Revista Latinoamericana de Metalurgia y Materiales, Vol 11, N° 1y 2, 1992 71

Failure Morphology of an Aluminium thennally sprayed coating exposed to 750 OCoxidation

1. C. Grigorescu, M. 1. Spécht, R. Alvarez, J. Teran, J. Medina INTEVEP, S. A. (R&D Center ofVenezuelan Oil Industry), Los Teques, and E. Blanco PEQUlVEN, Caracas, Venezuela.

Abstraet

This paper presents the failure analysis of an Al sprayed coating applied on a welded steelstructure. After welding and aluminizing, a stress relieving treatment was carried our át 750oC,producing the coating exfoliation in randomly distributed zones. Coating samples from the exfoliatedparts, as well as from the apparently undamaged areas showed a microstructure with tongue-like Fe-Alintermetallic compounds, projecting from a thin Allayer and penetrating into a relatively thick layer ofiron oxides.

At Laboratory scale, Al coated samples were heat treated at 7500C, in oxidizing and inertatmosphere, during 1,3,6 and 24 hours. Initially, both inert and oxidizing heat tratments producedFe-Al solid solution and intermetallic compoynds tongue-like shaped. In oxidizing atmosphere theintermetallic corripounds remained separated by brittle and porous iron oxides, while in inert atmosphere,the tongue like structure tended to develop into a continuous layer of Fe-Al solid solution .

In order to inhibit the formation of the failure microstructure constituted by Fe-Al intennetalliccompounds and Fe oxides, a modified construction sequence, i.e. welding - stress relieving -aluminizing, should be followed. Otherwise, an inert atmosphere heat treatment alternative should beconsidered.

Key words: aluminizing, thermal spray coating, metallic coatings, oxidation.

Resumen:

En este articulo se presenta el análisis de falla de un revestimiento de termorrociado de Alaplicado sobre una estructura de acero soldada. Después de la soldadura y aluminazado se realizó untratamiento térmico a 750°C que produjo la exfoliación del revestimiento en zonas distribuidasaleatoriamente. Muestras de las partes exfoliadas y de las zonas no afectadas mostraron unamicroestructura con formaciones de compuestos intermetálicos Fe-Al que se proyectaban desde la capade Al penetrando en una capa relativamente gruesa de óxidos de Fe.

Se hicieron ensayos de laboratorio con muestras revestidas calentandolas a 750°C en atmosferaoxidante e inerte durante 1,3,,6, y 24 horas. Inicialmente los tratamientos en ambos tipos de atmosferaprodujeron solución sólida Fe-Al y compuestos intermetálicos de formas parecidas a las mencionadasarriba .. En atmosfera oxidante los compuestos intermetálicos permanecieron separados por una capafrágil y porosa de óxidos de Fe, mientras que en atmosfera inerte, las microestructuras descritas tendíana desarrollase en capas continuas de solución sólida Fe-Al.

Para inhibir la formación de microestructuras de falla constituidas de intermetálicos Fe-Al yóxidos de Fe, se modifico la secuencia se construcción haciendo la soldadura, luego el tratamientotérmico de recocido y por último el aluminizado. De otra forma se debe considerar un tratamiento enatmosfera inerte de la estructura.

Palabras claves: Aluminizado, revestimiento por termorrociado, revestimiento s metálicos,oxidación.

Introduction

The aluminum thermally sprayed coatingsare recommended for the oxidation control ofindustrial equipments operating at relatively hightemperatures such as boilers, heat exchagers, andsulphur oxide converters (1, 2, 3). An aluminiumoxide film formed on the surface of the coatingmicro-constituents is suppose to ptotect the steelsubstrate againts oxidation, even at temperaturenear and above the aluminium melting point (450to 950°C). Beside, in the same temperature range,the improvement of coating adhesion is expected

LatinAmerican Journal o/ Metallurgy

and attributed to the formation of an intermetalliccompound layer in the interface with the substrate(1).

This paper presents the case history of thefailure of an Al thermally sprayed coating appliedon a low alloy steel and exposed to 750°Coxidation. In these conditions, a compositemicrostructure of Ge oxides and Fe-Al intennetalliccompounds deve1oped, which drastically, decreasedthe adhesion and corrosion control performance ofthe modified coating.

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Background

The failure occurred during the constructionof the metallic structure of a sulphur oxideconverter, made up by a cylindrical vessel of 10mdiameter and 12m height, with columns, bars andplates as intemal components.

