Engineering Failure Analysis 23 (2012) 82–89

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Engineering Failure Analysis

journal homepage: www.elsevier .com/locate /engfai lanal

Analyses of defects on the surface of hot plates for an electric stove

Aleš Nagode a,⇑, Grega Klancnik a, Heidy Schwarczova b, Borut Kosec a, Mirko Gojic c,Ladislav Kosec a

a University of Ljubljana, Faculty of Natural Sciences and Engineering, Aškerceva c. 12, SI-1000 Ljubljana, Sloveniab University of Central Europe in Skalica, Kralovska 386/11, 90901 Skalica, Slovakiac University of Zagreb, Faculty of Metallurgy, Aleja narodnih heroja 3, HR-44103 Sisak, Croatia

a r t i c l e i n f o a b s t r a c t

Article history:Received 24 January 2012Received in revised form 7 February 2012Accepted 12 March 2012Available online 23 March 2012

Keywords:Hot platesCast ironCoating failuresOxidesWhiskers

1350-6307/$ - see front matter � 2012 Elsevier Ltdhttp://dx.doi.org/10.1016/j.engfailanal.2012.03.001

⇑ Corresponding author.E-mail address: ales.nagode@omm.ntf.uni-lj.si (A

Brown stains were observed forming after the black-oxide coating on the surfaces ofgrey-cast-iron hot plates made for an electric stove. These stains were metallographicallyexamined using a scanning electron microscope (SEM) equipped with an energy-dispersiveX-ray spectrometer (EDXS). For the phase identification we also employed X-ray diffraction(XRD) analyses. The results indicated that the stained surfaces of the hot plates were cov-ered with whiskers of hematite (Fe2O3), while on the surfaces of the non-defective hotplates only magnetite (Fe3O4) was present. The thermodynamic calculations confirmedthe possibility of the formation of hematite (Fe2O3) as a result of the oxidation of magnetite(Fe3O4) if the partial pressure of oxygen is increased during the black-oxide-coatingprocess.

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1. Introduction

At the start of our investigation we received aesthetically damaged hot plates made of grey cast iron. These hot plates,which were in the unused condition, were produced for electric stoves. However, on the top surface of the received hot plates(with a diameter of 180 mm) we observed brown stains (Fig. 1a). Since the surface of a flawless hot plate is a dark-grey col-our (Fig. 1b), the visual appearance of the received hot plates was defective and thus inappropriate for use. The goal of ourresearch was to examine the stains on the surface of the hot plates and to explain the reason for their occurrence.

2. Fabrication of the hot plates

In order to better understand the problem a brief description of the production of the hot plates is necessary. These hotplates, for use on electric stoves, are made of grey cast iron. The grooves on the top surface are made by turning. An impor-tant step in the hot plates’ production is the black-oxide-coating process [1], which is used to produce a protective, darkoxide layer consisting predominantly of magnetite (Fe3O4) on the surface of the hot plate. Besides the protection against cor-rosion and abrasion, the dark oxide layer is also decorative and is stable at high temperatures. The process involves immers-ing the workpiece in a boiling aqueous alkaline solution of oxidising salts. This is the most popular method of black-oxidecoating; however, a black-oxide film can also be produced by annealing in an oxidising atmosphere made up of combustiongases flowing over the workpiece. This unconventional route was the method employed for the hot plates we received.Namely, the hot plates were put in a furnace where wood was burned to produce the appropriate atmosphere. The

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. Nagode).

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temperature in the furnace was 650 �C. The entire process in the furnace lasts for eight hours; however, the maximum tem-perature of 650 �C is only reached after 4 h.

3. Investigation methods

For the investigation of the surface morphology and the microstructure analysis as well as for the fractography of the hotplates a Jeol JSM 5610 scanning electron microscope equipped with an energy-dispersive X-ray spectrometer (EDXS) wasused. For the microstructural analysis the samples were cut off from the non-defective and defective hot plates and metal-lographically prepared (ground and polished). For the structural analysis of the oxide film [2,3] on the surface of the dam-aged and undamaged parts of the hot plate we conducted an X-ray diffraction analysis. For this purpose a PANalytical X’PertPRO X-ray diffractometer (radiation wavelength Cu Ka1 = 1.5406 Å) with a Johansson monochromator for the flat sampleswas used. Both XRD spectra were recorded under the same conditions.

