Author Contributions
Conceptualization, Y.-H.L. and G.-I.K.; methodology, Y.-H.L. and G.-I.K.; validation, Y.-H.L., G.-I.K., K.-M.K., S.-J.K., W.-C.K. and J.-G.K.; formal analysis, Y.-H.L. and G.-I.K.; investigation, Y.-H.L. and G.-I.K.; resources, Y.-H.L. and G.-I.K.; data curation, Y.-H.L., G.-I.K., K.-M.K. and S.-J.K.; writing—original draft preparation, Y.-H.L. and G.-I.K.; writing—review and editing, Y.-H.L., G.-I.K., K.-M.K., S.-J.K., W.-C.K. and J.-G.K.; visualization, Y.-H.L. and G.-I.K.; supervision, J.-G.K.; project administration, J.-G.K.; funding acquisition, W.-C.K. and J.-G.K. All authors have read and agreed to the published version of the manuscript.
Figure 1.
A schematic illustration of the heat transport pipe used in a DH system.
Figure 1.
A schematic illustration of the heat transport pipe used in a DH system.
Figure 2.
Photographs of the failed pipe: (a) water leakage resulting from pitting in the failed pipe, (b) external surface of the failed pipe around the leakage area.
Figure 2.
Photographs of the failed pipe: (a) water leakage resulting from pitting in the failed pipe, (b) external surface of the failed pipe around the leakage area.
Figure 3.
Surface image of the pitting part: (a) pitting corrosion area of the leakage (the outside of the pipe), (b) pitting corrosion area near the leakage (the outside of the pipe), and (c) pitting corrosion area near the leakage (the inside of the pipe).
Figure 3.
Surface image of the pitting part: (a) pitting corrosion area of the leakage (the outside of the pipe), (b) pitting corrosion area near the leakage (the outside of the pipe), and (c) pitting corrosion area near the leakage (the inside of the pipe).
Figure 4.
Cross-sectional images of the pitting part: (a) pitting corrosion where the leakage occurred, (b) pitting corrosion near the leakage, and (c) pitting corrosion near the leakage.
Figure 4.
Cross-sectional images of the pitting part: (a) pitting corrosion where the leakage occurred, (b) pitting corrosion near the leakage, and (c) pitting corrosion near the leakage.
Figure 5.
The optical microstructures of specimens: (a) specimen A (pitting corrosion region), and (b) specimen B (uniform corrosion region).
Figure 5.
The optical microstructures of specimens: (a) specimen A (pitting corrosion region), and (b) specimen B (uniform corrosion region).
Figure 6.
Topography and surface potential of pearlite (specimen A): (a) topography of pearlite, (b) surface potential of pearlite, (c) topography inside of pearlite, and (d) surface potential inside of pearlite.
Figure 6.
Topography and surface potential of pearlite (specimen A): (a) topography of pearlite, (b) surface potential of pearlite, (c) topography inside of pearlite, and (d) surface potential inside of pearlite.
Figure 7.
The OM, and SEM/EDS analyses of the pitting corrosion near the location of the leakage; (a) OM analysis, (b) SEM analysis (yellow box), and (c) EDS mapping analysis (yellow box).
Figure 7.
The OM, and SEM/EDS analyses of the pitting corrosion near the location of the leakage; (a) OM analysis, (b) SEM analysis (yellow box), and (c) EDS mapping analysis (yellow box).
Figure 8.
The EPMA analysis of a cross section where pitting corrosion occurred in the failed pipe.
Figure 8.
The EPMA analysis of a cross section where pitting corrosion occurred in the failed pipe.
Figure 9.
Open-circuit potential of specimen A (pitting corrosion region) and specimen B (uniform corrosion region) with immersion time in the groundwater at 60 °C.
Figure 9.
Open-circuit potential of specimen A (pitting corrosion region) and specimen B (uniform corrosion region) with immersion time in the groundwater at 60 °C.
Figure 10.
