Corrosion behaviour of various model alloys with NaCl–KCl coating

Materials Chemistry and Physics 93 (2005) 217–223

Corrosion behaviour of various model alloys with NaCl–KCl coating Y.S. Li a,b,∗ , M. Spiegel a , S. Shimada b a b

Max-Planck-Institut f¨ur Eisenforschung GmbH. Max-Planck-Straße 1, D-40237 D¨usseldorf, Germany Division of Materials Engineering, Hokkaido University, North 13, West 8, 060-8628 Sapporo, Japan Received 13 April 2004; received in revised form 3 February 2005; accepted 17 March 2005

Abstract The hot corrosion behaviour of various Fe–Cr, Fe–Al and NiAl model alloys was investigated in air with a surface deposit of NaCl–KCl melt. The results showed that Al is very beneficial by improving the corrosion resistance of Fe–Al alloys with increasing Al content, while Cr exhibits a detrimental effect on corrosion of Fe–Cr alloys. Metallic Ni remains relatively stable and NiAl alloy presents the best corrosion resistance comparing with the other materials. The degradation mechanism of these materials was clarified with respect to the different thermal–chemical stability of metallic Cr, Al and their oxides and chlorides, particularly by considering the high reactivity of Cr and C2 O3 with alkali chloride salts. The corrosion trend was basically very similar to the behaviour caused by individual KCl or NaCl. © 2005 Elsevier B.V. All rights reserved. Keywords: Alloy; Hot corrosion; Chlorination; NaCl; KCl

1. Introduction Sodium and potassium impurities presented in the form of chloride or sulfates are very corrosive constituents under certain combustion conditions such as waste incinerators and biomass-fired boilers [1,2]. Early failure of the thermal components frequently occurs due to the complex reactions between the metallic materials and the hostile combustion environment. It is important to clarify the degradation mechanism of the high temperature materials by typical salts, so as to reduce the tube material consumption by developing more protective structural materials and coatings, with an ultimate goal to increase the energy recovery efficiency. The influence of individual KCl, NaCl and their mixtures with heavy metal chlorides or sulfates on the corrosion behaviour of a series of alloy systems has been studied in detail so far [3–9]. It is generally realized that Cr is not an effective element for corrosion resistance improvement of Fe-base and Ni-based alloys due to chloride salt attack. In contrast, alumina-forming alloys exhibit promising candidate materials considering the bettter high temperature corroison resis∗

Corresponding author. Tel.: +81 117067113; fax: +81 117066576. E-mail address: yuanshi [email protected] (Y.S. Li).

0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.03.015

tance of alumina over chromia and their properties such as lower cost, low density, high strength and good wear resistance [10–16]. This study described the synergitic effect of a NaCl–KCl melt deposit on the degradation of some Fe–Cr, Fe–Al and Ni–Al alloys. The results were compared with what have been previously obtained from individual KCl or NaCl.

2. Experimental The materials used in this study were pure Ni, NiAl and several Al/Cr modified Fe-based alloys, with nominal chemical compositions listed in Table 1. All the materials were machined into specimens with dimensions of about 10 mm × 15 mm × 1.5 mm, then ground to 600# SiC paper and subsequently cleaned in a supersonic bath of acetone. The dried chloride powders were firstly weighted in desired 1:1 mole ratio then subjected to a mechanical milling in an agate mortar to obtain well-mixed reagents. Only one side of the major surfaces of each dried sample was coated with a 60 mg cm−2 NaCl–KCl mixture by piling the sample surface uniformly with the powder mixtures. The chloride mixture has a eutectic point of 659 ◦ C and becomes molten

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Y.S. Li et al. / Materials Chemistry and Physics 93 (2005) 217–223

Table 1 Nominal chemical composition of the materials (at.% except Fe–Cr in wt.%)

Ni NiAl Fe–10Al Fe–20Al Fe–45Al Fe–21Cr–5Al Fe–35Cr

Fe

Al

– 90 80 55 bulk bulk

50 10 20 45 5 –

Ni bulk 50 – – – –

Cr – – – – 21 35

Fig. 2. Mass losses for various materials exposed beneath a NaCl–KCl (1:1 molar ratio) deposit in air for 48 h at 670 ◦ C.

dispersive analysis (EDX), X-ray diffraction (XRD) and electron probe microanalysis (EPMA). The mass changes of the samples were measured before the test and after removal of the corrosion products by chemical etching in an alkaline KMnO4 solution at 80 ◦ C and inhibited HCl solution, respectively.

