INFLUENCE OF EXPERIMENT CONDITIONS TO THE RESULTS OF CORROSION TESTS

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1 INFLUENCE OF EXPERIMENT CONDITIONS TO THE RESULTS OF CORROSION TESTS Miroslava KLÁROVÁ a, Pavel HAŠEK a, Oldřich SALVA a, Petr TVARDEK b a VŠB-TU Ostrava, 17.listopadu 15, Ostrava - Poruba, Česká republika, miroslava.klarova@vsb.cz Abstract b ArcelorMittal Ostrava a. s., Vratimovská 689, Ostrava - Kunčice, Česká republika, petr.tvardek@arcelormittal.com In this paper, attention is focused on the corrosion resistance of materials used for ladle lining. For Experiments were selected refractory materials, which are used for critical areas of ladles, therefore are highly resistant materials. Test materials were therefore based on magnesium-carbon, magnesium-spinel with the addition of carbon and aluminium-magnesium-carbon. As a corrosion medium were used two slags from secondary metallurgy. The first one was strong acid and came from the beginning of the process of steel refining. The second one was alkaline and it came from the end of the process of steel refining. Given the possibility of testing in pilot and laboratory conditions, it was possible to evaluate the corrosion resistance tests of those materials and also compared the conditions of the experiments and evaluates their impact on the results of corrosion tests. Pilot test was characterized by samples size, steel bath with melted slag on its surface, oxidation atmosphere and MgO melting pot. Laboratory test was performed only with slag, in inert atmosphere of Argon and in corundum melting pot. Understanding was that the materials containing Al 2 O 3, which are used mainly in places where there is minimal contact with the slag, prove less resistant. When comparing the conditions of both experiments, it was assumed that the results of each are very few correspond, and greater wear of the samples are reflected in pilot measurements, where the course of the experiment was much more intense and more remind real operation. Keywords: corrosion, ladle, slag, magnesium-carbon, steel 1. EXPERIMENTAL METHODS AND TESTED MATERIALS Tests of ladle refractories were carried out in pilot plant and laboratory conditions at workplaces Research department of ArcelorMittal Ostrava and Department of Metallurgy VŠB - TU Ostrava. In both cases it was used principle of dipping trabecular static method, which is one of the basic tests determining the resistance of materials against corrosion. Conditions of pilot measurements were simulated so that the most similar to real operation. The basic device was the medium frequency induction furnace with weight of charge 40 kg. For security reasons, in the oven was installed highly dense isostatic pressed MgO crucible. Charge was formed by steel bath with a layer of slag on the surface, just as in practice. Refractories samples with size 40x40x200 mm3 were dipped into the bath. Exposure time was 30 and 60 minutes and the temperature was kept at a constant value of 1600 C. The atmosphere was oxidizing. Conditions of laboratory measurements have not been so easy to adapt to the reality and for our tests have been set as follows. The basis of the measuring apparatus was corundum Al2O3 crucible, which was heated using graphite heating tube by water-cooled inductor. Charge formed slag in quantities of 100 g. To the size of the device respond the dimensions of the samples 10x10x120 mm3. Crucible was closed by the magnesium lid with holes for thermocouple, sample and gas inlet. Exposure time was always 15 minutes and the temperature was maintained at 1600 C. Atmosphere was argon. 1

2 Refractories which were tested are used in the lining of impact points and lines slag ladles. Generically they were shaped magnesium-carbon material containing carbon 5 to 12 %, and then it was magnesium-spinelcarbon material, and aluminium-manganese with carbon and castables based on Al2O3. As corrosive media were selected slag from the beginning and the end of the process of secondary metallurgy. The first one is highly acidic slag, whose main component is SiO2, with basicity 0.66 and the second high-contrast, basic slag, whose main component is CaO, with basicity The composition of both media based on their function to perform in steel refining. 2. EVALUATION OF RESULTES In the pilot plant test was the emphasis primarily to assess the impact of exposure time on corrosion samples. In laboratory tests, the time of exposure was in all cases the same and the emphasis was placed mainly on the assessment of the impact of slag to the samples. The obtained results were divided into the following sections. Surface changes visible to the naked eye Mass changes The effect of immersion time on materials wear Evaluation of materials and corrosive media 2.1 Surface changes visible to the naked eye Surface changes are the first thing we can evaluate on samples after corrosion tests. These include in particular the changes in the samples profile, loss of material when compared to immersed part of samples and not immersed part of samples, grains of samples sticking out after washing off the surface layers of samples and the emergence of typical corrosion necks. In the pilot plant test occurred to the surface wear of materials during prolonged exposure, even formed necks in some cases, or there was an overall decline in submerged parts of the samples around the perimeter and changing the profile of the samples. Selected extreme cases are summarized in Table 1. The largest decreases were recorded in samples D, then C, and then fell both samples A and B. Table 1 Extreme cases of wear samples at pilot plant tests. Sample after 30 minutes exposition Sample after 60 minutes exposition 2

