metal corrosion and its impact on glass tempering furnace design

Metal Corrosion and its Impact on Glass TemperingFurnace Design

Peter Tiernan*

Materials and Surface Science Institute, University of Limerick, Limerick, Ireland

Michael D. Naughton

Department of Manufacturing and Operations Engineering, University of Limerick, Limerick, Ireland

A reliable well-designed tempering furnace is considered to be the cornerstone of any modern glass-processing facility.This paper addresses a series of engineering anomalies encountered during the commissioning of such a glass-processingfurnace. Following the installation of a furnace in a European facility, small black deposits were noticed on both the silica-fusedrollers used to transport the glass through the hearth of the furnace and on the processed glass surface itself. EDAX andscanning electron microscopy investigations indicated conclusively that the deposits were primarily constituted of sodiumsulfate and trace elements consistent with stainless steel (chromium, iron, and nickel). Traditionally, high-density glass fiberwas used to insulate the roof walls and side walls of tempering furnaces; however, it was noticed in this particular case thatrolled stainless steel sheeting (SS316) was used. Chemical and X-ray diffraction analyses were used to pinpoint the origin of thedeposits. It was determined that poor material selection choices taken during the design stage of the furnace in question were atfault. The combination of stainless steel and sulfur dioxide (SO2 is used as a lubricant to prevent scuffing) at elevated tem-peratures (46501C) generated droplets of sodium sulfate, which condensed due to the convectional flow of the heat currentswithin the oven. These droplets scorched the glass surface and destroyed the fused silica rollers. As a recommendation, theusage of stainless steel and other nonrefractory metals should be avoided in the design of any future glass tempering furnaces.

Introduction

Tempering is a thermal process used to strengthenglass. The process is predicated on the generation of

Int. J. Appl. Ceram. Technol., 7 [5] 687–696 (2010)DOI:10.1111/j.1744-7402.2009.02375.x

Ceramic Product Development and Commercialization

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r 2009 The American Ceramic Society

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compressive stresses that supress preexisting stress cor-rosion and microdefects that cause breakage. Glass canbe broadly classified into two categories: (1) float glass(untempered glass) is weak and will shatter readily(because of the high concentration of microdefects1)and (2) tempered glass. A tempering furnace forms thebackbone of any modern glass-processing facility. Theselected processing conditions vary wildly between glasstypes (float glass, coated glass, low E glass, etc.) and aredependent on the properties of the glass in question (em-issivity, reflectivity, density, porosity, and conductivity).

After a section of glass arrives in a glass-processingfacility, it is firstly cut and sized to shape. During thecutting process, the glass may be arrised or chamfered toprotect the operators and end users. After arrising, thesized units are placed onto the loading bed of the tem-pering furnace. An example of a standard temperingfurnace is presented in Fig. 1. All tempering furnacesuse a combination of heat and circulated air to bring thesheet glass to the desired processing temperature. Oncethe glass is cooled, it is removed from the unloadingarea. The majority of the modern furnaces use fusedsilica rollers to transport the glass through the furnacesection. Sulfur dioxide (SO2) is used to lightly coat(dope) the silica rollers to prevent glass scuffing andmarking.

Within the furnace, heat is applied in order to raisethe glass specimen above its glass transition temperature,Tg. At ambient temperature, glass behaves similar to anelastic solid; however, at elevated temperatures, it be-comes viscoelastic and behaves like a linear viscoelasticmaterial. The internal stresses and homogenous distri-bution can be assessed using a double interferometricmethod developed by Devos et al.2 The viscoelastic be-havior of glass has been modelled by To et al.3 using

Equation 1(a and b):

SijðtÞ ¼Z t

0

G1 t � tð ÞdeijðtÞ

dtdt; ð1aÞ

�sðtÞ ¼Z t

0

G2 t � tð Þd�eijðtÞ

dtdt; ð1bÞ

In the above equations, Sij and eij are the derivativeparts of the stress and strain components sij and eij while�s�e are the spherical components of sij and eij, respectively.

The quality of the tempered glass is dependent onthe efficiency of the heat transfer coefficient and thehomogeneous distribution of the strengthening residualstresses. Air circulation is essential for heat transfer anddistribution and modern numerical simulation has as-sisted in reducing waste products.4

Glass breakage during the cooling period has alwaysbeen a problem for glass processors.5 This has been ad-dressed and modeled theoretically by Shutov et al.6 us-ing a model derived by L. M. Kachanov, presented inEquation 2.

dz

dt¼ � sn

max@@z

R t0 s

nmaxdt

� � ; ð2Þ

where z is a coordinate read from the glass surface, t isthe current time, and smax 5s1.