3. Results

Fig. 1. NaCl–KCl phase diagram.

under employed temperature [17], as seen in Fig. 1. During corrosion, the salt melt can spread out and cover the whole surfaces of each sample, supporting a hot corrosion process [7]. The samples were then horizontally placed inside an alumina crucible for the exposure experiment, which was kept at 670 ◦ C in an air atmosphere for 48 h, using a horizontal furnace equipped with a quartz working tube. After corrosion, metallographic cross sections were prepared by dry grinding of the corroded sample in order to prevent chloride products being removed from the scale. The morphological and compositional analyses were carried out using scanning electron microscopy (SEM) with energy-

Fig. 2 shows the mass loss of the different materials after exposure beneath a molten NaCl–KCl deposit for 48 h at 670 ◦ C. It can be seen that Fe–35Cr suffers from the greatest mass loss, while Ni shows less mass loss than the Fe–Cr alloys and NiAl shows the smallest loss. For Fe–Al alloys, an increase in Al content results in a significantly improved corrosion resistance. The surface scale formed on pure Ni is composed of porous NiO (Fig. 3a). This layer shows an irregular thickness distribution in parallel with the matrix and local attack deep inside the matrix. For NiAl alloy, the salts are not completely consumed after the exposure test and still remain largely on the alloy surface. Aluminium oxide is presented primarily at the salt/substrate interface, underneath an Al-depleted Ni layer is observed with some voids (Fig. 3b). The two lower Al-content Fe–Al alloys (Fe–10Al and Fe–20Al) form a double-layered scale, including an outermost layer consisting of nearly pure Fe2 O3 and an inner layer

Fig. 3. Cross sections of pure Ni (a) and NiAl alloy (b) exposed beneath a NaCl–KCl deposit in air for 48 h at 670 ◦ C.

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Fig. 4. Cross sections of Fe–10Al (a), Fe–20Al (b) and Fe–45Al (c and d) exposed beneath a NaCl–KCl deposit in air for 48 h at 670 ◦ C.

of a mixture of iron and aluminium oxides (Fig. 4a and b). In addition, some KCl is detected in the inner layer for Fe–10Al, but not for Fe–20Al. Aluminium oxide formed in the inner layer is significantly greater for Fe–20Al than for Fe–10Al. Three layers have grown on the Fe–45Al alloy (Fig. 4c); the outermost layer is a mixture of residual salt and aluminium oxide, the intermediate layer being a mixture of the oxides of Fe and Al plus some bright metallic iron particles, and the innermost layer in contact with the matrix is very rich in iron with depletions in aluminium. The presence of this innermost layer implies that the aluminium has been largely removed from this area. The amount of the residual salt on the surfaces of Fe–Al alloys becomes greater with an increasing Al concentration. For Fe–10Al, very little salt remains on the corroded sample surface but, in contrast, voluminous salt crystals are visible on Fe–45Al surface (Fig. 4d), as confirmed by XRD analysis (Fig. 5a). A pre-oxidation treatment

Fig. 5. XRD patterns of the surface reaction products exposed beneath a NaCl–KCl deposit in air for 48 h at 670 ◦ C. (a) Fe–45Al and (b) Fe–20Al with pre-oxidation treatment.

on Fe–Al alloys was found to affect the following corrosion process significantly. For example, one Fe–20Al sample is firstly air heated for 10 h at 900 ◦ C to form a dense and continuous alumina scale, then subsequently subjected to the salt exposure test under the same condition as described above. The pre-formed alumina scale is scarcely destroyed by the salt and can remain its original structure (Fig. 5b). Moreover, this alumina scale seems to have a very poor wetability with the salt, which shows an isolated distribution on the oxide surface (Fig. 6). This implies the very noble character of alumina with respect to the salt attack. The scale formed on Fe–35Cr is rather thick and porous (Figs. 7 and 8). After 10 h exposure (Fig. 7a), the surface products consist mainly of Fe2 O3 and the chromates of sodium and potassium, while no chromium oxide is detected. After reaching 48 h, only Fe2 O3 remains at the scale–gas inter-

Fig. 6. Surface image of pre-oxided Fe–20Al followed exposed beneath a NaCl–KCl deposit in air for 48 h at 670 ◦ C.

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Fig. 9. Cross section of Fe–21Cr–5Al exposed beneath a NaCl–KCl deposit for 48 h at 670 ◦ C.

Fig. 7. XRD patterns of reaction products formed on Fe–35Cr exposed with a NaCl–KCl deposit at 670 ◦ C. (a and b) as corroded surface scale after 10 and 48 h, respectively, (c) inner side of the spalled external scale after 48 h.

of the scale in contact with the matrix; small amounts of potassium and chromium plus oxygen are incorporated in this layer.

