Leakage analysis of 10# steel heat exchanger tube

A 10# steel heat exchanger tube with an outside diameter of 25mm and a wall thickness of 2.5mm had a steam leak in service. The leaking of heat exchanger tube was macroscopically analyzed, chemical composition testing, metallurgical examination, and energy spectrum analysis. The results show that: the chemical composition of the heat exchanger tube meets the requirements, but the corrosion is serious; the oxide layer is thick and dense, close to the oxide matrix with the highest oxygen content. It can be concluded that the leakage of the heat exchanger tube is mainly due to the rolling process; the oxide skin is pressed into the inner surface and is seriously corroded and perforated. Corresponding improvement measures and suggestions are put forward.

0. Introduction

The heat exchanger is mainly used for heat transfer between fluids, widely used in the chemical, petroleum, and food industries. A 10# steel heat exchanger tube with an outer diameter of 25mm and a wall thickness of 2.5mm (the fin material is aluminum alloy) has a steam leakage after 3 months of use. In this paper, the causes of heat exchanger tube leakage are found through macro-inspection, chemical composition testing, metallographic inspection, and energy spectrum analysis, and suggestions for improvement are made.

1. Macro-analysis

The surface of the leaking part of the heat exchanger tube has corrosion holes, as shown in Figure 1(a), the tube’s cross-section is severely corroded, and there are some corrosion bulges on the inner wall of the tube. The longitudinal cross-section of the tube is shown in Figure 1 (b); the inner wall has many corrosion products, mainly rust; there are open corrosion bulges, cleaned up after the discovery of corrosion pits underneath, that is, the material pitting corrosion occurs. Some small bumps were difficult to clean off when cleaning the inner wall with an iron brush. In the raised part of the transverse sample, grinding cross-section oxide is black, see Figure 1 (c), with a total thickness of about 2.5mm.

2. Physical and chemical test

2.1 chemical composition analysis

HW2000G type ignition infrared carbon and sulfur analyzer and SPECTROMAX-FV type vertical direct-reading spectrometer detected the chemical composition of the leakage of heat exchanger tubes; the results are listed in Table 1; its chemical composition is in line with the requirements of GB/T 9948-2013.

Figure.1 Macroscopic morphology of the surface of the leaking heat exchanger tube (a), corroded inner wall (b), and oxide cross-section (c).

Table.1 Chemical composition of the leaking tube (mass fraction, %)

Item	C	Si	Mn	P	S	Cr	Cu
Measured value	0.11	0.216	0.423	0.018	0.008	0.04	0.015
Standard value	0.07-0.13	0.170-0.370	0.350-0.650	≤0.025	≤0.015	≤0.15	≤0.200

2.2 Metallographic examination

The morphology of the cross-section of the un-etched pipe is shown in Fig. 2. Figure 2 (a) of the upper half of the oxide can be seen in two layers of oxide; one layer is about 0.70 mm thick; the other layer is about 0.58 mm thick. The uppermost layer of oxide is the iron brush is difficult to clean the raised parts, indicating that the upper and lower layers of oxide combined firmly. Figure 2 (b) for the other parts of the oxide, the thickness of about 0.72mm, and the substrate tightly combined, the hardness of 264HV0.2, the hardness of the substrate is 119HV0.2.

Fig.2 Oxide on the cross-section of the leaking tube

Specimen corrosion metallographic photographs are shown in Figure 3. Figure 3 (a) in the oxide layering phenomenon is more obvious; Figure 3 (b) of the upper half of the oxide, the matrix organization of ferrite and pearlite, the ferrite grain size of about 8, relatively small, the heat treatment state of the steel pipe is normalized state. Figure 3 shows no decarburization of the matrix near the oxide, indicating that the oxide has nothing to do with normalizing heating.

Fig.3 Low (a) and high (b) magnification of the leaking pipe cross-section

2.3 Spectral analysis

The chemical composition of the oxide shown in Fig. 2(b) was detected by energy spectrum analysis, and the results are shown in Fig. 4 and Table 2. The results of the energy spectrum analysis show that the oxide is mainly iron oxide, and the closer to the substrate, the higher the oxygen content and the lower the content of chlorine and sulfur.

Figure.4 Energy spectra of the detected parts of the oxide components (a) and parts A (b), B (c), and C (d) in the oxide.

Table.2 Chemical composition of the oxides (mass fraction, %)

Position	0	Si	S.	Cl	Ca	Mn	Fe
Part A	23.5	0.36	0.12	0.29	0.07	0.36	69.53
Part B	28.94	0.22	0. 10	0.07	0. 01	0.03	65.69
Part C	36.16	0.05	0.07	0.09	0. 14	0.32	56.36

3. Comprehensive analysis

From Fig. 2 and Fig. 3, it can be seen that the cross-section of the leaking tube has a 0.70mm thick oxide, fine grain, the oxide layer, and the matrix combined with no decarburization phenomenon, which can indicate that the oxide is not produced under the high temperature of the steel pipe. Figure 2 (a) shows that the oxide has a superposition phenomenon, and the combination is strong. Figure 1 (c) shows the raised part of the black tissue, and the scale is white material, so the oxide is not a condensate scale. Assuming that pitting has occurred in this area, the upper part of the oxide here is very dense, the corrosion products generated by pitting cannot be discharged, and pitting is difficult to occur, so it is not the appearance of pitting. In addition, the specimen is placed in the atmosphere for 3 months; Figure 1 (c) of the section, in addition to the black oxide, other parts of the red rust, high temperature oxidation of Fe3O4 has good corrosion resistance. Therefore, the denser oxides shown in Fig. 2 may be caused by pressure processing.
The energy spectrum analysis of the oxide shows that: the closer to the substrate, the oxygen content is higher, and the iron content is about close to the substrate, the lower. In contrast, normal corrosion produces oxides lower in oxygen content the closer they are to the substrate. Steel pipe corrosion in the steam environment is most likely to be related to the oxygen in the condensate; the leakage of the pipe wall surface should be an oxygen-poor environment, so the formation of this layer of oxide has nothing to do with oxygen corrosion. Chlorine and sulfur elements detected by spectral analysis may be due to the hot rolling of phosphorus removal is not clean and chlorine and sulfur ions in water contamination. The sum of black oxide and matrix thickness in the steel pipe is exactly equal to the wall thickness, so the black oxide in the cross-section of the steel pipe is not caused by corrosion but by the rolling process of the steel pipe.
Steel pipe is pressed into the oxide skin in the normalizing process part of the loosening, which produces a gap with the matrix, easy to lead to the matrix to produce crevice corrosion. Oxide skin and the base metal constitute a primary cell, thus accelerating the steel pipe to produce galvanic corrosion. In addition, pitting oxides easily lead to steel pipe pitting corrosion. This corrosion exacerbates the breakage of the steel pipe and eventually leads to perforated leakage due to severe surface corrosion and pitting.
From the above analysis can be concluded: steel pipe in the production process will not be removed from the surface of the oxide skin pressed into the inner surface of the pipe wall so that the inner surface of the steel pipe oxidation inclusions are more and more serious corrosion, resulting in perforation and leakage.

4. A few suggestions

• (1) Strengthen the heat exchanger tube wall flaw inspection, strict control of the quality of the steel pipe, and rolling process of regular replacement of the top head.
• (2) before using the heat exchanger pipe for a comprehensive cleaning to prevent galvanic corrosion. In addition, strengthen the management of boiler water quality, control the heat exchanger tube condensate generation, and timely evacuation of condensate.

Author: Song Rui