MTS Water Quality Guide - Manual (Page 13)

Heat Exchanger Care and Water Quality Guide Heat Exchanger Care 13
• Dezincification: This problem occurs in copper-zinc alloys containing less
than 85 percent copper when they are in contact with water having a high
oxygen and carbon dioxide content, or in a stagnant solution. The effect
tends to accelerate as temperature increases or pH decreased below 7.
Dezincification creates a porous surface in which the zinc is chemically
removed from the alloy. The remaining copper has a sponge-like
appearance. Dezincification is prevented by using a brass with lower zinc
content or a brass containing tin or arsenic to inhibit the chemical action, or
by controlling the environment causing the problem.
• Galvanic (electrical current) corrosion: This type of corrosion occurs when
dissimilar metals are joined in the presence of an electrolyte, such as acidic
water. Galvanic corrosion usually produces a higher rate of reaction on the
less noble (chemically inert) metal.
• Crevice corrosion: This type of corrosion originates in and around hidden
and secluded areas, such as between baffles and tubes, or under loose scale
or dirt. A localized cell develops and the resulting corrosion appears as a
metal loss with local pits, often giving the impression that erosion is taking
place. This condition is in contrast to a vibration failure in which the metal
is sharply cut and there are no pits. Relatively stagnant conditions must exist
for crevice corrosion to occur.
Crevice corrosion can often be controlled by making sure that fluid flow
velocities are adequate to prevent stagnation or the accumulation of solids.
Mechanical failure with chemically induced corrosion
Heat exchanger failure is often not from a single cause, but a combination of
more than one condition. Quite often, a mechanical problem combined with a
corrosion problem produces a quicker failure than either of them alone. There are
two common types of combination mechanical and corrosion failures: erosion-
corrosion and corrosion-fatigue.
• Erosion-corrosion: any corrosion is greatly accelerated if the protective
films are worn away by excessive velocity, suspended solids, or mechanical
vibration. Erosion-corrosion is usually found in the entrance area of
chambers, below the inlet nozzles, at the point of contact of two surfaces
such as where baffles and tubes meet.
• Corrosion-fatigue: in this dual failure mode, stresses associated with
fatigue are the result of externally applied mechanical loads, such as
vibrations from machinery, expansion or contraction because of temperature
cycles, or light water hammer. In most environments where only corrosion
occurs, corrosion products and films block or retard further attack.
However, in corrosion-fatigue, cyclic stresses rupture the protected areas
and make them permeable; this action subjects open areas to accelerated
corrosion.