4. Discussion face (Fig. 7b), underneath is a mixture of iron and chromium oxides (Fig. 7c). The EPMA and EDX data coincide well with this analysis. It is naturally assumed that the chromates formed in the initial stage of corrosion have been incorporated into the rapidly growing external scales with a fine dispersion. In particular, a chromium chloride layer is identified at the scale–substrate interface, as shown by EDX in Fig. 8c and d. Multi-layered scales are formed following corrosion of Fe–21Cr–5Al (Fig. 9). An outermost thin layer contains an iron oxide Fe2 O3 and an intermediate thin layer is composed of more aluminium and chromium oxides but less iron oxides. Iron was the dominant component in the innermost layer

The corrosion results described above imply clearly that a high Al content for Fe–Al and NiAl alloys is very effective in improving the corrosion resistance against NaCl–KCl melt attack, while Cr plays a detrimental role under the same conditions. This behaviour is close to what has been known on the individual attack by KCl and NaCl. The corrosion protection of an alloy against salt melt attack depends on the chemical stability of both the kinds of metal and their compounds such as oxides and chlorides. In fact, a breakdown of the protective oxide readily occurs by dissolution into the melt, and the degradation rate can be spe-

Fig. 8. Images of Fe–35Cr exposed beneath a NaCl–KCl deposit at 670 ◦ C for 48 h. (a) Surface morphology, (b) general view of the cross section, (c) magnified view of the scale–matrix interface and (d) EDX result of the chloride area in (c).

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Fig. 11. Equilibrium partial pressures of NaCl and KCl.

chromate can originate from the several complex reactions Cr 2 O3 + 4MCl(l) + 5/2O2 = 2M2 CrO4 + 2Cl2

(1)

Cr2 O3 + 2MCl(l) + 2O2 = M2 Cr 2 O7 (l) + Cl2

(2)

Cr + 2MCl(l) + 2O2 = M2 CrO4 + Cl2

(3)

where M is Na or K. Further, these chromates and chlorine act as oxidants Cr + M2 CrO4 + Cl2 = Cr 2 O3 + 2MCl + 1/2O2

Fig. 10. Thermodynamic stability diagrams for M–Cr–Cl2 –O2 at 650 ◦ C at mole ratio M/(M+Cr) = 0.05 where M is K or Na. (a): Na–Cr–Cl2 –O2 and (b): superimposed M–Cr–Cl2 –O2 with solid line for K and dashed line for Na. The atmospheric condition is marked as area A.

cially fast if the oxide has a high solubility [18,19]. Recent research has confirmed that Cr2 O3 has a much higher solubility in NaCl–KCl melt as chromate than do iron and nickel oxides [20]. Thus, Cr component can cause deterioration of the corrosion resistance on encountering chloride salts in an oxidizing atmosphere. However, the dissolution mechanism by Cr2 O3 cannot fully account for the rapid degradation rate of Fe–Cr alloy, considering the material is actually separated from the atmosphere by the melt and the oxide formation is limited by the oxygen diffusion across the melt. Besides, enhanced corrosion of chromia-forming materials also occurs in the cases of solid or gaseous chloride salt [3–8,21–23]. Under this circumstance, some other important factors should be taken into account. Fig. 10 compares the phase stability diagrams for Na–Cr–Cl–O and K–Cr–Cl–O at 650 ◦ C. It can be seen that NaCl and KCl show close reactivities with Cr and, in each salt case, chromium oxide is thermodynamically favored to lose its stability and is transformed into chromate under experimental condition, as marked as area A. Such thermodynamic prediction is confirmed by studies of corrosion of pure Cr and Fe–Cr alloys beneath solid NaCl deposits and solid KCl deposits as well as by their gaseous phases [3,5]. Basically, the

(4)

In fact, pure chromium reacts much faster with Na(K)Cl than does Cr2 O3 and Eq. (4) stands for the major kinetic factor for the degradation of Cr-containing alloys [5]. Meanwhile, Cr can be selectively oxidized into its chloride at the scale–matrix interface and dissolves into the melt by reaction Eq. (5) Cr + Cl2 (diss.) = CrCl2 (diss.)

(5)

Cl2 (diss.) means the dissolved chlorine in the metal that has originated from reaction (1–3). The existence of a thick chromium chloride layer at the scale–matrix interface, see Fig. 8c, supports this assumption. Driven by the concentration gradient established across the melt and also due to the high vapour pressure, chromium chloride will diffuse outwards from the matrix–melt interface to the melt–gas interface, where higher oxygen potential prevails and chromium chloride precipitates as Cr2 O3 2CrCl2 (diss.) + 3/2O2 = Cr 2 O3 + 2Cl2

(6)

Obviously, this reaction is fast in terms of a liquid phase migration process. During the corrosion procedure, the eutectic composition of chloride melts varies with exposure time, because of the different consumption rates of NaCl and KCl. On the one hand, this is caused by the different evaporation rates of the two salts, as seen in Fig. 11, a slightly higher vapor pressure for KCl than that for NaCl. On the other hand, a kinetic reactivity distinction between the two salts with the alloys leads to a gradual enrichment in either NaCl or KCl in the melt.

