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Evaluation of pH under-deposit using the dual-cell technique in flow conditions – The case of CuNi 70:30 in high bacteria contaminated freshwater

pubblicazioni - Presentazione

Evaluation of pH under-deposit using the dual-cell technique in flow conditions – The case of CuNi 70:30 in high bacteria contaminated freshwater

In questo lavoro sono presentati i rusltati di un’attività sperimentale condotta in acqua reflua con campioni di CuNi 70/30, una lega utilizzata comenemente in scambiatori di calore industriali. La valutazione del’effetto del cambiamento di pH nell’intervallo tra 2-12 è stato l’obiettivo principale della sperimentazione condotta.

Copper alloys are widely used in cooling circuits due to their corrosion resistance, mechanical workability, and excellent electrical and thermal conductivity. Copper was believed to be immune to microbial corrosion because its toxicity to the biological organisms, but it is well known that this material suffers of this kind of corrosion. Biofilms modifies the local environment at the biofilm/alloy interface; the local environment can be radically different from that of the bulk medium in terms of pH, dissolved oxygen, and different ions concentrations that build at the interface coming from the metal and/or the electrolyte as a direct action of the bacteria.

We investigate the effect of pH changes on CuNi 70:30 samples clean and covered with a biofilm. Experiments were carried out in a hydraulic circuit built in a wastewater treatment plant, after the depurative process and before the sanitization treatment. The methodology consisted in measuring the coupling current between two copper alloy samples (electrodes), one covered by biofilm and other clean (no biofilm); both immersed in the hydraulic loop and with the same electrolyte characteristics. A current, measured with a zero resistance ammeter, will be detected between the two electrodes (biofouled vs. clean sample).

Our hypothesis is: if the current is prevalently due to a lower pH under the biofouled interface (biofouled sample vs clean one), it will become gradually smaller as the pH in the cell compartment with the clean sample is decreased. The results showed that, decreasing the pH in the cell compartment containing the clean sample has a significant impact on the coupling current, as it is expected. The measurement performed indicated that a zero current between the two electrodes cannot be reached at pH higher than 4. This may indicate that CuNi passive film, under the biofilm samples, may change not solely due to the pH changes; i.e. pH is not the only variable affecting the corrosion formed under biofilm. The possible complex role of the pH on the cathodic and anodic reactions is discussed.

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