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Corrosion Damage and Countermeasures of Hastelloy C22 Amide Heater

Preface:

The methyl methyl ester unit of an acrylonitrile factory was built with the technology provided by a German company. Among them, the Hastelloy C22 amide heater ruptured and leaked many times after being used for 10 months, and it happened again about 20 days after each welding repair leaked, had to be replaced after only a year of operation.

In order to clarify the form, nature, and cause of damage and rupture, propose repair and improvement technologies, carry out relevant test and analysis work, and participate in the whole process of repair and transformation. This article focuses on restoration and renovation techniques. This work has important reference value for the construction and operation of methyl methyl ester plant. This work shows that even if high-nickel alloys such as Hastelloy C22 with high chromium and molybdenum content and excellent corrosion resistance in oxidizing, reducing and oxidizing-reducing media are used, if the manufacturing process is not proper, it is still unavoidable. Early failure.

1 Overview of Amide Heater:

The amide heater has a double-coil structure (see Figure 1), with a total of 10 layers, the core diameter of the outer coil tube is φ1508 mm, and the core diameter of the inner coil tube is φ1258 mm. The specification of the coil is φ114 mm×3 mm, the total length is 90,000 mm (connected by dozens of girth welds), the material is Hastelloy C22, and the welding material is ENi Cr Mo-10. The inner diameter of the coil embedded container is φ1042 mm, the wall thickness is 24 mm, and the material is S31603 (316L).

Amide heater operating parameters: inside the tube: working pressure 0.5MPa; working temperature 105.5/156°C; medium: amide solution, containing sulfuric acid, ammonium bisulfate, water, acetone, intermediate products, and other organic substances, with strong reducing and oxidizing properties[ 2] , especially the problem of high-temperature dilute sulfuric acid corrosion; inside the container: working pressure 0.95 MPa, working temperature 210/184 ℃, medium steam. The corrosion margin of heat exchange tubes and containers are both 0.

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Figure 1 Appearance of amide heater

The components of the heat exchange tubes have been tested to meet the requirements of relevant standards, as shown in Table 1.

Table 1 Chemical composition of heat exchange tube base materials and welding materials %

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2 Damage detection and analysis

2.1 Damage detection

The test found that the coil had serious corrosion damage, which caused rupture and leakage.

The details are as follows.

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Figure 2 Appearance of corroded grooves in the heat-affected zone 1 (1)

The heat-affected zone on both sides of the circumferential weld of the coil is severely corroded and thinned, forming an annular corrosion groove. The width of the groove is about 10-15 mm, and the remaining wall thickness at the bottom is only 0.1-0.5 mm. Holes appear one after another in the severely corroded parts, resulting in leakage. Almost all butt ring seam areas of coils have such corrosion damage, see Figures 2 and 3.

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Figure 3 Appearance of corroded grooves in the heat-affected zone 2 (2)

The local weld metal corrodes, forming pit-like or strip-like depressions, see Figure 4.

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Figure 4 Corrosion appearance of weld seam

(3)The pipe under the ring for repair (the ring covering the outer wall of the leaking pipe, the two ends of which are connected to the pipe by fillet welds) is severely corroded, the wall thickness is severely thinned, and large strip-shaped corrosion grooves and holes appear , or even fester, see Figures 5 and 6.

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Figure 5 Corrosion appearance under the collar 1

