BFE10-1-1 cupronickel tube has good anti-fouling performance and heat transfer performance, and its corrosion temperature sensitivity is low. The hydrogen cooler of a ultra-supercritical thermal power unit was leaking after 4A (year) operation. After disassembling, it was found that the inner wall of heat exchanger tubes was corroded in many places, and several of them were leaking through holes. The leaking failed heat exchanger tube is made of BFe10-1-1 cupronickel? 19 mm and 1mm, the flow medium in the pipe is closed circulating water, Ph is 8.5ー9.5, flow velocity is 1.2 ms-1, working pressure is 0.2 MPA, working temperature is 40ー50 °C. In order to find out the failure reason of BFe10-1-1 Cupronickel Tube, the author has carried on a series of physical and chemical examination and analysis to it in order to prevent the similar accident from happening again. 1. Macroscopic observation of physical and chemical examination

Fig. 1 shows the macroscopic appearance of the failed tube. It can be seen that there is obvious crack at the leaking part of the tube. The crack is radiating and no obvious plastic deformation is observed. Three samples were taken from the non-leaking part of the failed pipe by chemical composition analysis, and analyzed by PDA7000 total quantitative element analyzer. The results are shown in Table 1.

Visible failure of tubes in accordance with Gb/t 5231-2012? Grade and chemical composition of processed copper and copper alloys? Requirements for composition of BFe10-1-1 cupronickel. 2. After being polished by water mill, the tubes were corroded by 100mL hydrochloric acid + 5g copper chloride + 100mL alcohol, and then the microstructure and crack morphology of the tubes were observed by Carl Zeiss Observer A1m metallographic microscope, as shown in figure 2.

Figures 2A) and B) show that the microstructure of the failed tube is a single-phase solid solution. Figures 2C) and D) show that there are many cracks in the leaking part of the tube. 3. After 3 times ultrasonic cleaning (the cleaning medium is pure anhydrous alcohol) , the fracture and crack morphology of the sample at the leaking part of the failure pipe were observed by Carl Zeisssigma 300 thermal field emission Scanning electron microscope, as shown in Fig. 3.

Fig. 3A) the fracture surface is rock sugar, Fig. 3B) there is obvious coating inside the crack. EDS analysis of the fracture surface and the coating inside the crack is shown in Table 2.

It can be seen that the material in the fracture surface and the inner layer of the crack is mainly corrosion product, in which the mass fraction of chloride is higher and some sulfides are contained. Perform surface scan analysis of the leaking part of the failed pipe, as shown in figure 4.

t can be seen that the elements of carbon, nickel and iron in the failure tube matrix have obvious regional distribution and are mainly distributed at the grain boundary of the Matrix. 4. The inner coating of the failed tube is analyzed by SEM, as shown in Fig. 5, and the inner coating of the failed tube is analyzed by EDS. The results are shown in Table 3.

It can be seen that there is an obvious coating on the inner wall of the failed pipe, which is mainly composed of sediment and corrosion products. 5. It is analyzed and discussed that BFe10-1-1 Cupronickel tube has good corrosion resistance and is not easy to be corroded under general conditions. The surface of bfe10-1 cupronickel tube will be seriously damaged only when the flow velocity of seawater medium reaches 3 ms-1, rapid increase in corrosion rate. There is obvious sediment cover on the inner wall of the leaking heat exchanger tube, which easily leads to the difference of oxygen concentration and media concentration between the inner surface of the inner wall of the deposit and the circulation medium, then oxygen concentration difference corrosion is formed under the sediment. The anoxic Tube Wall under the sediment is the anode which corrodes the primary battery, and the oxygen-rich area around the sediment is the cathode. Due to the alkalinity of the cooling water in the cupronickel tube of the heat exchanger, the cupronickel Tube (anode) under the sediment is oxidized to CU2 + and CU2 + , and then hydrolyzed to CU2O under the condition of oxygen shortage, which makes the solution in the corrosion pit acidic and then intensifies the corrosion, the reaction equation is as follows: CU-E = Cu + (1) Cu-2e = Cu 2 + (2)2CU + + H2o = Cu 2O + 2h + (3) Cu 2 + + + H2O + 2e = Cu 2O + 2h + (4) , cl-further corrodes the FE elements at the grain boundary, and the FE elements enriched at the grain boundary are continuously lost to form the vacancy, and the vacancy is continuously formed and merged, resulting in the initiation of intergranular cracks, finally, the tube was corroded and cracked at grain boundary. 6. Conclusions and suggestions are made that there are obvious deposits on the inner wall of BFe10-1-1 Cupronickel tube of the heat exchanger, and oxygen concentration difference corrosion is formed under the deposits, which leads to pitting corrosion on the inner wall of the Tube, the continuous formation and combination of vacancies lead to the initiation of intergranular cracks, which eventually lead to intergranular corrosion cracking of pipes. It is recommended that the heat exchanger be maintained and cleaned when the unit is shut down, the residual medium in the heat exchanger tubes be blown out and filled with nitrogen gas, and the corrosion of the inner wall of the heat exchanger tubes produced in the same batch by the same manufacturer be checked, to ensure the safe operation of the unit. 

Source: Corrosion and protection

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