Failure analysis of Incoloy 800 water immersion heating element

Driven by regulations and restrictions of using fossil fuels and reducing carbon dioxide emissions, electrical heating is becoming a good choice for many domestic and industrial applications. High temperature oxidation along with corrosivity of the service environment bring many challenges to heating element designers and service engineers. Many factors, including design, overheating, sheath tube failure, corrosion, and overheating, cause heating elements to fail in use. Failure analysis is one method for enhancing a product's performance, reliability, and design. In this paper, a metallurgical failure analysis of Incoloy 800 water immersion heating element was conducted. Optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Vickers microhardness (Hv), and X-Ray radiography were used to analyze the different components of the failed item. Stress corrosion cracking (SCC) was identified as the cause of the failure. The chloride containing water caused the failure to begin by pitting on the Inconel 800 sheath material's outer surface, and then propagated as stress corrosion cracking (SCC).


Introduction and background
Heating elements are widely used in many domestic and industrial applications.Applications comprise ovens, dishwashers, water heaters, and furnaces.Heating elements can be classified into two main types, exposed heating elements and sheathed coiled resistive heating elements.The latter is normally comprised of a high electrical resistant wire such as nichrome (80% nickel, 20% chromium), cupronickel (Cu-Ni alloy), or kanthal (Fe-Cr-Al).The wire is situated in a metallic tube or sheath that is typically made of cast iron, stainless steel, copper, or incoloy [1].The tube or the sheath is filled with an insulating powder that surrounds the wire.The powder provides thermal and electrical insulation and keeps the wire centered.Due to its high thermal conductivity and low electrical conductivity, ceramic materials like compressed magnesium oxide (MgO) powder are frequently employed for this purpose.
Heating element failures may cause serious property loss, fire, electrical shock and even injuries [2].Literatures have attributed heating element failures to different causes such as design, sheath tube failure, fatigue, open circuit, and corrosion [2][3][4].Areas such as heat transfer, modeling, and thermodynamics analysis have been investigated.However, the effect of metallurgical factors and corrosion have not gained much attention.Shayan et al. have studied the failure mechanism of a steel plate used as a heating element in a steam power plant.The authors found that corrosive acid condensates at the dew point on the heating surfaces which eventually reacts with the metal and leads to corrosion and fouling related problems.In addition to the experimental anlysis that was carried out on the heating element, temperature distribution across the air heater matrix was simulated using a software called Fluent.The analysis was used to calculate the acid dew point temperature (ADPT).There results show that the calculated sulfuric ADPT, the simulated temperature distribution, and the actual corroded location exposed to the acid were in good agreement [5].Xu et al. used finite element method (FEM) to simulate the thermal stress and whether the failure of Incoloy 800HT pipe was caused by thermal stresses during service.The distribution of stresses and temperature in the inner wall of the pipe was obtained.The simulation reveals that residual stresses in the axial, radial, and circumferential direction were too low to cause the failure.The authors concluded that the heat stresses of the pipe working at 1032 °C exceeded the yield stress of the material and caused the failure [6].
Stress corrosion cracking (SCC) is one of the causes of significant failures of engineering components such as heater sleeves, steam generator tubes, pipelines, and pressure vessels [5,7,8].SCC refers to a cracking and failure phenomenon in which a typically ductile metal loses its ductility when subjected to a mechanical stress in the presence of a specific corrosive environment.For SCC to happen, three amin factors are required, a vulnerable metal microstructure, a mechanical tensile stress, and a particular corrosive environment, all these factors must happen simultaneously [9].
