failure analysis of a stainless steel pipeline

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Failure analysis of a stainless steel pipeline M. Suresh Kumar, M. Sujata, M.A. Venkataswamy, S.K. Bhaumik * Failure Analysis and Accident Investigation Group, Materials Science Division National Aerospace Laboratories, Bangalore 560 017, India Received 30 April 2007; accepted 14 May 2007 Available online 26 May 2007 Abstract In this study, the failure of a stainless steel (SS) pipeline supplying hydrogen to a hydro cracking reactor of a petrochem- ical industry is investigated. Leakage was observed in the pipeline during operation. Study revealed that the failure was by chloride stress corrosion cracking. The source of chlorine was found to be the glass wool that was wrapped on the pipeline for thermal insulation purpose. Use of SS foil beneath the thermal insulator facilitated condensation of chloride ions. The protective SS foil was destroyed by pitting corrosion followed by which the pipeline failed by stress corrosion cracking. A detailed analysis of the failure is presented in this paper. Ó 2007 Elsevier Ltd. All rights reserved. Keywords: Stainless steel pipeline; Corrosion under insulation (CUI); Pitting corrosion; Chloride stress corrosion cracking 1. Introduction Corrosion failures of pipeline and equipment under thermal insulation are of major concern in petroleum and chemical process industries. Generally, the insulation is provided to (a) save energy, (b) control process temper- atures, and (c) prevent hot working environment. Since the insulated metal surface is not amenable for inspec- tion, corrosion under insulation (CUI) occurs in an insidious manner. If adequate attention not paid, CUI imposes a major problem to these industries and often proves to be too expensive in terms of frequent repairs and shut down time. The failures may also result in the release of hazardous chemicals/gases to environment [1]. In these cases, the disastrous consequences are of more serious concern than that of the economic losses. The majority of the pipelines used in petrochemical and chemical industries are made of either plain carbon steels or 3XX series stainless steels (SS). The failures in carbon steels are mostly manifested due to generalized or localized loss of material while those in SS components occur by pitting corrosion or by stress corrosion cracking (SCC) [2]. The initiation of CUI usually occurs, because of ingression of water/moisture, high oper- ating temperatures, and presence of corrodants such as SO 2 (in the form of H 2 SO 4 ) and chlorides [2]. Despite the fact that most engineers and designers in the chemical industries are aware of CUI, and stan- dard recommended practices for the control of CUI are available, failures due to CUI are still a common 1350-6307/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.engfailanal.2007.05.002 * Corresponding author. Tel.: +91 080 25086277; fax: +91 080 25270098. E-mail address: [email protected] (S.K. Bhaumik). Engineering Failure Analysis 15 (2008) 497–504 www.elsevier.com/locate/engfailanal

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Page 1: Failure Analysis of a Stainless Steel Pipeline

Engineering Failure Analysis 15 (2008) 497–504

www.elsevier.com/locate/engfailanal

Failure analysis of a stainless steel pipeline

M. Suresh Kumar, M. Sujata, M.A. Venkataswamy, S.K. Bhaumik *

Failure Analysis and Accident Investigation Group, Materials Science Division National Aerospace Laboratories, Bangalore 560 017, India

Received 30 April 2007; accepted 14 May 2007Available online 26 May 2007

Abstract

In this study, the failure of a stainless steel (SS) pipeline supplying hydrogen to a hydro cracking reactor of a petrochem-ical industry is investigated. Leakage was observed in the pipeline during operation. Study revealed that the failure was bychloride stress corrosion cracking. The source of chlorine was found to be the glass wool that was wrapped on the pipelinefor thermal insulation purpose. Use of SS foil beneath the thermal insulator facilitated condensation of chloride ions. Theprotective SS foil was destroyed by pitting corrosion followed by which the pipeline failed by stress corrosion cracking. Adetailed analysis of the failure is presented in this paper.� 2007 Elsevier Ltd. All rights reserved.

