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How to Prevent Solar Heat Exchange System Corrosion?

Views:1     Author:Site Editor     Publish Time: 2019-04-23      Origin:Site

Indirect solar hot water system, the heat transfer cycle system needs to use the heat transfer medium, of the heat transfer system corrosion and prevention as many flat solar manufacturers concerned about the problem. This paper will analyze the causes of corrosion from many aspects and put forward preventive measures.


Metal material is damaged by the action of surrounding medium, which is called metal corrosion. During corrosion, chemical or electrochemical polyphase reactions take place on the interface of metal, so that metal is transferred to the oxidation (ion) state, that is, the metal elements are oxidized to form compounds. This corrosion process is generally carried out in two ways: chemical corrosion and electrochemical corrosion. Chemical corrosion refers to the corrosion caused by the chemical reaction between the metal surface and the surrounding medium. Electrochemical corrosion refers to the corrosion of metal materials in contact with an electrolyte solution through electrode reaction.


Currently, most indirect solar hot water systems are made of copper, brass, carbon steel, stainless steel and aluminum. The corrosion of each material is described below.

 

01 Copper corrosion and prevention

The copper used in the heat exchange system is divided into red copper and brass. Red copper is usually pure copper. Brass is copper zinc alloy. Copper has good thermal conductivity, welding, flexibility and corrosion resistance, brass color beautiful, with good process and mechanical properties, high thermal conductivity, corrosion resistance in the atmosphere, fresh water and sea water, easy cutting polishing.


In the electrochemical sequence, the standard electrode potential of copper is + 0.35v, and its thermodynamic stability is relatively high, without depolarization reaction of hydrogen. It is listed as one of the corrosion-resistant metal. But in the corrosive medium with high humidity, serious corrosion will occur in copper. Copper alloys exhibit higher corrosion resistance than pure copper alloys. Copper alloys are mainly uniformly corroded in general media, and are highly sensitive to corrosion in the solution with ammonia. There are also local corrosion forms such as galvanic corrosion, pitting corrosion and wear corrosion. Dezincification of brass is a unique form of corrosion in copper alloys.

 

Preventive measures

1) keep the solution moderately alkaline and avoid acidification; 2) add high efficiency copper special organic corrosion inhibitor to reduce the corrosion rate; 3) avoid the presence of ions in the solution of the heat conducting medium that can cause the complexation reaction of copper, that is, avoid the existence of NH4+ plasma in the system; 4) reduce the oxygen content in the solution of heat conducting medium.

 

02 Corrosion prevention of carbon steel

Prone to corrosion of carbon steel in acidic conditions, relatively stable in alkaline conditions, but several corrosion can occur as follows: 1) uniform corrosion, characterized by contact with the medium of thermal conductivity of the metal surface is almost at the same rate of corrosion, a handful of corrosion of carbon steel, such as 10 crmoal for this status, this kind of corrosion cannot avoid completely, but can reduce the corrosion rate; 2) local corrosion, manifested as pitting corrosion or honeycomb corrosion, is caused by the effect of local microbattery; 3) galvanic corrosion. If the solar energy system is jacket type heat transfer, the carbon steel and copper pipe may form galvanic corrosion, which is caused by the potential difference between the two metals with different electrode potentials when they contact each other in the electrolyte solution of the thermal medium, and the corrosion is caused by the formation of the macro battery.


Preventive measures

1) Control the pH value of the heat conducting medium between 9 and 11 to ensure the stable environment of carbon steel; 2) choose carbon steel grades that are prone to uniform corrosion, and avoid carbon steel grades that are prone to pitting corrosion; 3) special corrosion inhibitors for carbon steel are added to prevent corrosion; 4) when connecting with copper pipe, try to be insulated to reduce the probability of galvanic couple corrosion.


03 Corrosion and prevention of stainless steel

It is generally believed that stainless steel has good corrosion resistance, but in the presence of chloride ions, this concept was completely overturned. The basic characteristic of corrosion of stainless steel in alkaline heat conducting medium is local corrosion. Corrosion is caused by the damage of chloride ions to the passivation film of stainless steel.


