KCl Induced Corrosion of a 304-type Austenitic ... - Springer Link

Oxidation of Metals, Vol. 64, Nos. 1/2, August 2005 (© 2005) DOI: 10.1007/s11085-005-5704-3

KCl Induced Corrosion of a 304-type Austenitic Stainless Steel at 600◦ C; The Role of Potassium J. Pettersson,∗ H. Asteman,∗ † J.-E. Svensson,∗ and L.-G. Johansson∗ ‡ Received July 21, 2004; revised March 11, 2005

The influence of KCl on the oxidation of the 304-type (Fe18Cr10Ni) austenitic stainless steel at 600◦ C in 5% O2 and in 5% O2 + 40%H2 O is investigated in the laboratory. The samples are coated with 0.1 mg/cm2 KCl prior to exposure. Exposure time is 1–168 h. Uncoated samples are exposed for reference. The oxidized samples are analyzed by ESEM/EDX, XRD and AES. The results show that small additions of potassium chloride strongly accelerate high temperature corrosion, the oxide thickness being up to two orders of magnitude greater after exposure in the presence of KCl. The rapid corrosion is initiated by the formation of potassium chromate through the reaction of KCl with the protective oxide. Chromate formation is a sink for chromium in the oxide and leads to a loss of its protective properties. The resulting rapidly growing scale consists of an outer hematite layer with embedded K2 CrO4 particles and an inner layer consisting of spinel oxide, (Fe,Cr,Ni)3 O4 . Little or no chlorine is found in the scale or at the scale/metal interface. KEY WORDS: oxidation; KCl; deposit; breakaway corrosion; 304L; chromate.

INTRODUCTION The power industry in Scandinavia is using biomass as fuel since the early 1990s. The use of biomass fuel for generating electricity is based on ∗ Department

of Environmental Inorganic Chemistry, Chalmers University of Technology S-41296, Sweden. † Max-Planck-Institut fur ¨ Eisenforschung GmbH, Max-Planck-Straße 1, D-40237, ¨ Dusseldorf. ‡ To whom correspondance should be sent. e-mail: [email protected] 23 0030-770X/05/0800–0023/0 © 2005 Springer Science+Business Media, Inc.

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Pettersson, Asteman, Svensson, and Johansson

sustainability arguments, i.e. biomass is an endless, renewable resource that gives no net contribution of CO2 to the atmosphere. In comparison to traditional fuels such as coal and natural gas, the combustion of biomass generates a flue gas characterized by high concentrations of water vapour and relatively high amounts of alkali and chlorine. In contrast, the sulphur content is typically low.1 The corrosivity of this environment tends to be high, causing problems with fireside corrosion of the superheaters. Accordingly, many of the power plants that switched from fossil fuels to biomass have experienced severe corrosion problems. It is well known that the high temperature corrosion of FeCr steel is greatly enhanced by alkali chlorides, e.g. KCl.2−8 This effect has been explained by a mechanism, “chlorine cycle”, involving the formation of molecular chlorine that generates volatile transition metal chlorides at the scale/metal interface.4−11 In a similar mechanism proposed by Lee et al., volatile metal chlorides were suggested to play a crucial role in the high temperature corrosion of stainless steel in HCl/Cl2 containing atmospheres.12 Y. Shinata13 has proposed another explanation for the corrosivity of alkali chlorides based on the formation of eutectic melts of NaCl/Na2 CrO4 that enhance corrosion. Previous work in this field has mainly considered the role of chlorine in the corrosion process. However, the initiation of corrosion, i.e. the breakdown of the chromium-rich protective oxide which is present initially on chromia forming steels, can not be explained by previously presented mechanisms, e.g. the “chlorine cycle” mechanism. The present work investigates how small amounts of KCl(s) influence the high temperature corrosion of the austenitic stainless steel 304L (Fe18Cr10Ni) at 600◦ C. Especially, we address the initiation of corrosion i.e. the breakdown of the normally protective oxide. The present results show that the oxide scales formed after short exposures in the presence of KCl are rich in potassium while they contain very little chlorine. A mechanism is proposed, focusing on the role of potassium, which explains the initiation of corrosion by the reaction of KCl with the protective oxide to form potassium chromate. EXPERIMENTAL PROCEDURES Sample Preparation The material investigated is the austenitic stainless steel, 304L, for chemical composition see Table I. The geometrical area of the samples was 5.56 cm2 , (15 × 15 × 2 mm3 ). A hole (φ = 1.5 mm) was drilled for handling. Before exposure the samples were grinded to 1000 grit SiC and

