Studies of the mechanism of metal dusting of 10CrMo9-10 steel after 10 years of operation in the semi-regenerative catalytic reformer
Introduction
Corrosion of metals and alloys in industrial environments with high carbon activity—the so-called metal dusting, is a phenomenon known for more than 60 years. This phenomenon occurs mainly in thermo-chemical factories, metallurgical, petrochemical and refining industries [1], [2], [3], [4], [5], [6], [7]. A common feature of these environments along with the high carbon activity is the temperature in the range of 450–800 °C [8]. The first data on damage related to metal dusting was published by Camp in 1945 [9]. Superheater feedstock unit was damaged in a refinery naphtha reforming installation. In 1950, Burns made a series of case studies of corrosion of metal alloys under different environments in refinery plants [10]. He showed that in the case of processing crude oil with low sulfur content of 0.2% weight, significant corrosive losses in crude oil distillation unit devices operating at temperatures up to about 400 °C may occur. This problem has not been elucidated, however, it is likely that the rapid progress of metal loss was caused by the corrosion due to high activity of carbon atoms.
In recent years, much attention was paid to the metal dusting phenomenon in petrochemical installations. It is a common problem in the production of synthesis gas by means of reforming process, where natural gas is replaced by a mixture of CO, H2, CO2 and H2O. The synthesis gas is then used to produce hydrogen, ammonia, methanol and liquid hydrocarbons. In order to increase the efficiency of production, the processes are performed at lower share of water vapor and greater participation of carbon monoxide. Such modification of the process creates a more aggressive environment, the lower oxygen partial pressure and the higher carbon activity, promoting the development of metal dusting process [11]. In the natural gas reforming installations nickel alloys and iron alloys, with the content of alloying elements, chromium and nickel are used among others. In case of these alloys, the character of corrosion is particularly dangerous because it is selective. Forming pits penetrate through the wall element in a short time. Pitting may occur within a few days, leading to considerable damage. An example of such “instant” failure, which occurred after the three-week operation, was presented by Eberle and Wylie, who analyzed the damage to the components of the boiler made of 25Cr-20Ni (310) and 18Cr-8Ni (347) steels operating under the synthesis gas in the range of 400–950 °C [12].
The fact that metal dusting is still an unsolved problem is proven by many publications appearing every year [27], [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42].
In the literature, several physical models explaining the course of metal dusting of Fe-base alloys can be found. For example, according to Natesan and his team [24], metal dusting proceeds in several stages. First, carbon deposits on the iron surfaces and supersaturates the ferrite. In the next step, Fe3C forms at the surface. Cementite formation is accompanied by an increase in volume which results in the generation of crystalline defects. Further, carbon diffuses through the Fe3C. Numerous defects in the crystal lattice of cementite improve the diffusion of carbon. Carbon atoms accumulate at cementite defects causing the Fe3C particles to separate into small particles and to move away from the metal. In this model, the driving force of metal dusting is the free energy difference between good and poor crystalline carbon. Their physical models help to explain metal dusting without decomposition of Fe3C.
Another physical model was presented by Grabke [17]. According to this mechanism, within the temperature range 400–650 °C, active carbon atoms are dissolved in ferrite, causing its supersaturation. Next, cementite nucleation starts at the carbon supersaturated ferrite. Fe3C increases mainly on the metal surfaces but also at the grain boundaries. Cementite layer forms a barrier for further penetration of carbon into the metal because of its low diffusion coefficient in cementite. Therefore, the carbon activity on the surface increases and the graphite nucleation takes place locally on the surface. Consequently, the carbon activity is reduced to aC = 1 which causes instability of cementite. In the next stage cementite breaks down into graphite and iron. The carbon atoms originating from the decay of Fe3C, join the graphite planes and iron atoms diffuse out through the graphite, and when the concentration reaches 3–4%, agglomerate forms from fine particles of Fe having a diameter of about 20 nm. Carbon, formed subsequently from the gas atmosphere, penetrates into the fine particles of iron. The carbon diffuses through the Fe particles in places where graphite nucleates easily. In these places, graphite begins to increase often in the form of fibers. The rate of growth of the carbon fibers and, thus, coke formation rate depends on the velocity of the carbon atoms in the iron particles.
In turn, Young and his group proposed still another physical model of metal dusting of pure iron and low alloy steels. According to them [25], the mass transfer process involved is the diffusion of gas through porous coke, and permeation of carbon through the cementite phase of the scale. Diffusion process is driven by activity gradients produced by contact with the gas. Carbon permeation supports transformation of iron into cementite at the alloy-scale interface, and inward scale growth. Carbon diffusion also leads to graphite formation near the surface. The resulting volume increase causes scale disintegration and the generation of dust.
Chun and Ramanarayanan [26] proposed their own metal dusting model of steel with low chromium content. According to their model, metastable surface M3C growth and its subsequent decomposition upon carbon deposition is the main corrosion mechanism. If the nature of the carbon deposit on the surface of M3C is mostly graphitic, the graphite planes are oriented more or less perpendicularly to the dissociating M3C. Then iron atoms arising from M3C dissociation intercalate into the graphite and diffuse outward to the carbon-supersaturated environment where they coalesce and catalyze filamentous-carbon formation. In the case when carbon deposit on the surface of M3C is mostly amorphous, stress-induced fracturing of M3C can occur in some regions since amorphous carbon does not provide a pathway for iron atoms from dissociated M3C to escape. During the formation and growth of M3C, stresses accumulate and lead to the breakup of M3C above a critical stress level. Once initial fracture of M3C has occurs, corrosion preferentially continues in the same region.
