The operating environment of flue gas desulfurization system is extremely harsh. The flue gas contains acidic substances such as SO₂, SO₃, HCl and HF, which will form strongly corrosive condensate under low temperature and high humidity conditions. The pH value can be as low as 2-3, and the Cl⁻¹ concentration can be as high as 20,000-60,000 ppm。 The traditional expansion joint frame adopts carbon steel lined glass flakes to prevent corrosion, which is prone to problems such as coating damage and matrix corrosion during long-term operation.

The introduction of the flue gas expansion joint liner 2205 is precisely to solve this pain point. 2205 duplex stainless steel is a kind of ferritic-austenitic duplex stainless steel whose chemical composition is mainly 22%Cr-5.3%Ni-3.2%Mo-0.16N。 This unique microstructure makes it have the good toughness of austenitic stainless steel and the high strength and stress corrosion resistance of ferritic stainless steel.

2. Core performance advantages of 2205 duplex stainless steel

1. Excellent chloride ion pitting resistance

In the environment of flue gas desulfurization, chloride ion is the main culprit of pitting and stress corrosion cracking of metal materials. Due to the content of 3.2% molybdenum and 22% chromium, the pitting equivalent resistance (PREN) of flue gas expansion joint liner 2205 is higher than that of 304 stainless steel (about 19) and 316L stainless steel (about 25)。

The results show that 304 stainless steel is very prone to pitting or even full corrosion in desulfurization simulation solution at 40-60℃, while 2205 duplex stainless steel exhibits good corrosion resistance and is suitable for flue gas desulfurization equipment。 This characteristic allows it to withstand the acidic corrosive environment in wet desulfurization systems for a long time.

2. Excellent resistance to stress corrosion cracking

Austenitic stainless steels (such as 304 and 316L) are prone to stress corrosion cracking (SCC) in tensile stress environments containing chloride ions. The ferrite-austenite duplex structure of 2205 duplex stainless steel effectively prevents the crack propagation path, and its SCC resistance is significantly better than that of ordinary austenitic stainless steel。 This characteristic is particularly important for expansion joints that are subjected to thermal stress and pressure fluctuations.

3. High strength and good machinability

The yield strength of 2205 duplex stainless steel is approximately twice that of 316L, which means that when lining 2205 with flue gas expansion joints, equivalent strength can be achieved using thinner sheets, thus reducing structural weight. At the same time, it has good processing performance and welding performance, and can use laser welding integral molding technology to manufacture complex structural parts。

4. Economic advantages

Compared to higher grade materials such as 904L Super Austenitic Stainless Steel, Hastelloy C-276, Titanium, the cost of 2205 Duplex Stainless Steel is more manageable. Under the premise of meeting the requirements of desulfurization flue gas corrosion protection, the flue gas expansion joint liner 2205 provides the best cost performance. In practical engineering, "lining" structure can be adopted-carbon steel is used for the main body of the frame, and 2205 steel plate is lined on the internal contact surface with flue gas, which not only ensures corrosion resistance, but also greatly reduces material cost。

3. Application scenario of 2205 lining expansion joint

1. Flue expansion joint at inlet of desulfurization tower

The flue gas temperature at the inlet of the desulfurization tower is about 120-180℃, and it contains high concentration of SO₂ and dust. This is a bad working condition where corrosion and erosion coexist. The flue gas expansion joint liner 2205 can withstand both acid corrosion and dusty gas flow, and its service life far exceeds that of the carbon steel lining scale scheme. The latest patent technology applied by TISCO shows that the original flue end material is upgraded to 2205 duplex stainless steel, and the integral molding technology of laser welding is adopted. The flue surface does not need to be anticorrosive, and the anti-scale performance is improved by more than 95%。

2. Clean flue expansion joint

The net flue gas temperature at the outlet of the desulfurization tower drops to 45-55℃, which is in a completely saturated state, and the condensate is extremely corrosive. The flue gas expansion joint liner 2205 is used in this area, which can effectively resist chloride ion pitting and crevice corrosion.

3. Bypass flue expansion joint

For the desulfurization device equipped with a bypass system, the bypass flue is impacted by high temperature flue gas when the bypass is opened, and is in a low temperature and humid environment when the bypass is closed. This alternating working condition requires extremely high thermal shock resistance and corrosion resistance of materials, and 2205 liner is an ideal choice.

4. Key points of structural design of 2205 lining expansion joint

1. Liner Structural Form

Typical structures of the flue gas expansion joint liner 2205 are divided into two types:

• FULL ALLOY CONSTRUCTION: The frame is manufactured in 2205 duplex stainless steel as a whole. The advantage is the most thorough corrosion resistance, but the cost is higher。
• Inner lining composite structure: The main body of the frame is made of carbon steel (Q235B), and the internal contact surface with flue gas is lined with 2205 steel plate, and the thickness is usually 2-4mm. Carbon steel and 2205 can be connected by plug welding or explosive compounding. This structure considers both anti-corrosion performance and cost control, and is the most widely used scheme at present。

2. Guide tube design

The guide tube is the component that is directly washed by smoke inside the expansion joint, and must be made of wear-resistant and corrosion-resistant materials. The design key points of the flue gas expansion joint liner 2205 guide tube include:

• The small end of that guide tube is facing the direction of flue gas flow, and the large end is facing the outlet
• The double straight-edge guide tube structure is adopted to prevent the gas from washing the guide tube caused by the change of the inner diameter of the compensator
• Intermittent welding is carried out on the windward side, while the leeward side is not moved and the expansion gap is kept

3. Cooperation with skin

For non-metallic expansion joints, the frame of the inner liner 2205 is used in conjunction with a fluororubber skin. The 2205 frame provides structural strength and a corrosion-resistant base, and the fluororubber skin provides flexible compensation and sealing functions. The matching surfaces of the two need to be finely treated to ensure no seamless gaps and no liquid accumulation.

Key Points of Construction and Quality Control

1. Welding process

The welding of 2205 duplex stainless steel needs special attention:

• Strictly control line energy input to avoid harmful phase generation in welding heat-affected zone
• Select matching 2209 welding wire
• After welding, it is recommended to carry out acid pickling passivation treatment to restore the corrosion resistance

2. Liner Fit Quality

For composite structures, it is necessary to ensure that there is no hollow drum and no gap between the 2205 lining plate and the carbon steel matrix. Permeation testing or ultrasonic testing can be used to verify the fit quality.

3. Connection with the original system

Transitional measures (such as setting anti-corrosion coating or lengthening the liner length) shall be taken at the connection between the liner 2205 of the flue expansion joint and the carbon steel flue to prevent galvanic corrosion.