4. Results and discussion

4.1. Scanning electron microscopy (SEM)

The surface of the non-defective hot plate was observed with the scanning electron microscope (SEM) (Fig. 2a). The oxidegrains grown during the annealing at 650 �C are clearly visible. The developed pattern reflects the grain orientation of the

Fig. 1a. Defective surface of the hot plate.

Fig. 1b. Non-defective surface of the hot plate.

Fig. 2a. Surface of non-defective hot plate; SEI.

Fig. 2b. Surface of defective hot plate; SEI.

Fig. 3a. Microstructure of non-defective hot plate (cross section), BEI.

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Table 1EDXS analysis of oxide layers of non-defective hot plates (in wt.%).

Site of interest Fe O Mn Si Cr

1 93.3 6.7 – – –2 93.9 5.1 1.0 – –3 88.4 6.0 0.6 4.6 0.44 95.4 1.2 0.3 2.7 0.45 97.1 – 0.4 2.2 0.3

Table 2EDXS analysis of oxide layers of defective hot plates (in wt.%).

Site of interest Fe O Mn Si Cr

1 93.9 6.1 – – –2 93.9 6.1 – – –3 95.0 0.9 4.1 – –4 88.0 6.0 0.7 4.9 0.45 94.9 1.5 0.5 2.6 0.56 97.5 – 0.5 1.7 0.3

Fig. 3b. Microstructure of defective hot plate (cross-section), BEI.

A. Nagode et al. / Engineering Failure Analysis 23 (2012) 82–89 85

base material. In contrast, the secondary-electron image of the defective hot plate shows that the surface with brown stainsis covered with whiskers (Fig. 2b) [4–7].

The SEM equipped with the EDXS was also used for the microstructural analysis of the cross-sections of the hot plates. Abackscattered electron image (BEI) of the flawless hot plate shows, chemically, two different oxide layers on the base mate-rial (grey cast iron), i.e., the outer (light contrast) and the inner (dark contrast) oxide layers (Fig. 3a). However, a detailedEDXS analysis (Table 1) shows that the outer oxide layer is composed of pure Fe-oxide in the upper part and FeMn-oxidein the lower part.

The EDXS analysis of the inner oxide layer (dark contrast) confirms the presence of Fe, Mn and Cr; however, the increasedcontent of Si indicates that the inner layer is composed of Si-rich Fe(MnCr) oxide. A similar distribution and composition ofthe oxide layers was also found in the case of the defective hot plates (Fig. 3b); however, the EDXS analysis confirmed thatthe whiskers developed on the outer oxide layer are from pure Fe-oxide as well (see Table 2).

In both Figs. 3a and 3b the internal oxidation of the matrix around the graphite flakes near the surface can be seen. Theinternal oxidation actually consists of iron oxidation (xFe + yO ? FexOy) as well as graphite oxidation (2C + O2 ? 2CO orC + O2 ? CO2). The formation and growth of the Fe-oxide in the subsurface is promoted by oxygen penetration throughthe graphite boundaries with the metallic matrix [8,9]. In a comparison between the non-defective (Fig. 3a) and defective(Fig. 3b) hot plates it looks like the oxide layers, in the case of the defective part of the hot plate, spalls, while in the caseof the non-defective part of the hot plate it does not. Therefore, the defective hot plate was fractured after the samplewas cooled in liquid nitrogen. The fracture surface (Fig. 4) of the hot plate covered by the whiskers shows that all the oxidelayers exhibit good adhesion with the other oxide layers as well as with the metal. Thus, no difference in the adhesion of theoxide layers between the defective and the non-defective parts of the hot plates was observed.

Fig. 4. The fractured surface of the defective hot plate.

