Role of the Phosphate Ion as Corrosion Inhibitor of Copper and Brass in Chlorinated Tap Water

L. YOHAI; M. VÁZQUEZ; M.B. VALCARCE

Niza

Encuentro; 61st Annual Meeting of the International Society of Electrochemistry; 2010

ISE

Copper and copper alloys are materials largely used in drinking distribution systems where sodium hypochlorite is frequently used to reduce the bacterial content. This biocide could increase copper and brass corrosion in drinking water, favouring the Cu2+ release to the medium, even inducing the development of localized corrosion [1, 2]. Corrosion inhibitors are widely used to prevent the dissolution of copper. In this application, the inhibitors should be innocuous for human consumption and they should not present a negative impact on the environment. A compound that is frequently employed for this purpose is the sodium orthophosphate [3]. In this context, it is important to evaluate the effectiveness of this inhibitor in presence of sodium hypochlorite that is the most commonly used biocide in drinking water. All the experiments were carried out using artificial tap water (ATW) pH 7.6, comparing copper and brass electrodes.  The inhibitor (10 mg l-1 as P) was tested in the presence of two biocide concentration (2 and 5 mg l-1 as Cl2). The corrosion resistance was investigated following the polarization resistance (Rp) and carrying out anodic polarisation curves after two hours at the corrosion potential (Ecorr). The results were complemented with a weight-loss tests performed during 90 days. When were compared with the behaviour in ATW, copper and brass in ATW + NaOCl presented lower Rp values and a more active Ecorr, without showing a defined pitting potential. At long immersion times, both metals undergo generalized dissolution, and in the case of brass, dezincification could be observed. In ATW + 10 mg l-1 P + NaOCl, Ecorr moves towards more noble potentials with an increment in the Rp values, in comparison with the behaviour in ATW for both metals. Also, the difference (Epit –Ecorr) increases 2 times for copper and 3 times for brass. When the inhibitor is added to ATW + NaOCl, the weight-loss tests show the inhibition of copper and brass generalized dissolution. Brass presented pitting in ATW and dezincification in ATW + NaOCl, but the presence of phosphate ions was able to inhibit both types of localized attack. References [1] D. Atlas, J. Coombs and O. T. Zajicek; Water Research 16 (1982) [2] N. Boulay and M. Edwards; Water Research 35 (2001) 683 [3] M. Edwards, L. Hidmi and D. Gladwell; Corrosion Science 44 (2002) 1057-1071