Abstract: This article discusses all aspects of the indirect solar water heating system and analyzes the causes of corrosion and proposes countermeasures. It fully describes the performance and measurement standards of the special heat conducting medium for solar energy, and concludes that the heat transfer medium should be It is a kind of energy exchange fluid that combines environmental protection, long-term effect, scale inhibition, antisepsis, corrosion inhibition, antifreeze, anti-boiling, low foam, thermal stability and high thermal conductivity; This article conducts detailed comparison and identification in order to provide useful reference and help.

0 Preface

In recent years, indirect solar water heaters have developed rapidly, and the market share of ordinary vacuum tube solar water heaters has declined rapidly. The reasons for this are as follows:

1. China’s large number of people’s national conditions determine the road to high-rise housing in the process of urbanization, which not only guarantees the housing needs of the people’s livelihood, but also saves land and improves the efficiency of land use. The large-scale construction of high-rise residential buildings has limited the solar energy machine. The use of this option can only be made on the option of flat (or split) solar energy.

2. China's air pollution is serious. The combination of solar energy and buildings is an important measure for energy saving and emission reduction and improvement of the atmospheric environment. Because flat solar energy has a planar structure, good pressure-bearing performance, system stability, safety and durability, and easy to build beautiful buildings The characteristics of the unity of sex and other factors determine its trend of occupying the mainstream.

3. After several decades of development, flat-plate solar water heaters have become more mature and improved, and thermal efficiency has been greatly improved. China's flat-panel solar water heaters have ushered in a golden period of development.

With the rapid development and popularization of indirect (platen or vacuum tube) solar hot water systems, many problems lie ahead, such as how to select the type of corrosion of metals in indirect heat exchange systems, metal materials, and heat exchange systems. How to prevent corrosion and how to choose a suitable heat transfer medium. This article will explain the indirect solar heat exchanger system corrosion and solutions from the following three aspects:

1. Analyze the metal materials existing in the entire system one by one to explain the cause of corrosion and propose preventive measures.

2. List the performance and metrics that the heat transfer medium used in the heat exchange system should have.

3. Comparing different kinds of heat transfer media (antifreeze) to provide useful reference and help for everyone when choosing working media.

1 Heat Exchanger System Corrosion and Prevention

Metallic materials are eroded by the surrounding medium (solar system medium includes both heat-conducting medium and air) and is called metal corrosion. In the case of corrosion, a chemical or electrochemical heterogeneous reaction occurs at the interface of the metal to cause the metal to enter into an oxidized (ion) state. Essentially, the simple metal is oxidized to form a compound, such as the rust of the iron product and the occurrence of white spots on the surface of the aluminum product. The copper green on the surface of copper products and the blackening on the surface of silverware are metal corrosion. This corrosion process is generally performed in two ways: chemical etching and electrochemical corrosion. Chemical corrosion refers to the corrosion caused by the direct chemical reaction between the metal surface and the surrounding medium. Electrochemical corrosion refers to the corrosion of metal materials (alloys or impure metals) that come into contact with the electrolyte solution and react through the electrode.

At present, most of the indirect solar water heating systems are made of copper, brass, carbon steel, and stainless steel. A few manufacturers use aluminum die-casting collectors to save costs. However, this is not the mainstream and there are many hidden dangers. The following is a description of the corrosion of each material.

2 Copper corrosion and prevention

Copper used in the heat exchange system is copper and brass, copper is usually pure copper, brass is copper-zinc alloy (copper content of more than 80%). Copper has good thermal conductivity, weldability, flexibility and corrosion resistance, brass color is beautiful, has a good process and mechanical properties, high thermal conductivity, corrosion resistance in the atmosphere, fresh water and sea water, easy cutting and polishing.

In the electrochemical sequence, copper has high thermodynamic stability and does not undergo depolarization of hydrogen. It is listed as one of the corrosion-resistant metals, but in high humidity and corrosive media (such as air containing sulfur dioxide, oxygen Water, oxidizing acids, and liquids that contain complex ions such as CN- and NH4+ that form complex ions with copper, cause severe copper corrosion. Copper alloys exhibit higher corrosion resistance than pure copper. Copper alloys are mainly based on uniform corrosion in normal media, and have strong stress corrosion susceptibility in solutions containing ammonia. There are also local corrosion forms such as galvanic corrosion, pitting corrosion, and wear and corrosion. Brass dezincification is a unique form of corrosion of copper alloys. For solar heat exchange systems, the probability of such corrosion in brass is very small.

In view of the above, prevention of copper corrosion should take the following measures:

1. Keep the solution mildly alkaline to avoid acidification.

2. The high humidity of the environment cannot be avoided. It is necessary to add an efficient copper-specific organic corrosion inhibitor to reduce the corrosion rate.

3. Avoid heat-conducting medium solution containing ions that can cause complex reaction of copper, that is to say, avoid the presence of NH4+ ions in the system.

4. Reduce the oxygen content in the heat transfer medium solution.

3 Carbon Steel Corrosion and Prevention

Carbon steel is prone to corrosion under acidic conditions and is stable under alkaline conditions, but there are several possibilities for corrosion:

1. Uniform Corrosion: This corrosive form manifests itself in corrosion at the same rate on the entire surface of the metal in contact with the thermally conductive medium. A few carbon steels corrode this state. This kind of corrosion cannot be completely avoided, but the corrosion rate can be reduced.