The structure was fabricated of an ASTM A387 grade 22 steel (O,lC; 0,49Mn; 0,03P; O,OIS;0,29Si; 2,25Cr; 0,93Mo). The inner face of thecylindrical sections and all the internal componentswere sand blasted with a grit 25 silica sand,aluminium coated by thermal spraying and sealedwith an aluminium-silicon paint; for the metaldeposition both flame and electríc are proeesseswereused. The minimun thickness of the Alcoating was 0,2 mm and the total coated ares wasabout 2000 m2.

The coated parts were assembled andsubsequently, the welded joints were thermallysprayed in situ and sealed. A stress relieving heattreatment was perfonned at about 7500C during 24hours in oxiding atmosphere by using porpane gascombustion. The heating was monitored bytwenty four thermocouples placed in differentpoints in the converter and a near uniformtemperature of 740 ± 15°C was recorded.

A visual inspection after the heat treatmentindicated that about 40% of the internal partscoating was exfoliated. The investigation of thefailure cause was required as well as the evaluationof the remnant coating condition in order to definethe restoration procedure.

Experimental

Field and laboratory tests were carried outaceording the following steps:1. Identification of the failure cause, evaluating aremovable internal component from the mostdamaged section of the converter (fig. 1). Samplesfor metallographical, chemical and surfacemorphology analysis were taken from theexfoliated area and the zone with remnant coatingof this part. As a comparison, a similar spare part,not exposed to the heat treatment in the convertorwas evaluated. Samples from this spare part wereheat treated at laboratory scale in an oxidizingatmosphere fumace at 7500C, during 24 hours.Il. Inspection of the apparent1y undamaged surfaceof theconverter wall and roof (757m) using astatistical method which was defined according to adouble sampling plan - simplified inspection, withan inspection level 1, considering the defectivefraction 1,5% (4,5).The inspection was carried outin two stages following, respectively, two quelity

criteria: coating - substrate adhesion atmacroscopical level and microstructural damage.

Figure 1In the first stage, on the surface to be evaluated,

seventeen random samp1e areas of one squaremeter were traced; in each one, thirteen ASTM B571/ISO 2063 adhesion scribe-grid .tests wereperformed. This test consisted in scribing arectangular pattern using a sharp pointed too1 topenetrate the coating to the substrate. The distancebetween the scribe-lines was about 3mm with anapproximate area of the grid pattern of 4 cm (6,7).In the second stage, fragments of all the coatingwere removed frorn the scribe-grid tested areas;these were submitted to micro-morphologicalanalysis on the spalled face and the cross section.Ill. Study of the failure mechanism by heat treatingthe Al coated samples in oxidizing and inert gas(argon) atmosphere.These samples were preparedby flame and electric are spraying, according to theprocedures recommended by the manufacturers ofthe spraying equipments and applied during theconstruction of the converter. The heat treatmentwere performed at 750 for 1, 3, 6, 12 and 24hours.

The morphology of the samples consideredin this work was observed by using optical andscanning electron microscopes (OM and SEM).Qualitative and semi-quantitative chemical analysiswere perfonned with and X ray energy dispersionsystem (EDX) attached to SEM.

ResuIts

Failure CauseThe morphological and chemical analysis of

the heat treated samples revealed drastic changes inthe coating microstructure. In the originaluntreated samples, a laminar microstructure,typical for the thermally sprayed coatings, wasobserved, where aluminium was the only metalliccomponent (Fig. 2a), the thickness of this coatingwas in the order of 0,2 mm.

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Figura 2

After the oxidizing heat treatment, performed bothin the field and laboratory, the coating showed amodified microstructure, characterized by tongu-like-rnicro-constituents, perpendicular to thecoating-substrate interface, quasi regulardistributed (Fig. 2b), the thickness of the modifiedcoating was in the order of 0,42 mm. In theseconstituents, the mean values of the componentsweight concentration, as detected by EDX, were:52,6% Al, 46,9% Fe, 0,5% Cr; this chemicalcomposition corresponds to an aluminum-ironintermetallic compoynd (Fe2AIs - type), with Cr insolid solution (8, 9, 10). The element distributionin the modified coating is illustrated in Fig. 3. Themicrohardness mean value for the Al-Fe phase was878 HV300, this value is similar to the one reponedby Niimoni [8] for the intermetalic compoundFe2AIsll phase.

The tongue-like intermetallic compounds werespaced by laminar layers of iron oxides, parallel tothe coating-substrate interface. The oxidespresented a relatively high density near the steelsubstrate while toward the external surface,elongated pores were observed. Themicrohardness of the oxide layer was 406 HV300

and for the substrate, 129 HV300 The exfoliationof the modified coating was induced by thecoalescence of the laminar porous in the Fe oxidesreach zones, producing the fracture of the tongue-like .intermetallic compounds wich acted asreinforcements of the remaining coating (Fig. 2b).A typical surface morphology in the exfoliatedzone is illustrated in Fig.4, where well definedsmooth areas, resulted by the brittle fracture of theAl-Fe tongue-like constituents are surrounded byiron oxide matrix wich presented whiskers typicalfor wustite (FeO).

