investigation and analysis of high temperature corrosion

J. Marine Sci. Appl. (2016) 15: 86-94

DOI: 10.1007/s11804-016-1347-9

Investigation and Analysis of High Temperature Corrosion and Degradation of Marine Boiler Combustion Swirler

RS Virdi and DG Thakur*

Defence Institute of Advanced Technology (DU), Pune 411 025 (MS), India

Abstract: The present paper investigated and analyzed swirler material consisting of mild steel which was subjected to service for the period of one year in a 30 MW marine boiler. Due to the presence of high temperatures in the furnace coupled with the corrosive marine environment swirler material showed accelerated degradation and material wastage. An investigation into the feasibility of manufacturing the existing swirler with an alternate material or coating the swirler material with a thermal barrier coating was undertaken. Based on their properties and performance, SS 304 and SS 316 were proposed as the replacement materials for the swirler. The other alternative of coating the existing swirlers with a form thermal barrier coating to observe for any improvement in their performance at elevated temperatures was also tested. Stellite, which is a Ni-Co based coating, was carried out on the MS samples and the same were exposed to same temperatures mentioned above. The performance of the available options was evaluated with respect to the grain structure of the material, the hardness value of the materials and deterioration at elevated temperatures. Investigation showed the proposed materials/ coatings like SS 304, SS 316 and Stellite coating revealed that SS 316 is the material best suited for high temperature application. Keywords: marine boiler, corrosion, degradation, high temperature, swirler, stainless steel Article ID: 1671-9433(2016)01-0086-09

1 Introduction1

The resurgent interest in structural materials with the technological importance pertaining to the area of marine application has provided the necessary impetus for the development of new class of materials. A comprehensive understanding of different aspects of the ocean such as wind, water currents, tides, salinity, temperature and oxygen concentration is compulsory for structural trustworthiness in high performance ships and offshore structures. Corrosion, marine microfouling, velocity of flow and aeration are thought to be the most imperative variables leading to age-related structural degradation of numerous steel structures of ships.

Boilers operated on a ship present a major challenge in terms of operation and performance as marine environment is considered as the most corrosive natural environment.

Received date: 2015-06-26 Accepted date: 2015-12-28 *Corresponding author Email: [email protected]

© Harbin Engineering University and Springer-Verlag Berlin Heidelberg 2016

Combustion of heavy furnace fuel oils containing vanadium and sulphur compounds affects the performance of boiler materials (Castrejon et al., 2006). High temperature corrosion of the boiler combustion components is a very prevalent phenomenon in marine boilers. Production of corrosive compounds by burning of Furnace Fuel Oil (FFO) and the chloride based air result in accelerated corrosion of the combustion components of a boiler (López-López et al., 1999). The combustion components primarily consist of registers, dampers, oil burners, atomizers and air swirlers. Air swirlers are most susceptible to high temperature corrosion as it is closest to the flame in the boiler furnace. This research aims at finding alternate materials best suited for application at high temperatures in a marine environment.

Boiler materials performance is largely affected by the conditions in which the fuel oil combustion takes place. High temperature oxidation of super heaters and reheaters is due to the excess of air during the combustion process and is further accelerated by the presence of vanadium and sulphur compounds. The generation of SO3 also takes place and hence the corrosion in the low temperature zone of the boiler occurs due to the presence of sulphuric acid (López-López et al., 2002; Wong-Moreno et al., 2002). On the other hand, if the combustion system is well designed and in good condition, operation at low air excess is possible, otherwise a lot of partially burnt particles could be generated, reducing boiler efficiency and resulting in degradation of boiler materials due to carburization process (Castrejon et al., 2006). Degradation of combustion system components displaces up the minimum oxygen excess required for its operation under good conditions, favouring the high temperature and low temperature corrosion of boiler materials. As the viscosity, asphaltene, sulphur and vanadium content increases, the problem is magnified. Marine experience in burning heavy fuel oil shows that degradation of the combustion system has been an old problem even using conventional materials like mild steel and carbon steels. The purpose of this research work was to find evidence of the degradation mechanisms involved in several corroded burner components, namely air swirler.

