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Open recirculating cooling water treatment corrosion control |
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Impingement Waters in turbulent flow, especially those high in dissolved and suspended solids or dissolved and entrained gases, often damage passive oxide films, causing extensive local corrosion. The abrasiveness of the flowing water physically tears metal away from the structure and results in the horseshoe shaped pits characteristic of this attack. The pits are aligned in a manner of a "horse walking upstream"; shown in the following Illustration.
In evaluating the effects of entrained gases, the size of the gas bubble often determines the extent of pitting. Larger bubbles destroy the oxide layer, while a small bubble might bounce off the metal surface or actually replenish a passivating oxide film. The most severe attack occurs at the inlet to heat exchanger tube bundles, points of restricted flow, sharp bends and elbows, etc. Although many materials are subject to impingement, the most serious problems occur in copper and its alloys. In general, flow through copper tubes in a heat exchanger should be reduced if total suspended solids or gas concentrations are high. Admiralty brasses, aluminum bronzes and cupro-nickel alloys are more resistant to impingement. In non-critical heat transfer applications, impingement is often controlled by the addition of ferrous sulfate. At 1 ppm of ferrous ion, a reddish film a few mils thick builds on the metal surface and protects it from further attack. |
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