Stainless steel, high strength aluminum alloys, nickel, and copper alloys are among the materials vulnerable to SCC.Heat resistant nickel alloys (HRA) are designed to play a dual role as heat and corrosion resistant alloy (CRA).Alloys such as 625, N06625, 718, Incoloy 800 are used for both high temperature and corrosion resistant applications [9].The Incoloy 800 family includes Incoloy 800, 800H, and 800 HT was designed to fill the gap for a high temperature and corrosion resistant alloys.The three alloys have almost identical chemical composition limits.However, the three alloys vary in carbon, aluminum, and titanium content.Alloy 800H and alloy 800HT have significantly higher creep and rupture strength than Incoloy 800.Incoloy 800 alloys were designed to withstand extreme corrosion conditions.However, research has shown that this class of alloys is susceptible to other types of corrosion, such as high temperature corrosion [10][11][12].
Dinu and Radu studied SCC of incoloy 800 in a chloride -containing aqueous solution.Accelerated electrochemical corrosion testing, scanning, and optical microscopy were used to characterize the corroded samples.The authors concluded that the susceptibility of SCC increases as chloride concentration increases.They also concluded that the SCC cracks that started at the pits were associated with high chloride concentration and propagate in a trans-granular mode.In the absence of pitting; however, cracks exhibit intergranular propagation mode [13].Similar results were obtained by Chen et al. when they investigated cavitation erosion of incoloy 862 in a chloride containing solution.They concluded that the erosion rate decreased with the decrease in chloride concentration.They also concluded that the fatigue failure mechanism starts with the formation of pits [14].Increase of corrosion susceptibility due to the increase in chloride concentration was also concluded by Pirvan et al. when they investigated the effect of water chemistry on corrosion resistance of incoloy 800 tubes [15].Corrosion behaviour of Incoloy 800 alloy at high temperature aqueous environment was investigated by Fulger et al.They concluded that water chemistry is an essential factor for corrosion behavior of Incoloy 800 alloy.They found that an increase of the impurities level of the water used increases the pitting susceptibility [16].
In addition to corrosion in aqueous environment, Incoloy alloys may suffer corrosion deterioration in the presence of some gasses at high temperature.Kim et al. investigated the corrosivity of Incoloy 800 alloy in the presence of N 2 /H 2 S gas mixture at high temperature.They found that the corrosion rate of Incoloy 800 increases with the addition of H 2 O and increases greatly with the addition of H 2 S. The two corrosion types observed were oxidation and sulfidation [12].
In general, Incoloy alloys are known for their corrosion resistance including SCC.However, based on the service conditions that alloys are subjected to, evidence from reports and scientific studies have demonstrated that Incoloy alloys are susceptible to corrosion failures due to SCC.Dinu et al. concluded that Incoloy 800 suffers SCC failure when exposed to 10% sodium hydroxide at 260 °C.The authors concluded that the presence of the oxide layer causes the initiation of trans-granular SCC that starts at the fissures in the oxide layer, while intergranular SCC initiation was associated with the absence of the oxide layer [17].
In this investigation, a metallurgical failure analysis of water immersion heating element manufactured from Incoloy 800 alloy has been conducted.The investigation involved examination of the damage using X-Ray radiography (XR), optical microscopes (OM), scanning electron microscope (SEM), and energy dispersive spectroscopy (EDS).

