UFC 3-240-13FN
25 May 2005
Table 4-10. Criteria for Corrosion Inhibitor Selection
Metal
Corrosion
pH Range
Inhibitor
(ideal)
Steel
Copper
Aluminum
Cathodic Inhibitor
Polyphosphate1
Excellent
Attacks
Attacks
6.5-8.5
Zinc salts2
Excellent
None
None
6.5-8.5
Polysilicate3
Excellent
Excellent
Excellent
7.5-10.0
Molybdate
Good
Fair
Fair
7.5-10.0
Anodic Inhibitor
Orthophosphate1
Good
Attacks
Attacks
6.5-8.5
Orthosilicate3
Good
Good
Good
7.5-10.0
Copper Corrosion Inhibitor
Azoles
Fair
Excellent
Fair
6.0-10.0
NOTES:
1. May require polymeric calcium phosphate dispersant.
2. Rarely used alone. Requires zinc solubilizer above pH 7.8.
3. Do not use if natural silica is 150 ppm or greater.
4-5.5.1
Effective Corrosion Control. Effective corrosion control requires
maintaining appropriate pH levels, plus adding maintenance dosages of chemical
corrosion inhibitors. Chemical corrosion inhibitors form a protective film or barrier on the
cooling system metal surfaces that have been cleaned prior to adding an initial high
dosage of inhibitor. The initial high dosage of inhibitor passivates (protects) the metal.
The appropriate dosage of corrosion inhibitor must be maintained continuously in the
cooling water to ensure continuing protection. Table 4-11 shows examples of various
passivation and maintenance dosage levels of corrosion inhibitors. Corrosion inhibitors
are divided into three classes: cathodic, anodic, and general filmers. Corrosion inhibitors
form a protective film on either the anode, the cathode, or over the entire metal surface.
The types of corrosion inhibitors are described below.
4-5.5.1.1
Anodic Inhibitors. Anodic inhibitors form a protective film coating on the
anodic metal (where the metal is lost) and thus directly control corrosion by preventing
the reaction that results in corrosion. Any unprotected areas will corrode at a much
faster rate than the protected areas, a factor that could result in pitting or localized
attack of the unprotected areas. Examples of anodic inhibitors include orthophosphate,
nitrite, and orthosilicates. Under certain conditions, molybdate can function as an anodic
inhibitor.
4-5.5.1.2
Cathodic Inhibitors. Cathodic inhibitors form a protective film coating of
the cathodic metal (where metal is not lost) and thus indirectly prevent corrosion by
interfering with the current flow required for the electrochemical reaction to proceed
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