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Figure 6 Corrosion appearance under the collar 2
(4) The overall corrosion and thinning of the coil is obvious. The wall thickness of many pipe sections is only about 1 mm, and the specific data are shown in Table 2.
(5) The corrosion of the inner coil (with higher operating temperature) is more severe than that of the outer coil, see Table 2 for specific data.
(6) In the same cross-section of the coil, the corrosion and thinning of the outer part with large deformation (stretching) is more severe than that of the inner part during processing and forming, see Table 2. Table 2 lists the wall thickness measurement values at about 40 mm on both sides of the inner and outer coil annular seams of a heater (maximum and minimum values, the former is located inside the coil, the latter is located outside the coil), and the annular seam number Arrange upwards sequentially from the bottom ring seam of the coil. It can be seen from the data that the overall corrosion and thinning of the heater coil is obvious; the corrosion and thinning of the inner coil is more serious than that of the outer layer; the same tube section, the outer wall thickness thinner is more serious than the inner.
Table 2 The measured value of the wall thickness at about 40 mm on both sides of the annular seam of the heater coil part
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2.2 Causes of Coiled Pipe Corrosion
The following results were obtained through detection, metallographic and energy spectrum analysis.
(1) Due to the action of welding heat in the heat-affected zone and the pipe under the ring sleeve for repair welding, carbides or intermetallic compounds rich in Cr and Mo are precipitated at the grain boundaries (heated in the range of 400~1150°C, M6C, M2C, M23C6, etc. are precipitated Carbide and a variety of Ni-Mo intermetallic compounds[3-7]), that is, the material is sensitized, and the corrosion-resistant elements (Cr, Mo) in the surrounding matrix are greatly reduced, resulting in deterioration of corrosion resistance and severe intergranular corrosion. This is exactly what metallographic and energy spectrum tests have seen, see Figures 7-10, Table 3.
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Fig.7 Metallographic phase of intergranular corrosion on the surface in the sensitization temperature zone
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Fig.8 Metallographic phase of intergranular corrosion on the surface in the sensitization temperature zone
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Fig.9 EDS analysis lines at the edge of corrosion voids
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Figure 10 Matrix EDS analysis lines
Table 3 Results of energy spectrum analysis
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(1)Since the thickness of the repair ring is larger than that of the covered coil (2~3 times), the heat generated during the repair welding process is greater than the welding heat of the original coil, and the secondary heating results in more serious corrosion of the coil under the ring, as shown in the figure 5, 6. It can be seen that under operating conditions, even a corrosion-resistant material with excellent performance such as Hastelloy C22 is difficult to avoid the risk of sensitization [8]. Tests have shown [9] that the C2000 alloy with higher Mo content (Cr 23%, Mo 16%) still has the problem of sensitization-corrosion.
(2) For the weld metal whose composition is the same or similar to that of the base metal, alloying elements such as Mo and Cr segregate when the deposited metal is solidified, especially in the rapidly cooled as-cast structure, the segregation between coarse dendrites will be more severe. serious. Therefore, the corrosion resistance of the weld metal is obviously lower than that of the base metal, and the parts with low Mo and Cr contents in the weld metal will have obvious corrosion.
(3) When the welding heat specification is too large and the grains in the weld seam and the heat-affected zone are large, the above-mentioned heat-affected zone is rich in Cr, and the precipitation of Mo phase and the segregation of corrosion-resistant elements between the weld metal dendrites will be more prominent. The corrosion resistance of the joint area is severely deteriorated.
(4) A gap is formed between the ring sleeve and the outer wall of the coil with penetrating defects below, which leads to the concentration of the corrosive medium in the gap and accelerates the corrosion in the sensitized zone.
(5) During operation, the inner coil receives more heat and has a higher temperature, while the outer coil has a lower temperature, resulting in more serious corrosion of the inner coil with a higher temperature than the outer coil.
(6) Deformation can promote corrosion, resulting in more obvious corrosion thinning in areas with larger tensile deformation on the outside than on the inside.
(7) Medium erosion corrosion, especially on the outer side of the ring pipe, the medium changes flow direction from time to time, and the erosion corrosion suffered is more significant.
(8) This set of equipment adopts the widely used acetone cyanohydrin method to produce methyl methacrylate (that is, the ACH method). The biggest problem of this process is that the medium contains sulfuric acid, and there is a problem of high-temperature dilute sulfuric acid corrosion [10]. High-temperature dilute sulfuric acid is a strong electrolyte and has strong corrosiveness. Its corrosion on metals is mainly carried out according to the electrochemical mechanism; the medium containing dilute sulfuric acid is basically reductive corrosion, which can change from reductive to oxidative with changes in concentration, temperature, and impurity factors. Sex [2]. To sum up, on the one hand, the medium of the amide heater is a strong corrosive medium; on the other hand, the corrosion resistance of the grain boundary in the sensitized area of the material is greatly reduced.

 3 Countermeasures 3.1 Ideas for dealing with corrosion damage

(1) It can be seen from the inspection and analysis that the method of adding an external casing cannot be used for repair. Because of the welding heat, secondary heating, and gap effect, the repaired area is corroded faster, and the coils below it are generally corroded and festered (see Figure 6); moreover, after each repair with casing, run Corrosion leakage occurred again in only about 20 days.

