THE EFFECT OF SILICON AND ALUMINUM ADDITIONS ON THE OXIDATIONRESISTANCE OF LEAN CHROMIUM STAINLESS STEELS

J. S. Dunning, D. E. Alman, and J. C. RawersUnited States Department of Energy

Albany Research Center1450 Queen Avenue, SW

Albany, OR 97321

ABSTRACT

The effect of Si and Al additions on the oxidation of lean chromium austenitic stainless steels has beenstudied. A baseline composition of Fe-16Cr-16Ni-2Mn-1Mo was selected to allow combined Si and Aladditions of up to 5 wt. pct. in a fully austenitic alloy. The baseline composition was selected using a netCr equivalent equation to predict the onset of G-ferrite formation in austenite. Cyclic oxidation tests in airfor 1000 hours were carried out on alloys with Si only or combined Si and Al additions in the temperaturerange 700°C to 800°C. Oxidation resistance of alloys with Si only additions were outstanding,particularly at 800°C. It was evident that different rate controlling mechanisms for oxidation wereoperative at 700°C and 800°C in the Si alloys. In addition, Si alloys pre-oxidized at 800°C, showed a zeroweight gain in subsequent testing for 1000 hours at 700°C. The rate controlling mechanism in alloys withcombined Si and Al addition for oxidation at 800°C was also different than alloys with Si only. SEM andESCA analysis of the oxide films and base material at the oxide/base metal interface were conducted tostudy potential rate controlling mechanisms.

INTRODUCTION

For increased thermal efficiency of fossil fuel power plants, there is a need for economic lean chromiumaustenitic steels with improved creep properties and oxidation/corrosion resistance compared toconventional stainless steels. It is not possible to reduce the Cr content of conventional stainless steelsand, either from theory or empirically derived relationships, select alloy elements and concentrations thatwill provide equivalent or improved oxidation resistance.(1)

Protection for Fe-Cr alloys, at 16 wt. % Cr or greater, is usually attributed to the formation of dense,adherent chromia through which the outward diffusion of Cr controls the oxidation process.(2)(3) Theoxidation resistance of non-Cr, alloys Fe-Al and Fe-Si depend on the formation of Si or Al oxides that areas protective, if not better than the oxide protection of Fe-Cr alloys.(4)(5)(6) Additions of Al and Si, whenalloyed together, have been reported to have synergistic effects on oxidation.(7)(8) It is apparent that minoralloying additions of Al or Si can play a significant role in the oxidation process of Fe-Cr alloys.Relatively small additions of Si can have a beneficial effect on oxidation resistance due to the reportedformation of a layer of SiO2 at the surface.(9–11) It has been suggested this film can act as a diffusionbarrier(9),(12) leading to lower oxidation rates or by promoting the formation of protective outer chromiafilms.(11),(13) The exact oxidation mechanism in any given Fe based alloy will depend on the protectiveoxide films that are stabilized, which, in turn, will depend on the exact concentration of the activediffusing additions Cr, Mn, Al, Si and O.

The Albany Research Center has a substantive background in developing low chromium oxidation andcorrosion resistant alloys as substitutes for conventional stainless steels.(14–22) This current paper presentsthe results of a study of lean chromium (16 wt. pct. Cr) stainless steels with Si additions or combined Siand Al additions. The purpose of the study was to measure the oxidation resistance and determine the

effect of Si and mixed Si and Al additions on the oxidation rates and on the underlying mechanismscontrolling these rates.

EXPERIMENTAL PROCEDURE

The formation of δ-ferrite in austenitic alloys was predicted using the concept of Cr equivalence of multicomponent alloys. In this research the Cr and Ni equivalence equations of Schneider and Pickering wereused together with a net Cr equivalence equation to predict the onset of ä-ferrite formation. Thus, for thealloying elements of concern:

Cr eq = Cr + 1.5 Mo + Si + 1.5 AlNi eq = Ni + 0.5 Mn

andnet Cr eq = Cr eq – 0.69l Ni eq

δ ferrite formation occurs when net Cr eq <10 percent.

The coefficients ahead of each alloying element are very composition sensitive and the coefficients for Siand Al in the above equations were selected and used specifically for the 16 Cr, 16Ni base composition.

Alloys were melted by vacuum induction or air induction melting with an argon shield. Ingots were rolledto 12.7 mm plate at 1075°C and then austenitized at 1200°C for 1 hour and fan cooled. For oxidationstudies, samples with nominal dimension of 25.4 × 25.4 × 7.6 mm were machined from the annealed plateand polished to a 400 grit finish.

Oxidation of the samples were carried out in laboratory air at 700°C and 800°C. Weight changes weremeasured at regular intervals after removing the samples from the furnace and cooling to roomtemperature. Chemical compositions were determined by wet chemical analysis and phase analysis wasconducted by x-ray diffraction (XRD). Local elemental analysis of the oxide films and the bulk metal atthe interface were conducted by electron spectroscopy for chemical analysis (ESCA).