3 In laboratory tests also occurred to influence the examined materials by both slags. In laboratory tests also occurred to influence the materials examined by both slags. Regarding the visual assessment, slag SP1 with higher Si content to create a continuous layer on samples, without serious damage of the surface samples. Slag SP2 demonstrated that, if adhered to the samples, it were the only places in the form of small islets. Even in laboratory tests, there were some extreme cases, which are shown in Table 2. Table 2 Extreme cases of wear samples in laboratory tests. Vzorek po expozici v kyselé strusce Vzorek po expozici v zásadité strusce 2.2 Mass changes Weight changes of samples proved to be less accurate manner for evaluate the corrosion. The pilot plant test values obtained can not be taken as authoritative, because these tests there were a strong flow and bubbling spa with induction heating. Moreover, during these tests was not to ensure a constant level of bath or immersion depth of samples. Slag had also packed during the test. Due to the open furnace workspace slag had also a tendency for the freezing and willingly lingered on samples, somewhere in very strong layers. For this reason, the relevance of differences in weight of samples before and after the test may be doubted. In laboratory tests the molten slag to avoid its intense flow, the sample was always immersed in a constant depth, for each sample was always ready a new charge and due to closed furnace workplace slag had not a tendency to freezing on the surface. From this perspective, the weighing differences could be taking into account on this test. It can be demonstrated by the concrete changes in the masses in mutual comparison of the two slags. Basic slag caused greater losses of refractory samples, on average by 3,6 wt. %. 2.3 The effect of immersion time on materials wear Influence of the immersion time was based on two basic assumptions, however during the experiment they were not clearly confirmed. The first assumption was that the samples immersed for 30 minutes they will show less wear than the samples immersed for 60 minutes. Whereas the one bath has been used over the entire series of 4 samples from concrete materials, samples immersed in the first should show the greatest 3

4 wear and specimens immersed in the last should show the smallest wear. Neither this assumption was proven. 2.4 Evaluation of materials and corrosive media Evaluation of materials The pilot plant test to be assessed in addition to the penetration of slag penetration of steel into the samples, as is apparent from the results, samples are not susceptible to penetrate the steel. However, the tendency to slag penetration had. All materials should at least a small amount of penetrated slag below the surface, but there was no damage to the surface or grains of materials. On the samples created slag layer, which has been shown that from the volume of most samples MgO moved into the slag, namely that the materials tend to dissolve in acid slag. In one case, the effect is manifested in the form of a continuous MgO layer at the interface slag/sample (magnesium-carbon material in the exposition 30 minutes). The magnesium-spinel material (D30 and D60) was MgO fined in the form of elongated grains, in other magnesium-carbon materials it was in the form of larger or smaller grains in the not entirely proven according to the immersion time. In overall consideration of all effects and the results can be said that by the wear was most affected material D, followed by the material C, B and A. The extreme wear of materials occurred more for longer periods of immersion, but it was not the rule. In laboratory tests, which took place without the presence of the steel bath was still possible in most of the samples immersed in acid slag found at least a small amount of iron. The iron was unknown in samples immersed to the alkaline slag. The reason for this phenomenon is the origin of acid slag, which in itself contains iron compounds and then they passed to the samples during corrosion tests. A basic slag due to its origin already has a very small amount of iron compounds and hence in the samples was not found. In the evaluation of concrete materials could be samples divided into two groups. Materials A - D are highmanganese materials content 5 to 12 % carbon and content 0 to 15% Al2O3. All these materials showed similar behaviour when the surface layer contained mainly phase Al + Ca, spinel MA or both, in acidic slag Fe too. Slag in the samples did not tend to penetrate. The alkaline slag sample surface was slightly damaged. E and F materials were high-aluminium containing MgO 3 to 13% and carbon content from 0 to 7%. These materials behave differently. The acidic slag occur deformed sample, or severe damage to the sample surface, in alkaline slag samples surface was damaged, with material F corundum move out and had accumulated beneath the surface. Evaluate of corrosive media In the case of slag SP2, which on the surface of the samples almost not adhered, there were losses of refractories and thus almost zero penetration of slag in the samples. For technical reasons, it was possible to test this slag only in laboratory conditions, so comparison of behaviour in the pilot plant is not possible. In the case of slag SP1 in its layer on the surface of the samples was in most cases MgO.Al2O3 alumina spinel (MA), or at least Al2O3 and usually more globular particles of iron. Iron in the slag mainly occurs because this slag still contains some tapping slag, which is in high Fe oxides characterized. As the MA spinel, its occurrence is given by the composition of original material and it is typical for samples D, E and F. However, it should be present mainly in the sample. Its occurrence in the slag or on the slag interface / sample shows its leaching of the samples. But in other samples of materials A, B and C the spinel was finding at slag or on the interface occurred as well. In these cases, the reason is mainly in reaction of the sample material with corundum crucible, due to the strong corrosive effect of highly acidic slag. 4

