ORI GIN AL PA PER

Al, Si, and Al–Si Coatings to Improve theHigh-Temperature Oxidation Resistance of AISI 304Stainless Steel

Morteza Zandrahimi • Javad Vatandoost •

Hadi Ebrahimifar

Received: 25 February 2011 / Revised: 17 May 2011 / Published online: 15 June 2011

� Springer Science+Business Media, LLC 2011

Abstract Oxidation resistance of chromium steels is due to the formation of

Cr2O3 on the surface. However, this surface layer destabilizes above 1,000 �C and

does not protect the metal. In this study, three types of coatings were applied to AISI

304 stainless steel (SS), and the microstructure and oxidation resistance of the

coatings were investigated. Aluminum coating, silicon coating, and the codeposition

of Al and Si were deposited on an SS substrate by the pack cementation method.

The microstructure of the samples was then examined by SEM and EDS, and phases

were identified by XRD. The oxidation resistance of these samples was studied in

air at 1,050 �C. The results showed that the best resistance to oxidation was

obtained, in order, from the codeposition of Al–Si, Al coating, and Si coating.

Keywords AISI 304 stainless steel � Pack cementation � Aluminum � Silicon �Codeposition of Al–Si � Oxidation resistance

Introduction

Besides various other applications, stainless steels (SSs) are widely used in exhaust pipes

to improve the service life of their components, especially the upstream part of the

exhaust line (manifold, down-pipe, converter shell), where temperatures can reach

1,100 �C [1]. Due to their ability to thermally grow a protective Cr-rich a-(CrxFe1-x)2O3

scale, ferritic (body-centered cubic) or austenitic (face-centered cubic) layer, SSs

exhibit good oxidation resistance in dry oxidizing atmospheres at high temperatures

[1, 2]. However, this layer is destabilized at temperatures above 1,000 �C and no

longer protects the metal [3]. Pack aluminizing has been widely applied to steels and

M. Zandrahimi (&) � J. Vatandoost � H. Ebrahimifar

Department of Metallurgy and Materials Science, Faculty of Engineering, Shahid Bahonar

University of Kerman, Jomhoori Eslami Blvd., Kerman, Iran

e-mail: m.zandrahimi@mail.uk.ac.ir

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Oxid Met (2011) 76:347–358

DOI 10.1007/s11085-011-9259-1

superalloys to improve their hot temperature oxidation and sulfidation resistance [4].

The coating can form Al2O3, which is highly resistant to oxidation and hot corrosion at

elevated temperatures and in hostile environments [3, 5, 6].

The aluminization of steels has been performed by many methods, including

pack cementation, hot dip, thermal spray, and vapor deposition [5]. The pack-

cementation method is cheap and reliable, and it is widely used in the coating

industries [5, 7].

It has been previously reported that the addition of Si to SSs could improve their

high-temperature oxidation resistance. The beneficial effects of Si on the high-

temperature oxidation resistance of SSs are twofold. First, with sufficient

concentration, Si can form a continuous vitreous silica layer between the metal

and scale interface. This silica layer has a low concentration of defects, allowing it

to become a good diffusion barrier and provide excellent oxidation resistance.

Second, the preferentially formed silica acts as the nucleation site for the subsequent

formation of chromia, which provides oxidation protection [8, 9].

The pack powder mixtures for codepositing Al and Si on metal substrates are

normally expected to consist of Al–Si master alloy powders as a depositing source.

Master alloy powders are used because they are considered to allow for the

adjustment of the pack activity of deposition elements to generate vapor phase

conditions favorable for codeposition [10–12].

In this study, mixtures of elemental Al and Si powders were evaluated as a

depositing source, rather than their alloys, as using these powders was not only more

economical, but also technically advantageous, providing an easy way to adjust

pack compositions and hence their depositing properties [10].

The effects of Al, Si, and Al–Si coatings on the oxidation resistance of AISI 304

SS has been investigated less than on other alloys. As such, the high-temperature

oxidation behavior of AISI 304 SS with and without coating was investigated in this

study.

Experimental Procedures

In this investigation, coatings of Al, Si, and the codeposition of Al and Si were

carried out on AISI 304 SS. The chemical composition of this steel is shown in

Table 1. Specimens measuring 10 9 10 9 5 mm were cut from a square bar. These

Table 1 Chemical composition

of SS AISI 304 in wt%Element Concentration (wt%)

Fe Bal

C 0.08

Cr 19.1

Mn 1.6

Si 1

P 0.045

Ni 8.2

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Table 2 The pack compositions used and the holding temperature/time

Number Powder composition Temp. (�C) Time (h)

1 Al10% ? NH4Cl5% ? Al2O385% 1,050 6

2 Si10% ? NH4Cl5% ? Al2O385% 900 5

3 Al10% ? Si10% ? NH4Cl5% ? Al2O375% 1,050 6

Fig. 1 SEM cross-section micrograph of aluminized sample (a) and EDS results showing theconcentration variations of Fe, Cr, Ni, and Al elements near the surface of the aluminized 304 SS (b)

