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Corrosion Protection
Water Treatment
Corrosion and Scale Control
Chemical Treatment
Requirements
Passivation
Biological Control
• BAC discourages acid dosing as means of scale control except for open circuit cooling
towers with remote sump applications or towers constructed from stainless steel. This
should be done at a point in the system where total mixing and dilution occur before
reaching the evaporative cooling equipment. The preferred injection point for chemical
scale and corrosion inhibitors is on the discharge side of the system circulating pump(s).
These chemicals should not be batch fed directly into the unit’s cold water basin or
water distribution system, as this can severely damage areas directly contacted.
• When chlorine is added to the system, free residual chlorine should not exceed 1 ppm,
except as noted in start-up and shutdown section on
Page 5
. Exceeding this limit may
accelerate corrosion.
Passivation
When new systems are first commissioned, special measures should be taken to ensure
that galvanized steel surfaces are properly passivated to provide maximum protection from
corrosion. Passivation is the formation of a protective, passive, oxide layer on galvanized
steel surfaces. To ensure the galvanized steel surfaces are passivated, the pH of circulating
water should be kept between 6.5 and 8.2 and calcium hardness between 50 and 600 ppm
(as CaCO
3
) for four to eight weeks after start-up, or until new zinc surfaces turn dull gray in
color. If white deposits form on galvanized steel surfaces after the pH is returned to normal
service levels, it may be necessary to repeat the passivation process. In case the pH can’t be
kept below 8.2, a secondary approach is to conduct a chemical passivation using inorganic
phosphate or film-forming passivation agents. Consult your water treatment specialist for
specific recommendation.
Biological Control
• The warm, oxygen and nutrient rich environment inside evaporative cooling equipment
provides an ideal environment for the growth of algae, slime, and other micro-organisms.
Uncontrolled, this can reduce heat transfer, promote corrosion, and promote the growth
of potentially harmful organisms such as Legionella.
•
To avoid biological contamination and minimize the risk of
Legionella,
initiate the
biocide treatment program at start-up and continue on a regular basis thereafter in
accordance with the treatment supplier’s instructions.
• Bleed/blowdown or chemical treatment used for corrosion and scale control alone is not
adequate for control of biological contamination.
• Introduce solid or granular biocides through a chemical “pot” feeder installed in parallel
with the system circulating pump. Diluted liquid biocides may be added directly to the
cold water basin.
NOTE:
Stainless steel cold water
basins and basins protected by
the TriArmor
®
Corrosion Protection
System or thermosetting hybrid
polymer do not require passivation.
However, if the upper structure is
galvanized steel, passivation is
required on the galvanized area
including any Hot Dip Galvanized
After Fabrication (HDGAF) coil(s).
NOTE:
ASHRAE Guideline 12:
Minimizing the Risk of Legionellosis
Associated with Building Water
Systems is available for free from
www.BaltimoreAircoil.com.
