LOCALIZED ATTACK OF COPPER AND BRASS IN TAP WATER. THE EFFECT OF PSEUDOMONAS

M.B. VALCARCE; M.VÁZQUEZ; S.R. DE SÁNCHEZ

Sao Pedro, Brasil

Congreso; 54th. Meeting International Society of Electrochemistry; 2003

Copper and Copper alloys are frequently used in tubing systems that distribute tapwater. In this application, microbiologically induced corrosion is a recurrentproblem. In this context, the present paper deals with the susceptibility to pittingcorrosion in copper and 70/30 brass under the influence of Pseudomonasfluorescens.In a first experiment weight-loss was evaluated in coupons after 90 days ofimmersion. Four containers were prepared with sterile tap water + 0.5% peptone,two for each material and one of them inoculated with the culture. The containerswere kept under agitation at 32ºC, renewing the medium every 7 days. After 90 daysof immersion, the coupons were cleaned and dried. Average weight loss is presentedin Table 1. As can be seen, the presence of Zn as an alloying element decreases themass loss, but the presence of bacteria enhances the attack in brass by a factor closeto 7, as compared to a factor of about 2 for copper. The appearance ofdezincification associated to the presence of bacteria was reveled by microscopicobservation, as well as by the disappearance of the current peak attributed to Zn inpotentiodynamic reduction curves.The susceptibility to pitting corrosion was also studied recording slow-ratevoltamperometric curves in order to determine the critical pitting potential and therepassivation potential. The curves were registered at 0.1 mV s-1 in tap watersimulating solution, with and without the presence of bacteria in the solution. Thescan direction was reversed at 2.18 104 A cm-2 in the case of brass and at 6.40 104 Acm-2 in the case of copper. The results, an average of three independentmeasurements, show that in the presence of bacteria the pitting potential movestowards more positive (noble) values for both materials and the difference betweenthe repassivation and the pitting potential also increases. When observed at themicroscope, bigger and deeper pits can be seen in the presence of microorganisms.The interpretation of this fast response tests, where a biofilm does not have time todevelop, can be misleading when compared to corrosion experiments over moreextended periods of time, particularly when dealing with systems containing livingmicroorganisms.