Cirillo and Isopi7 have modeled the heat transfer co-efficients for a small simulated glass tempering furnace.They used a computational fluid dynamics model (incor-porated into a visual basic algorithm) to predict variousblower applications. This assists with cost savings incen-tives as the blower motors used during the quench stage ofthe tempering process represent over one half of the totalglass-forming and tempering plant costs.7

Fig. 1. An example of a modern efficient tempering furnace (courtesy of EFCO Furnaces, UK).

688 International Journal of Applied Ceramic Technology—Tiernan and Naughton Vol. 7, No. 5, 2010

Daudeville and Carre8 also modeled the thermo-mechanical behavior of glass using the finite elementmethod (generalised Maxwell’s Model approach). Theydetermined that glass behaved in a viscoelastic mannerand that the residual stress shape or configuration de-pends on the strength of the thermal transfer coefficientand any defects on the edge of the glass.

However, the savings in waste (glass breakage) havequite often been offset by incorrect furnace material se-lection during the design stage. This paper addresses onesuch practical issue and highlights the importance of ma-terial selection for furnace roof design for glass tempering.

Background

On commissioning of a newly installed temperingfurnace, lint and dust particles were noticed on the sur-face of the formed/tempered glass. This eventually forcedthe decommissioning of the furnace as it was determinedthat a rework order would be uneconomically viable. Aconventional furnace design utilizes tightly bound insu-lated glass fiber (such as Fibrefraxt) sheeting as the walland roof material. This has one primary function, to re-tain the heat energy within the heart of the furnace.However, in this case, it was determined that a cheap al-ternative to the insulated glass fiber material was used.

During construction of the furnace, SS304 stain-less-steel sheeting was used to prevent the recurrence ofthe inherent problems associated with the lint/dustcompromising the glass. It prevented the lint from fall-ing from the roof structure by suspending the stainlesssheeting below the insulated glass fiber material.

This solution was further hampered by the fact thatthe stainless sheeting used in the roof structure began tobuckle and twist under the intense heat of the furnace. Theparent company recommended that the stainless steelsheeting should be welded to the support struts of theroof. This was carried out in accordance with the originalequipment manufacturers (OEM’s) specifications. Thecombination of incorrect roof material selection and sub-sequent reworking led to a number of technical problems,which are discussed in detail in ‘‘Technical Issues.’’

Technical Issues

After running the furnace for a short period, it wasnoticed that black deposits were forming on the fusedsilica rollers. These deposits appeared to have been im-

bedded into the substrate of the rollers. Examples of theblack deposits are presented in Fig. 2. The glass itself wasalso marked by what appeared to be black droplets. Thisaffected coated glass more readily as the droplets ap-peared to scorch the coating again, rendering the pro-cessed piece useless. An example of the black depositsfound on the glass surface is presented in Fig. 3.

X-Ray Analysis and EDAX on Glass Deposits

Samples of the (deposits) droplets from the glass wereanalyzed using EDAX and X-ray analysis. Two types of

Fig. 2. Black deposits on the fused silica rollers.

Fig. 3. Deposits found on the surface of the tempered glass(Sample A, top; Sample B, bottom).

www.ceramics.org/ACT Impact of Metal Corrosion in Furnace Design 689

samples were analyzed. The first sample (Sample A) wasin the form of dust/grit and some larger particles scrapedfrom the glass deposits. These were scanned separately toestablish whether there were noticeable differences be-tween them. The first was granular in the 200–750mmrange and speckled brown to black while the other con-sisted of thick brown flakes approximately 3 mm across.Similarly, Sample B was scraped from the glass black spotand was in the form of grit, again in the range of 200–750mm. X-ray scans were run on both samples (SamplesA and B). The results are presented in Figs. 4–6, whereasthe EDAX results are presented in Fig. 7. The peaklists and identifiable patterns for the results presented inFigs. 4–6 are shown in Tables I–III, respectively.