2 Experimental procedures

2.1 Materials and methods Air swirler exploited in a 30 MW marine boiler for

Journal of Marine Science and Application (2016) 15: 86-94 87

approximately one year was analysed for corrosion characteristics. Fig. 1 represents a Air Swirler made up of mild steel which is used in a marine boiler. Five burners are arranged in a clockwise manner on the furnace wall of the boiler.

Fig 1. Air Swirler used in marine boiler Fig.2 presents a Energy Dispersive X-Ray (EDX) analysis

showing composition of the mild steel. The boiler was operated on full load during the day and at 60% load during the night. The boiler used FFO as the primary fuel for its operation.

Fig. 2 EDAX spectrum for swirler material

The swirler obtained was cut into smaller sections for analysis from the fin and the hub areas. Overall the surface of the swirler was extensively corroded and degradation was excessive at the hub section because of its close proximity to the flame front. The samples cut were prepared for metallographic analysis and for observing grain growth pattern as per standards ASTM E112 (ASTM Designation E-112-96, 2004). The analysis was done using Biovis MP 2000 Optical microscope and ZEISS Scanning Electron Microscope (SEM). The samples of the hub and fin sections of the swirler were prepared for Optical Microscopy (Olympus Optical Microscope) and SEM (Zeiss Sigma Scanning Electron Microscope) analysis to determine their grain structure and grain size. The samples were polished as

per set procedures and the same were etched with Nital (100 ml Ethanol and 10 ml Nitric Acid). Fig. 3 represents raw and prepared samples.

In addition to the above, SS 304 and SS 316 which are known steels used for high temperature applications (Priss et al., 2014) were procured and samples were cut using Wire Cut EDM and prepared for SEM analysis for comparison of the grain structure of the swirler material and SS 340/316 material. The Stainless Steel of grade 304 and 316 was cut into smaller samples of 10 mm10 mm sections for analysis as shown in Fig. 4. The samples were heated in Lenton UK tubular furnace for a soaking period of one hour and two hours at 600 oC, 800 oC and 1000 oC (Atanda et al., 2010; Metals Handbook 9th ed., 1987; Pickering, 1976). The samples were then analysed for metallographic analysis and observing grain growth pattern.

Fig. 3 Sample of Hub section

Fig. 4 Samples cut from SS 316 cut from swirler plate

3 Results and discussion

3.1 Damage description of swirler From Fig. 1 it can be seen that extensive corrosion has

taken place on the swirler. The swirler made of mild steel shows clear deformation at certain places and the hub section of the swirler is in an advanced degradation state. Since the load to be carried by the swirler is its own weight, the deformation indicates that the operating temperature inside the furnace was on the higher side (ASTM Designation E-112-96, 2004).

Samples cut from the swirler were polished and prepared

RS Virdi, et al. Investigation and Analysis of High Temperature Corrosion and Degradation of Marine Boiler Combustion Swirler 88

for microstructure analysis by optical microscope and SEM. The polished samples were etched with NITAL (HNO3+Ethanol). The images obtained for hub from optical microscope is shown in Fig. 5 and SEM images for hub and fin section are shown in Figs. 6 and 7 respectively. In addition, another sample from and unused MS plate was cut and prepared form microstructure analysis for comparison with the swirler material. Image of the same is shown in Fig. 8.

Fig. 5 Optical microscope image of swirler material 200X

Fig. 6 SEM image of swirler hub section

Fig. 7 SEM image of swirler fin section

Fig. 8 SEM image of sample cut from MS plate

3.2 Grain growth process Existence of high temperatures resulted in the phenomenon

of grain growth. Grain growth is clearly visible in the swirler hub material as indicated by the irregular shaped edges of the grain in image at Fig. 6 as compared to fin section in Fig. 7 which are 10 cms apart. Comparison of the images at Fig. 6 and 7 with Fig. 8, all of which are at the same magnification, infers that grain growth has occurred in the most corroded zone i.e hub of the swirler material which was exposed to high temperatures for a long duration of time. The grain growth is of the order of 25% in the hub as compared to the fin. This clearly shows that the temperature inside the furnace was well beyond the recrystallization temperature of mild steel which resulted in the grain growth of the swirler material.

