According to what principle magnesium bar anticorrosion

According to what principle magnesium bar anticorrosion?

Sacrificial anode protection is one of the most commonly used metal corrosion prevention technologies. Magnesium sacrificial anode with zinc base, aluminum sacrificial anode is more excellent electrochemical performance and are widely used in high resistivity of the medium (such as water, soil, etc.) of the cathodic protection, metal anti-corrosion magnesium alloy sacrificial anode rod (hereinafter referred to as the magnesium anode rod) in the electric water heater tank protection also is very small, so the unification of the specifications of the magnesium anode rod, It is especially necessary to extend the protection time of the magnesium anode to the tank of the electric water heater.

2. The calculation basis of magnesium anode rod

2.1 Minimum protection current density

Minimum protection current density is the metal used in the fully protect the minimum current density, the type, the size of the numerical and metal surface have a great relationship and medium conditions, every square metre on current density change can be a small percentage from ma to several hundred amperes, during the implementation of cathodic protection, the protection current density must be careful, Should be fully based on practical experience or according to the literature.

We consulted the relevant data and obtained the following data, as shown in Table 1.

For electric water heater tank, generally speaking, there are hot dip galvanized protective covering water resistant resin type, enamel, stainless steel, the tank itself has good antiseptic function, but because it is a closed system, full of fresh water, still water hardness, PH value, oxygen, chloride, the influence of such factors as the service life of the tank can range from a few months to more than 20 years. Therefore, anti-corrosion cathodic protection is needed. We refer to Table 1 and conduct tests to determine the minimum protection current density of the tank of the electric water heater is 0.002-0.016mA/cm2.

Table 1 Steel in different medium conditions

Minimum protection current density

No. The ambient current density of the steel is mA/cm2

1. Place the steel tube in seawater from 0.01 to 0.016

Steel in rapids, polluted and fresh water with dissolved oxygen 0.005-0.016

3 Iron in hot water (bucket galvanized) 0.0011-0.0032

4 steel in still water 0.0055

5 iron in the water in the container 0.014

2.2 The current occurrence of magnesium anode rod is related to the specifications of magnesium anode rod. Each size anode has its current occurrence data, and some anodes have been standardized and serialized. However, the application time of magnesium anode rod electric water heater is short, and the current occurrence of each size anode has not formed a series, so domestic water heater manufacturers choose magnesium anode rod is blind. Now applied to the electric water heater on the magnesium anode rod is mostly too small to reach the desired anti-corrosion effect, magnesium anode rod current is far below the actual need, short use time, some magnesium anode rod used less than half a year on the corrosion off, the protective effect of the tank is almost small.

Through practice, we find out an experience in selecting the size of magnesium anode rod, that is, when the surface area of the inner bladder to be protected and the area of magnesium anode rod have a certain proportion, cathodic protection can achieve good results. When the magnesium anode is designed for a life of 2-3 years and the ratio of the magnesium anode to the tank surface area is 1:30, the current occurrence of the magnesium anode in fresh water is well suited to the minimum protective current density.

It is important to note that the magnesium anode rod life is determined to be 2-3 years. The reason is as follows: it is the bladder of electric water heater is to work below airtight condition, when temperature < At 80℃ (the temperature is the highest heating temperature of fresh water in the electric water heater), as the temperature increases, the corrosion rate increases, if the magnesium anode rod is designed too large, the water heater is actively smaller, the current density of the water heater tank is too large and the cost increases, and even the phenomenon of protection. The second is the magnesium anode rod with the use of time, the current efficiency will gradually decrease, the output current is reduced, thus reducing the protection effect of the electric water heater tank.

3. Selection and application of magnesium anode rod

Now electric water heater daily use of magnesium anode rod specifications have 16, 18, 20, 24, 25, etc., and electric water heater capacity is from 5 liters to 1000 liters, in order to better illustrate how to according to the water heater capacity (according to the shape of the tank can be calculated according to the manufacturers of the tank surface area) select magnesium anode rod specifications, The lengths and areas of magnesium anode rods of the same weight with different diameters are now listed in Table 2. Select the magnesium anode based on the surface area of the water heater and the data in Table 2.

For example: a tank for ￠360× 490 electric water heater, the surface area S tank =757368mm2 

Press S liner: S magnesium anode rod = 30:1

S magnesium anode bar = 757,368/30 =25245.6 mm2

The table shows that ￠20*400 magnesium anode rod is the closest, weighing 230g.

It is also possible to choose other diameter magnesium anode rod, using this method can be easily selected for various specifications of electric water heater magnesium anode rod.