(2) Use the method of cutting off all the ring seams and heat-affected zones (pipe sections) of the coil with serious corrosion damage, reassemble and weld. The reason is that the wide range of heat-affected zones on both sides of the weld has been corroded and thinned into grooves, or has leaked; and the material in this zone has been severely sensitized, so it is difficult to repair it with the usual repair welding method after removing defects by grinding.

(3) The overall solution treatment after welding is adopted. Under the current operating conditions, the biggest problem of the C22 coil heater is that, due to the action of welding heat, severe trench corrosion occurs in a large area in the area of the sensitizing temperature range. The overall solution treatment after welding can re-solutionize the rich Mo, Cr carbides and Ni-Mo intermetallic compounds precipitated during the welding process in the joint area, improve the composition segregation in the weld metal, and eliminate the adverse effects of cold deformation , Therefore, it is possible to fundamentally improve the joint area and the overall corrosion resistance of the coil.

(4) Technological transformation of the heater. In order to solve the overall serious corrosion and thinning problem of the amide heater and prolong its service life, it is proposed to increase the wall thickness of the coil from 3.00 mm to 6.00 mm after necessary demonstrations and tests (accounting shows that this change does not affect the original replacement. Thermal effect[1] ); In order to solve the serious intergranular corrosion problem in the sensitized zone, the overall solid solution treatment is carried out after the heater is welded.

 3.2 Coil welding repair
(1) Cut off the 60-80 mm long pipe section including the circular seam with a thin grinding wheel (aluminum oxide or silicon carbide grinding wheel), and the incision should be suitable for re-welding; check whether the corrosion grooves and other defects at the pipe mouth to be welded are removed Clean; clean and remove oil stains and scales within 30 mm of the pipe end, and polish until it reveals a metallic luster; trim the groove; reassemble as a whole (with the help of corresponding tools and fixtures) and adjust the shape and size of the coil; weld.
(2) Welded by manual tungsten gas shielded welding (TIG). Due to the on-site coil butt joint, it is difficult to choose laser welding with more concentrated energy [11-12].
(3) Welding material: Welding wire ERNi Cr Mo-10 (2.5 mm) whose composition is similar to that of the base metal is used. If the welding wire whose content of Cr and Mo (mainly Mo) is appropriately higher than that of the base metal is selected as the filler metal, such as ERNi Cr Mo-14, the influence of interdendritic segregation of the weld metal will be reduced, see Table 4.
Table 4 Chemical composition of welding materials
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(4) Shielding gas: For the welding pool, argon gas with purity [Ar] ≥ 99.99% can be used for protection; helium or He-Ar (80%~20%) can also be used for protection on the front side, and argon gas for protection on the back side ; Protected with helium, the arc energy density will be higher, and the temperature of the welding pool will be higher, which is conducive to improving the quality of the weld and is also conducive to the purification of the weld metal. This time, argon gas protection was used for the repair, 8~14L/min on the front side; 8~10 L/min on the back side (the flow rate of the shielding gas should be adjusted appropriately according to the color of the deposited metal surface (silver white, golden yellow)).
(5) Adopt a smaller welding heat specification to prevent excessive growth of weld metal and heat-affected zone grains, reduce the width of the joint sensitization temperature zone, and reduce the residence time at the sensitization temperature.
(6) Measures shall be taken to speed up the cooling rate of welded joints, and the layer temperature shall not exceed 60°C.
(7) Due to the poor fluidity of the deposited metal, it can be slightly swayed during welding; the arc is closed and filled, and carefully checked to avoid cracks at the burner; the slag is carefully cleaned between layers. Welding wire, grooves, tools, welder gloves, etc. must be kept clean during welding.
3.3 Tests related to post-weld solution treatment
(1) Preparation of test piece Test tube D0110×6 mm, welding wire ERNi Cr Mo-10, TIG butt welding. Solution treatment after welding at 1115~1130°C and heat preservation for 15 minutes.
(2) The room temperature tensile, bending and hardness tests were carried out on the solution treated parts, and the results are shown in Tables 5-7 respectively. The results of the bending test show that there is no problem with the atmosphere of the solution treatment furnace.
Table 5 Tensile test results of solution pipe welded joints at room temperature
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Table 6 Bending test results of solid solution pipe welded joints
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Table 7 Hardness test results of solid solution pipe welded joints