Potentiodynamic polarization curves of specimen A (pitting corrosion region) and specimen B (uniform corrosion region) in the groundwater at 60 °C.
Figure 10.
Potentiodynamic polarization curves of specimen A (pitting corrosion region) and specimen B (uniform corrosion region) in the groundwater at 60 °C.
Figure 11.
Surface and cross-sectional images of the specimens after galvanostatic polarization test: (a) surface image of specimen A (pitting corrosion region), (b) surface image of specimen B (uniform corrosion region), (c) cross-sectional image of the specimen A, and (d) cross-sectional image of the specimen B.
Figure 11.
Surface and cross-sectional images of the specimens after galvanostatic polarization test: (a) surface image of specimen A (pitting corrosion region), (b) surface image of specimen B (uniform corrosion region), (c) cross-sectional image of the specimen A, and (d) cross-sectional image of the specimen B.
Figure 12.
Failure mechanism of the failed low-carbon steel pipe based on aluminum inclusion and the larger phase fraction of the pearlite: (a) initial stage, and (b) later stage.
Figure 12.
Failure mechanism of the failed low-carbon steel pipe based on aluminum inclusion and the larger phase fraction of the pearlite: (a) initial stage, and (b) later stage.
Table 1.
Chemical composition of district heating water (ppm) used in a district heating system.
Table 1.
Chemical composition of district heating water (ppm) used in a district heating system.
pH | NaCl | Mg(OH)2 | CaCO3 | NH4OH |
---|
9.5 | 15.01 | 0.48 | 2.65 | 10.28 |
Table 2.
Chemical composition of synthetic ground water (ppm).
Table 2.
Chemical composition of synthetic ground water (ppm).
pH | CaCl2 | MgSO4∙7H2O | NaHCO3 | H2SO4 | HNO3 |
---|
6.8 | 133.2 | 59.0 | 208.0 | 48.0 | 21.8 |
Table 3.
Chemical compositions of the failed low-carbon steel pipe and KS D 3562 standard (wt. %).
Table 3.
Chemical compositions of the failed low-carbon steel pipe and KS D 3562 standard (wt. %).
Elements | C | Si | Mn | P | S | Al |
---|
Failed pipe | 0.08 | 0.02 | 0.42 | 0.011 | 0006 | 0.04 |
KS D 3562 | 0.25 | 0.35 | 0.30–0.90 | 0.04 | 0.004 | – |
Table 4.
Volume fraction of the pearlite and ferrite phases according to the specimen A (pitting corrosion region) and specimen B (uniform corrosion region) in the failed pipe.
Table 4.
Volume fraction of the pearlite and ferrite phases according to the specimen A (pitting corrosion region) and specimen B (uniform corrosion region) in the failed pipe.
| Pearlite (%) | Ferrite (%) |
---|
Specimen A | 13.68 ± 0.58 | 86.32 ± 0.58 |
Specimen B | 5.57 ± 0.34 | 94.43 ± 0.34 |
Table 5.
Surface potential of different phase area and their differences.
Table 5.
Surface potential of different phase area and their differences.
Phase | Position | Potential (mV) | Surface Potential Difference (mV) |
---|
Mean | Dev |
---|
Pearlite | A | 124.99 | 15.85 | 44.40 |
Ferrite | B | 80.59 | 13.97 |
Cementite in pearlite | C | 147.22 | 13.56 | 34.78 |
Ferrite in pearlite | D | 112.44 | 13.26 |
Table 6.
The electrochemical parameters resulting from the polarization measurements of specimen A and specimen B in the groundwater at 60 °C.
Table 6.
The electrochemical parameters resulting from the polarization measurements of specimen A and specimen B in the groundwater at 60 °C.
| Ecorr (mVSCE) | icorr (A/cm2) | Corrosion Rate (mm/yr) |
---|
Specimen A | −621.04 | 6.47 × 10−5 | 0.75 |
Specimen B | −704.63 | 3.33 × 10−5 | 0.39 |