2. Localized corrosion: manifested as pitting (plated or pitted corrosion of the metal surface, and expansion from the surface to the inside, formation of holes) or honeycomb corrosion (spots or pits connected together, the longitudinal development of similar honeycomb shape ), the formation of the cause is the role of local micro-batteries.

3. Galvanic corrosion: If the solar system is jacketed heat exchange, carbon steel and copper tube may form galvanic corrosion. This is due to the potential difference between the two metals with different electrode potentials in contact with the heat transfer medium. Macro battery.

Precaution:

1. Control the pH of the heat transfer medium between 9-11 to ensure a stable environment for the carbon steel.

2. Select carbon steels that are susceptible to uniform corrosion and try to avoid the use of carbon steels that are susceptible to pitting corrosion.

3. Add special carbon steel corrosion inhibitors to prevent corrosion.

4. When connecting with copper pipes, try to make insulation and reduce the probability of galvanic corrosion.

4 Stainless Steel Corrosion and Prevention

In everyone's opinion, stainless steel has a good anti-corrosion effect and its anti-corrosion performance is better than carbon steel. However, in the presence of chloride ions, this concept was completely negated. The basic corrosion characteristic of stainless steel in alkaline heat transfer medium is local corrosion, and the corrosion state is mainly two types, ie, slight erosion and crevice corrosion. Corrosion is caused by the large area of ​​passivation surface cathode region and the local non-passivated surface as the anode region of the occlusive battery system, caused by the chloride ions on the stainless steel passivation film damage.

For austenitic stainless steel (304 belongs to this type), chloride ion is the biggest killer, the cause of corrosion is the chloride ion damage stainless steel passivation film chromium oxide, because chloride ions are preferentially adsorbed by the metal and put from the metal surface Oxygen is expelled and soluble metal chlorides are formed, which leads to accelerated corrosion. As a result, small pits are formed on the base metal, and pitting corrosion occurs, that is, it automatically accelerates deeper. Crevice corrosion is the same as the pit corrosion mechanism because of the presence of occluding cells in the crevice, which leads to the corrosion phenomena of enrichment of chloride ions; this type of corrosion generally occurs in the gaps of flange gaskets, lap joints, and bolt nuts. The crevice corrosion at the gap between the heat exchange tube and the tube plate hole is closely related to the concentration of the stationary solution in the crevice. Once the crevice corrosion environment is available, the probability of inducing stress corrosion is high.

For the corrosion of austenitic stainless steel in the chloride ion environment, various authoritative books have explained that the chlorine ion content is very small and stress corrosion, pitting corrosion and intergranular corrosion will not occur. Therefore, the following measures should be taken for the prevention of corrosion of 304 stainless steel:

1. Strictly control the chloride ion concentration of the heat transfer medium.

2. Add special organic corrosion inhibitors to increase the stability of the passivation film or to facilitate the passivation of the damaged passivation film.

3. Use extra cathode current protection to suppress pitting corrosion.

5 Aluminum corrosion and prevention

Aluminum is an active metal that reacts easily with oxygen in the air to form aluminum oxide. Alumina forms a thin layer of gray, dense and thin film on the surface of aluminum alloy. This film is very strong and can make the metal inside and the outside The air is completely separated so that the inner aluminum is protected from the oxygen in the air. However, aluminum and aluminum oxide films can be corroded by reaction with many acidic or alkaline substances, and they are relatively stable in a neutral environment. As used in solar collector systems, the potential corrosion of aluminum is as follows:

1. Pitting: A unique form of anode reaction that is an autocatalytic process.

2. Uniform corrosion: In the medium with too large acid or alkaline, the oxide film on the surface reacts and dissolves, and the dissolution rate is uniform. The higher the solution temperature, the more severe the corrosion.

3. Intergranular corrosion: In the heat-conducting medium, the intergranular corrosion of the aluminum alloy mostly occurs inside the pit, the corrosion develops along the intergranular longitudinal direction, and sometimes forms a tunnel.

4. Galvanic corrosion: The potential difference between aluminum and stainless steel, carbon steel, and copper is large, so aluminum easily loses electrons and electrochemical corrosion occurs.

Precaution:

1. The pH value of the heat transfer medium should not be too large, and the control should be 7~8. This is the main reason why the antifreeze liquid or the heat transfer medium based on the automobile antifreeze liquid formula must be replaced in about two years.

2. Inspired by the use of rubber tubes in automobile radiators and engine systems, it must be insulated when connecting the heat collecting plate and the water tank, cut off the electronic channels forming the macro battery, and suppress the occurrence of galvanic corrosion. Otherwise, the aluminum flow collecting plate It will become a "magnesium rod" similar to an ordinary household electric water heater.

3. Add a suitable corrosion inhibitor to reduce the occurrence of corrosion. Although the above-mentioned effective measures have been taken, aluminum alloys are still the most easily corroded metals, and it is recommended to adopt them carefully.

As mentioned above, special heat-transfer media for solar water heating systems should add the following special corrosion inhibitors to the material of the system itself: copper corrosion inhibitor, brass corrosion inhibitor, carbon steel corrosion inhibitor, stainless steel Etchant, if necessary, add aluminum corrosion inhibitor to ensure that the pH of the solution is 8 to 11 and has a strong buffer capacity, strict control of chloride ion concentration, so that the thermal conductivity medium has a strong corrosion inhibition ability, so as to be effective Prevent and slow down the occurrence of corrosion. (Tian Shengjun Li Gang)

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