A bromine etching technique and scanning electron microscopy (SEM) were used to study themorphology of the oxide films at the oxide/metal interface.

RESULTS

Two series of alloys were melted. The first series was melted as 1 kg. charges. The nominal compositionsare shown in Table 1. The actual compositions, as determined by wet chemical analysis, are shown inTable 2. Table 2 also shows the magnetic response of the alloys indicating the possible presence of ferritein samples that were magnetic. X-ray diffraction of the austenitized plate confirmed slight traces of ferritein the 2Si-2Al composition. This composition was omitted from a second series of 4.5 kg. ingots. Allcompositions in the second series of ingots were non-magnetic.

Cyclic oxidation tests were conducted at 700°C and 800°C in air. Specimens of a conventional 18Cr-8Nitype 304 stainless steel were included as a standard. Results for the 1000 hour tests at 700°C are shown inFig. 1. The baseline Fe-16Cr-16Ni-2Mn-1Mo composition shows the highest rate of oxidation which issignificantly higher than the higher chromium 18Cr-8Ni standard. Alloys with Si only additions (Group Balloys) and Si combined with Al additions (Group A alloys) both showed weight gains approximately halfthat of the 18Cr-8Ni alloy. All data for Group A and Group B fell within the boundary curves shown in

Table 1. Nominal composition (wt pct)Alloy Fe Cr Ni Mn Mo Si Al

A bal 16 16 2 1 0 0B bal 16 16 2 1 3 0C bal 16 16 2 1 3 1D bal 16 16 2 1 2 0E bal 16 16 2 1 2 1F bal 16 16 2 1 2 2G bal 16 16 2 1 1 1

Table 2. Actual compositions (wt pct)Alloy Fe Cr Ni Mn Mo Si Al Magnetic response

A bal 15.7 16.2 1.5 0.6 0 0 NMB bal 16.1 16.3 2.0 0.7 2.9 0 NMC bal 14.9 16.0 2.0 0.6 2.9 0.8 SMD bal 15.4 16.0 2.0 0.7 2.0 0 NME bal 16.0 16.1 1.9 0.8 2.1 1.0 NMF bal 15.5 16.2 2.1 0.8 2.1 2.0 SMG bal 15.6 16.0 1.9 0.8 1.1 1.1 NM

Magnetic response: NM = non-magnetic; SM = slightly magnetic.

Fig. 1 for these alloys. All alloys in Group A and Group B show a significant improvement(approximately 2 times in terms of weight gain) in oxidation resistance compared with the higher Cr type304 stainless steel.

Figure 2 shows results for cyclic oxidation at 800°C for the same alloy series. It should be noted thatis a very large increase in temperature within this temperature range. The first point to note is that

Group A alloys (Si plus Al combinations) and Group B alloys behave differently at 800°C. Group A

Fig. 1. Oxidation Behavior at 700°C Fig. 2. Oxidation Behavior at 800°C.

alloys show weight gains approximately 2 times greater that the standard 18Cr-8Ni while Group B alloysshow weight gains 4 times less after 1000 hours at 800°C. Clearly, at 800°C, different oxidationmechanisms are operative in Group A (Si + Al) and Group B alloys (Si).

The two Group B alloys with 2 percent Si and 3 percent Si additions were oxidized for 175 hours at800°C prior to a 1000 hour exposure at 700°C with standard type 304 stainless and Group B alloys thathad not been pre-oxidized. Data are shown in Fig. 3. The Group B alloys (not pre-oxidized) and the type304 standard showed the same relationship as the earlier 700°C tests. However the pre-oxidizedspecimens showed zero weight gain after 1000 hours at 700°C.

Fig. 3. Oxidation Behavior of Pre-oxidized and Non-pre-oxidized Alloys at700°C

It is evident that Si is very effective in providing oxidation protection at 800°C. Aluminum disrupts thiseffectiveness at 800°C. Oxide film thickness in Group B alloys after 1000 hours at 800°C range from 2 to5 µ, while after pre-oxidation and 1000 hours at 700°C they range from 1 to 6 ì. In Group A alloys, after1000 hours at 800°C, oxide film thickness of 5 to 15 µ are observed.

Tables 3 and 4 show ESCA analysis of material adjacent to the oxide/metal interface for one Group Aalloy (2Si-2Al) and one Group B alloy (3Si) The specimens had undergone oxidation testing at 800°C for1000 hours. Analyses of the bulk material is shown together with analyses close to the interface of thebase metal and the oxide film. Analyses are shown in atomic percent for each element detected and isbroken down into atomic percent at various binding energies. As a rule, the lowest binding energyobserved indicates the elemental metal while higher binding energies represent oxides of varyingcomplexity. In both alloys, the oxide is rich in Cr and Mn leading to a corresponding depletion of Cr andMn in the base metal close to the interface. In the case of Mn, the depletion is almost complete. In the Sionly alloy, a significant buildup of elemental Si occurs in the base metal near the interface (10.6 atomicpct. vs 5.6 atomic pct. in the bulk metal). A smaller percentage of silicon is observed in the oxide film asan oxide. In the Si-Al alloy, a similar concentration of Si occurs close to the interface in the base metal,again as elemental silicon. There is greater participation of Si in the oxide film. A similar or even greater(14.8 atomic pct. vs. 4.4 atomic pct. in the bulk metal) concentration of Al occurs in the base metaladjacent to the interface. However, the Al concentration is present as an oxide. There is a correspondingoxygen concentration (16.7 atomic percent) associated with the aluminum. This is consistent with discreteinternal oxidation of aluminum resulting in discrete Al203 particles within the base metal and has been

