MMO vs. HSCI Anodes for the Cathodic Protection of Potable Water Storage Tanks
Impressed current cathodic protection (ICCP) has proven to be an efficient technique for the corrosion protection/mitigation of the internal surfaces of metallic storage tanks. The protection of internal surfaces differs from that of structures in natural environments because of some additional factors such as complex geometry, hydrodynamics, temperature, fluid composition and accumulation of H2 gas; all elements that require further study and consideration during the design phase. The anode choice is certainly a key step in determining the effectiveness of the protection system.
High Silicon Cast Iron anodes (HSCI) were introduced in the early 1950s as a replacement of graphite anodes, improving performance in terms of higher current outputs and lower consumption rates. Today, Ti/MMO anodes play a fundamental role in the market of ICCP anodes, because of their interesting physical-chemical properties such as metallic conductivity, low over-potentials, corrosion resistance and so on.
HSCI anodes are made of an iron cast alloy (14% Si, 4.5% Cr, 0.75% Mn and 0.95% C) whose surface results to be covered by a porous conductive hydroxide layer that limits the anodic dissolution in favor of the oxygen evolution reaction. The main advantages of such anodes are their low cost and widespread availability as, once the composition is known, any foundry can potentially manufacture them. However, an appropriate alloy composition is not enough to guarantee the desired electrode’s quality and durability, deeply affected by the casting process and the resulting microstructure. Further disadvantages related to HSCI are their brittle nature and heavy weight that pose transportation, handling and installation issues. Finally, potable water storage tanks require a disinfection system aimed at preventing/inhibiting the proliferation of bacteria, viruses and algae. Among all alternatives, chlorine containing compounds are widespread used as disinfecting agents, due to their cost/efficiency and superior bactericidal power. Different forms of disinfecting chlorinated compounds exist, from elemental chlorine, to hypochlorous acid, chlorite, hypochlorite, or chlorine dioxide; whatever the form, they are known to badly affect the stability of the passive layer. As a consequence, HSCI anodes (positively charged), yet stressed by the anodic current flow, could be further damaged by the presence of, although diluted, negatively charged, chlorinated compounds exerting their detrimental action on the hydroxide layer and shifting the oxygen evolution reaction towards an anodic electrode dissolution.
Among the alternatives, chloride dioxide is said to be the less corrosive chlorinated compound; however, a detailed scrutiny of the available technical and scientific literature points that there is no consensus about the aggressiveness of ClO2. Serious corrosion issues have been reported in bleach plants, where the operating conditions are far more demanding than those of water disinfection. In this latter case, tests performed at room temperature and ClO2 concentrations of few ppm did not highlight any significant corrosion process on cast iron samples, however it is worth noting that, to the author knowledge, the combined effect with anodic currents has never been investigated.
Furthermore, it seems important to underline that chlorine dioxide rapidly reacts with soluble forms of iron and manganese, forming inert precipitates; as a consequence, the few ppm of water dispersed ClO2, could be rapidly abated by the reaction with metallic ions released from the anodes, thus significantly reducing the disinfection efficiency.
On the other hand, while the use of activated titanium electrodes (Ti/MMO) faces a higher investment cost, it gives way to a series of important advantages. These electrodes are made of a titanium substrate, providing mechanical strength, metallic conductivity and corrosion resistance, coated with a thin layer of mixed metal oxides (MMO) that catalyzes the anodic reactions.
Similar to the HSCI electrodes, durability is a key factor for Ti/MMO; however, the latter are more easily tested and there are standard procedures based on accelerated life tests that can provide a relatively precise evaluation of the anode performance. Additionally, because of their nature, MMO anodes have longer service lives, being characterized by a consumption rate of about 10-4 Kg/Ay, while the corresponding value for HSCI anodes is about 0.5 – 1 Kg/Ay. This difference in consumption rates, along with their lower density (4.5 g/cm3 for Ti/MMO vs. 7 g/cm3 for HSCI) allows an overall reduction of transportation, installation and maintenance costs.
The MMO coating has a significantly higher corrosion resistance with respect to any other passive layer, additionally it does not suffer from the eventual presence of chlorinates species. In a properly manufactured Ti/MMO electrode, anodic consumption affects only the coating layer, and provided that the operating conditions comply with those recommended by the manufacturer, the dissolution rate is extremely slow, thus guaranteeing a long service life and no interferences with the ClO2 disinfection mechanism.
Given all the above, Ti/MMO are the best option for the impressed current cathodic protection of the internal surfaces of potable water storage tanks.
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