| Life of electric water heater depends on the life of tank, and corrosion will easily happen in the tank without any protection. So double corrosion protection methods are usually applied including daubing enamel and sacrifice anode alloy to protect the tank. Some electric water heaters have a device for measuring corrosion current.When detected corrosion current is apporching a pre-set threshold, alarm signal will occour to remind replacing new anode rod Actually, false alarm of detection occurs sometimes. This paper mainly analyses the reason of false alarm, and find out the corrosion law of magnesium alloy anode rod through experiments, and try to give optimal alarm scheme in the end.In order to real-time monitor the change of the corrosion current, a device for real-time corrosion current measurement is developed. It’s designed as versatile functions such as arbitrarily choosing channel combinations and measurement time interval, automatic drawing curves and dynamically displaying, collecting data on the computer and stored as data file.Previous investigation indicated that the main reason of the false alarm is directly related with water quality. Accelerated experiments were performed to understand how the four ions: Cl-, SO42-, Mg2+, Ca2+in the water influence on magnesium alloy corrosion. At first, the law of corrosion current changing with time was investigated in the high concentration solution. Results showed that the magnesium alloy corrosion in the tap water are mainly affected by anion (C1-, SO42-), Then the corrosion behavior of magnesium alloys in NaCl and Na2SO4 solution was further explored, such as corrosion rate, composition of corrosion products, the change of macro and organization micro morphologies, and then leads to a discussion of the corrosion mechanism of magnesium alloys in NaCl and Na2SO4 solution. Results showed that the corrosion of magnesium alloy is pitting and selective in NaCl solution, which results in uneven organization micro morphology of magnesium alloy, and corrosion product is easy to fall off, macro morphology is acicular, and corrosion rate is fast. However, the corrosion of magnesium alloy is pitting but general in Na2SO4 solution, which results in uniform organization micro morphology of the magnesium alloy, the corrosion product is not easy to fall off, macro appearance is explosion, and corrosion rate is slow.In an electric water heater, a resistance is usually added between heating element and tank steel to balance corrosion current among them. Experiments were performed for measuring the profile of the potential of tank and heating tube vs. various value of balance resistance with defferent mass fraction of magnesium alloy in different ion and concentration solution, concluding that the optimization range of balance resistance is 600Ω-750Ω in Nanjing water condition, and further concluding that water quality is better, balance resistance value range moves down correspondingly; water quality of the area is worse, balance resistance value range moves up correspondingly. In this article,620Ω was chosen as balance resistance in later experiments. As the corrosion of magnesium alloy are mainly affected by anion (C1-，SO42-),This article mainly research corrosion current and potential changes of magnesium alloy at different mass fraction in different low anion (Cl-, SO42-) solution, indicating that corrosion current change rule of magnesium alloy in low concentration solution is similar with the rule in high concentration solution. And the corrosion current of magnesium alloy at 90% corroded is 70% of the initial corrosion current in both high and low concentration ignoring the influence of deposit on the magnesium alloy corrosion. And the corrosion current when magnesium alloy at 90% corroded is 40% of the initial corrosion current when considering the influence of deposit on the magnesium alloy corrosion. Considering both with deposit and without deposit, alarm current is set at 40%-70% of the initial current.In the case of false alarm scheme optimization, two schemes including current ratio method and balance resistance adjustment method were suggested. The first method suggests setting different alarm current according to different water qualities, the final alarm current value is y=κ(0.00264ut/Ks+1.03909); Among them:κ is a coefficient of 0.4～0.7 (electrical conductivity is more than 100 μS/cm) or 0.8 (conductivity is less than 100 μS/cm); ut/=1+0.022(t-25); Ks is the conductivity in aqueous solution under 25℃. And the second method suggests adjusting the balance resistance to achieve the same alarm value. Decrease balance resistance at the good water quality; increase the balance resistance at the bad water quality. |
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