KCl Induced Corrosion of a 304-type Austenitic Stainless Steel

25

Table I. Chemical Composition of Alloy 304L in Weight-%

304L

Fe

Cr

Ni

Mn

Si

Mo

N

C

Balance

18.5

10.2

1.41

0.55

0.49

0.075

0.027

polished with 1 µm diamond spray until a mirror-like surface appeared. After polishing, the samples were degreased and cleaned in acetone and ethanol using an ultrasonic bath. KCl was applied by spraying the samples with a saturated solution of KCl in water/ethanol (20:80). The samples were dried with cool air and stored in a desiccator prior to exposure. The mass change of the samples was measured prior to and after exposure using a five decimal SartoriusTM balance. Exposures The exposures were performed in silica tube furnaces with ex-situ recording of the weight (Fig. 1). The temperature was kept at 600 ◦ C (±3◦ C). The experiments were carried out in 5% O2 and 5% O2 + 40% H2 O atmosphere and the flow rate kept constant at 2.5 cm/s. Nitrogen was used as carrier gas. The 40% water vapour concentration was produced by a humidifier. The dry flow rate was measured prior to exposure by a BIOS DC2 Flow Calibrator. The samples were mounted three by three on an alumina tray, containing 2 mm slits for support, and placed in the furnace parallel to the flow direction. To protect the silica tube from KCl(g) that

Fig. 1. Experimental setup for exposures in a horizontal silica furnace.

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might arise, a ∼ 20 cm long alumina inner tube, inside the silica tube was used. The experiments lasted 1, 24, 72 or 168 hr. CORROSION PRODUCT CHARACTERIZATION Environmental Scanning Electron Microscopy (ESEM/EDX) ESEM imaging was performed using an Electroscan instrument equipped with a Link eXl EDX system. The microscope was operated at 20 kV for secondary imaging and EDX analysis. X-Ray Diffraction (XRD) The crystalline corrosion products were analysed with Grazing-Incidence X-ray diffraction (GI-XRD) using a Siemens D5000 powder diffrac¨ tometer, equipped with grazing – incidence beam attachment and a Gobel mirror. Cu Kα radiation was used and the angle of incidence was 0.50◦ and 20◦ . The measurement range was 15◦ < 2◦ < 75◦ . Auger Electron Spectroscopy (AES) The Auger measurements were performed using a scanning Auger microprobe (PH1660). The electron beam was 10 kV and the beam current was about 160 nA. The depth profiles were obtained using a differentially pumped ion gun (Ar+ ) with acceleration voltage 4 kV. The raw data was refined using Multipak v6.0a software. The software makes it possible to evaluate peak shapes of differentiated Auger peaks. This feature can be used in order to distinguish between oxidized and metallic iron and chromium, as these elements exhibit significant chemical shifts for the oxidized state. RESULTS Gravimetry While the samples exposed in 5% O2 in the absence of KCl oxidize very slowly, the corresponding exposures in 5% O2 + 40% H2 O give rise to relatively rapid corrosion, the mass gain after 168 h being about 25 times that registered in dry conditions (see Fig. 2). Exposure to 5% O2 in the presence of KCl resulted in much greater mass gains compared to the corresponding runs without KCl (Fig. 2). The highest corrosion rates were registered in the 5% O2 + 40% H2 O + KCl environment. However, the accelerating effect of KCl is less marked in 5% O2 + 40% H2 O, the


2

Mass gain (mg/cm )

1.0

27

O2 + H2O + KCl O2 + H2O

0.8

O2 + KCl O2

0.6 0.4 0.2 0.0 0

24

48

72

96

120

144

168

Exposure time (h) Fig. 2. Mass gain versus exposure time for samples exposed at 600◦ C in the absence/presence of 0.10 mg/cm2 KCl.