Table 3. ESCA Data for 2Si-2Al Alloy at 800°CComposition base Composition (atomic %)

ElementWeight % Atomic % Base metal Oxide film

Coments

29.2 2.7Fe 59.9 57.3

7.1 1.7

Low Fe conc., near interfacereflect conc. In base of silicon,aluminum, oxygen, carbon

8.1Cr 16.1 16.5

1.5 21.0Cr depletion in base, heavyconcentration in oxide film

6.7 1.2Ni 16.3 14.8

No significance

Mn 2.3 2.20 9.0

Complete Mn depletion in basenear interface, heavyconcentration in oxide film

1.3 0.2Mo 1.1 0.6

No significance

9.6 1.0Si 2.1 3.96

4.5Si concentration as oxide inbase metal near interface

Al 2.2 4.414.8 2.3

Al concentration as oxide inbase metal near interface

16.7 48.8O – –

3.25.1 4.3

C – –No significance. Buildup of Cdata typical of ESCA

Table 4. ESCA Data 2Si Alloy at 800°CComposition base Composition (atomic %)

ElementWeight % Atomic % Base metal Oxide film

Comments

45.9 3.8Fe 62 60.2

9.7 3.410.4 2.3

Cr 16.1 16.81.3 18.3

Cr depletion at interface inbase metal heavyconcentration in oxide film

9.6 1.6Ni 16.3 15.1

2.0No significance

Mn 2.0 1.90 11.3

Complete Mn depletion inbase metal heavyconcentration in oxide film

Mo 0.7 0.40 0.1

10.6 1.1Si 2.9 5.6

1.9Heavy Si concentration in basemetal near interface

Al – –

4.4O – –

1.3 50.52.7 5.8

C – –0.8

No significance. Buildup of Cdata typical of ESCA

observed in prior oxidation studies with low chromium alloys with Al and Si additions of thismagnitude.(18)

A definitive interpretation of these results would be premature. Oxidation protection for a 16 wt. pct. Cralloy would usually be attributed to a dense, adherent chromia film controlled by the outward diffusion ofchromium. However, silicon additions are obviously either acting as a diffusion barrier or in some mannerfavoring the formation of a thin, adherent chromia layer. On the other hand, Al is disrupting thismechanism.

Several studies are underway to provide more information to determine how Si additions can best be usedwith these alloys. Two fortuitous factors will be of great importance in these studies; (a) the sharp changein oxidation mechanism occurring between 700°C and 800°C, and (b) the mechanistic changes betweenSi only and Si-Al alloys.

Preliminary SEM studies have been useful in determining different oxide film thickness in the alloys. Inaddition, the use of brominated solvent techniques to study oxide structures right at the oxide metalinterface have suggested some intriguing possibilities. Brominated solvents are used to etch away themetal from beneath the oxide film and reveal the three dimensional structure of the oxide film right at theinterface. Significant differences are observed between the Si only and Si-Al alloy interfaces. A typicalmicrograph of a Si only composition (3 wt. pct. Si) after 1000 hours at 800°C is shown in Fig. 4. Withinthe grain boundaries at the interface, the scratch marks of the original 400 grit surface finish are clearlyvisible. The main oxide is very rich in Cr with little Fe or Mn. Also clearly visible, is a network of Si richmaterial at the grain boundaries and Si rich pegs at the grain boundary triple points. It is possible thatthese structures, not detected in Si-Al compositions, point to the effect of Si being attributable to an effecton grain boundary diffusion. Studies are continuing to answer some of these questions.

Fig. 4. Scanning Electron Micrographs of oxide surface at the oxide/metal interface afterbromide etching of the metal.

CONCLUSIONS

x A significant change in rate controlling mechanism in the oxidation of lean chromium alloys with Siadditions occurs in alloys oxidized at 700°C and 800°C.

x Oxidation characteristics in alloys with Si additions are excellent at 800°C.

x These oxidation characteristics at 800°C are disrupted by small Al additions.

x Alloys with Si additions, pre-oxidized at 800°C, show no significant weight gain during subsequent1000 hour oxidation tests at 700°C.

x These marked transitions in oxidation behavior open avenues to research to objectively determine themechanisms by which very small silicon additions can result in very significant gains in oxidationresistance. This information can be of great value in utilizing this technology most efficiently and inproviding basic alloy design data.

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