What are the common corrosion phenomena in heat exchanger units?

- Aug 07, 2018-

The common phenomena of corrosion of heat exchanger units are as follows:

1.1 The nature of metal corrosion in heat exchangers

In nature, most metals are often in the form of ores, or metal compounds, and corrosion is the process by which a metal returns to its natural state. For example, iron is mostly hematite in nature (the main component is Fe2O3), and the main component of iron corrosion product, rust, is Fe2O3. It can be seen that the corrosion process of iron is returned to its natural state--ore the process of. Therefore, the essence of metal corrosion is the process in which a metal undergoes a reaction to return to a compound state in a certain environment.

1.2 Types of metal corrosion of heat exchangers

The phenomenon and mechanism of metal corrosion are complex and the scope involved is very wide. According to the reaction mechanism, metal corrosion can be divided into chemical corrosion and electrochemical corrosion. The electrochemical corrosion caused by the electrochemical action of the metal surface and the electrolyte solution is a common and common corrosion. Electrochemical corrosion usually occurs in the form of localized corrosion such as stress corrosion cracking, pitting corrosion (small pore corrosion), crevice corrosion.

1.3 Causes of corrosion of heat exchanger tubes of heat exchanger refrigeration unit

It is closely related to factors such as impure water quality, atmospheric pollution to water, inner wall condition and water flow rate. Local erosion and pitting of the inner wall of the pipe often occurs due to the presence of fouling deposits (iron oxide, calcium deposits, sludge, etc.) on the inner wall of the pipe.

1) Local erosion of the inner wall of the water side pipe

When cold water (or cooling water) containing dirt, air (SO2 dissolved in the atmosphere), or sulfurous acid gas flows rapidly through the metal pipe, it will break through a corrosion-resistant protective film on the inner surface of the pipe. On the surface where the tube protective film is peeled off, due to the difference in metal ion concentration, electrochemical corrosion of the anode (the portion with less metal ions) and the cathode (the portion with more metal ions) is formed, and the inner surface of the tube which has such corrosion is concave. With small spots.

2) Pitting and pitting of the inner wall of the water side pipe

Corrosion on the water side of the heat exchanger tube is mostly caused by uneven coverage of scale or other deposits. For copper and its alloys, when the surface of the inner wall of the pipe is provided with porous sludge and mud sand, it will cause another electrochemical corrosion of the battery with poor oxygen concentration. For example, when cold water or cooling water contains 0.1 mg/L of copper ions, a small battery is generated, which causes copper ions to be plated on the iron and causes pitting and pitting corrosion, that is, copper plating [2].

3) Corrosion of the surface of the water tube side tube sheet

The heat exchanger is located on the surface of the side tube plate of the water chamber, especially the surface around the tube will also produce electrochemical corrosion, which is plaque-like.

4) Heat exchanger tube stress corrosion cracking

Brass heat exchangers are in certain environments (eg in water, water vapour, the atmosphere) and may cause stress corrosion cracking under stress conditions. It should be noted that the refrigerant ammonia is an etchant that ruptures copper alloys (brass and bronze). For brass, its crack resistance is enhanced with increasing copper content.