For austenitic stainless steel, the chloride ion is the biggest killer, the cause of corrosion of chlorine ion chromium stainless steel passivation membrane oxidation, because of chloride ion has a strong by metal adsorption capacity, they preferred by metal adsorption, and the oxygen from the metal surface, the formation of soluble metal chlorides, accelerated corrosion, in the base metal generated on the pore diameter of 20 ~ 30 microns of the etch pit, pitting corrosion once formed, namely automatic acceleration to the depths. The mechanism of crevice corrosion is the same as that of pitting corrosion. Such corrosion generally occurs in the cracks of flange gaskets, lap joints, bolts and nuts, as well as the gaps between heat exchange pipes and tube plates. Gap corrosion is closely related to the concentration of static solution in the gap. Once there is a gap corrosion environment, the probability of inducing stress corrosion is relatively high.


For the corrosion of austenitic stainless steel in the environment of chloride ion, all kinds of authoritative books have strict requirements, the content of chloride ion should be < 25 10-6, otherwise it will occur to stress corrosion, pore corrosion and intergranular corrosion.

 

Preventive measures

1) Strictly control the concentration of chloride ion in the heat conducting medium, which must be < 25 10-6; 2) special organic corrosion inhibitors are added to increase the stability of the passivation film or to facilitate the re-passivation of the damaged passivation film; 3) the impressed cathode current protection is adopted to suppress the hole corrosion.

 

04 Corrosion prevention of aluminum

Aluminum is a kind of active metal, easily react with oxygen in the air to generate aluminum oxide, aluminum oxide in the aluminum alloy surface junction a layer of gray dense film, this layer of film is very strong, can make the metal inside and the outside completely separated, so as to protect the internal aluminum is no longer by the oxygen in the air erosion. However, aluminum and alumina films can react with many acidic or alkaline substances and be corroded, which is more stable in a neutral environment. In the solar thermal system, its potential corrosion has the following types: 1) pitting, is a unique form of anode reaction, is a kind of autocatalytic process; 2) uniform corrosion, in the acidic or alkaline too much medium, the surface of the oxide film reaction dissolution, uniform speed, the higher the temperature of the solution, the more severe the corrosion; 3) intergranular corrosion. In the heat conducting medium, intergranular corrosion of aluminum alloy mostly occurs in the inside of the corrosion pit, and corrosion develops along the intergranular direction in both directions, sometimes forming a tunnel; 4) galvanic corrosion: the standard electrode potential of aluminum is -1.18v, and the potential difference with the three metals of stainless steel, carbon steel and copper is more than 0.25v. Electrochemical corrosion occurs because aluminum is easy to lose electrons.


Preventive measures

1) The pH value of the heat conducting medium should not be too high, and should be controlled within 7~8; 2) when connecting the heat collecting plate and the water tank, it must be insulated to cut off the electronic channel forming the macro battery and inhibit the occurrence of galvanic corrosion; 3) appropriate corrosion inhibitors are added to reduce the occurrence of corrosion. However, aluminum alloys are the most corrosive metal in the solar industry, and caution is recommended.


Above is soaked in the solution of heat conduction medium corrosion on the surface of the metal, for the expansion of the reserved space inside the jacket or brass parts, because of the existence of oxygen, the probability of corrosion to occur in this part of a larger, the only measures is to use the inert nitrogen will be replacement and completely sealed air heat exchange system, isolated from oxygen to reduce the possibility of corrosion.


Above all, solar hot water system for heat conduction medium formula in the design should be aimed at the system itself has the material add the following special inhibitor: copper corrosion inhibitor, carbon steel corrosion inhibitor and corrosion inhibitor, stainless steel, brass corrosion inhibitors ensure solution pH value between 8 ~ 11 and has stronger buffering capacity, control of chloride ion concentration in 20 x 10-6 below, make the heat conduction medium has a strong ability of corrosion.


The corrosion resistance of the heat conducting medium shall meet the following requirements:

Glassware corrosion (88 ℃ + 2 ℃, compared to 336 + h, specimens changes value: mg/tablet)

Copper - 5 ~ + 5

Brass - 5 ~ + 5

Carbon steel - 10 ~ + 10

Stainless steel (304) -10~+10

Cast aluminum - 30 ~ + 30


There is generally no such material in the flat-panel solar system. If there is, this index can be tested. From the range of this index, it can be seen whether the cast aluminum is easy to be corroded.


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