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coupons were ground through 800-grit SiC paper, cleaned, and dried. Pack powder

mixtures were prepared by weighing out and mixing appropriate amounts of

powders of Al2O3, Al, Si, and NH4Cl. The particle sizes of the Al2O3, Al, and Si

powders were less than 50, 75, and 20 lm, respectively. The substrate samples and

pack materials were placed in an aluminide crucible, which was closed with an

aluminide lid using an aluminide-based cement. The crucible was placed into an

electric tube furnace, which was heated to 250 �C and held at this temperature for

1.5 h to remove moisture from the pack. The furnace was circulated with argon, and

the temperature was raised to a final coating depositing temperature and held there

for the required time. The furnace was then cooled to room temperature at its natural

rate by switching off the power supply while maintaining the argon gas flow. The

pack compositions and the holding temperature are shown in Table 2. After pack

cementation, the samples were removed from the pack and ultrasonically cleaned in

ethanol to remove any embedded pack material.

The microstructure and chemical composition of cross-sections of the coated

specimens were analyzed using scanning electron microscopy (SEM) (CamScan

Fig. 2 XRD analysis of aluminized sample (a) for first layer and (b) for second layer

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MV320) with energy dispersive spectroscopy (EDS). The different phases of the

surface layers were determined with an X-ray diffraction (XRD) technique using

Cu-Ka radiation at 40 kV and 30 mA (Phillips X’Pert). For the oxidation tests, two

groups of samples were used: bare steels and coatings. These samples were exposed

to air at 1,050 �C for the evaluation of oxidation resistance. The weight gains of the

specimens were determined by measuring specimen weight before and after

oxidation.

Fig. 3 SEM cross-section micrograph of siliconized sample (a) and EDS results showing theconcentration variations of Fe, Cr, Ni, and Si elements near the surface of the aluminized 304 SS (b)

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Results and Discussion

Aluminizing

The coating formed on the surface of SSs by the aluminizing process consisted of

two layers. A cross-section micrograph of the aluminized sample is shown in

Fig. 1a. As can be seen, the total thickness of the coating was about 450 lm. The

concentration profiles of Al, Cr, Fe, and Ni are presented in Fig. 1b. This figure

shows that the quantity of Al at the edge of the second layer was about 42% at.

Figure 2a shows the X-ray diffraction pattern of the first layer of the aluminized

sample. The diffraction pattern indicates the formation of AlFe, Al0.4Fe0.6, AlNi,

and AlCr3.

The results of X-ray diffraction of the second layer of the coating are shown in

Fig. 2b. As illustrated, peaks of AlFe, AlFe3, Al78Fe24, and AlCr2 were identified in

the coating. There were fewer phases observed than in the Fe–Al equilibrium phase

diagram, which might be due to problems with nucleation and growth in most of the

phases [3]. Intermetallics such as Fe3Al and FeAl are unique materials for structural

applications because of their excellent high-temperature oxidation and corrosion

properties [13, 14]. For 304 SS, a pack of low Al such as 35 wt% produces

intermetallic phases such as FeAl, which are favorable for corrosion protection [15].

Therefore, the existence of the FeAl and Fe3Al phases shows that the outward

diffusion of substrate elements has occurred.

Siliconizing

An SEM cross-section micrograph of the siliconized samples is shown in Fig. 3a. It

can be seen that the total thickness of the coating is about 100 lm. The

concentration profiles of Si, Cr, Fe, and Ni are illustrated in Fig. 3b. This figure

shows that the amount of Si at the edge of the coated layer is about 63 at.%. As can

be seen, the Si concentrations vary steadily across the depth of this layer to the

Fig. 4 XRD analysis of siliconized sample

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surface. Figure 4 shows the X-ray diffraction pattern of the layer of the siliconized

sample, exhibiting the presence of Fe3Si, Fe2Si, Cr3Si, and Ni2SiO4. Although the

substrate was austenitic SS, its characteristic peaks were small, indicating that a

rather thick siliconized layer was formed.

Codeposition of Al and Si

With adequate control of the pack powder composition, it was found that

codeposition of Al and Si could also be achieved at 1,050 �C for 6 h (see Table 2).

Fig. 5 SEM Cross-section micrograph of Al–Si coating (a) and EDS results showing the concentrationvariations of Fe, Cr, Ni, Al and Si elements near the surface of the aluminized 304 SS (b)

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Figure 5a, b shows a cross-section SEM image and the concentration profiles of Al,

Si, Fe, Cr, and Ni in the coating layers. Two uniform and distinct layers can be

observed. The total thickness of the layers is about 350 lm; the outer and inner

layers are about 170 and 180 lm, respectively. The element concentration profiles

confirmed that both Al and Si elements were deposited from the vapor phase and

diffused into the substrate, which led to the formation of two uniform coating layers

rich in Al and Si. Figure 6 presents an XRD diffractogram measured from the

as-coated surface shown in Fig. 8. It can be seen that the characteristic peaks are

Al3FeSi, Cr3Si, AlNi2Si, Ni2SiO4, and AlFe.

