Industrial water contains impurities that cause industrial water to corrode most metals. The corrosion of metals in water is electrochemical corrosion, which is divided into general corrosion and local corrosion. What is the common brewery equipment problems during brewing?
If the corrosion is distributed on the entire metal surface, it is called general corrosion, but it can be uniform or uneven.
If the corrosion damage is mainly concentrated in a certain area, and other parts are not corroded, this type of corrosion damage is called local corrosion. Local corrosion is more dangerous than general corrosion. The smaller the corrosion area, the deeper the pitting corrosion, and the greater the damage. .
In fact, the main cause of heat exchanger perforation is often pitting corrosion. Impurities in water have the greatest impact on corrosion from dissolved oxygen in the air; the other most common corrosion-causing substance is dissolved salts in water, the most important of which is chloride.
1. Dissolved oxygen corrosion
This corrosion is caused by the action of carbon steel and oxygen dissolved in water to form iron oxides. This type of corrosion is often uneven general corrosion. However, if the water quality is hard, local corrosion may also occur.
2. Galvanic corrosion
If some parts of the equipment are made of different metal materials, and they are connected to each other and placed in water, the electrode potential of the metal of different materials is different to form a galvanic battery, and the corrosion produced at this time is a galvanic cell. corrosion. The most common example is the electrical coupling between carbon steel and copper to accelerate the corrosion of steel.
3. Crevice corrosion
Crevice corrosion is caused by the formation of a particularly small gap between the metal and the covering (metal or non-metal), so that the medium in the gap is in a stagnant state, and there is a harmful anion (Cl-) in this medium. A form of corrosion.
For example, the flange connection surface, the rust layer and the bottom of the scale layer of the heat exchanger may be produced, and the perforation of the heat exchanger is often caused by crevice corrosion.
4. Pitting corrosion
Pitting corrosion is a special kind of local corrosion, which causes small holes in the metal, which can perforate the equipment in severe cases. Pitting corrosion mainly occurs in materials that can be self-passivated (including passive films) such as aluminum, titanium, and stainless steel in media containing dissolved oxygen and hazardous anions (mainly Cl-).
Generally speaking, metals that are sensitive to pitting corrosion are particularly prone to crevice corrosion when there are crevices. Among all the materials, stainless steel is the most sensitive to pitting corrosion.
5. Stress Corrosion Cracking (SCC)
The cracking of metal caused by the combined action of tensile stress and corrosive medium is called stress corrosion cracking, which is traditionally represented by SCC. For stainless steel, titanium alloy, aluminum alloy, and sometimes even carbon steel and other materials in the gas medium containing Cl-, due to the effect of stress, it is easy to produce stress corrosion cracking.
As shown in the figure below, a vertically installed stainless steel heat exchanger, the cooling water is outside the tube, and the process medium is inside the tube. Because of the unreasonable design, the upper part of the pipe cannot be filled with cooling water and there is a blind spot.
During the flow of cooling water, due to the splashing effect of the cooling water, the dead angle becomes a dry and wet alternate part. Even though the salinity of the cooling water is very low, the salinity of the cooling water is continuously increased due to the continuous concentration of the cooling water at this dry-wet alternate part. The tensile stress and high temperature of the expanded tube part prompt the tube to quickly crack at the gap.
6. Wear and corrosion
Due to the high moving speed of the medium, or the high relative moving speed of the medium and the metal component, the local surface of the component is severely damaged by corrosion. This type of corrosion is called abrasion corrosion.
Abrasion corrosion is the combined result of the mechanical erosion of the corrosion products already generated by the high-speed fluid on the metal surface and the diffuse corrosion of the newly exposed metal surface.
Abrasion corrosion can be divided into two types: turbulent corrosion and cavitation corrosion:
Turbulent corrosion occurs in some specific parts of the equipment, such as the inlet end of the heat exchanger tube. Due to the sudden increase of the flow velocity, turbulence is formed there, which causes the metal surface to be greatly disturbed (shear stress), thereby causing corrosion.
Cavitation corrosion is also called cavitation corrosion. When the fluid and metal components move relative to each other, eddy currents are generated in a local area of the metal surface, which is accompanied by the rapid formation and destruction of vapor bubbles on the metal surface, resulting in an impact on the metal surface. corrosion.
7. Hydrogen hazards
It represents the damage to metals due to the presence of hydrogen or the interaction with hydrogen, including: hydrogen bubbling, hydrogen embrittlement, decarburization, and hydrogen corrosion. For example, carbon steel and low-alloy steel may undergo hydrogen embrittlement in geothermal water containing hydrogen sulfide.
8. Microbiological corrosion
Microbiological corrosion is a special type of corrosion, it is difficult to exist alone, and often occurs at the same time as electrochemical corrosion. The main cause of microbial corrosion is due to the accumulation of sludge. The metal surface covered by the sludge is an oxygen-poor zone, and the metal is locally corroded due to the action of the oxygen concentration battery.
In addition, due to the reproduction of microorganisms, a special corrosive environment is created, which intensifies corrosion. For example, the river bottom sludge deposited in the cooling water system contains sulfate-reducing bacteria, which is a very corrosive bacteria that can reduce sulfate to sulfide.