High temperature corrosion of 12%Cr-Al cast steels

Surface Technology, 22 (1984) 387 - 396

387

HIGH T E M P E R A T U R E C O R R O S I O N OF 12%Cr-A1 CAST STEELS B. ZAGHLOUL, N. NASSIF and A. A. ABDUL AZIM

Central Metallurgical Research and Development Institute, National Research Centre, Dokki, Cairo (Egypt) (Received December 12, 1983)

Summary The influence of aluminium, titanium and niobium on the high temperature oxidation o f F e - 1 2 C r steel was investigated. The steels were tested at 850 °C before and after they were coated with NaC1, Fe203 or Fe203NaC1 mixtures. Titanium and aluminium were found to be highly beneficial. Excellent corrosion resistance under all environments was shown by the steel containing 0.8% Ti. Niobium was detrimental to the corrosion resistance especially in Fe203-20%NaC1. The results were interpreted in the light of the protective nature of the films formed by chromium, titanium and aluminium.

1. Introduction A local Egyptian iron ore contains a relatively high content of NaC1 (up to 1.3%) which presents serious corrosion problems in the sintering plant. The present work was initiated to search for a steel which can withstand the conditions prevailing in the sinter plant. A candidate steel for the sinter grates has to fulfil the following requirements: (1) adequate corrosion resistance at the sintering temperatures ( 7 5 0 - 950 °C) particularly in Fe203 ore contaminated with NaC1 and adequate oxidation resistance in air; (2) considerable resistance to thermal fatigue and thermal shock to withstand thermal cycles usually varying from 100 to 800 °C because of the nature of the sintering process; (3) reasonable high temperature strength; (4) high resistance to mechanical wear to withstand the sand blasting effect of high velocity air passing between the grates and carrying dust and fine sinter; (5) good resistance to corrosion when wet to resist any deposition of acid or alkali during downtime. Chromium is the element generally added to steel to increase its corrosion resistance [1, 2]. This effect can be enhanced by the simultaneous addition of other elements, such as silicon, aluminium or a combination of the two. To combine high resistance to corrosion at high temperatures and high strength with good wear resistance, several additions to 12% Cr steel were tried. Aluminium was added in the order of 2% to improve the 0376-4583/84/$3.00

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388 corrosion resistance at high temperatures. Also, simultaneous additions of titanium and niobium were tried to increase the creep strength and the wear resistance. To simulate the working conditions the salt~oated oxidation test was used. 2. Experimental details The heats were melted in a basic lined 15 KVA high frequency induction furnace; the nominal chemical composition of the heats is shown in Table 1. Each heat was cast into rods 25 mm in diameter in sand moulds. The composition was maintained constant for all the heats at 0.2% C, 0.5% Mn, 0.5% Si, 12% Cr and 4% A1 except for heat 1 which did not contain aluminium and heat 3 which had 1.0% Si. Niobium and titanium were added in three concentration levels for each. The choice of these levels of additions was based on previous investigations [3, 4]. The test coupon had a diameter of 25 mm and a thickness of 0.4 mm, giving a surface area of 12.5 cm 2. The specimens were coated with NaC1, Fe203 or NaC1-Fe203 mixtures using the coating m e t h o d described in detail elsewhere [5]. The oxidation tests were carried out at constant temperatures of 850 and 950 °C for periods of up to 100 h. TABLE 1 Nominal composition of the steels Steel designation

1 2

3 1N 2N 3N 1T 2T 3T

A m o u n t (wt.%) o f the following elements C

Si

Mn

Cr

0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2 0.2

0.5 0.5 1.0 0.5 0.5 0.5 0.5 0.5 0.5

0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

12 12 12 12 12 12 12 12 12

Al

Nb

Ti

--

--

--

4 4 4 4 4 4 4 4

--0.5 0.75 1.00 ----

-----0.2 0.4 0.8

N, niobium steel; T, titanium steel. 3. Results 3.1. O x i d a t i o n o f u n c o a t e d s t e e l

Figure 1 shows mass gain versus time curves for the nine steel samples (see Section 2) heated at 950 °C for 100 h. It can be seen that the gain in mass is a linear function of time, the slope being very high for steel 1. The corrosion rate decreases in the order I > 3 N > I N > 2N> 1T>2>2T, 3 > 3T, excellent corrosion resistance being shown by steel 3T.