Metallography and microstructural analysis
Sections cut from the failed sheath were prepared for metallographic investigation by hot mounting, grinding, polishing, and electrochemical etching in NaOH solution.The grinding step was performed using SiC emery paper on a sequence of 240, 320, 400, 600, and 1000 grit.Polishing was performed using alumina suspension solution with a particle size of 9, 6, 3, and 1 μm.An optical microscope equipped with image analyzer software and magnification power up to 1000X was used for optical microscopy analysis.Stereo microscope was used for macro analysis using lenses in the range of 10-25X.

Microhardness testing
Samples already prepared for metallographic analysis were tested using Vickers microhardness machine equipped with a 136° pyramidal diamond indenter and a load of 10 KG.Different deformed and undeformed zones of Incoloy 800 sheath tube were tested.The purpose was to correlate microhardness to residual stresses inherited in the sheath material and to track residual stresses in the deformed and non-deformed zones.

Chemical analysis and scanning electron microscopy
Chemical analysis from sections cut from the failed sheath tube were analyzed using a Spectro test spectrometer equipped with an ultraviolet spark probe under argon gas.SEM examination was conducted using Tescan field emission scanning electron microscope (FESEM) equipped with EDS analysis for microanalysis.

X-ray radiography
X-ray radiography (XR) was used to show the configuration and construction features of the heating wire inside the tube.Additionally, X-ray was used to locate local fusion or any debris or materials that remain as the result of any localized melting inside the tube.Due to the shape complexity Five positions were chosen to ensure that all the heating element components were exposed clearly to the X-ray beam.

Visual inspection
The failed item is shown in Fig. 1.It is a 7KW, 600V double loop water immersion heating element that failed at multiple locations.The outer surface of the heating element did not show evidence of mechanical damage, dents, or severe deformation.The longitudinal section of the tube was perfectly circular with an outer diameter (OD = 12.07 mm).The tube's heater is bent intentionally to form a double U shape.Obviously, deforming the tube would have created localized stresses that would have affected the circularity of the tube.The tube dimeter at the bend was 10.09 mm.
It is obvious that the area of interest is the area where the tube is bent at which the perforations and localized rust/pitting were observed.Figures 2 and 3 show the main area of interest.

Non-destructive testing (Radiography)
Figure 4 shows a scanned X-ray image of the double loop area (area of interest).In addition to positioning the sheath damage's location, the image shows that the heating element is subjected to severe damage.The X-ray images show that the heating element components (heating wire and sheath) damage was associated with the looped area.
It is noted that there is debris inside the sheath tube as a result of localized melting as shown in Fig. 5. Figure 6 shows a typical cross section through the heating element.The section shows the heating filament, heating tube/sheath, and a compact powdery material that was confirmed later using EDS to be magnesia (MgO).

Chemical analysis
The chemical analysis was performed on drillings removed from the sheath tube using the analysis method ICP-ASTMD1976-12Mod.The result is given in Table 1.It is confirmed that the tube material matches Incoloy 800 alloy.The composition determined from the chemical analysis was later confirmed using EDS analysis.

Optical microscopy
Sections for metallographic analysis were taken from the perforated area, unaffected area, and from locations where localized corrosion product was observed.The optical micrograph of a perforation in Fig. 7 clearly demonstrates that the majority of the attack is related to the OD.On the contrarily, the ID has not been as greatly impacted.It appears that the failure started with corrosion/pitting on the outer surface.More evidence that supports this conclusion can be seen from Fig. 8.It is shown that the corrosion attack concentrated on the outer surface which initially has a thickness of 824.8 μm, while the last segment of the tube has a thickness of only189.7 μm.Corrosion conditions combined with possible erosion due to fluid flow might have led to this severe thinning.This thinning probably started with pitting as shown in Fig. 9.
Figures 10 and 11 show low and high magnification polished sections taken from perforation A, respectively.It shows branched transgranular attack typical of stress corrosion cracking (SCC).The crack propagation shows that the cracking at this location has started from inside the tube and then propagated to the outer surface.It is worth noting that the perforations were associated with the bend area.It is obvious that this area of the tube was mechanically deformed to produce a U/loop shape.
The microstructure of the tube material has an austenitic structure as shown in Fig. 12. Careful examination of the microstructure shows no evidence of sensitization, defects, or any abnormal growth of secondary particles that might have contributed to the failure.