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(3) Metallographic test

The tissues of various parts of the butt ring seam area of the C22 pipe after solution treatment are shown in Figures 11-15. Figures 11 and 12 show the weld metal structure, that is, austenite. The former (near the fusion line) is cast columnar crystals; the latter (weld core) is equiaxed dendrites, but the grain boundaries and dendrite junctions are relatively Fuzzy, many grain boundaries have been dissolved. Figure 13 shows the microstructure of the fusion zone. The equiaxed grain boundaries on the base metal side are incomplete, and most of the columnar or dendritic grain boundaries on the weld side have been dissolved. Figure 15 shows the microstructure of the substrate, which is equiaxed and twinned, and the grain boundaries are fine and not obvious. Obviously, there is little precipitation at the grain boundaries [13].

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Fig. 1 1 Solid solution pipe weld 1 100×

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Figure 1 2 Solid solution pipe weld 2 100×

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Figure 1 3 Fusion line of solid solution pipe 100×

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Figure 1 4 Heat-affected zone of solid solution pipe 100×

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Figure 1 5 solid solution tube substrate 100×

For comparison, the organization of the corresponding position (weld metal, fusion zone, base material) of the running pipe is shown in Figures 16-20. Through comparison, it can be seen that the weld seam, heat-affected zone and base material structure of the solid solution pipe are thicker than the corresponding zone of the operating pipe, and the grain boundary of the operating pipe is thicker and clear; Partial disappearance of the grain boundaries in the grain region; most of the columnar grain boundaries on the side of the weld in the superheated zone of the solid solution tube disappear, and the austenite grain boundaries on the side of the base metal are incomplete. The above shows that after solid solution treatment after welding, the microstructure has changed significantly, and the precipitates in the joint sensitization temperature zone can be redissolved, which also has a significant effect on the weld metal composition, microstructure homogenization and elimination of cold deformation [13].

Based on the above tests, it can be concluded that after the solution treatment after welding, the uniformity of the composition and structure of the welded joint area of the C22 pipe has been fundamentally improved. From the hardness test, solid solution is also of great benefit in eliminating the influence of cold deformation.

3.4 Project implementation

(1) For the severely corroded operating pipes, two coil heaters were repaired using the method described in Section 3.2.

(2) Retrofit the C22 amide heater using the method described in Section 3.3. That is, the wall thickness of the remanufactured coil is increased to 6.0 mm; the coil is assembled and welded by TIG process; after welding, the overall solution treatment is carried out, and the heating temperature is 1115~1130 °C. An electric heating furnace is used for solution treatment. If gas heating is used for heat treatment, heating and cooling in a reducing atmosphere should be strictly controlled [14], and the flame should not be in direct contact with the component, otherwise, the metal may be embrittled and even cracked; dry nitrogen and argon can also be used for Protective atmosphere: The sulfur content in the combustion atmosphere should be strictly controlled and confirmed by relevant testing [14-16] to avoid severe high temperature (>635°C) sulfidation corrosion. The modified amide heater has been running for more than two years without any problems and works well.

4 Conclusion

(1) The rupture and leakage of Hastelloy C22 coil amide heater in the methyl ester unit within a short period of time is mainly caused by severe corrosion damage in the welding zone.

(2) Due to the action of welding heat, the material in the welded joint area precipitates carbides or intermetallic compounds rich in Cr and Mo, making the surrounding matrix poor in corrosion-resistant elements, which is the main factor leading to severe corrosion. Therefore, under the operating conditions of the heater, Hastelloy C22 still has the risk of “material sensitization”. Deformation and erosion accelerate corrosion.

(3) The method of adding casing should not be used for repair. Because of the welding heat, secondary heating, and interstitial effect, the repaired area will be corroded faster. It is recommended to use the method of cutting the circular seam and its heat-affected zone, reassembling, and welding for repair.

(4) Regardless of welding production or repair, post-weld solution treatment is the key link to solve serious corrosion problems. It will enrich the Mo in the joint area, re-solutionize the Cr precipitated phase, and homogenize the composition and structure of the joint area. Eliminate the adverse effects of cold deformation, so it will fundamentally improve the corrosion resistance of the joint area and the overall coil.

 
 
Post time: Jul-15-2023