389

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5

c"

20

40

60

80

100

Time , h

Fig. 1. Oxidation of various steels in air at 950 °C: e, steel 1; ×, steel 2; ®, steel 3; D, steel 1N;A, steel 2N; *, steel 3N; m, steel 1T; A, steel 2 T ; - - - - - - , steel 3T.

3.2. Oxidation o f Fe203-coated steel samples T h e mass gain versus time plots o b t a i n e d f o r the F e 2 0 3 - c o a t e d steel samples h e a t e d at 8 5 0 °C are s h o w n in Fig. 2. T w o groups o f curves can be identified. A f t e r a p r e l i m i n a r y small gain in mass during the first 20 h, steels I T , 2 T and 3 T attain an a l m o s t c o n s t a n t value, which is m a i n t a i n e d f o r t h e n e x t 80 h. T h e initial rise is smallest f o r steel 3 T and the c o r r o s i o n rate during the following p e r i o d is very slight f o r steel 2 T and is a l m o s t zero f o r steels 3T and I T . T h e o t h e r g r o u p o f curves (steels I N - 3N and 1 and 3) s h o w an initial loss in mass f o l l o w e d b y a very small mass gain. Figure 3 shows t h e results o b t a i n e d f o r t w o representatives o f the t i t a n i u m g r o u p (1T and 2T) and o n e r e p r e s e n t a t i v e o f the n i o b i u m g r o u p w h e n t h e F e 2 0 3 - c o a t e d specimens are h e a t e d at 950 °C. T h e t h r e e curves reveal a parabolic increase in mass w h i c h leads t o a c o n s t a n t value. This is a t t a i n e d rapidly and is very low f o r steel 2T; steels 1T and 2N s h o w somew h a t higher s t e a d y state values.

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Fig. 2. Oxidation of various steels coated with Fe203 at 850 °C: e, steel 1; ®, steel 3; 6, steel 1N; ~, steel 2N; *, steel 3N; m, steel 1T;4, steel 2 T ; . , steel 3T. 1,25

1.00

0.75 e-

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Fig. 3. Oxidation of various steels coated with an Fe203 film at 950 °C: •, steel 2N; I , steel 1T; ,~, steel 2T.

Metallographic examination of two representatives of the titanium and niobium group steels {Figs. 4(a) and 4(b) respectively) reveals attack along the grain boundaries for the titanium group steel and some voids in the bulk alloy for the niobium group steel. 3. 3. Oxidation o f NaCl-coated steel samples The mass gain versus time plots obtained for the NaCl-coated samples heated at 850 °C for 100 h are shown in Fig. 5. The curves generally reveal

391

(a)

(b)

Fig. 4. Micrographs of 12%Cr-Al steel containing (a) 0,8% Ti and (b) 1.0% N b coated with Fe203 and oxidized at 850 °C for 100 h. (Magnifications, 55)<.)

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3

0

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f

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ALA/ •

,

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--

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*=* - - -

i

f

2O

40

60

80

I00

Time , h

Fig. 5. Oxidation of various steels coated with an NaCI film at 850 °C" e, steel 1; x, steel 2; o, steel 3; r~,steel 1N; •, steel 2N; *, steel 3N; m, steel 1T; 4, steel 2T;., steel 3T.

392 a paralinear b e h a v i o u r and the c o r r o s i o n rate is highest for steel 1 and is almost zero f o r steel 3T. F o r the rest o f the samples, the c o r r o s i o n rate decreases in the o r d e r 3N > 1N > 2N ~ 2 > 3 > 2 T > 1T, i.e. it is highest f o r t h e n i o b i u m g r o u p and lowest f o r the t i t a n i u m group. Micrographs were o b t a i n e d f o r representatives o f the t i t a n i u m , the n i o b i u m and t h e a l u m i n i u m - f r e e groups (Figs. 6(a), 6(b) and 6(c) respectively) c o a t e d with NaC1 and h e a t e d at 8 5 0 °C f o r 100 h. It can be seen t h a t the a t t a c k is least for the t i t a n i u m and greatest f o r the aluminium-free group. H o w e v e r , signs o f internal o x i d a t i o n are m o r e evident f o r the tit a n i u m group t h a n f o r t h e t w o o t h e r groups.

(a)

(b)

(c) Fig. 6. Micrographs of (a) 12%Cr-A1 steel with titanium added, (b) 12%Cr-Al steel

with niobium added and (c) aluminium-free steel coated with NaC1 and oxidized at 850 °C for 100 h. (Magnifications, 58×.)