It can also be seen from the SEM images of MS samples prior heating and post heating that grain growth has occurred. In case of the MS samples which have not been heated the grain boundaries are well defined and clear. For the samples which have been exposed to temperature i.e 600 oC and 800 oC for 02 hours, grain growth of 57% as compared to MS sample prior heating is clearly visible as depicted in Figs. 9 and 10. The grain boundaries are irregular and not defined at all. It can be inferred that for materials such as Mild Steel grain growth takes place at high temperatures which can result in intergranular corrosion and reduce the service life of the equipment.

Fig. 9 SEM image of treated MS at 600 ˚C for 2 hr


Fig. 10 SEM image of treated MS at 800 ˚C for 2 hr

3.3 Analysis of SS 304 and 316 samples Samples cut from SS 304 and 316 plates were polished

and prepared for metallographic analysis by Optical Microscope and SEM. Fig. 11 shows unpolished and polished SS 304 samples. The samples were etched with Carpenter’s SS etch which consists of FeCl3, CuCl2, HCL, HNO3 and Ethanol. The images produced from SEM and Optical Microscope for SS 304 and SS 316 samples are shown in Figs. 12, 13, 14 and 15 respectively.

Fig. 11 SS 304 polished and unpolished samples

Fig. 12 SEM image of SS 304 sample prior heating

Fig. 13 Optical Microscope image (200X) of SS 304 prior heating

Fig. 14 SEM image of SS 316 prior heating

Fig. 15 Optical micrograph of optical microscope image (200X) of SS 316 prior heating

The samples were then heated in a tubular furnace for

post heat treatment analysis. The soaking period for the samples was one hour and two hours (Atanda et al., 2010) . The samples were heated at temperatures of 600 oC, 800 oC and 1 000oC (Metals Handbook 9th ed., 1987; Pickering, 1976). The specified heating temperatures were selected based on the recrystallization temperature of steel which is between 500−700 oC (Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original


grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility). The samples were again prepared and etched for imaging with Optical Microscope and SEM. The images produced for the heated samples by Optical Microscope and SEM are shown below in Figs. 16 to 27.

Fig. 16 Optical micrograph of SS 304 and SS 316 heated for

1Hr at 600 oC at 200X






2Hrs at 600 oC at 200X





Fig. 22 SEM micrograph of SS 304 and SS 316 heated for





1Hr at 1000 oC at 1500X







2Hr at 1000 oC at 1500X

It can be clearly seen from the images obtained from SEM and Optical Microscope that the phenomenon of grain growth is almost negligible or reduced to a minimum post exposure to heat in the case of SS 304 and 316 with the performance of 316 being the better out of the two SS samples because of the clearly defined and fine grain structure (Metals Handbook 9th ed., 1987).

3.4 Measurement of grain Size The grain size for all the samples including swirler Fin,

Hub, MS, SS 304/316 prior heat treatment and after heat treatment was measured.

From graph at Fig. 28, it can be seen that excessive grain growth was observed in the samples of fin and hub removed from the swirler used onboard. The grain growth was noted to be around 25% in the hub sample as compared to the fin sample.

Fig. 28 Bar graph for grain sizes for MS samples before and after heat treatment

Similarly, the MS sample which was untreated had the grain size as 32.6 µm as compared to the MS samples which were heated at 600 oC and 800 oC for 02 hrs which had a size of 42 and 52 µm respectively clearly indicating grain growth. The grain growth was of the order of 57% in the case of treated MS sample heated to 800 oC as compared to the MS sample which was untreated.