86 A. Nagode et al. / Engineering Failure Analysis 23 (2012) 82–89

4.2. X-ray diffraction analysis (XRD)

The X-ray diffraction analysis was used for a phase identification of the surface of the non-defective and the defective hotplates. The analysis showed that the only difference between the XRD pattern of the defective and the non-defective surfaceof the hot plate lies in the ratio of the peak intensities between the hematite (Fe2O3) and the magnetite (Fe3O4). Namely, adetail from the XRD spectrum (two theta = 32�–two theta = 36�) of the defective surface shows a higher peak-intensity ratiobetween the hematite (Fe2O3) and the magnetite (Fe3O4) (Fig. 5a) in comparison to the X-ray diffraction pattern of the non-defective surface (Fig. 5b). Since the surface of the defective hot plate is covered with whiskers this result indicates that thewhiskers are composed of hematite (Fe2O3).

4.3. Thermodynamic calculations

Iron forms three types of oxides with oxygen: FeO-iron(II), Fe2O3-iron(III) and a double iron (III)-iron(II) oxide Fe3O4. Thereactions with oxygen can be described by the following equations:

2Feþ O2 ¼ 2FeO; DG0298 ¼ �490:28 kJ ð1Þ

4=3Feþ O2 ¼ 2=3Fe2O3; DG0298 ¼ �494:86 kJ ð2Þ

3=2Feþ O2 ¼ 1=2Fe3O4; DG0298 ¼ �507:62 kJ ð3Þ

Nevertheless, the presence of an oxide interface (Fe/FeO, Fe/Fe2O3 FeO) means it is important to study the oxidation effectfor industrial purposes. The reactions for the study of an oxide interface are:

6FeOþ O2 ¼ 2Fe3O4 ;DG0298 ¼ �564:4 kJ ð4Þ

4Fe3O4 þ O2 ¼ 6Fe2O3; DG0298 ¼ �389:2 kJ ð5Þ

4FeOþ O2 ¼ 2Fe2O3; DG0298 ¼ �504:5 kJ ð6Þ

The oxidation of an iron surface is directly related to the partial pressure of oxygen pO2in the furnace. In an oxidative

atmosphere the Fe2O3 phase represents the most stable oxide present on the iron surface (Eqs. (5) and (6)). The reductionof oxides like Fe2O3 to pure iron is dependent on the partial pressure of oxygen pO2

in the furnace. This reduction can beachieved through Fe3O4 or through FeO (Eqs. (5) and (6)). The calculation of the isobars was made using the standard Gibbsenergy of formation, see Table 3.

The calculation of the equilibrium constant K was made with Eq. (7):

K ¼ e�ðDG0=RTÞ ¼ay

MuOv

axFe � pO2

ð7Þ

Fig. 5. A detail of the XRD spectrum: (a) defective surface; (b) non-defective surface.

Table 3Standard Gibbs energy per mole of substance (in kJ/mol).

Substance DG0298

Substance DG0298

O2 0 Fe2O3 �742.29Fe 0 Fe3O4 �1015.23FeO �245.14

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where the activity of the formed oxide is taken to be 1. The partial pressure of oxygen pO2is then calculated with Eq. (8):

pO2¼ 1

K � axFe

ð8Þ

The values of axFe were taken to be 1 at the temperature of interest for the pure iron. The calculated values of the

equilibrium partial pressures of oxygen can be calculated to be much higher if axFe is close to zero (taking into account that

other elements are noble and they do not form any oxide). In this way the formed oxides in the regions of smaller iron

Table 4Equilibrium partial pressure of oxygen for the formed interfaces (in atm).

Interface Fe/Fe3O4 Fe/Fe2O3 Fe/FeO FeO/Fe3O4 Fe3O4/Fe2O3

T (�C) pO2

100 1.53E�69 1.53E�67 1.34E�67 1.30543E�52200 3.73E�53 1.89E�51 5.94E�52 9.18542E�39300 1.59E�42 5.25E�41 8.42E�42 9.8506E�30400 4.39E�35 1.09E�33 1.1E�34 2.36663E�23500 1.35E�29 2.76E�28 2.01E�29 4.0737E�30 1.45539E�18600 2.28E�25 1.7899E�25 8.38806E�15700 3.82E�22 7.5984E�22 9.54087E�12800 1.64E�19 6.5399E�19 3.4936E�09

Fig. 6. Partial pressure of oxygen versus temperature for iron oxide formation.