5 Overall, the strong acid slag SP1 has caused the emergence and accumulation of spinel at the interface or in the slag layer on the surface of samples. Also MgO from samples were present in the slag. This slag penetrated to the subsurface areas of samples, while the surface of the samples was not significantly affected. Unlike basically slag SP2, which not penetrated but causing loss of material samples in an average of 3.6 wt.%. In conditions of the pilot plant slag penetrated into all samples, but there was damage to the surface or grain materials. Like in the laboratory here slag SP1 also creates the surface layer on samples. 3. CONCLUSION Summary of results Achievements pilot plant tests can be summarized as follows: Samples of refractories are not susceptible to steel penetration Slag penetrated into all samples at least in small amount Surface layer of adhered slag showed higher amount of MgO, so most of the samples had tendency to dissolve into the slag In several cases, the slag caused extreme wear, which is reflected in visible loss of material and the emergence of corrosive necks Magnesium-spinel material characterized corundum move out and had accumulated near the slag/sample interface When comparing the material with each other, can be said about the pilot plant tests with an acid slag, that as the least resistant material showed the material D, followed by the material C, B and A. Exposure of the samples was reflected in greater wear of the samples with longer exposure, but this trend has not been demonstrated in all cases. Achievements of laboratory tests can be summarized as follows: Acid slag create on the surface of samples protective layer, penetrated to the samples Basic slag almost not adhered on samples surface, but in contact with her occurred to greater surface wear Occurred to the formation and accumulation of MA spinel below the surface of samples Occurred to the leaching of samples material into the slag, or at least to its diffusion from the centre toward the surface of the sample When comparing all the materials together, can be said about the laboratory corrosion tests with strongly acidic and strongly alkaline slag, that high-magnesium material with carbon content 5 to 12% are suitable for contact with the slag. In contrast to materials with increasing content of Al2O3 or directly to its base, which are already much less suitable. In the case of pilot plant test occurred in a bath to free convection and therefore can be seen on samples partially submerged in the melt level-corrosion in the form of the necks, as demonstrated in Table 1. Illustrates in Table 1 also shows that there was wall corrosion of samples in the form of re-profiling of the samples and from their shape can be concluded that there were changes in melt density. In our case, there 5

6 was usually an increase in melt density due to the increasing concentration of the sample in solution in the melt, which corresponds to the shape of samples. In laboratory tests can be seen more clearly one of the basic types of corrosion only in one case, which is shown in Table 2 on the left. A comparison of the conditions of both experiments, it is clear that the pilot tests are much more similar to real operation and therefore the results can be regarded as a more conclusive. The results of both types of corrosion tests, however showed, that the magnesium-carbon materials, or magnesium- carbon with low Al2O3 are more resistant to the used corrosive media and thus seem more suitable for use in lining slag lines and impact points of ladles, while the materials Al2O3-based, it is advise only for stressed places without contact with the slag. ACKNOWLEDGEMENTS This report was elaborated in the scope of the grant project no. FI-IM5/185, which were carry out through the financial assistance of Department of Industry and Trade Czech Republic. LITERATURE [1] KUTZENDÖRFER,J. Koroze žárovzdorných materiálů. Řada P-1 Praha: Silikátová společnost, s. ISBN [2] RÁBL, V. Žárovzdorné vyzdívky. Protokol č. LP/08 V042/145. Ostrava: ArcelorMittal Ostrava a.s., červenec s. [3] RÁBL, V. Žárovzdorné vyzdívky. Protokol č. LP/09 V052/191. Ostrava: ArcelorMittal Ostrava a.s., říjen s. [4] RÁBL, V. Žárobetonové vyzdívky. Protokol č. LP/08 V078/274. Ostrava: ArcelorMittal Ostrava a.s., listopad s. 6