25 May 2005
between the cathodic and anodic metals. The corrosion reaction rate is governed by the
size and type of the cathode relative to the anode. Even when cathodic areas are not
completely covered by the protective film, corrosion will occur, but usually more slowly
and uniformly than when using anodic inhibitors alone. The occurrence of localized
corrosion or pitting attack is greatly reduced. Examples of cathodic inhibitors include
zinc salts, polyphosphates, and polysilicates. Under most conditions, molybdate will
function as a cathodic inhibitor.
General Inhibitors. General inhibitors produce a protective film on the
surfaces of all metals. These corrosion inhibitors are organic compounds, such as
certain phosphonates, amines, and other nitrogen chemicals. They may be used in
cooling water systems.
Corrosion Inhibitors for Specific Metals. Some corrosion inhibitors
provide corrosion control for specific metals. The protection of copper and copper alloys
requires the use of azoles, such as tolyltriazole (TTA), benzotriazole (BZT), and
butylbenzo-thiazole, which can be added to the system separately from, or as part of a
blend of, other treatment chemicals.
Galvanic or Crevice Corrosion. Control of galvanic or crevice corrosion
is achieved primarily by engineering and mechanical design. These types of corrosion
can also be partially controlled by physical and chemical considerations. A dielectric
coupling (insulator), used to separate two different metallurgies, can help to prevent
galvanic corrosion. From a chemical perspective, adjusting pH and using proper
corrosion inhibitors will reduce corrosion.
Deposit Corrosion. Control of deposit corrosion requires maintaining
deposit-free metal surfaces. This applies to any form of deposit, such as scale,
biomass, corrosion products, or foulants. The occurrence of most types of deposits is
prevented by dispersants, scale inhibitors, or biocides, along with the maintenance of
adequate flow velocities. Routine, adequate cleaning of filters and tower sumps will help
. MIC is best prevented
by stopping conditions that foster biological
growth and by using an effective microbiological control program. A MIC prevention
program includes adequate control (prevention) of deposits and fouling, avoidance of
low-flow and dead-leg conditions, and implementation of a consistently effective
microbiological program (see paragraph 4-4.4).