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The storage tanks of water heaters are usually made out of steel with a lining of glass inside them. Water will corrode exposed steel, so the glass lining prevents or delays corrosion. The tanks also have magnesium anode rods. The anode rod will slow down corrosion of the steel tank by corroding in its place. [7]
Over time the galvanic anode continues to corrode, consuming the anode material until eventually it must be replaced. Galvanic or sacrificial anodes are made in various shapes and sizes using alloys of zinc, magnesium, and aluminum. ASTM International publishes standards on the composition and manufacturing of galvanic anodes. [10] [11] [12]
In brief, corrosion is a chemical reaction occurring by an electrochemical mechanism (a redox reaction). [1] During corrosion of iron or steel there are two reactions, oxidation (equation 1), where electrons leave the metal (and the metal dissolves, i.e. actual loss of metal results) and reduction, where the electrons are used to convert oxygen and water to hydroxide ions (equation 2): [2]
Alloys of metals commonly used for sacrificial anodes include zinc, magnesium, and aluminium. This approach is commonplace in water heaters and many buried or immersed metallic structures. Cathodic protection can also be applied by connecting a direct current (DC) electrical power supply to oppose the corrosive galvanic current.
When two metals touch each other and water is present, electrolysis occurs. [2] One well known example is the reaction between zinc (Zn) and iron (Fe). Zinc atoms will lose electrons in preference to the iron as they are more electropositive and therefore zinc is oxidized and corrodes. Zn(s)→ Zn 2+ (aq) +2e (oxidation)
The terms anode and cathode are not defined by the voltage polarity of electrodes, but are usually defined by the direction of current through the electrode. An anode usually is the electrode of a device through which conventional current (positive charge) flows into the device from an external circuit, while a cathode usually is the electrode through which conventional current flows out of ...
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