Keywords: Stainless steel pipeline; Corrosion under insulation (CUI); Pitting corrosion; Chloride stress corrosion cracking

1. Introduction

Corrosion failures of pipeline and equipment under thermal insulation are of major concern in petroleum andchemical process industries. Generally, the insulation is provided to (a) save energy, (b) control process temper-atures, and (c) prevent hot working environment. Since the insulated metal surface is not amenable for inspec-tion, corrosion under insulation (CUI) occurs in an insidious manner. If adequate attention not paid, CUIimposes a major problem to these industries and often proves to be too expensive in terms of frequent repairsand shut down time. The failures may also result in the release of hazardous chemicals/gases to environment [1].In these cases, the disastrous consequences are of more serious concern than that of the economic losses.

The majority of the pipelines used in petrochemical and chemical industries are made of either plain carbonsteels or 3XX series stainless steels (SS). The failures in carbon steels are mostly manifested due to generalizedor localized loss of material while those in SS components occur by pitting corrosion or by stress corrosioncracking (SCC) [2]. The initiation of CUI usually occurs, because of ingression of water/moisture, high oper-ating temperatures, and presence of corrodants such as SO2 (in the form of H2SO4) and chlorides [2].

Despite the fact that most engineers and designers in the chemical industries are aware of CUI, and stan-dard recommended practices for the control of CUI are available, failures due to CUI are still a common

1350-6307/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.engfailanal.2007.05.002

* Corresponding author. Tel.: +91 080 25086277; fax: +91 080 25270098.E-mail address: [email protected] (S.K. Bhaumik).

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occurrence. A number of factors influence the CUI of pipelines and equipment in service. These are (i) impro-per design of pipeline/equipment and thermal insulation, (ii) improper selection of material and insulation, (iii)high operating temperatures, and (iv) inadequate inspection procedure [3]. Statistics show that majority of thefailures resulting from CUI are due to improper choice of materials. This paper deals with failure of a stainlesssteel (SS) pipeline wherein improper choice of protective metal foil material/insulation material resulted inchloride induced SCC.

2. Background

During operation, a leak was noticed in the pipeline supplying hydrogen to a hydro cracking reactor of apetrochemical industry. The pipeline was made of type 321 SS. The pipeline was wrapped with a protectivelayer of SS foil followed by glass wool insulation. On removal of the glass wool, perforations were noticedin the SS foil at the location of failure. A tight crack was noticed in the failed region of the pipeline. The nor-mal operating temperature and pressure of the pipeline are 72 �C and 195 kg cm�2, respectively.

3. Failure identification

3.1. Visual and stereo-binocular examination

Fig. 1 shows the cut portion of the pipeline where leakage was observed. Visual examination revealed cor-rosion on the external surface. The corroded regions were covered with brownish powdery deposit. Examina-tion under a stereo-binocular microscope revealed severe pitting all over the surface of the pipe (Fig. 2).

3.2. Fluorescent dye penetrant inspection (FPI)

FPI revealed cracks in the pipe. The appearance of the cracks under the ultraviolet (UV) light is shown inFig. 3a. Clusters of tight cracks were detected in the failed region of the pipeline. The cracks were interlinkedand they were oriented mostly in the longitudinal direction of the pipe (Fig. 3b).

Fig. 1. The failed region of the SS pipeline.

Fig. 2. Pitting corrosion on the external surface of the failed SS pipe.

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Fig. 3. (a) Cracks under UV light (after FPI) and (b) close-up view of the region marked in (a).

M. Suresh Kumar et al. / Engineering Failure Analysis 15 (2008) 497–504 499

3.3. Fractography

The tube was longitudinally cut into two halves and the half containing the cracks was subjected to tensileload in the transverse direction to facilitate opening of the cracks. The appearance of the cracks during theprocess of opening is shown in Fig. 4. On the external surface, several cracks had joined together to form alarge crack in the longitudinal direction. But on the inner surface, the cracks were discrete in nature. This

Fig. 4. Cracks on (a) external surface and (b) internal surface of the SS pipeline.