The reason for the grain growth can be attributed to the existence of very high temperatures inside the furnace to which the swirler was exposed to prolonged period of time.

It can be clearly seen that from the chart at Fig. 29, that the grain sizes of the SS 316 samples which were exposed to high temperatures were in the range of 30 to 40 µm which is

consistent with the grain size of SS 316 samples which were not heated i.e. 39 and 35 µm respectively. Both SS 304 and SS 316 had negligible grain growth.

The above data infers that the phenomenon of grain growth is prevalent in MS material whereas SS 304 and SS 316 had minimal or no grain growth whatsoever indicating that SS 304/316 is a better material for high temperature applications. The performance of SS 316 was found to be the better of the two SS materials view the grain sizes was almost same in the case of SS 316 samples which were treated then the ones which were untreated. Hence, SS 316 is the better suited material for high temperature applications.

Fig. 29 Bar graph for grain sizes for SS 316 samples before and after heat treatment

3.5 Coating for High Temperature Application The MS samples cut from MS plate which were untreated

were coated with Stellite coating by the process of Thermal Spraying to check for any improvement in the performance of the material at high temperatures. Pictures of MS samples before and after coating are shown below in Fig. 30.

Fig. 30 MS sample before and after coating

3.6 Analysis of coated samples The MS samples post Stellite coating were heated in the

Lenton, UK Tubular Furnace at 600 oC, 800 oC and 1000 oC for 01 and 02 hours to observe any change in the grain structure of the coating. The samples were then analysed under Scanning Electron Microscope and the images produced are shown below in Figs. 31 to 33. Images of Coated samples which were untreated are also shown for comparison and to check for any changes in the coated surface structure.

The SEM images of the coated samples prior and post


heating revealed nil or very minimal change in the surface structure thereby indicating that the coating was able to withstand high temperatures without deterioration.

The coated samples were also subjected to EDX analysis and the results obtained post EDX testing is as below in Fig. 34.

From the EDX analysis it was revealed that the major constituents of the coating material were Nickel (Ni), Chromium (Cr), Silicon (Si) and Cobalt (Co). The Stellite coating was primarily Co-Ni based which is meant for high temperature applications and can be used as an additional coating in case of the swirler being used in boilers.

3.7 Microhardness testing of samples The Hub and Fin samples cut from the swirler, SS304/ SS

316/ MS uncoated/ MS coated samples which were treated at high temperatures were then analysed for change in their microhardness values by the use of a Microhardness Tester of Wilson make having model number TUKON 1202. The microhardness test was carried out with a scale weight of 500 gms and dwell time of 10 seconds for all the samples. Indentation images are shown in Fig. 35.

Fig. 31 SEM micrograph of coated sample prior treatment and post treatment at 600 oC for 1 hr at 500X

Fig. 32 SEM micrograph of coated sample treated at 600 oC for 2 hr and 800 oC for 1 hr at 500X

Fig. 33 SEM micrograph of coated sample treated at 800 oC and 1000 oC for 2 hr at 500X

Fig. 34 EDX spectrum of coated MS sample heated at

1000 oC for 2 hrs

(a) (b)

Fig. 35 Vickers indentation of SS 316 heated at 800 oC for (a) 01hr and (b) 02 hrs

The above graph Fig. 36 shows that the hardness values

for the SS 304 and SS 316 samples were more or less consistent before and after heating, this shows that the performance of these materials were superior then MS. Out of the two, SS 316 showed a better consistency in terms of hardness values. Hence, it can be seen that SS 316 is the best out of the selected materials for high temperature applications. The reduction in hardness of SS 304 and SS 316 which were heated to 1000 oC can be attributed to heating beyond recrystallization temperature of steel which resulted in reorientation of grain boundaries.

