How do heat exchanger manufacturers repair corrosion?

Time2024-11-28

The determining factor for the choice of materials is their economic viability. The materials for the pipes include stainless steel, copper-nickel alloy, nickel-based alloy, titanium, and zirconium. Welded pipes are used in all cases except where welding pipes cannot be used in industrial applications. Corrosion-resistant materials are only used for the tube side, while the shell side material is carbon steel.

1.Heat exchanger manufacturersMaterial selection

The deciding factor for the choice of materials is their economic viability. The materials for pipes include stainless steel, copper-nickel alloy, nickel-based alloy, titanium, and zirconium. Welded pipes are used in all cases except where welding pipes cannot be used in industry. Corrosion-resistant materials are only used for the tube side, while the shell side material is carbon steel.

2. Metal corrosion of heat exchangers

2.1 Principle of metal corrosion Metal corrosion refers to the damage caused to metals under the chemical or electrochemical action of the surrounding medium, often in conjunction with physical, mechanical, or biological factors, meaning the damage caused to metals by their environment.

2.2 Common types of corrosion damage in heat exchangers

2.2.1 Uniform corrosion This is the macro-level uniform corrosion damage that occurs over the entire surface exposed to the medium or over a larger area.

2.2.2 Galvanic corrosion When two metals or alloys with different potentials come into contact and are immersed in an electrolyte solution, a current flows between them. The corrosion rate of the positively charged metal decreases, while the corrosion rate of the negatively charged metal increases.

2.2.3 Selective corrosion The phenomenon where a certain element in an alloy preferentially enters the medium due to corrosion is called selective corrosion.

2.2.4 Pitting corrosion Pitting corrosion, also known as localized corrosion or point corrosion, is concentrated on individual small points on the metal surface and has a greater depth.

2.2.5 Crevice corrosion Crevice corrosion occurs in the crevices and covered areas of the metal surface, leading to severe localized corrosion.

2.2.6 Erosion corrosion Erosion corrosion is a type of corrosion accelerated by the relative motion between the medium and the metal surface.

2.2.7 Intergranular corrosion Intergranular corrosion preferentially attacks the grain boundaries and areas near the grain boundaries of metals or alloys, while the grains themselves experience relatively little corrosion.

2.2.8 Stress corrosion cracking (SCC) and corrosion fatigue SCC is the fracture of materials caused by the combined action of corrosion and tensile stress within a certain metal-medium system.

2.2.9 Hydrogen damage In an electrolyte solution, damage caused by hydrogen permeation can occur due to corrosion, acid cleaning, cathodic protection, or electroplating.

3. The effect of cooling media on metal corrosion

The most commonly used cooling media in industry are various natural waters. There are many factors that affect metal corrosion, with several main factors and their effects on commonly used metals:

3.1 Dissolved oxygen The dissolved oxygen in water acts as an oxidizing agent in the cathodic process, thus generally promoting corrosion. When the concentration of oxygen in water is uneven, it forms an oxygen concentration cell, causing localized corrosion. For carbon steel, low-alloy steel, copper alloys, and certain grades of stainless steel, dissolved oxygen is the most important factor affecting their corrosion behavior in water.

3.2 Other dissolved gases In the absence of oxygen in water, CO2 will lead to corrosion of copper and steel but does not promote corrosion of aluminum. Trace amounts of ammonia corrode copper alloys but have no effect on aluminum and steel. H2S promotes corrosion of copper and steel but has no effect on aluminum. SO2 lowers the pH of water, increasing its corrosiveness towards metals.

3.3 Hardness Generally speaking, increased hardness of freshwater reduces corrosion of metals such as copper, zinc, lead, and steel. Very soft water is highly corrosive, and copper, lead, and zinc should not be used in such water. Conversely, lead is corrosion-resistant in soft water but experiences pitting in hard water.

3.4 pH value Steel experiences less corrosion in water with pH > 11, while corrosion increases when pH < 7.

3.5 Ion effects Chloride ions can damage the surface of passivated metals such as stainless steel, inducing pitting or SCC.

3.6 Scale effects CaCO3 scale in freshwater. CaCO3 scale layers are detrimental to heat transfer but beneficial in preventing corrosion.

4. The effect of the heat transfer process on corrosion

The corrosion behavior of metals is different under conditions with and without heat transfer. Generally, heat transfer exacerbates metal corrosion, especially under conditions of boiling, vaporization, or overheating. The impact of heat transfer also varies among different media or different metals.

5. Anti-corrosion methods

Understanding the various causes of corrosion in heat exchangers and reasonably selecting anti-corrosion measures is essential to achieve efficient utilization of equipment.