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indicates that the cracks have initiated on the outer surface and propagated progressively through the thick-ness. There was no corrosion on the internal surface of the pipe.

A typical appearance of the fracture surface is shown in Fig. 5. The fracture surface was very uneven and itwas covered with corrosion products. The gross fractographic features presented a brittle mode of fracture.

The fracture surface was further examined under a scanning electron microscope (SEM) for identificationof the mode of fracture. Examination revealed that the cracks had propagated mainly by transgranular modeof fracture. The fracture surface also showed presence of enumerable number of secondary cracks along thegrain boundaries (Fig. 6).

3.4. Analysis of corrosion product

The corrosion products on the fracture surface as well as those on the outer surface of the pipe were ana-lyzed by energy dispersive X-ray (EDX) analyzer attached to the SEM. Analysis showed presence of sulphurand chlorine in the corrosion products (Fig. 7). Examination revealed numerous pits on the external surface ofthe pipe (Fig. 8). The chlorine concentration in these pits was found to be as high as 1.0 wt%.

3.5. Metallographic study

Suitable samples were cut from the cracked regions of the pipe, mounted on the cross section, metallo-graphically prepared and observed under an optical microscope. The samples showed presence of branchingcracks (Fig. 9). These cracks had emanated from corrosion pits present on the external surface of the pipe. Thecracks had propagated in both intergranular and transgranular mode (Fig. 10). The pipe material revealed amicrostructure typical of austenitic stainless steel. There was no carbide precipitation at the grain boundaries(Fig. 11).

Fig. 5. Typical appearance of the fracture surface.

Fig. 6. SEM fractograph showing transgranular fracture and secondary cracks along the grain boundaries.

Page 5: Failure Analysis of a Stainless Steel Pipeline

Fig. 7. EDX spectrum of the corrosion products.

Fig. 8. Corrosion pits on the external surface of the pipeline.

Fig. 9. Branching cracks emanating from a corrosion pit on the external surface.

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Fig. 10. Transgranular and intergranular crack propagation.

Fig. 11. A typical microstructure of the parent material.

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4. Failure analysis

The external surface of the pipe was covered with a brown, powdery scale. It had rough appearance andcontained enumerable number of corrosion pits. Cracks were found to have originated from these pits andhad penetrated the pipe wall. The cracks had propagated mainly in transgranular mode, and they werebranching in nature. These features are typical of stress corrosion cracking (SCC). Stress corrosion cracks gen-erally undergo extensive branching and proceed in a direction perpendicular to the stresses contributing totheir initiation.

In general, four conditions are necessary for SCC to occur: (a) susceptible material (b) sustained tensilestress, either residual or applied, (c) aggressive environment containing specific ions, and (d) presence ofan electrolyte (water/moisture) [2]. It is known that austenitic stainless steels are susceptible to SCC andhence they need protection for satisfactory working under certain environmental conditions. The SCC inwrought stainless steels is usually transgranular in nature, if the microstructure is not sensitized. In sen-sitized microstructure, SCC invariably results in intergranular cracking. The microstructural examinationconfirmed that the pipe was properly heat-treated and there were no microstructural abnormalities. Hence,the SCC in the pipeline was caused due to environmental effects that resulted in transgranular mode offracture.

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Compositional analysis showed that chlorine ion was responsible for the initiation of pitting corrosionon the external surface and subsequent SCC in the pipeline. A chloride concentration of less than 10 ppmis sufficient to cause SCC in austenitic stainless steel if the metal temperature is >60 �C [2]. The operatingtemperature of the failed pipeline was 72 �C, which is ideal for chloride induced SCC. The source ofchloride was identified to be the insulation material, i.e., glass wool. Chemical analysis revealed thatthe glass wool contained about 200 ppm of chloride. Chlorides do not cause SCC unless an aqueousphase is present and therefore, it is certain that there was a condensation of water at the hot metalsurface.

