and owner, not the least of which was rusty laundry.
An investigation revealed that the leaks were confined only
to the basement area. The water-supply system consisted of
2-in. galvanized pipes that acted as a header. Copper tubing,
coupled into this header, distributed the water to the different
rooms.
Other discoveries were that the leaks occurred in
galvanized pipes near the junction of the steel and copper.
Plastic insulating couplings were to have been placed between
the junctions but were not. Perforation occurred because of
gross galvanic corrosion, resulting in red water.
To eliminate this problem, the copper was separated from
the steel by a plastic coupling.
Another example involved a central Minnesota communi
ty the galvanized steel hot-water tanks would last, at the
maximum, two years, in which time they became perforated.
An analysis of the corrosion products showed a high con
tent of copper; a survey showed that a large quantity of cop
per tubing was used in the water systems. The water was cor
rosive enough to attack the copper thereby putting copper
ions into solution. The copper ions then replaced the iron of
the hot-water tanks, which led to pitting and final perforation
of the tank.
A. C. HOLLER
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