Figura 3,

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Based on these evidences, ir could be statedthat the failure was produced by the micro-morphological modifications of the Al coating-steelsubstrate system, consisting in the formation anpermeable structure of Fe-Al intermetalliccompounds and Fe oxides. As the microstructuralmodification appeared after the oxidizing heattreatment both at field and laboratory scale, it wasascertained that the cause of the failu~e was theconstruction processes sequence used, i.e.welding-aluminizing-heat treating\ In a correctsequence, the aluminizing should be carried outafter the stress relieving heat treatment of thewelded joints.

Inspection of the Apparently Undamagedeoating

For the first stage of the inspection ofremnant coating on the wall and roof, the

of one square meter evaluated were founddefective, as at least three fragments removedfrom each one presented the typical failure micro-morphology.

Consequently, the quality of the remnantcoating was considered rejectable, and thecomplete removal and restoraticn wasrecommended.

Failure MechanismAl205 intermetallic compounds form

almost instantaneously when solid iron or ironalloys comes into contact with molten aluminum.As reported by M. Niimoni, this phase wasmarkedly tongue-like shaped when it was obtainedby the immersion of pure iron in a moltenaluminum bath.

The sharpness of the tongues decreases andthe interrnetallic compound tended to developedinto a band-like morphology whe alloying elementssuch as e, Cr and Mn were added or the bath wasstirred; this effect was attributed, in both cases, tomodifications in the kinetic of iron mass transportinto aluminum (8, 9, 10).

The thermally sprayed coating analyzed inthe present work was exposed to temperaturesabove the aluminum melting point during the heattreatment. Therefore, the aluminum could achievea transitory liquid state followed by the formationof solid Fe-Al intermetallic compounds. It appearsthat practically all the material of the originalcoating was redistributed in the discreet tongue-likeintermetallic compounds, increasing the thicknessof the modified coating in about 100%.

It is worth 10 note that in samples exposedto heat treatments of short duration (lh), that theintermetallic compounds have similar shapes anddimensions both in inert and oxidizingatmosphere. However , the modified coatingspresented structural differences since theintermetallic compounds are spaced by void spacesin the former case (nmatrmsph:retreúing)and by ironoxides in the last one (mmpareFigs.5 and6).

rejectable/acceptable levels were quealitativelyrelated to the occurrence or not of the fracture inthe coatingsubstrate interface, under the scribe-testconditions and using macroscopical appreciations.No exfoliation took place during the 169 adhesiontests performed in different sites of this surface.This result allowed to state with a 99% confidencelevel that the rnaxirnurn possible extension of thewall area with lack of adhesion between coatingand substrate was of 1,5%.

Furthermore, considering the quite uniforrntemperature and atmosphere in the converter duringthe heat treatment, it was supposed that theunexfoliated areas could present microstructuralmodifications, similar to (hose observed in thefailed part. Therefore, in a second inspectionstage, the microstructure was defined as quelityparameter; the defective microstructure consisted inAl-Fe tongue-Iike cornpounds in an iron oxidematrix. The sampling method indicates that if thedefective feature appears in a sample, at least othertwo samples have to be evaluated; if the cumulatibenumber of defects is two, the quality is consideredrejectable In the present case the first three samples

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- ~ \ After the primer formation o~ .Fe~Alintermetallic compounds skeleton, the modificationprocesses of the coatings followed different waysdepending on the heat treatment atmosphere. So,in the case of the exposure to inert gas, solid statediffusion between the substrate and theintermetallic compounds generated a layer of Fe-Alsolid solution formed on the top of the steelsubstrate (Fig. 5). From the practical point ofview the presence of this layer with an aluminumconcentration from Oto 47% could be desirable asit assure a good protection against corrosion withsulphur oxides (11).

Therefore, failure prevention could beassured by performing the heat treatment in inertatmosphere, without altering the employedfabrication sequence i.e. welding-aluminizing-stress relieving; inert gas heating was feasible sincethe converter could be sealed.

During the heat treatment carried out in airthe intermetallic compound skeleton maintained analmost stable morphology, independently of thedurations of heat treatments. A thin solid solutionfilm formed, surrounding the tips of theintermetallic compounds in contact with steel; thethickness of this layer did not increased in time.Beside, the progressive oxidation of the substratetook place with the formation of iron oxide layerswith variable density, being denser close thesubstrate while large pores appeared toward theexternal surface. These differences were probablyproduced by a variable concentration of oxygen.As the oxygen penetration depth increased, the

complete separation between the tips of the tongue-like intermetallic compounds and the steel substrateoccurred (Fig. 6). . .

