Corrosion Types of Heat Exchangers - YUHONG GROUP

【OVERVIEW】 Due to the impurities contained in industrial water, most metals are corroded by industrial water. The corrosion of metals in water is electrochemical corrosion, and electrochemical corrosion is divided into general corrosion and local corrosion. If corrosion is distributed over the entire metal surface, it is called general corrosion, but it can be uniform or non-uniform. If the corrosion damage is mainly concentrated in a certain area, and other parts are not corroded, this type of corrosion damage is called localized corrosion. Localized corrosion is more dangerous than generalized corrosion. The smaller the corrosion area, the deeper the pitting, and the greater the damage. In fact, the main cause of heat exchanger perforation is often pitting corrosion. The most important effect of impurities in water on corrosion is dissolved oxygen in the air; another most common corrosion-causing substance is dissolved salts in water, the most important of which is chloride. The types of corrosion of metals in water generally fall into the following categories:


Dissolved Oxygen Corrosion

This corrosion is due to the reaction of carbon steel with oxygen dissolved in water to form iron oxides. This type of corrosion is often uneven general corrosion. However, if the water quality is hard, localized corrosion may also occur.


Galvanic corrosion

If some parts in the equipment are made of different metal materials, and they are connected to each other and placed in water, a galvanic battery will be formed due to the different electrode potentials of metals of different materials, and the corrosion generated at this time is galvanic corrosion. The most common example is the galvanic coupling between carbon steel and copper to accelerate the corrosion of steel.


Crevice Corrosion

Crevice corrosion is caused by the formation of a very 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, it may occur on the flange connection surface of the heat exchanger, under the rust layer and the scale layer, etc. Perforations in heat exchangers are often caused by crevice corrosion.


Pitting Corrosion

Pitting is a special type of localized corrosion that causes small holes in the metal and, in severe cases, can perforate equipment. Pitting corrosion mainly occurs in materials that can self-passivate (including passivation 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 in the presence of crevices. Of all the materials, stainless steel is the most susceptible to pitting corrosion.


Stress Corrosion Cracking (SCC)

The cracking caused by metal under the combined action of tensile stress and corrosive medium is called stress corrosion cracking, which is customarily represented by SCC. For stainless steel, titanium alloy, aluminum alloy, and sometimes even carbon steel and other materials in the gas medium containing Cl-, it is easy to produce stress corrosion cracking due to the effect of stress. A vertically installed stainless steel heat exchanger as shown in the figure below, 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 dead angle. During the flow of cooling water, due to the splashing effect of cooling water, the dead corners become dry and wet alternate parts. Even if the salinity of the cooling water is very low, in this alternating dry and wet part, the salinity is continuously increased due to the continuous concentration of the cooling water.


Wear 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 corroded and damaged. This type of corrosion is called wear corrosion. Wear corrosion is the combined result of the mechanical scouring action of high-velocity fluids on the corrosion products already formed on the metal surface and the diffuse corrosion action on the newly exposed metal surface. Wear corrosion can be divided into turbulent corrosion and cavitation corrosion. Turbulent flow corrosion occurs in some specific parts of the equipment, such as the inlet end of the heat exchanger tube. Due to the sudden increase in the flow velocity, turbulent flow is formed there, which causes the metal surface to be greatly disturbed (shear stress), thereby causing corrosion. Cavitation corrosion is also known as cavitation. It is when the fluid and the metal components move relative to each other, eddy currents are generated in the local area of the metal surface, so the bubbles are rapidly generated and collapsed on the metal surface, thereby impacting the metal surface and causing corrosion.


Hydrogen Hazard

It represents the damage to the metal due to the presence or interaction of hydrogen, including: hydrogen bubbling, hydrogen embrittlement, decarburization, and hydrogen corrosion. For example, hydrogen embrittlement may occur in carbon steel and low alloy steel in geothermal water containing hydrogen sulfide.


Microbial corrosion

Microbial corrosion is a special type of corrosion, which is difficult to exist alone, and often occurs at the same time as electrochemical corrosion. The main reason for microbial corrosion is the accumulation of sludge. The metal surface covered by the sludge is an oxygen-depleted area, and the metal suffers local corrosion due to the action of the oxygen concentration cell. In addition, due to the reproduction of microorganisms, a special corrosive environment is created, which aggravates the corrosion. For example, river bottom sludge deposited in cooling water systems contains sulfate-reducing bacteria, highly corrosive bacteria that reduce sulfate to sulfide.