It was determined that Sample A (dust) had mini-mum identifiable peaks. It had a low match score withtridymite, a form of SiO2. EDAX was run to see whether

the cations present could be identified. One run indicateda presence of Si with some sulfur (S) and traces of otherelements. A second run on the same dust sample (i.e., at adifferent location) showed more sulfur, some silicon (Si),calcium (Ca), titanium (Ti), iron (Fe), and trace amountsof other elements. The relative amount of sulfur in thisEDAX run seemed anomalous. As the X-ray scan was notstrongly crystalline and some of the material appearedglassy, the presence of silicon was not surprising. Theother elements did not seem to be present in sufficientquantity to be strongly evident in an X-ray scan.

A separate analysis on Sample A (large particle) wasagain not strongly crystalline. Quartz (SiO2) appearedto have a reasonable match with the identified peaks.These were similar to the peaks identified in the otherpart of Sample A. EDAX in this instance indicated ahigher level of chromium (Cr), iron (Fe), and nickel(Ni) than in the previous sample. This conclusively in-dicated the presence of stainless steel.

Position [°2Theta]10 20 30 40 50 60 70 80 90

Peak List

85-0419

Position [°2Theta]10 20 30 40 50 60 70

Cou

nts

0

4

16

36

64

Fig. 4. X-ray analysis of Sample A – dust particle (top) and theplot of the identified phases (bottom). Peak list and identifiablepatterns are presented in Table I.

Fig. 5. X-ray analysis of Sample A – large particle (top) and theplot of the identified phases (bottom). Peak list and identifiablepatterns are presented in Table II.


The X-ray scan of Sample B did not indicate astrong degree of crystallinity. This made it difficult toidentify the phases present. A form of quartz gave thebest peak search match. EDAX again indicated the

presence of stainless steel (Fe, Ni, and Cr). It alsoshowed higher levels of sulfur, potassium, calciumwith some silicon, and traces of other elements.

Position [°2Theta]10 20 30 40 50 60 70

Cou

nts

0

4

16

36

64

Fig. 6. X-ray analysis of Sample B – large particle (top) and theplot of the identified phases (bottom). Peak list and identifiablepatterns are presented in Appendix B3.

Table I. Peak List and Identifiable Patterns for X-Ray Analysis of Sample A—Dust Particle (Fig. 4)

Pos. (12Th.) Height (cts)FWHM(12Th.)

d-spacing(A) Rel. int (%)

Tip width(12Th.) Matched by

Peak list: Sample A—dust23.1106 12.84 0.3779 3.84864 74.44 0.3840 85–041933.5136 17.25 0.3779 2.67399 100.00 0.384035.8315 13.59 0.5038 2.50615 78.80 0.5120 85–041954.5960 6.14 0.9216 1.67961 35.59 0.7680

Visible Ref. code Score Compound name Displacement (12Th.) Scale factor Chemical formula

Identified patterns list: Sample A—dust� 85–0419 11 Tridymite 0.000 0.135 SiO2

Fig. 7. EDAX on Samples A and B.


The relatively high presence of sulfur combinedwith potassium raised a query about the source of theseelements. Si and Ca cations are consistent with glassmaking while Fe, Ni, and Cr are consistent with thepresence of stainless steel.

Scanning Electron Microscopy (SEM) and EDAXAnalysis on the Roller Deposits

SEM and EDAX analyses were carried out on threesections of the fused silicon rollers used to transport theglass through the tempering furnace. The SEM image ispresented in Fig. 8. With reference to Fig. 8, Area 2corresponds to the unaffected area of the roll, Area 1corresponds to the center of the visible black deposit,while Area 3 corresponds to the periphery of the depos-ited substance. The EDAX evidence presented in Fig. 9suggested that Area 1 was mainly constituted by sodium

Table II. Peak List and Identifiable Patterns for X-Ray Analysis of Sample A—Large Particle (Fig. 5)




Peak list: Sample A—large particle27.6137 8.57 0.6298 3.23042 76.01 0.6400 86–156436.1350 5.72 0.7557 2.48579 50.71 0.768041.6208 5.29 0.4408 2.16996 46.96 0.4480 86–156454.6550 11.27 0.3840 1.67793 100.00 0.3200


Identified patterns list: Sample A—large particle� 85–1564 18 Quartz low 0.0000 0.297 SiO2

Table III. Peak List and Identifiable Patterns for X-Ray Analysis of Sample B—Large Particle (Fig. 6)




Peak list: Sample B—large particle26.1045 32.21 0.2519 3.41365 100.00 0.2560 75–138128.9757 28.96 0.2519 3.08159 89.89 0.2560 75–138132.9416 11.78 0.3779 2.71910 36.58 0.3840 75–138142.9929 15.09 0.7680 2.10209 46.85 0.6400


Identified patterns list: Sample B—large particle� 75–1381 38 Coesite 0.0000 0.776 SiO2

Fig. 8. Scanning electron microscopy image of the depositedmaterial present on the fused silica rollers (Area 1, Area 2, andArea 3).