addition of KCl increasing the mass gain after 168 hr by 1.7×. In the presence of KCl, the mass gain is rapid in the beginning and slows down with exposure time. In contrast, the samples exposed in 5% O2 + 40% H2 O in the absence of KCl exhibit roughly linear mass gains. ESEM The ESEM images in Fig. 3a–c show samples exposed in 5% O2 in the absence of KCl for 1, 24 and 168 hr. In accordance with the mass gain results, the oxide is thin and protective in all cases, only a few oxide islands being present. In 5% O2 + 40% H2 O in the absence of KCl (Fig. 3d–f), the situation is markedly different. After 1h the oxide is still thin and protective but after 24 hr local breakaway corrosion has occurred, oxide islands forming on the centre of the alloy grains. After 168 hr the oxide islands have increased both in number and size while the surface close to the alloy grain boundaries is still protected by a thin oxide. Samples exposed to 5% O2 in the presence of KCl for 1 hr exhibit numerous small oxide islands (Fig. 3g). AES shows that the oxide between the islands is much thicker compared to the corresponding exposure in the absence of KCl, (see below). After 24 hr the number and size of the oxide islands have increased while the oxide between the islands has grown and become more irregular (Fig. 3h). After 168 hr the ESEM image (Fig. 3i) shows large oxide islands, the surface morphology being rough with visible cracks in the oxide.

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Fig. 3. ESEM images with a magnification of 500×. The samples are oxidized for 1, 24 and 168 hr at 600◦ C in (a–c) 5% O2 (d–f) 5% O2 + 40% H2 O (g–i) 5% O2 , 0.10 mg/cm2 KCl (j–l) 5% O2 + 40% H2 O, 0.10 mg/cm2 KCl.

The most heavily corroded samples are the ones exposed to 5% O2 + 40% H2 O in the presence of KCl (Fig. 3j–l). Already after 1 hr the entire surface is covered by a thick scale containing numerous small particles. After 24 hr hematite whiskers appear and the scale thickness has


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grown (see below, Fig. 9). The oxide appears to be thinner over the alloy grain boundaries. Extending the exposure time from 24 to 168 hr produced little change in the ESEM images except that the oxide close to the alloy grain boundaries becomes thicker and that more hematite whiskers form. EDX For reference, an unexposed sample with added KCl was analyzed by EDX and AES. As expected, large KCl crystallites (10–150 µm) were present. Between the crystallites, the amount of potassium and chlorine was below the limit of detection. EDX analysis of a sample with added KCl after 1 hr exposure to 5% O2 + 40% H2 O shows that most of the KCl crystallites remain on the surface (Fig. 4). The elemental maps show that potassium has spread over the surface, indicating that KCl (s) has partly reacted. The EDX maps also show that the reacted potassium, which is concentrated in small particles, is not associated with chlorine. The potassium-rich particles are relatively poor in iron while the chromium signal remains at the same intensity as in the surrounding area. The potassium- and chromium-rich particles are considered to consist of potassium chromate (see XRD analysis below). The “hollowed out” shape of the remaining KCl crystals is clearly a consequence of the reaction with the surface. The fact that chlorine is only detected in the remaining unreacted KCl particles implies that it has left the surface in gaseous form. The remaining KCl crystals are surrounded by large amounts of iron-rich oxide. After 168 hr, no unreacted KCl remains on the surface. Small amounts of potassium are detected close to the position of the original KCl particles while no chlorine was detected on the surface. XRD The samples exposed in 5% O2 in the absence of KCl form a chromium-rich oxide of corundum-type corresponding to the solid solution (Fe,Cr)2 O3 . This phase was detected at all exposure times. The solid solution (Fe,Cr)2 O3 , was also detected after 1 hr in the corresponding exposure in the presence of water vapour. In contrast, longer exposures times (≥24 hr) in that environment mainly produce α-Fe2 O3 · α-Fe2 O3 is also the main corrosion product in the presence of KCl, with K2 CrO4 and unreacted KCl being detected after short exposure times (1 hr). After long exposure times in the presence of KCl, only α-Fe2 O3 was detected. There were no indications of Cr2 O3 or transition metal chlorides, (e.g. FeCl2 , FeCl3 , CrCl3 ).

