Configurazione e parametri di esercizio di un sistema di protezione catodica a correnti impresse all’interno di scaldaacqua domestici (Service parameters and configuration of a cathodic protection system using impressed current technique inside domestic boilers)

The necessity for highly durable electric boilers used for domestic purposes implies the study of cathodic protection systems with higher efficiency against corrosion phenomena, responsible for the failure of both the steel tank for the accumulation of hot water, internally coated with a vitreous enamel, and of the heating-elements sheathing made of stainless alloys. Among the different methods used for the corrosion protection of these metallic materials, other than the possible superficial treatments, there is the possibility to apply a cathodic protection. In the case of domestic boilers, this type of protection can be performed both by sacrificial anodes and by an impressed current technique. In this work, a cathodic protection system, using an impressed current supplied with an activated titanium anode, commonly used inside domestic boilers, was studied (Fig. 1). In more detail, the efficiency of the cathodic protection was examined as a function of the anode "low" (B) and "high" (A) position, of the current supplied by itself and of the water hardness (Tab. 1). With this aim, different workstations were installed in order to monitor the potential and partial current values corresponding to the several metallic parts which make up the boilers submitted to an accelerated "lifecycles" test, where the service pressure was higher than the atmospheric pressure ("closed" boilers, "C" Tab. 1) and therefore the same as that present in common domestic applications (3 ÷ 4 bar). In some cases, a given number of tests were executed with "opened" boilers ("A" Tab. 1) on the top, operating at atmospheric pressure; as a consequence, the potential measurements inside these last boilers were performed by immersion of a long-stemmed saturated calomel reference electrode. Instead, in the case of common "closed" boilers the potential measurements were performed by an Ag\AgCl reference electrode installed in the water tank walls (Fig. 1). All the boilers used in this investigation were submitted to an accelerated "life-cycles" test which consisted in cyclic hot water withdrawal when the temperature value set in the thermostat was reached, thanks to the opening of an electromagnetic valve suitably connected with the electric power supply of the heating-element used. Subsequently, with replacement of water in the tank with new cold water, the thermostatic probe reached a minimum temperature which caused the reactivation of the heating-element in parallel with the electromagnetic valve closure thus determining the onset of a new heating cycle. With all these steps, the test continued with a series of subsequent "hot-cold" cycles. From this investigation, the cathodic protection was effective for the different metallic parts of the boiler within a few days from the beginning of the test. This efficiency is higher with increasing water hardness, with the titanium anode farther from the heating-elements base and when the current of the cathodic protection system is highest (Table 2 and Fig. 10). Furthermore, if this experimentation is performed with "opened" boilers, the data obtained from the tests are not suitable for studying their behaviour during their real service life, where "closed" boilers are normally used. In fact, where some experimental conditions resulted critical for the cathodic protection of the "closed" boilers, these same conditions gave no problems for the cathodic protection of the "opened" boilers.