Oxidation Resistance of the Coatings

The oxidation resistance of SS is due to the formation of Cr2O3 on the surface.

However, this surface layer is destabilized at high temperatures (above 1,000 �C)

and no longer protects the metal [3].

Fig. 6 XRD analysis of Al–Si coating

Fig. 7 Weight change curves in air at 1,050 �C for Al, Si and Al–Si coatings and substrate

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Figure 7 shows the weight change of bare and coated samples oxidized in air at

1,050 �C for 100 h. The oxidation curve of the coatings shows two stages. In the

first stage, there is a sharp increase in oxidation, which is due to nucleation and the

formation of the oxide layer. However, as the oxidation progresses, the protected

oxide thickens; therefore, the oxidation rate decreases in this stage and, as a result,

the shape of the curve becomes parabolic. As seen in this figure, all three coatings

enhanced the oxidation resistance of the steel. The codeposited sample coated with

Al and Si had the lowest weight gain, which means that it exhibited the best

oxidation resistance among the coatings.

Scanning electron microscopy cross-sectional images of coated samples after

100 h of oxidation at 1,050 �C are shown in Fig. 8. As can be seen, there is some

spallation in the siliconized specimen (Fig. 8b), while the aluminized and

Fig. 8 SEM cross section image of the oxidized aluminized (a) siliconized (b) and aluminized–siliconized (c) samples after 100 h isothermal oxidation at 1,050 �C

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codeposited samples exhibited good resistance against spallation and cracking

(Figs. 8a, c) after 100 h of isothermal oxidation. Oxide-scale spallation is a common

problem in materials exposed to high temperatures. Stress generated during

oxidation can delaminate the oxide from the metal because the oxide scales and the

underlying coating invariably have different thermal expansion coefficients [10].

Figure 9 shows an XRD analysis of the surface of the uncoated steels oxidized at

1,050 �C for 100 h. The surface of the bare steel is covered by iron oxides (Fe2O3

and Fe3O4). The low oxidation resistance of the bare steel (Fig. 7) depends on Cr for

the formation of Cr2O3, but Cr2O3 destabilizes at high temperatures (above

1,000 �C) and converts to volatile oxide (CrO3). Therefore, Fe is oxidized rapidly

due to the lack of a protective layer on the surface. Figure 10 shows an XRD

analysis of the surface of the aluminized specimen protected by aluminum oxide

Fig. 9 XRD analysis of bare substrate after 100 h oxidation at 1,050 �C

Fig. 10 XRD analysis of Al coating after 100 h oxidation at 1,050 �C

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(Al2O3), indicating that the good oxidation resistance of the aluminized sample

(Fig. 7) is due to the formation of Al2O3 in the coating. Figure 11 shows the X-ray

diffraction pattern of the siliconized sample oxidized at a temperature of 1,050 �C

for 100 h. As shown, the phases formed after oxidation were FeSiO3, Fe2SiO4,

Ni2SiO4, Cr2SiO4, SiO2, and Fe2O3. This shows that the Si coating did not

completely protect the substrate from high-temperature (1,050 �C) oxidation after

100 h. The XRD results for the pack-treated sample codeposited with Al and Si

after 100 h oxidation at 1,050 �C show the presence of Al2SiO5, Al2O3, Cr2SiO4,

Ni2SiO4, and AlFeO3 (Fig. 12). This coating had the lowest weight gain and

therefore exhibited better high-temperature oxidation resistance (Fig. 7), due to the

presence of Si (or SiO2), which blocked the rapid diffusion of oxygen through the

aluminide layer and thus retarded the oxidation of the substrate [16].

Fig. 11 XRD analysis of Si coating after 100 h oxidation at 1,050 �C

Fig. 12 XRD analysis of Al–Si coating after 100 h oxidation at 1,050 �C

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Conclusions

The coating obtained by aluminizing the AISI 304 SS consisted of two layers. The

total thickness of the layers was about 450 lm. The outer layer consisted of the

AlFe, AlFe3, Al78Fe24, and AlCr2 phases, and the first layer consisted of the AlFe,

Al0.4Fe0.6, AlNi, and AlCr3 phases.

The coating obtained by siliconizing the AISI 304 SS consisted of one layer. The

thickness of the layer was about 100 lm and consisted of the Fe3Si, Fe2Si, Cr3Si,

and Ni2SiO4 phases.

The coating obtained by the codeposition of Al–Si on the AISI 304 SS consisted

of two layers. The total thickness of the layers was about 350 lm, and the coating

consisted of the Al3FeSi, Cr3Si, AlNi2Si, Ni2SiO4, and AlFe phases.

The oxidation resistance of the AISI 430 steel increased with all the coatings, but

the weight gain of the codeposited Al–Si sample was lower than that of the other

coatings, indicating that it provided the highest corrosion resistance among the three

coatings.

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