Zinc
By James McDonald, PE, CWT
Originally Published: CSTN – January 2003
Chemical Formula: Zn
Chemical Name: Zinc
Molecular Weight: 65.39
Purpose: Cathodic corrosion inhibitor
Applications: Open recirculating or once-through cooling water systems
Functionality: Rapidly forms a film of zinc hydroxide, zinc carbonate, or
zinc phosphate at cathodic sites on steel surfaces which minimizes the
reaction at the anode where the metal loss actually occurs.
Dosage Range: 0.2 to 3.0 ppm. Because the zinc film is not very durable,
zinc is usually not used alone. It is found in many synergistic blends which
take advantage of its rapid film-forming abilities.
Tests: Hach Zincon Method using ZincoVer 5 Reagent Powder Pillows and
cyclohexanone and a spectrophotometer or colorimeter.
Concentration Effects:
Too Low: Corrosion rates will increase.
Too High: Toxic to the environment. May precipitate out at pH's > 8.0.
High levels can adversely affect biological oxidation waste treatment
systems.
Microbiological Effects: Has appreciable toxicity to a number of aquatic
organisms. Accumulates in shellfish. Its use and discharge are often
restricted near coastal areas.
Temperature Effects: As temperature increases, solubility decreases.
pH Effects: In the absence of specific stabilizers or sequestrants, zinc will
precipitate in the bulk water at pH 8.0 and above. Sequestrants have been
developed which can extend zinc solubility up to pH 9.0.
Solubility: Has restricted solubility in bulk water systems. Increasing pH
decreases solubility. Dispersant polymers are often used to stabilize zinc and
maintain or enhance its corrosion inhibition properties.
Reactivity: Rapid film forming abilities.
Chemistry
397
Incompatibilities: High pH.
Other Information: Synergistic with orthophosphate, polyphosphate,
phosphonate, molybdate, sequestrants, polymers.
Chemistry
398