Models available in the literature [13], [14], [15], [16], [17], [18], [24], [25], [26] describing the mechanisms of metal dusting for low- and high-alloyed steels are based on laboratory tests, which do not reflect the operating parameters of these materials in industrial plants. Most of these studies, in order to ensure quick corrosion, were carried out in the atmospheres of much higher carbon activity than the atmosphere found in industrial installations, and durations were much shorter than the typical operating time of heat resistant steel in these plants. Long-term operation at high temperatures causes changes in the microstructure of these steels, including precipitation of carbides and intermetallic phases, carbide changes and changes in the distribution of alloying elements between matrix and precipitates (depletion of alloying elements in the matrix) which may affect the corrosion mechanism. Meanwhile, adopted durations of experiments conducted in laboratory conditions were too short for these phenomena to occur, making it impossible to analyze their impact on the mechanism of metal dusting. Thus, the models describing the mechanisms of metal dusting corrosion, developed on the basis of laboratory tests do not fully describe the mechanisms of steel degradation occurring in industrial installations. These mechanisms can be determined on the basis of the study of materials operating under industrial conditions, while conducting laboratory tests in conditions reflecting the operating parameters of these materials in industrial plants would require time in the range of tens of thousands hours.
The analysis of the mechanism of metal dusting in an industrial environment is difficult due to the lack of up-to-date traceability changes in the structure of the material. Another problem is the determination of carbon activity. Calculation of the carbon activity is simple when, in laboratory conditions, exogenous gas is used with a specific content of components such as CO, CH4, CO2, H2O. Then the most important reactions occurring during the process are:2CO → C + CO2,CH4 → C + 2H2,CO + H2 ↔ C + H2O,
If the hydrocarbon concentration is high, the following formula can be used to calculate the carbon activity:In the case of semi-regenerative reforming unit, feed is rich in dozens of hydrocarbons, and most of them can be reacted to activated carbon by contact with the surface of the steel. Depending on the hydrocarbon, the following reactions may occur:C2H2 → 2C + H2,C3H8 → C + 2CH4,C3H8 → C2H4 + CH4 → C + 2CH4,C3H8 → C2H2 + CH4 + H2 → C + 2CH4,C2H4 → C + CH4,
In addition, each of these reactions has its own free energy and the kinetics of the reaction. The resulting products of these reactions are next carbon atoms, shorter hydrocarbons which can still be subject to further reactions. For this reason, it is impossible to determine the carbon activity in the gas atmosphere.
Metal dusting corrosion was identified in the furnace installation of semi-regenerative catalytic unit in a petroleum refinery in Gdansk (Grupa Lotos S.A.). After 10 years of operation of the furnaces on the inner surface of the pipe made of 10CrMo9-10 (P22 according to ASTM) steel, containing 2.25 wt% Cr and 1 wt% Mo, a thin layer of coke formed, containing graphite, iron and alloy carbides. The analysis of corrosion mechanism pointed to the metal dusting. Therefore, the carbon activity of the gas atmosphere had to be greater than singularity. The corrosion was even in nature, and the pipe wall thickness remained in the range of dimensional tolerance of products. The corrosion rate was not catastrophic in nature, which is usual in environments involving hydrogen, steam and carbon monoxide.
The purpose of this paper is to present a mechanism of metal dusting for 10CrMo9-10 steel operated for 10 years in the semi-regenerative catalytic reformer.
Section snippets
Experimental procedure
The semi- regenerative catalytic reforming installation was operated in refinery in Gdansk (Poland). Typical diagram of such an installation is shown in Fig. 1a.
Four furnaces were in the reactor unit of the plant. The pipes of the fired heater were made of steel with a nominal content of chromium 2.25 wt% and molybdenum 1 wt% (P22 steel according to ASTM and 10CrMo9-10 according to PN-EN). After 10 years of operation from second furnace from zone in which there was the highest temperature,
Results
Fig. 3a shows a cross-section of the pipe wall. A carburized microstructure is visible below the surface of the inner pipe. Its thickness is about 2 mm. Study of the microstructure using SEM revealed the morphology of the surface layer. The inner surface of the pipe is covered with a coke layer whose thickness ranges from 5 to 15 μm. Directly beneath the inner tube, the thin, continuous layer of carbide having a thickness of about 1–2 μm (Fig. 3b) occurs. Observations at higher magnification
Discussion
Analysis of the test results of samples of steel taken from the reformer furnace tubes after 10 years of operation showed that there are significant differences between the metal dusting mechanism developed in the laboratory, and the mechanism of destruction of 10CrMo9-10 steel exploited in industrial conditions. Elements, which are not included in the previously proposed models of corrosion, include: processes of formation of MxCy′ alloy carbides with an increase of carbon activity in the
Conclusion
The results allow the formation of the following conclusions:
- 1.
The mechanism of metal dusting of 10CrMo9-10 steel operating at semi-regenerative catalytic reforming unit differs from the models developed on the basis of laboratory tests.
- 2.
As a result of an increase of the carbon activity in the steel, MxCy′ alloyed carbides (mostly M23C6) precipitate in the first place which is not taken into account by theoretical models, however, at a later stage these carbides may convert to the metastable M3C
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