The SS pipeline was wrapped with a protective layer of SS foil followed by which it was thermally insulatedby a layer of glass wool. On removal of the glass wool at the failed region of the pipeline, perforations wereobserved in the SS foil protective layer. It is, therefore, imperative that SS foil had undergone pitting corrosionfirst. After this, the protective layer was ineffective and the chlorides from the glass wool could easily migrateto the hot surface of the SS pipeline. As a result, concentrated chlorides had condensed/accumulated in local-ized regions on the outer surface of the pipeline leading to SCC failure.

A concentration of 200 ppm of chlorides in the glass wool at a temperature of about 70 �C would causeextensive pitting in the SS foil. Once this protection is destroyed, the pipeline beneath the protective layeris affected. Stainless steel is a bad thermal conductor and hence, there existed a temperature gradient whereinthe SS foil was at a lower temperature than that of the outer surface of the pipeline. This facilitated preferen-tial migration of the chlorides to the hot surface of the pipeline and resulted in condensation/accumulation ofchlorides in localized regions at much higher concentrations.

5. Conclusions and recommended corrective actions

The SS pipeline had failed by chloride induced stress corrosion cracking. The source of chlorides was theglass wool insulation. In the sequence of failure, the protective layer of SS foil had undergone pitting corrosionfirst. Subsequently, the chlorides from the glass wool had preferentially migrated and condensed on the exter-nal surface of the SS pipeline resulting in chloride induced stress corrosion cracking.

In general, the insulation materials contain chlorides. The concentration of chloride may vary depending onthe type of insulation material chosen. On exposure to moisture, such insulation materials release chlorides,which in turn result in pitting/stress corrosion cracking in the SS pipeline. Under the present operating con-ditions, a leachable chloride level as low as 10 ppm can cause failure by SCC. From this perspective, an addi-tional SS foil as protective layer is ineffective and therefore, cannot eliminate the vulnerability of the SSpipeline failure.

In view of the above, the following recommendations are made to prevent the recurrence of similar failures.

(a) Prevention of chloride leaching from the thermal insulation would be the most efficient remedial action.This can be achieved by adopting one or more of the following.(i) Use of insulation materials low in chlorides [2,4,5].

(ii) Substitution of chloride containing glass wool by inhibited asbestos or calcium silicate insulation.(iii) Waterproofing the insulation.(iv) Coating the pipeline prior to insulation.

(b) Instead of wrapping the SS pipeline with SS foil, use of aluminum foil would reduce the risk of corrosion[2,6]. This is a preferred practice when the operating temperatures are within 60–500 �C, because of thefollowing reasons.(i) The aluminium foil provides a physical barrier that prevents the saturated chloride solution from

reaching the hot stainless steel surface.(ii) Due to its high thermal conductivity, aluminium would be at same temperature as that of the stain-

less steel pipe and hence the chloride solution would shift to the foil rather than to the stainless steel.(iii) Aluminium also provides cathodic protection to stainless steel in the presence of chlorides and

thereby prevents pitting and stress corrosion cracking.

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Acknowledgements

The authors thank Head, Materials Science Division and Director, NAL for granting permission to publishthis paper. The help received from Mr. M. Madan for microstructural study and Mr. C.R. Kannan for NDT isgratefully acknowledged.

References

[1] John J. Mc Ketta, executive editor. Encyclopedia of chemical processing and Design; vol. 57: 1996. p. 343.[2] Corrosion under thermal insulation. Corrosion, ASM handbook, Metals Park (OH): American society for metals; vol. 13: 1987. p.

1144–8.[3] Peter III Lazaar. Factors affecting corrosion of carbon steel under thermal insulation. ASTM STP 1983;880:11–26.[4] Brown BF. Stress corrosion cracking control measures. US Department of commerce. National Bureau of standard; NBS monograph

156; p. 57.[5] Sumbry Louis, Jean vegdahl E. Prevention of chloride stress corrosion cracking under insulation. ASTM STP 1983;880:165–77.[6] Richardson James, Fitzsimmons Trevor. Use of aluminium foil for prevention of stress corrosion cracking of austenitic stainless steel

under thermal insulation. ASTM STP 1983;880:188–98.