The freezing point of ethylene glycol-water antifreeze is not linear with the mass fraction of ethylene glycol. The freezing point of its aqueous solution does not decrease completely with the increase of concentration, but increases when the concentration exceeds 70%. In the process of preparation, reasonable selection should be made from reality to meet the requirements of frost resistance and economy. Generally, antifreeze can be prepared according to the lowest temperature in this area and the data in the reference table 1. In the south of China, the proportion of ethylene glycol is generally 40%, while in the cold north, it is more appropriate to take the proportion of ethylene glycol with a mass fraction of about 50%. Automobile cooling system is generally composed of copper, aluminum, cast iron, steel and solder. Ethylene glycol antifreeze will cause corrosion of cooling system materials in long-term work, and the corrosion media are water and ethylene glycol. People are familiar with the corrosion of water to metals. Ethylene glycol will not cause obvious corrosion to materials at room temperature, but with the increase of temperature, ethylene glycol will be oxidized, which will increase the acidity and generate a variety of corrosive substances. The precipitation of these corrosive substances will reduce the thermal conductivity of the engine, which will easily block the cooler pipeline and lead to overheating of the engine. Therefore, corrosion inhibitors must be added to antifreeze. Moreover, the corrosion inhibitor to be added has the characteristics of small dosage and good corrosion inhibition effect.
Corrosion inhibitors can be divided into two categories: one is inorganic composite corrosion inhibitors, including metasilicate, phosphate, tungstate, borate, nitrite, molybdate and benzoate. As an inorganic composite corrosion inhibitor, it can form a dense passivation film on the surface of metal, blocking or preventing hydrogen and acid penetration of metal. Such as adding 0. 05% ~ 0.03% sodium metasilicate can prevent the corrosion of aluminum parts. The other is an organic composite corrosion inhibitor, which is often used as an antirust additive in antifreeze. Triethanolamine, benzotriazole, mercaptobenzothiazole and organic phosphate are commonly used, which have good corrosion inhibition performance. For example, just add 0. 1% ~ 0.5% benzotriazole can prevent the corrosion of copper parts. Metal parts with different compositions need different corrosion inhibitors. Sodium nitrite has a good corrosion inhibition effect on steel and cast iron, but it causes pitting corrosion on solder. Triethanolamine, phosphate and organophosphate have good corrosion inhibition on ferrous metals, but they are corrosive to brass and copper. Silicate is an excellent corrosion inhibitor for aluminum, but it is easy to hydrolyze and form a large number of flocculent precipitates. Although the cost of these single inhibitors is low, it is difficult to meet the performance requirements of antifreeze in many aspects. At present, the antifreeze on the market adopts a compound formula with synergistic effect of various corrosion inhibitors, and the corrosion inhibition rate is very high. At present, the corrosion inhibitors used in foreign patents are composed of many kinds of corrosion inhibitors, and the corrosion inhibition effect is ideal. It is worth mentioning that benzotriazole and mercaptobenzothiazole are special corrosion inhibitors for copper and brass.
Judging from the patents of antifreeze in recent years, at least half of the corrosion inhibitors in the literature contain silicates. Although silicate is unstable and easy to precipitate gel during use, it has a good protective effect on Al, Cu, Fe and other metals, and it is cheap, easy to obtain and completely non-toxic, so it has been used ever since. In order to avoid gel formation, a small amount of silicate stabilizer must be added to antifreeze. There are many reports about stabilizers, mainly siloxane, Si2N and Si2P compounds. Various corrosion inhibitors have different effects on different metals. A corrosion inhibitor has an antiseptic effect on the same metal, but it may have little or no antiseptic effect on another metal. In the presence of a variety of metals, in order to achieve a good overall anti-rust effect, it is necessary to use a variety of corrosion inhibitors to compound. Generally speaking, the total amount of corrosion inhibitor is 0. 5% ~ 5% antifreeze stock solution. The temperature is low in winter. In order to make the car continue to be used at low temperature in winter, some substances that can lower the freezing point of water are added into the engine coolant as antifreeze, so that the cooling system will not freeze in low temperature weather. Therefore, people sometimes refer to coolant as "antifreeze" or "non-antifreeze".