Views: 0 Author: Site Editor Publish Time: 2026-01-13 Origin: Site
Heat exchangers used in medical waste incineration systems operate under extremely harsh conditions characterized by high-temperature flue gas, acidic components, particulate matter, and frequent start-up and shutdown cycles. These conditions often lead to severe corrosion failures, significantly reducing equipment service life and operational reliability. Through macroscopic inspection and microscopic analysis, the corrosion mechanisms affecting heat exchanger tubes were systematically investigated. The main corrosion forms identified include crevice corrosion, erosion–corrosion, stress corrosion cracking, and high-temperature molten salt corrosion. Corresponding causes and improvement measures are discussed.
Medical waste incineration generates flue gas containing acidic gases such as hydrogen chloride (HCl) and sulfur dioxide (SO₂), as well as dust and volatile metal salts. Heat exchangers installed downstream of incinerators are essential for waste heat recovery but are highly susceptible to corrosion damage. Understanding the corrosion mechanisms is a prerequisite for material selection, structural optimization, and operational improvement.
Based on macroscopic observations and microscopic examination, the corrosion of heat exchanger tubes can be classified into the following forms.
During operation, the heat exchanger is subjected to high stress and elevated temperature. The stresses mainly originate from:
Tensile stress, caused by welding and cold working residual stresses
Thermal stress, resulting from temperature differences between the tubes and the shell, axial and radial temperature gradients in the tube sheet and shell, and local temperature variations caused by uneven flow distribution or fouling
In this system, the pressure difference between the tube side and shell side is only about 0.05 MPa, indicating relatively low operating pressure. Additionally, the heat exchanger underwent overall post-weld heat treatment, so residual fabrication stress is limited. However, due to the very high tube-side temperature and large temperature gradients, thermal stress is the dominant source of stress.
Stress corrosion cracking is highly environment-dependent and results from the combined action of stress and corrosive media, often triggered by specific ions. For austenitic stainless steels, the presence of chloride ions significantly increases susceptibility to SCC.
Since the flue gas contains acidic gases such as HCl and SO₂, acidic condensate inevitably forms during each start-up and shutdown cycle. This condensate flows into and accumulates in the annular gaps around tube ends, where it remains stagnant and becomes concentrated. Combined with high operating temperatures, these conditions create an ideal environment for stress corrosion cracking. This explains why severe corrosion occurs between tube sections, while the upper water-cooled tubes show little or no corrosion.
The heat exchanger adopts a flanged connection between the water-cooled section and the air-cooled section. As a result, gaps exist at the flange joints between the upper water-cooled tubes and lower air-cooled tubes.
During long-term operation, particulate matter carried by the flue gas deposits in these gaps, forming narrow crevices on the metal surface. When the system shuts down, acidic gases such as HCl and SO₂ condense and remain trapped within these crevices.
Because the solution inside the crevice is stagnant, dissolved oxygen is gradually depleted and cannot be replenished, while oxygen outside the crevice remains available. This difference creates an oxygen concentration cell, causing rapid dissolution of metal ions inside the crevice. Chloride ions migrate continuously into the crevice to maintain charge neutrality. As metal chlorides hydrolyze, the pH value inside the crevice drops further, accelerating localized corrosion at the joint between the two tube sections.
In addition, crevice corrosion is also prone to occur at welded joints between tubes and tube sheets.
The tube-side configuration consists of connected water-cooled and air-cooled tubes, resulting in unavoidable gaps at the joint. Under high-velocity flue gas flow, turbulence is generated at these joints. Turbulence increases the frequency of contact between the corrosive medium and the metal surface and imposes additional shear stress, leading to erosion–corrosion.
Furthermore, the presence of dust particles in the flue gas amplifies the shear stress effect, significantly accelerating material loss and corrosion damage.
Hydrogen chloride in flue gas causes severe corrosion to stainless steel below the acid dew point. Above the dew point, corrosion decreases until temperatures exceed approximately 260 °C, where high-temperature reactions between HCl gas and steel intensify again. Austenitic stainless steels exhibit relatively good resistance to dry HCl corrosion in the temperature range between the acid dew point and 260 °C.
However, in the temperature range of 400–600 °C, reactions between chlorides and metals become most aggressive. In this heat exchanger, chlorine in the flue gas readily reacts with iron to form low-melting-point compounds such as FeCl₃ and FeCl₂, which exist in a molten state within the boundary layer and cause high-temperature molten salt corrosion of the tube wall.
Additionally, volatile low-melting-point compounds such as ZnCl₂ and PbCl₂ present in the flue gas can condense and adhere to the inner surfaces of heat exchanger tubes in liquid form during operation. These molten salts not only intensify high-temperature corrosion but may also lead to tube blockage.
Based on the above analysis, the following recommendations are proposed to mitigate corrosion in medical waste incineration flue gas heat exchangers.
Given the high-temperature flue gas environment, the heat exchanger materials should be selected by considering not only resistance to high-temperature corrosion but also resistance to:
Stress corrosion cracking
Pitting and crevice corrosion
Erosion–corrosion
Mechanical and thermal fatigue
During welding, filler metals and welding consumables with good ductility, high purity, and strong crack resistance should be selected.
The welded joint between heat exchanger tubes and tube sheets is a critical corrosion-prone area due to concentrated residual and thermal stresses, which may cause stress corrosion cracking and corrosion fatigue, leading to leakage. Therefore:
Tube ends should be properly ground and cleaned before welding
Welding zones must be free from contaminants
From a weld quality perspective, internal bore welding structures are recommended, as they effectively reduce stress corrosion cracking and crevice corrosion at tube-to-tube-sheet joints
Structural design should minimize dead zones and crevices to prevent chloride ion stagnation and local concentration, thereby reducing the risk of localized corrosion.
A sacrificial anode protection method is recommended, using carbon steel tube sheets lined with stainless steel. In chloride-containing environments, carbon steel acts as the anode and stainless steel as the cathode. This configuration provides cathodic protection to the stainless steel, effectively slowing corrosion at tube-to-tube-sheet connections.
Corrosion inhibitors can be added directly into the incinerator. Additives capable of forming high-melting-point compounds with ash components should be selected to prevent the formation of low-melting-point composite chlorides and sulfates.
Commonly used additives include CaO and MgO. These compounds:
Neutralize acidic gases, reducing acid-induced corrosion
Suppress molten salt corrosion by increasing ash melting points
The corrosion of flue gas heat exchangers in medical waste incineration systems is the result of multiple interacting mechanisms, including stress corrosion cracking, crevice corrosion, erosion–corrosion, and high-temperature molten salt corrosion. Through appropriate material selection, improved welding and structural design, sacrificial anode protection, and the use of corrosion-inhibiting additives, the service life and reliability of heat exchangers can be significantly enhanced.
