As the steel liner has a certain degree of metal properties, in order to prevent corrosion, water heater liner are set in the magnesium rod (the main component of magnesium), known as the anode rod. This is the use of electrochemical principles to prevent corrosion of the liner is an effective measure, known as the cathodic protection method - sacrificial anode to protect the cathode. Because magnesium metal activity than iron, in the case of magnesium rods and ordinary steel exposed at the same time, magnesium rods first replaced the corrosion of steel, so as to achieve the purpose of protecting the steel liner. After the corrosion of magnesium anode rod surface attached to a large number of solid material, the main components of scale, rust and magnesium anode rod itself corrosion of Mg (OH) 2 precipitation.
Water containing minerals such as calcium (Ca) magnesium (Mg) salts is called "hard water". River water, lake water, well water and spring water are hard water. Water is the river, lake or well water through the settlement, remove the sediment, sterilization after sterilization is also hard water. After the water boils, part of the water evaporates, and the well-dissolved calcium sulphate precipitates. The original dissolved calcium bicarbonate (Ca (HCO3) 2) and magnesium bicarbonate (Mg (HCO3) 2) decompose in boiling water to release carbon dioxide (CO2) into insoluble calcium carbonate (CaCO3) and hydrogen Magnesium oxide (Mg (OH) 2) is also precipitated, and MgCO3 is sometimes formed (because Mg (OH) 2 is more difficult to dissolve than MgCO3, Mg (OH) 2) is generally formed, thus forming scale. So after a period of time, magnesium anode surface will cover the scale.
magnesium induced coprecipitation procedure
Magnesium rod, liner and water to form the original battery, the original battery liner for the cathode, magnesium rod for the anode, the end result is the anode is consumed, while the cathode is protected.
The reaction of the battery poles is as follows:
Anode: 2Mg-4e = 2Mg2 +
Cathode: 2H2O + O2 + 4e = 4OH-
Mg2 + and OH- both into the solution, to a certain amount, combined with the formation of Mg (OH) 2 precipitation, attached to the anode rod.
Iron and steel liner after a certain period of time, although the protection of magnesium anode, or there will be some corrosion, iron ions into the solution, and OH- combined to produce FeOH) 3, the further formation of rust (Fe2O3), a large part of the rust attached to the liner On the other part of the brown solid material on the anode bar.
Rust generation process is as follows:
Fe2 ++ 2OH- = Fe (OH) 2
4Fe (OH) 2 + O2 + 2H2O = 4Fe (OH) 3
2Fe (OH) 3 + nH2O = 2Fe2O3 · nH2O + 3H2O
From the above analysis we can see that the use of magnesium sacrificial anode for some time, the surface will form these solid material mixed coating, need to regularly replace (2-3 years) to extend the life of the liner, that is, the use of water heater life.
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