Fig. 36 Hardness values of SS samples


Analysis of the above chart Fig. 37 revealed that the coated MS samples had higher hardness values then their MS counterparts which were not coated. The hardness of SS was 60% more than the MS samples. Hardness value of the coated samples was even more than their SS counterparts and can be used on SS material to provide a hard surface coat to withstand higher temperatures. The Coated samples showed a marked increase in the hardness values of the order of 200% as compared to MS thereby indicating that the Stellite coating produced a high temperature resistant coating on the MS material which could withstand applications at high temperature in a marine environment.

Fig. 37 Hardness values of coated samples

3.8 XRD analysis The various samples viz Fin, Hub, MS, SS and coated

samples were also exposed to X-Ray Diffraction in the XRD machine to check for crystalline or amorphous structure. The data/ graphs obtained have been produced below and shown in Fig. 38. In all the graphs, the parameter on X-axis is the sweep angle 2θ in degrees and the parameter on Y-axis is the Intensity or the total area under the peak.

The graphs of SS 304 and SS 316, both the ones which have been treated and the untreated ones, have distinct peaks of high intensity at around 44o, 51o and 74o on the 2θ axis. This indicates that the structure of the Stainless Steel samples were crystalline even post the heating operation. The grains were well defined and the structure was more or less symmetric in case of SS. The material was able to withstand high temperatures without deteriorating in terms of grain growth and hardness.

3.9 Exploitation at high temperatures Flame contact resulting in high temperatures and local

fusion is the primary reason for high rate of material degradation. Existence of fused spots indicates that the temperature was in the range of 700−800 oC which is beyond the re-crystallisation temperature of Mild Steel. The rate of corrosion is high in the fused zones during and after flame contact.

During flame contact, some areas of the swirler like the hub are exposed to very high thermal gradient, with temperature in some zones as high as 1 000 oC (Fauchais and Vardelle, 2012). In these zones grain growth occurs and can result in catastrophic rate of corrosion (Chaudhari, 1994).

Fig 38 X-ray diffraction for SS 304 and SS 316 heated at 1000 oC for 2 hrs

3.10 Operating conditions and corrosion rate Degradation of the swirler is not continuous and can vary

from negligible to maximum depending on operating conditions. A section of swirler can be consumed in a matter of hours or some days under the extreme condition of excessive flame contact. The life of the swirler can be prolonged by ensuring proper flame propagation inside the furnace (Virdi and Thakur, 2015).

3.11 Operating conditions favouring degradation Occurrence of flame contact is because of the following: 1) Improper air circulation resulting in backwards

movement of flame (Atanda et al., 2010). 2) Leakage of fuel from the burner resulting in localized

burning of fuel on the swirler surface. Existence of deposits inside the hub area, between the

burner and the swirler, indicates that fuel leaks were one of the reasons for degradation of the material.


4 Conclusions

The conclusions obtained from the research are as follows:

1) Existence of high temperatures and the presence of corrosive marine environment resulted in the accelerated degradation of the combustion components.

2) The existing swirler material i.e mild steel in this specific application in the 30 MW marine boiler was found to be unsuitable for prolonged use at elevated temperatures. Also, the degradation mechanism was accelerated due to high temperature corrosion which could prove to be catastrophic

3) The performance of proposed alternate materials SS 304 and SS 316 post treatment at high temperatures was examined with respect to grain growth and hardness.

4) SS 304 and SS 316 materials showed nil or minimal grain growth. The structure of the metals was clearly defined; the grains were well oriented and distributed in a fine matrix.

5) Hardness of SS 304 and SS 316 remained mostly unaltered post heat treatment with performance of SS 316 being better out of the two.

6) Hardness of SS samples was around 60% more than Mild Steel and remained unaltered even post exposure to temperatures beyond its recrystallization temperature range.

7) Structure of SS 304 and SS 316 remained crystalline, as revealed by XRD, even post exposure to heat indicating suitability for use at elevated temperatures.

8) Stellite coating of the MS samples produced a hard impenetrable layer on the metal which could withstand high temperatures without change in its microstructure or hardness.

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investigation and analysis of high temperature corrosion

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