Scanning electron microscopy
Sections from the perforations, corroded areas, and the filament were examined using SEM/EDS.Figure 14 shows an SEM micrograph of zone B. The perforation showed   probably occurred at an early stage compared to other perforations since the molten/deposit material has likely been ejected after the tube has burst.Higher magnification SEM images of the same damage shows signs of deformation on the tube's outer surface as shown in Fig. 15. Figure 16 shows the fracture surface of the damage on zone B on which EDS spectra were acquired from an area close to the perforation.The EDS result is shown in Table 2.
The EDS results show presence of chloride, which is known as pitting induced species.Presence of high chromium and nickel is expected for the tube material, while presence of oxygen is an indication of oxidation (corrosion product) on the tube surface.
Figure 17 and Tables 3 and 4 show an SEM micrograph taken from inside the perforation and the corresponding EDS results, respectively.Spectra 1 which was acquired from the fused material shows presence of some of the alloying elements of the heating filament wire such as Cr and Ni in addition to Mg and Oxygen.This composition is an indication of a mixture between the molten filament and the filling material.Spectra 2 which was acquired from the white powdery material shows mainly Fig. 8 Thinning on the tube outer surface Fig. 9 Pitting on tube outer surface two elements, oxygen, and magnesium indicating that the substance is magnesium oxide (Magnesia).Magnesium oxide is a common material for applications that require good thermal conductivity and is widely used in heating elements [18].
Similar to the damage in zone B, zone A shows that the perforation grew and propagated in the longitudinal direction of the tube/sheath.Moreover, they both have similar sharp fracture edges along part of the damaged areas.Figure 18 illustrates a shear lip characteristic on the fracture surface of damage zone A, which denotes a ductile failure.The sheath material was loaded to beyond its ultimate tensile strength, high temperature would have reduced the strength considerably.Incolly 800 is an iron -nickel chromium alloy with moderate strength.When extruded, the alloy's tubing exhibits a yield strength of 183 MPa and a tensile strength of 524 MPa at room temperature.However, its mechanical properties are affected by higher temperature.For instance, at 650 C, the yield and tensile strength decreases to 124 MPa  10 and 362 MPa, respectively (https:// www.speci almet als.com).Figure 19 shows an SEM micrograph illustrating the edge of damage zone A and the locations where EDS spectra were acquired, the results are shown in Table 5. Spectrum 2 which was obtained from a location far from the perforation reflects the tube's chemistry Cr, Ni, and Fe.A greater amount of chloride was also detected inside the perforation which suggests a possible role of chlorides as a pitting initiation factor.