88 A. Nagode et al. / Engineering Failure Analysis 23 (2012) 82–89

concentration are thermodynamically less stable than those calculated for pure iron or an iron-based alloy. The calculatedpartial pressure of oxygen pO2

values are gathered in Table 4. The values for the Fe/Fe3O4 interface (and the corresponding Fe/FeO and Fe/Fe2O3) are given up to 500 �C, resulting from having a (Fe + Fe3O4) two-phase region stable up to 570 �C. At highertemperatures (FeO + Fe), (FeO + Fe3O4) and the single-phase FeO region is stable according to the FeO–Fe2O3 phase diagram.Between Fe3O4 (FeO�Fe2O3) and Fe2O3 the two-phase region (Fe3O4 + Fe2O3) is expected to exist up to 1596 �C [10].

If a hot plate is heated from room temperature to approximately 400 �C it will be oxidised if the partial pressure of oxygen(pO2

) in the furnace is higher than 10�35 atm. At 400 �C and partial pressure of oxygen pO2¼ 10�35 atm an iron alloy/Fe3O4

interface is formed. The formation of magnetite is described with Eq. (3) as a combination of Eqs. (1) and (2). Magnetite isstable up to 10�23 atm, where the Fe3O4/Fe2O3 interface is formed. If the partial pressure of oxygen pO2

is higher than that anew iron oxide, i.e., hematite, is expected to form. The oxidation in our case was at 650 �C. The thermodynamic predictionsuggests that at 650 �C magnetite will be present if the partial pressure of oxygen pO2

in the furnace is lower than 10�14 atm.Below this pressure the Fe2O3 is not stable. At the same temperature FeO is also expected if the partial pressure of oxygen islower than 10�23. To obtain only magnetite on the hot-plate surface the partial pressure of oxygen pO2

should be between10�14 and 10�23 atm.

For a better visual representation a pressure–temperature diagram was constructed, Fig. 6. The iron or iron-based alloywill form an oxide layer by crossing the pressure–temperature curves of the Fe/Fe3O4 (and the corresponding Fe/FeO and Fe/Fe2O3) or Fe/FeO. The existence of a single FeO region is practically at the same pressure and temperature as the FeO/Fe3O4

and Fe/FeO interface, from 570 �C and as such is difficult to observe. Nevertheless, the Fe3O4 stability range is between Fe/Fe3O4 or FeO/Fe3O4 and Fe3O4/Fe2O3.

5. Conclusions

At the start of this investigation we received hot plates with brown stains on their surfaces. These stains occurred after ablack-oxide-coating process in a furnace fired with wood. Scanning electron microscopy showed that the stains are caused

A. Nagode et al. / Engineering Failure Analysis 23 (2012) 82–89 89

by whiskers that were growing from the top oxide layer on the surface of the hot plates. An EDXS analysis confirmed that theoxide on the surface of the grey cast iron consists, chemically, of three different types of oxides, i.e., the outer oxide layer ispure Fe-oxide, below it is a narrow layer of FeMn-oxide, while the inner layer consists of Si-rich Fe(MnCr) oxide. An X-raydiffraction pattern of the defective surface of the hot plate shows a higher Fe2O3/Fe3O4 peak-intensity ratio according to theX-ray diffraction pattern of the non-defective hot plate. This indicates that the whiskers are from hematite. The results of thethermodynamic calculations revealed that at 650 �C only magnetite (Fe3O4) will be present if the partial pressure of oxygenpO2

is between 10�14 and 10�23 atm. If the partial pressure of oxygen pO2is higher than 10�14 atm hematite (Fe2O3) is ex-

pected to be formed. For the proper oxidation (uniform growth of Fe3O4 oxide) of grey iron a better control of the partialpressure of oxygen at a certain temperature is needed. This could be achieved by using a more suitable fuel than the onethat is currently used. The drastic change in oxygen concentration occurred in the last stage of the oxidation, resulting inthe formation of hematite only on the surface of pre-existing magnetite.

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