3.4. Oxidation o f the steel samples coated with Fe203-NaCl mixtures T h e steels c o a t e d with an Fe203-10%NaC1 m i x t u r e and h e a t e d at 8 5 0 °C for 100 h (Fig. 7) reveal an initial s u d d e n j u m p which is highest f o r steel 3N and lowest f o r steel 3. T h e initial rise leads to an a l m o s t zero c o r r o s i o n rate (steels 3 and 3T) or to a small linear increase in mass (steels 3N and 1N). T h e c o r r e s p o n d i n g results o b t a i n e d f o r f o u r steels c o a t e d with an Fe203-20%NaC1 m i x t u r e are s h o w n in Fig. 8. Samples 3 and 3 T show a

393

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0.5

f

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f 0

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A

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20

i

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40 Time , h

i

i

60

i

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80

100

Fig. 7. Oxidation of various steels coated with Fe20~--10%NaCt at 850 °C: o, steel 3; el, steel 1N; *, steel 3N; -, steel 3T.

5

4

%

2

1

. . . . . . . . . . -0 . . . .

I

I

20

I

-~-

I

. . . . . . .

I

I

40 60 Time , h

• . . . . . . .

I

l

80

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-

--

I

I00

Fig. 8. Oxidation of various steels coated with Fe203-20%NaC1 at 850 °C: ®, steel 3; ~, steel 1N; *, steel 3 N ; . , steel 3T.

394 similar behaviour to that observed for the 10% NaC1 mixture. In contrast, steels 3N and 1N reveal a high continuous mass gain throughout the experiment, reaching more than 5 g dm -2 compared with 0.25 g dm -2 for steel 3. The micrographs obtained for steels 3T and 3N coated with Fe203-20%NaC1 and heated at 850 °C for 100 h {Figs. 9(a) and 9(b) respectively) show internal attack in both cases. However, the thickness of the attacked layer is greater for steel 3N than for steel 3T.

(a) (b) Fig. 9. Micrographs of 12%Cr-A1 steel containing (a) 0.8% Ti and (b) 1.0% Nb coated with Fe203-20%NaC1 and oxidized at 850 °C for 100 h. (Magnifications, 58×.)

4. Discussion

4.1. Oxidation of uncoated alloys The foregoing results show t h a t the resistance to high temperature oxidation is highest for alloys containing titanium in addition to chromium, aluminium and silicon. The resistance increases with increases in the titanium content from 0.2% to 0.8%. Titanium is known [6] to be beneficial, promoting protective oxide scales. The role of titanium has been related to the ability of titanium-containing alloys to form a c o m p o u n d oxide, CrTi203 spinel. On the basis of his work on Co-25%Cr alloys containing 2.5% - 10% Ti, Dahshan [6] suggested t h a t titanium reduced the oxygen activity at the surface of the alloy, preventing the formation of cobalt-containing oxides, which might otherwise disrupt the scale. A comparison of the corrosion rates of steel 1, on the one hand, and steels 2 and 3, on the other hand, reveals the beneficial effect of aluminium. It is well known [7] that addition of aluminium improves the scaling and oxidation resistance of ferrous alloys. The main reason for this increase in oxidation resistance is the formation of a thin and non-porous tightly adherent protective oxide film of A1203. X-ray examination of the white scale formed on Fe-A1 alloys containing more than 14% A1 showed that it was often entirely alumina [8, 9].

395 Addition of niobium counteracts the beneficial effect of aluminium as implied b y the relatively high oxidation rates of steels 1N, 2N and 3N. Alloy 3N containing the highest amount of niobium suffers most. These observations are supported by the results of earlier studies which showed that the addition of niobium to C o - C r base alloys was detrimental to the oxidation resistance. Niobium [10, 11] does n o t form c o m p o u n d oxides which help in reducing the oxygen activity; a layer of Cr203 externally does n o t readily form under such conditions.