Discussion
The visual examination shows that the immersion heater had several areas where the outer sheath had been perforated and the heating wire had failed.In addition to the perforations, there were also areas identified as having blisters of corrosion product on the surface that were visible by eye.Some other corrosion sites on the outside of the tube were visible only by using the stereomicroscope.The appearance of the smaller corrosion sites on the OD Fig. 12 As etched microstructure of the sheath material Fig. 13 SCC trans granular crack propagation mode was similar to that seen at areas adjacent to the major perforations.It appears that the corrosion attack started at the external tube surface which preceded the perforations.
Microstructural analysis shows clear evidence that stress corrosion cracking (SCC) is the main failure mechanism.The cracks initiated from inside the sheath and propagated toward the outside surface.Stress due to bending of tubing, temperature increase, and high Cl concentration from the water led to crack initiation and propagation in the form of SCC.For SCC to occur,  conditions such as a susceptible material, stress, and corrosive environment need to be present.Although Incoloy 800 is renowned for its strong resistance to SCC, it is not impervious to this kind of damage when present in corrosive environments like those that contain chloride [19,20].It is clear that the tube was under severe conditions when SCC developed.Local deformation and high chloride containing environment created favorable circumstances for SCC.Similar conclusion was reached by Dinu and Radu when they investigated SCC of Incoloy 800 in similar conditions [13].Optical micrographs (Figs. 10 and 11) show that SCC starts from pits, and it is observed that corrosion conditions led to pit formation which then followed by SCC initiation, propagation, and then final failure.This conclusion is consistent with others' findings.Figure 13 shows that stress corrosion cracks propagated as trans granular cracks.Similar results were obtained by Dinu et al. in which they found that SCC that starts with pitting propagate as trans granular cracks, while SCC that starts at oxide film follow intergranular propagation mode [13].
Factors that potentially affect SCC susceptibility were evaluated by comparing metallurgical differences.Researchers investigated the effect of some metallurgical variables such as microstructural characteristics, grain size, precipitates, and circumferential residual stress of  [23].Microhardness measurements taken from the cracked zones of the sheath tube showed higher values compared to the area where no SCC was observed.Hardness values at the SCC zone were recorded to be 215 HV compared to only 126 HV from the unaffected zone.The microhardness data is shown in Table 5.It is worth noting that the perforations and the SCC both were associated with the bent area of the sheath.This would lead us to conclude that the deformation of the sheath material (bending) has created residual stress that was high enough to contribute to the SCC initiation.
The only way the electrolyte could reach the inside surface is through pits that formed on the outer surface.Although there was enough evidence of SCC identified at the inside surfaces of the sites of the major perforations, there was no such damage apparent in cross sections away from these sites.This led us to believe that the SCC was likely the result of localized conditions generated because of the perforation and ingress of water.The material in the sheath tube should be resistant to SCC in water under typical conditions and the lack of SCC on the outside surface supports this.It appears that water chemistry is a factor in the generation of SCC on the inner tube surface.References identified halogens/chlorides as  a potential source of both SCC and corrosion in this alloy system [4,19,20].Chemical analysis performed close to the perforations by EDS showed elevated levels of Cl − in most analyses.The elevated chloride levels may be due to concentration under the scale formed on the outside surface, leading to active pitting.The chlorides would continue to concentrate once the water entered the inside of the tube, resulting in the SCC observed.Extensive analysis of different pits at different growth stages using the stereomicroscope showed that pitting is the main corrosion mechanism.The investigation focused on the pits still had a scale present and did not show sign of rupture.Figure 20 shows an SEM micrograph of a scale covered zone chosen for further analysis.Multiple cycles of cleaning using Evaporust in an ultrasonic cleaner to remove the corrosion product without disturbing the underlying were used.The examination showed that what appears as a scale from outside shows perforation in roughly the center of the damaged area as shown in Fig. 21.

Conclusion
1. Corrosion of the outside surface of the tube led to the perforation of the tube at multiple areas allowing small amounts of water to enter the insulation.2. In some areas the ingress of water was significant enough to create conditions for SCC that caused further degradation of the sheath.3. SCC was observed near the perforations, but not in the other areas of the tubes.The SCC appeared in all cases to originate on the inside surface of the sheath and propagated toward the outside surface.The SCC would have required specific conditions to be a factor in the failure.Specifically, there would have needed to be water present in the tube for SCC to occur.SCC was the secondary mechanism that occurred after the sheath tube had perforated.

Fig. 1
Fig. 1 Heating element in as received condition (a) Heating element overall dimensions and (b) Heating element components and location of failures

Fig. 2 Fig. 3
Fig. 2 Area of interest shows locations of damage (zones)

Fig. 4 XFig. 5
Fig. 4 X ray scanned image shows the looped area

Figure 13
Figure 13 shows an optical micrograph of the Incoloy 800 alloy in which multiple SCC have initiated at what looks like pits and then propagated in a trans-granular mode (TG).

Fig. 6
Fig. 6 Cross section of the investigated heating element

Fig. 7
Fig. 7 Corrosion attack on the tube outer surface

Fig. 10 Fig. 11
Fig. 10 Low magnification optical micrograph shows stress corrosion cracking of the tube material

Fig. 19
Fig. 19 EDS analysis of damage zone A

Fig. 20
Fig. 20 SEM micrograph shows a rust covered zone

Table 1
Chemical composition of the tube material

Table 2
EDS spectra acquired from damage zone B Fig. 17 EDS spectra acquired from inside the perforation (damage zone B)

Table 3
EDS analysis of material inside damage zone B

Table 4
EDS from damage zone A

Table 5
Microhardness measurement on sheath/tube material