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Table 2 Melting points and eutectic temperatures (◦ C) for specific systems (mol%) NaCl FeCl2 CrCl2 NiCl2 K2 CrO4 KCl–CrCl2 KCl–FeCl2 KCl–K2 CrO4 NaCl–50KCl KCl FeCl3 CrCl3 AlCl3 K2 Cr2 O7 NaCl–CrCl2 NaCl–FeCl2 KCl–K2 Cr2 O7 NaCl–KCl–FeCl2

801 677 845 1001 980 470 355 650 659 772 300 947 192 398 437 370 367 380

Since the individual melting points of the two salts are much higher than the experimental temperature, the NaCl–KCl salt may deviate from a molten state with time unless the liquid phases are replenished. In fact, the liquid phase attack can be sustained due to the formation of the intermediate chromate phases. Previous studies have confirmed that potassium or sodium chromate can both directly form on Cr-containing materials involving molten, solid and gaseous alkali chloride salts, and transform from pre-oxidized chromium oxide scale [3,5,22–28]. Therefore, many new liquid phases, like KCl–K2 CrO4 , NaCl–Na2 CrO4 , are easily favoured in a wide composition range in the salt mixture, as listed in Table 2 [29]. As a direct consequence, protective chromia films are difficult to establish under a NaCl–KCl melt. Instead, a liquid phase assisted hot corrosion can continue for a long time. The Cr-free NiAl and Fe–Al alloys showed excellent corrosion resistance against NaCl–KCl melt attack, and such a protection is further improved with increasing Al content. On the surface, this behaviour seems to conflict with the fact that aluminium chloride has a much lower melting point and a higher vapour pressure than chromium chloride does. According to the dissolution model proposed in Eqs. (5) and (6), aluminium should have a similar, or even much worse, performance in comparison with Cr. In fact, several other important factors should be taken into account to explain such phenomenon. From a thermodynamic point of view, Al is the most reactive element among the investigated alloys, thus Al chloride or oxide forms preferentially over the other elements such as Ni, Fe and Cr. At the melt–substrate interface, the oxygen potential is low while the chlorine potential is relatively high and, consequently, liquid aluminium chloride forms there and transports outwards afterwards, leaving an aluminum depleted layer. Near the melt–atmosphere interface, the transition from aluminum chloride to its oxide occurs, which needs only relatively low oxygen pressures because of the very stable nature of aluminum oxide. Therefore, the preferential re-

moval of aluminum from the matrix is enhanced, basically in the form of highly volatile liquid chloride. This process seems to be beneficial to help establish a protective alumina scale due to a fast aluminum supply. Comparing the two oxides of chromium and aluminium, the latter shows greater inert to chloride salt than the former. As described previously, chromium oxide can react with KCl–NaCl to form chromates and further combine these chloride salts to cause continued liquid phase enhanced hot corrosion. On the contrary, alumina is very stable with alkali chloride salt, as confirmed by one fact that no obvious attack occurs any more on pre-oxidized Fe–20Al alloy and, also, by the co-existence of alumina layer with large amount of salt remainder on Fe–45Al alloy. Most likely, for high Al alloys, rapid corrosion only occurs in the early stage; then the attack slows down when larger alumina is established on the surface. It has been widely accepted that Ni-rich alloys suffer from less attack in chlorine-containing environments than carbon steels and stainless steels. As base metals, nickel is more inert in comparison with Fe [14,30]. As a combined effect, NiAl alloy shows the least corrosive attack.

5. Conclusions This study presents the degradation behaviour of various Fe–Cr, Fe–Al and NiAl alloys beneath a NaCl–KCl melt coating in air at 670 ◦ C. A higher Cr content leads to an enhanced metal consumption thus Cr is thought to play a detrimental role in the corrosion process. On the other hand, Al addition is very effective in improving the corrosion resistance for Fe–Al alloys; Ni is less reactive with chloride salt and NiAl alloy suffers the least attack. The different corrosion performances of the Al and Cr modified alloys primarily result from the high reactivities of Cr and Cr2 O3 with the alkali chloride whereas Al2 O3 is relatively inert.

Acknowledgements One of the authors (Y. Li) wishes to express his appreciation to the Max Planck Institute for Iron Research (D¨usseldorf, Germany) and JSPS (Japan) for providing fellowships.

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