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Fig. 4. EDX images of a KCl treated sample exposed at 600◦ C in 5% O2 + 40% H2 O. The magnification is 1000×.

AES After 24 hr in 5% O2 in the absence of KCl, a 40 nm chromiumrich oxide has formed (Fig. 5). In the corresponding exposure in 5% O2 + 40% H2 O the oxide is somewhat thinner. In this case the oxide was depleted in chromium, especially at the gas/oxide interface (see Fig. 6). The KCl-treated samples developed thicker oxides. After 1 hr in 5% O2 + KCl, the oxide thickness was about 140 nm (Fig. 7), the outer part consisting mainly of iron oxide. The atomic concentration of potassium is about 10% at the surface and 3–4% in the interior of the oxide. Chlorine was only detected at the scale/metal interface, the maximum


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atomic concentration being about 1.5%. After 1hr in 5% O2 + 40% H2 O + KCl the oxide had a thickness of about 425 nm. Again, potassium was enriched at the scale surface, the maximum potassium content reaching ∼10%. The potassium content decreases as we move into the scale. The outermost 250 nm of the oxide is iron-rich while the inner part of the oxide contains equal amounts chromium and iron. Small amounts of chlorine were found in the scale, the chlorine concentration being close to the limit of detection (about 1%). After 24 hr in 5% O2 + KCl the oxide has grown to about 2500 nm (Fig. 8), about 60 times thicker than in the corresponding exposure in the absence of KCl. The outer 2000 nm of the oxide are iron-rich while the Cr/Fe ratio increases on approaching the metal/oxide interface. Again, potassium is enriched at the surface, reaching 10% in some areas. The chlorine level was below the detection limit throughout the entire oxide. After 24 hr in 5% O2 + 40% H2 O + KCl the oxide thickness is 6000 nm (Fig. 9). The AES depth profile reveals an outer chromia-depleted oxide, shown by XRD to be of the corundum type, with the approximate composition (Fe0.8 , Cr0.2 )2 O3 . The bottom 1/3 of the oxide contains roughly equal amounts of chromium, iron and nickel. Similar layered scales have been reported to form on FeCr alloys in O2 + H2 O environments in this temperature range.14 In the cases referred to, a combination of electron diffraction and TEM showed that the inner oxide consisted of a spinel ((Fe,Cr)3 O4 or (Fe,Cr,Ni)3 O4 depending on the alloy). Based on the similarities in composition and morphology, it is suggested that the inner part of the layered scales found in the present study is made up of spinel type oxide of the type (Fe,Cr,Ni)3 O4 . Also in this case, potassium is found on the surface. Traces of chlorine, at the limit of detection, were found at the oxide/metal interface. DISCUSSION While alloy 304L forms a protective oxide in dry oxygen at 600◦ C, mixing water vapour with oxygen creates a much more corrosive environment (see Figs. 2 and 3). This is in accordance with recent reports by Asteman et al. and others concerning the effect of water vapour additions on the high temperature corrosion resistance of FeCr alloys.15−17 The greater corrosivity of the O2 + H2 O environment only becomes apparent after an induction period. Initially, oxidation in O2 + H2 O environment follows the same course as in dry oxygen with the formation of a chromium-rich protective oxide. Theoretical calculations and experimental evidence show that the chromium-rich oxide formed on FeCr alloys reacts with water vapour and oxygen under these conditions, forming volatile chromiumoxyhydroxide.15,18

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Fig. 5. AES depth profiling for sample exposed at 600◦ C in 5% O2 for 24 hr.

