RAT - Preventing Erosion in Submerged Nozzles
Tests performed by Ogata et al.1 have confirmed that an increased ZrB2 content reduces corrosion. However, the results vary considerably for different slag compositions due to the strong assault of boric acid (B203), which builds during the oxidation of the ZrB2 on the stabilized Zr02. This reaction releases a stabilizer additive and modifies the Zr02 into an unstabilized material, which is attacked more vigorously by the slag.
In accordance with the experiments carried out by Ogata et al.1, tests were carried out with comparable slag types. A commercial composition of Zr02-C was chosen as the matrix material. The Zr02 was exchanged with ZrB2 in 2:1 and 1:2 ratios. Additionally, a metallic component, alumina (Al), was added to enable the B203 to react to a boride and therefore prevent the attack on the Zr02 stabilizer. The Al works as an antioxidant and has already proven itself in combination with boron compounds in magnesium oxide-carbon (Mg0-C) refractory material.
ExperimentsFive test samples were produced in conditions simulating actual production at Didier Werke AG in Germany. The composition of the samples is listed in Table 1. The Zr02, partly stabilized with calcium oxide (Ca0), was used in two grain fractures (<500 µm and <100 µm). The <100 µm proportion was substituted with <100 µm of ZrB2 (Samples 2-5). Three percent Al was added as additional metallic component in Samples 3 and 5.
The test samples, with a cross sectional area of 20 by 20 mm, were fixed in a lid of refractory concrete and suspended for 135 minutes in the 1600°C melt. They were submerged at a depth of 50 mm.
To evaluate the results, the corroded surface area of the test piece was compared before and after the experiments through digital picture analysis. The test pieces were also metallographically examined.
The tests and analyses of the results were carried out by DIFK in Bonn, Germany.
Results and DiscussionThe rate of corrosion in the various tests is shown in Figure 1. For both slags, the corrosion reduces as the test number increases. The best results were delivered by the compositions with a Zr02:ZrB2 ratio of 1:2 and an addition of 3% Al. Compared to the matrix test (Sample 1), the corrosion rate is almost halved in tests with both slags (Sample 5), and Slag 2 demonstrates stronger aggression than Slag 1.
Improving Corrosion ResistanceThe fact that adding ZrB2 to Zr02-C material prevents corrosion in the slag region of submerged nozzles has been described on many occasions.1,2 Previous tests have shown, however, that these improvements are only minor, especially with a slag composition like that of Slag 1. For this reason, ZrB2, which is relatively expensive compared to other materials, is only being used for submerged entry nozzles in special cases within industry, such as the strongly corrosive Slag 2.
However, as the results in Figure 1 demonstrate, a clear increase in corrosion resistance is provided for both slags used. The higher the proportion of ZrB2 in the Zr02-C material, the less rapid the corrosion, and the greater the positive effect of the metallic antioxidant.
Adding ZrB2 to the Zr02-C material provides a reduction of the wettability through the slag, thereby preventing its penetration into the microstructure. Additionally, it provides a moderate build-up of B203 through oxidation, which slows the attack on the graphite. Adding Al further reduces the oxidation of ZrB2 and, thus, its transformation into the thermo-dynamically unstable Zr02.
Substituting Zr02 with ZrB2 in a ratio of 1:2 and simultaneously adding Al provides an almost 100% improvement in the corrosion resistance against the slag, making the use of ZrB2 a sensible cost-effective solution.
AcknowledgementsThe authors would like to thank the Didier Research Institute Wiesbaden for manufacturing the test pieces. We would also like to thank Dr. K. Mavrommatis (Inst. für Eisenhüttenkunde, RWTH Aachen), Dr. M. Ollig and Dipl.-Ing. G. Routschka (both from DIFK GmbH, Bonn) for their technical support throughout the study.
For More InformationFor more information about the ZrB2 additives, contact Dr. Ing. Siegfried Prietzel, Elektroschmelzwerk Kempten GmbH, P.O. Box 1526, D-87405, Kempten, Germany; (49) 831-5618-506; fax: (49) 831-5618-495; or e-mail firstname.lastname@example.org.
*Dr.-Ing. K. Hunold and Dr.-Ing. S. Prietzel; Elektroschmelzwerk Kempten GmbH
**Dipl.-Ing. U. Kross, Prof. Dr.-Ing. habil. J. Pötschke and Dipl.-Ing. M. Thiesen, DIFK (Deutsches Institut für Feuerfest und Keramik) GmbH