4.2. Oxidation o f Fe203-coated steels The oxidation behaviour of the Fe203-coated alloys heated at 850 °C may be explained on the basis of the shielding effect caused by the oxide coating. Reduction in the oxygen activity at the alloy surface allows the rapid establishment of a protective Cr203 layer, the formation of which is completed, for alloys containing titanium, during the first 20 h, whereupon the oxidation rate drops to zero. Regarding the rest of the alloys, the delay in the establishment of the film allows for the escape of carbon resulting from the decomposition of carbides. The voids left behind are clearly shown by the micrograph obtained for steel 3N. The corresponding micrograph obtained for steel 2T does n o t show such voids. The behaviour of the Fe203-coated alloys at 950 °C demonstrates the beneficial effect of titanium, the effect being enhanced by increases in the titanium content in the alloy. The change in the oxidation process from linear to parabolic may be ascribed to the slow diffusion through the coating. 4.3. Oxidation o f NaCl-coated steels Alloy 3T containing 0.8% Ti develops a totally protective film after a b o u t 30 h. During this initial period, the oxidation is parabolic and proceeds at a relatively high rate compared with the corresponding Fe203coated or uncoated samples. The paralinear behaviour shown by the other alloys may be ascribed to the initially developed cracks. Cracking of the initially formed film is most pronounced for alloy 1 (which does n o t contain aluminium or titanium) and for the alloys containing niobium. The beneficial effect of aluminium and titanium and the deteriorating effect of niobium are again clearly demonstrated. The relatively high corrosion rates for the NaCl-coated alloys may be correlated with the establishment of local cells with the molten NaCl as the electrolyte and oxygen as the cathodic reactant. 4.4. Oxidation o f FezO3-NaCl-eoated steels The oxidation rates obtained for alloys coated with Fe~O3-10%NaC1 mixtures are generally low probably because the shielding effect is stronger than the attack b y NaC1. The deteriorating effect of niobium reflects itself

396 in the initial non-stationary corrosion, and is far more p r o n o u n c e d for the alloy containing 1% Nb than f or t h a t containing 0.5% Nb. In contrast, the beneficial effect of titanium and aluminium is well illustrated by the zero corrosion rate prevailing for steels 3T and 3 after 20 h and 2 h respectively. Increasing the NaC1 c o n t e n t of the oxide coating does n o t have any significant ef f ect on the behaviour of the alloys containing titanium a n d / o r aluminium. Although the mass gain increased f r om 0.15 to 0.6 g d m -2 and from 0.1 to 0.2 g d m -2, this initial gain was followed by a zero corrosion rate. Th e detrimental effect of niobium results in relatively high corrosion rates f o r steels 1N and 3N; the rate for steel 3N is higher than that for steel 1N. The voids shown by the micrographs obtained for steels 3N and 3T may be ascribed to the escape of carbon during the initial stages. The marked increase in the corrosion rate caused by the i n t r o d u c t i o n o f 20% NaC1 into the Fe203 may be correlated with the establishment of local cells. Molten NaC1 may percolate through the oxide coating, forming a c o n t i n u o u s film beneath it. The ferric ions probabl y leached by the m ol t en NaC1 act as cathodic depolarizers, counteracting the shielding effect of the coating.

5. Conclusions (1) The resistance of F e - 1 2 C r - 4 A 1 alloys coat ed with NaC1, Fe203 or Fe203-NaC1 to oxidation and corrosion at high temperatures is markedly enhanced by additions of titanium and impaired by additions of niobium. (2) In Fe203-NaC1 coatings, the ferric ions are leached by the m o l t e n NaC1 and enhance corrosion by acting as cathodic depolarizers.

References

1 L. L. Shreir (ed.), Corrosion, Vol. 1, Newnes, London, 1963, p. 756. 2 L. Colombier and J. Hochmann, Stainless and Heat Resisting Steels, Arnold, London, 1967, p. 326. 3 J. E. Truman and K. R. Pirt, Br. Corros. J., 11 (1976) 188. 4 B. Zaghloul, T. Shinoda and R. Tanaka, in B. H. Kear, D. R. Muzyka, J. K. Tien and S. T. Wlodek (eds.), Proc. 3rd Int. Syrup. on Superalloys, Metallurgy and Manufacture, Seven Springs, PA, 1975, Claitor's Publishing Division, Baton Rouge, LA, 1976, p. 265. 5 D, M. Johnson, D. P. Whittle and J. Stringer, Corros. Sci., 15 (1975) 649. 6 M. E. Dahshan, Trans. Jpn. Inst. Met., 22 (1981) 25. 7 A. M. Portevin, E. Pretet and H. Jolivet, J. Iron Steel Inst., London, 130 (1934) 219. 8 H. Von Schwarze, Mitt. Forschungsinst. Vet. Stahlwerke Ag., Dortmund, 2 (1932) 263. 9 C. Sykes and J. W. Bampfylde, J. Iron Steel Inst., London, 130 (1934) 389. 10 J. E. Truman and K. R. Pirt, Br. Corros. J., 13 (3) (1978) 136. 11 G. N. Irving, J. Stringer and D. P. Whittle, Corros. Sci., 15 (1975) 337.