Fig. 9. EDAX corresponding to Area 1, Area 2, and Area 3.


sulfate deposit (34% of sodium). Some metallic ele-ments were also detected. With reference to Area 3, ironwas detected at the periphery of the deposited material.

It was decided to further investigate the suspectedsodium sulfate deposits using a different section of thefused silica roller, to further verify the results of theEDAX from Area 1 of Fig. 8. The results of the SEMand corresponding EDAX are presented in Figs. 10 and11, respectively. It is evident from the EDAX resultsthat this deposit is also mainly constituted by sodiumsulfate.

Discussion and Conclusion

With reference to the deposited substances foundon the fused silica rollers, the results from the EDAXinvestigations (Figs. 9 and 11) indicated high levels ofsodium sulfate. This in itself is not unusual with glasstempering. Sodium sulfate is a by-product of the reac-tion of SO2 (injected inside the glass furnace) and theglass itself. Under normal circumstances, this sodiumsulfate forms a dry lubricant, which sticks to the fusedsilica rollers and acts as a prevention barrier. This pre-vents the glass from scuff marks and can be identifiedby a slight discoloration of the rollers. Under certainprocessing conditions, usually associated with an over-concentration of SO2 combined with a suitable activa-tion temperature, sodium sulfate (Na2SO4) will form.This sodium sulfate normally dissipates through the

roof and wall insulation and escapes to the atmosphere.However, this was not possible in this case due to theexistence of the stainless steel shroud. Instead, the sodafrom the sodium sulfate reacted with the fused silicarollers and crystallised into cristoballite. As fused silicaand crystoballite have different coefficients of thermalexpansion, the fused silica rollers cracked and the nodulelodged itself within the substrate of the rollers. The cry-stallisation and roller deterioration is irreversible.

During the chemical reaction of SO2 to sodiumsulfate, the sodium sulfate condensation also entrappedfine iron particles present inside the furnace from (1) thebucking of the stainless steel roof material from the highprocessing temperatures and (2) the presence of the fillerwelding material used during the OEM’s engineeringmodification order. This is evident in Fig. 7 and Area 3of Fig. 9.

It was also experimentally discovered that the drop-lets found on the surface of the glass were primarilyconstituted of sodium sulfate with some traces of chro-mium, iron, and nickel (Fig. 7). This is consistentwith the presence of stainless steel. Again, the sodiumsulfate could not dissipate due to the stainless steel roofmaterial and, in this circumstance, the sodium sulfatedroplets could not fuse themselves to the silica rollersas the glass was in contact with the rollers. Instead, thedroplets condensed and scorched the coating of theglass. Unfortunately, the scorching degradation is alsoirreversible.

As the corrosion distribution appears to be ran-domly distributed throughout the roof of the furnace,the question must be raised about the sporadic and sto-chastic nature of the corrosion process. The catalyst forstainless steel 304 corrosion may rest with the high-ratedissolution of inclusions during the fabrication processitself.9 Williams9 has concluded that pitting has its gen-esis in the solidification process owing to the theory thatsulfide particles solidify after the parent metal does.Using this assumption, Ryan et al.10 postulated thatrandom inclusions of sulfur give rise to sulfur-coatedcapped zones within which a sulfide chloride-rich acidsolution could develop through inclusion or dissolution.The parent metal then dissolves to form a corrosionpit.10 This may account for the statistical distribution ofthe pitting corrosion seen in the roof of the furnace inquestion.

A further compounding problem with using stain-less steel as a roof/wall material in a glass-temperingfurnace was observed. The sheeted stainless steel was

Fig. 10. Scanning electron microscopy image of the suspectedsodium deposit found on one of the fused silica rollers. (Alternatetest.)


welded to prevent the stainless steel from buckling, asdescribed in ‘‘Background.’’ Unfortunately, the weldedmaterial had started to corrode. The combination ofheat, SO2, and two dissimilar materials (welding fillermaterial and stainless steel) led to dissimilar metal cor-rosion. This is presented in Fig. 12.

