ABSTRACT

Numerous breakthroughs have occurred in the formulation of non - chromate environmentally safe green inhibitors for cooling water systems. The application of green compounds to the area of corrosion and scale inhibition would result in the reduction/elimination of many of the more toxic inhibitors. Types of cooling water inhibitors are reviewed. Some indications of practical applications are provided with comments on environment acceptability. Mechanism of inhibition is discussed. Cooling water treatment programs are recommended by looking at water chemistry, system characteristics and environmental limitations.

INTRODUCTION

Cooling water systems are integral part of most of the industries and their performance is seriously affected by corrosion of materials and scale deposits. Use of inhibitors in controlling corrosion/scales in cooling water systems is an established technology. A number of inorganic/organic inhibitors have been used in the last sixty years. Cooling water inhibitors are reviewed, emphasis being on the description of recent environment friendly inhibitors.

Chromates

Chromates, inhibitors of choice in I960's, are passivating inhibitors and offer protection of ferrous and non-ferrous alloys by incorporation into the oxide layer. They are effective over a wide pH range (6-11) and temperature range (38-66"C). Chromate initially is adsorbed on the metal followed by oxidation of iron to form a fixture of Fe:O3 and Cr203 Chromate is normally an anodic inhibitor in the concentration range of 200 - 1000 mg/l 2 and acts as a cathodic inhibitor in the concentration range of 15 - 25 mg/l 3 The use of chromate as an inhibitor is being phased out clfiefly because of its toxicity to environment and expensive ffeatment of blowdown.

Zinc

Zinc increases the cathodic polarization and hence inhibit corrosion of steel. Their action is attributed to precipitation of zinc hydroxide on the cathodic areas as a result of locally elevated pH 4. Protective film readily dissolves on decreasing pH. However, it is effective in combination with other inhibitors, like, chromates, molybdates, polyphosphates, phosphonates, etc. s. Moreover, presence of zinc in the blowdown necessiates its treatment.

Nitrites

They are effective in aqueous solutions of NaC1. Sodium nitrite, passivating inhibitor, is effective only at concentrations equal to that of chloride and should exceed sulfate by 250 - 500 mg/l . Concentrations of 300 - 500 mg/l are required under optimal conditions 7. It mainly inhibit anodic reaction and forms a very thin protective oxide film of Fe203 + f-Fe203 on the metal surface 8. Bacterial decomposition of nitrite discourages its use in open recirculating cooling water systems 9.

Molybdates

Sodium molybdate is an anodic inhibitor and is effective for the protection of carbon steel in the pH range of 5.5 - 8.5. It promotes the formation of-f-Fe203 only in the presence of oxygen. When used alone a concentration of 200 - 1000 ppm is needed to minimise the corrosion of mild steel in the presence of chlorides lo. Molybdates synergises in the presence of nitrites, chromate, zinc, etc. t l

Phosphates

They require the presence of oxygen for their inhibitive action and are effective in a narrow pH range (6.0 --6.5). Monosodium phosphate is least protective while trisodium phosphate is the most protective species. Corrosion inhibition efficiency decreases with increase in temperature.

Polyphosphates

Polyphosphates, PP, used as cathodic inhibitors for the last thirty years, are very effective against galvanic corrosion and prevent formation of calcium carbonate and calcium sulfate scales 12 pp are complexing agents and with a div

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