Cr2 O3 (s) + 2H2 O(g) + (3/2)O2 (g) → 2CrO2 (OH)2 (g)

(1)

This causes chromium depletion of the oxide and in the alloy substrate, causing the breakdown of the protective chromium-rich corundum type oxide. The protective oxide is replaced by a poorly protective scale consisting of an outer hematite layer and an inner spinel-type layer. The hematite layer appears to grow by outward cation transport while the spinel layer grows inwardly. The present study is in accordance with the previous work, indicating that vaporization of chromiumoxyhydroxide does take place in O2 + H2 O environment, resulting in the depletion of the oxide in chromium (see Fig. 6). To maintain the protectiveness of the oxide, the supply of chromium to the oxide (by diffusion in the alloy) must match the evaporation


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Fig. 6. AES depth profiling for sample exposed at 600◦ C in 5% O2 + 40% H2 O for 24 hr.

rate (see schematic view in Fig. 10a). At 600◦ C, chromium diffusion in the alloy is dominated by grain boundary transport. Accordingly, chromium depletion of the oxide in O2 + H2 O environment depends on the distance to an alloy grain boundary, the oxide on the centre of an alloy grain tending to be more severely chromium depleted. As a result a thick, poorly protective, oxide forms on the alloy grain centres (Fig. 10b). The present study shows that the high temperature corrosion of alloy 304L is strongly accelerated by small amounts of KCl(s) (Figs. 2, 3). The effect is especially strong in dry O2 where the oxide is protective in the absence of KCl. The greatest corrosion rate was observed in O2 + H2 O + KCl environment. The influence of KCl on corrosion in O2 + H2 O environment was strong especially after short exposure times.

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Fig. 7. AES depth profiling for KCl treated sample exposed at 600◦ C in 5% O2 for 1 hr.

The acceleration of the high temperature corrosion of stainless steel by alkali chlorides is well-known.2−8 According to Grabke and Spiegel,3,4 the corrosivity of alkali chlorides towards FeCr alloys is connected to the formation of molecular chlorine at the oxide surface. The authors suggest that Cl2 (g) diffuses through cracks and pores in the oxide scale, forming volatile metal chlorides in the reducing environment at the oxide/metal interface, e.g.: Fe(s) + Cl2 (g) → FeCl2 (g)

(2)

The gaseous metal chlorides then diffuse through the oxide, forming metal oxide in the outer part of the scale where the partial pressure of oxygen is higher, releasing Cl2 (g): 2FeCl2 (g) + (3/2)O2 (g) → Fe2 O3 (s) + 2Cl2 (g)

(3)

The molecular chlorine formed in this way is then suggested to diffuse inwards through the scale and reacts with the metal again, completing


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Fig. 8. AES depth profiling for KCl treated sample exposed at 600◦ C in 5% O2 for 24 hr.

the cycle. In this scheme (commonly referred to as the “chlorine cycle”), alloy oxidation is catalyzed by molecular chlorine. The chlorine cycle has been used to explain, e.g. the accumulation of transition metal chloride at the scale oxide interface observed by many investigators. It may be noted that for metal chloride formation to proceed the oxygen activity at the metal/oxide interface must be low while the activity of Cl2 must be high (See reactions 2 and 3). An apparent problem with the mechanism is that it appears to assume that the oxide scale is permeable to molecular chlorine and gaseous metal chlorides while it is impermeable to molecular oxygen. As Cl2 and MeCl2 are both considerably larger than the O2 molecule, it is not obvious why this should be the case. It may be noted that the initiation of corrosion, i.e. the breakdown of the chromium-rich protective oxide which is present initially, is not explained by the “chlorine cycle” mechanism. The present study focuses on early stages (1 → 168 hr) of high temperature corrosion in the presence

36


Fig. 9. AES depth profiling for KCl treated sample exposed at 600◦ C in 5% O2 + 40% H2 O for 24 hr.

of KCl(s). Regarding the relevance of the “chlorine cycle” to the present work, it may be noted that we find very little chlorine in the oxide or at the oxide/metal interface. In contrast, large amounts of potassium are present throughout the scale after exposure. As shown by XRD and EDX (see Fig. 4), much of the potassium is present in the form of potassium chromate, K2 CrO4 . Based on these observations we propose that rapid corrosion is initiated by the reaction of potassium chloride with chromium oxide in the scale, forming potassium chromate and HCl or Cl2 : Cr2 O3 (s) + 4KCl(s) + (5/2)O2 (g) ⇔ 2K2 CrO4 (s) + 2Cl2 (g)

(4)

Thermodynamic calculations show that under our experimental conditions the formation of chlorine dominates only in extremely dry conditions (