It is not unusual for SS305 to corrode in the pres-ence of sulfur. Abd El Meguid et al.11 proved conclu-sively that SS304 corroded readily in the presence of lowconcentrations of SO2�

4 ; SO2�3 ; and S2O

2�3 using elect-

rochemical potentiodynamic anodic polarizations. It wasdiscovered that the addition of SO2�

4 ; SO2�3 ; and S2O

2�3

significantly promoted the pitting propagation kinetics.It is also not unusual for stainless steel to corrode inharsh operating environments. Van Bennekom12 re-ported corrosion in a stainless steel conditioning tower

Fig. 11. EDAX illustrating the chemistry of the build-up removed from one of the fused silica rollers. (Alternate test.)

Fig. 12. Dissimilar metal corrosion within the furnace.


of a cement-processing facility in South Africa and at-tributed it to the presence of sulfur and elevated oper-ating temperatures.

As a recommendation, the use of stainless steelshould be avoided within the heating zones of glass-tempering furnaces. High-quality trusted brands of in-sulated wall and roof material should be utilised withinthe furnaces. The level of SO2 should also be limitedwithin the glass-tempering furnaces. Over-doping willlead to the creation of sodium sulfate (Na2SO4), whichis detrimental if trapped within various airtight zones ofthe furnace. Periodical airing of the furnace (by raisingthe upper section) will assist in preventing a build-up ofNa2SO4. If the use of stainless steel is unavoidable forwhatever reason, stainless steels with high chromiumcontents should be used.

References

1. S. M. Wiederhorn, ‘‘Influence of Water Vapor on Crack Propagation inSoda–Lime–Silicate Glass,’’ J. Am. Ceram. Soc., 50 [8] 407–414 (1967).

2. D. Devos, M. Duquennoy, E. Romero, F. Jenot, D. Lochegnies, M.Ouaftouh, and M. Ourak, ‘‘Ultrasonic Evaluation of Residual Stresses inFlat Glass Tempering by an Original Double Interferometric Detection,’’Ultrasonics, 44 923–927 (2006).

3. Q. D. To, Q. C. He, M. Cossavella, K. Morcant, A. Panait, and J. Yvonnet, ‘‘TheTempering of Glass and the Failure of Tempered Glass Plates with PinLoaded Joints: Modelling and Simulation,’’ Mater. Des., 29 943–951 (2008).

4. F. Monnoyer and D. Lochegines, ‘‘Heat Transfer and Flow Characteristics ofthe Cooling System of an Industrial Glass Tempering Unit,’’ Appl. ThermalEng., 28 2167–2177 (2008).

5. A. G. Shabanov, A. I. Shutov, and V. P. Markov, ‘‘Problems in QuenchingThin Glass and their Solutions,’’ Glass Ceram., 8 10–12 (1991).

6. A. I. Shutov, P. V. Popov, V. L. Todorov, and V. G. Strukov, ‘‘Reducing theBreakage Rate in Glass Tempering,’’ Glass Ceram., 54 [9–10] 309–310 (1997).

7. F. Cirillo and G. M. Isopi, ‘‘Glass Tempering Heat Transfer CoefficientEvaluation and Air Jet Parameter Optimization,’’ Appl. Therm. Eng., 291173–1179 (2009).

8. L. Daudeville and H. Carre, ‘‘Thermal Tempering Simulation of Glass PlatesInner and Edge Residual Stresses,’’ J. Therm. Stresses, 21 667–689 (1998).

9. D. E. Williams and Y. Y. Zhu, ‘‘Explanation for Initiation of Pitting Cor-rosion of Stainless Steel at Sulfide Inclusions,’’ J. Electrochem. Soc., 147 1763–1766 (2000).

10. M. P. Ryan, D. E. Williams, R. J. Chater, B. M. Hutton, and D. S. McPhail,‘‘Why Stainless Steel Corrodes,’’ Nature, 415 770–774 (2002).

11. E. A. Abd El Meguid, N. A. Mahmoud, and S. S. Abd El Rehim, ‘‘The Effectof Some Sulphur Compounds on the Pitting Corrosion of Type 304 StainlessSteel,’’ Mater. Chem. Phys., 63 67–74 (2000).

12. A. van Bennekom and J. H. Potgieter, ‘‘An Examination of the Cause ofExtensive Corrosion of the Shell of a 3CR12 Conditioning Tower in a Ce-ment Plant,’’ Anti-Corrosion Met. Mater., 47 [3] 152–156 (2000).


metal corrosion and its impact on glass tempering furnace design

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