As remarked by Baxter and Reiter Inmicrostructural studies carried out on preoxidizedsteel substrate-plasma coated with aluminum, thespallig of the coating is more frequent in theinterface between iron oxide and steel, and theadhesion resistance in the interface between both isrelated with the presence of Fe-Al intermetalliccompounds as well as the chemical characteristicsof the iron oxides. So, the presence ofinterrnetall ic compounds, even discreetlydistributed, increases the adhesion resistance atlevel equal or superior than the one demonimatedas acceptable in tests canied out by scratching withsharp steel tools (12). Therefore, the differentdegrees of coating adhesion found in the convertercould be attributed to differences in the size andchemical characteristics of oxide layers.

Conclusions

The exfoliation of the aluminium thermally-sprayed coating was due to the microstructuralmodification which consisted of a Fe-Alintermetalic phase, distributed in a thick and brittleiron oxide layer. The modification of the coatingmicrostructure occured during the field post-welding heat treatment at 750OC. The extension ofthe damage was general even in those areas wherethe modified coating showed a fairly goodadhesión to the substrate.

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The results of the microstructural analysisof field and laboratory samples showed that at750°C, the Fe-Al intermetallic compounds intongue-like skeleton, were highly permeable. Inoxidizing atmosphere it allows the progressiveoxidation of the substrate and finally the exfoliationof the modified coating. In inert atmosphere, acontinue band-like layer tend to form in theinterface with the substrate by solid state diffusion.

Due to the high dimensions of theconverter, variable temperature and atmospherecould develop during the heat treatment, producingdifferences in the oxidation degree of the substrate.Also, variable stress state in the coating-substrateinterface was expected, with more probable stressconcentration in the profiled bars, than in the wall,leading to the preferential exfoliation of the former.

Recornrnendations

As consequence of the failure analysis andinspection, the removal and restoration of thewhole Al coating was recommended. For newsimilar installations, the spray aluminizing shouldbe performed after the stress relieving heattreatment. Also, it should be carried out furtherstudies on the diffusion kinetics in the Al-steelsystem, in oxidizing and inert atmosphere for abetter understanding of the mechanism.

Figura 6

Reference

1. Cochran, W. C., "Thermally Sprayed AlCoatings on Steel", Metal Progress, Dec. 1982, p.37-40.

2. Birchfield, I. R., "Sprayed Al covers BoilerTubes" , Welding Design and Fabrication, July1986, p. 46-49.3. Modi, M. D., "Corrosion Protection by Zn/AIMetal Spraying", Corrosion&Maintenance, March1978, p. 62-67.

4. ASTM B 697-81, "Selection of Sampling Plansfor Inspection of Electrodeposited MetallicCoatings and Related Finishes onProducts"

5. Covenin 598-75, "Planes de Muestreo Unico,Coble y Múltiple con Rechazo" Comisión deNormas Industriales, Caracas, Venezuela 1975.

6. ASTM B 571-85, "Standard Tests Methods forAdhesion of Metallic Coatings".

7. ISO 2063, "Metallic Coatings-Protection of Ironand Steel.against Corrosion Metal Spraying of Zincand Aluminium".

8. Niimoni, M. and Y. Veda, "On the alloy LayersFormed by the Reaction between Ferrous Alloysand Molten Aluminium", Trans. of the Jpn, Inst.of Metals, Vol 23, No. 11 (1982), p. 109-717.

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9. Niimoni, M., Y. Veda and M. Sano,"Dissolution of Ferrous Alloys into MoltenAluminium", Trans. of the Jpn. Inst. of Metals,Vol 23, No. 12(1982), p. 109-717.

10. Niimoni, M., Y. Veda and M. Sano,"Dissolution of Ferrous Alloys into MoltenAluminium into Molten Aluminium under ForcedFlow", Trans. of the Jpn. Inst. ofMetals, Vol 25,No. 6 (1984), p. 429-439.

11. Fellman, D., Y. Bienvenu, M. Foucault,"Corrosion a Haute Température sous Atmomsperede Soufre Vapeur Contenant de l'Oxygene desRevetements Riches en Aluminium", Mémoires etEtudes Scientifiques Revue de Métallurgie, Jan.1983, p. 37-47.

12. Baxter, C. F. G. and H. Reiter, "Theadherence of Plasma Sprayed Aluminium to Stee1",8th Int. Thermal Spraying Conf., Miami BeachFla., 1976, p. 271-280.

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