Refractories, Corrosion testing

STATE OF THE ART REFRACTORY CORROSION TEST WORK FOR THE NONFERROUS METALS INDUSTRY... 

State of the Art Refractory Corrosion Test Work for the Nonferrous Metals Industry.231... 

The simplest corrosion test is based on placing the respective melt into a hole drilled in a brick of the respective refractory. After thermal exposure, the brick is cut cross-wise and the depth of melt infiltration, enlarging of the hole or other signs of corrosive attacks at the interface are assessed on the cross section. 

Flayden [52] conducted a controlled series of alkali corrosion tests of commercial refractories using the crucible cover technique illustrated in Fig. 13. This test involved placing a refractory sample over a crucible containing a premeasured amount of alkali carbonate and heating the assembly to 1350°C for 12 h. A furnace containing a load of test samples is... 

Phase II - Pilot Plant Testing. This phase of coal gasification materials evaluation involves exposure of metals and refractories in selected test locations in current pilot plants. Corrosion testing in this effort began during 1974 with four pilot plants and now involves six HYGAS, CONOCO COAL, Synthane, BI-GAS, Steam-Iron, and Battelle. 

This study is a result of a collaboration between Stillwater Mining Company and RHl AG aimed at the determination of refractory products to improve the lining life in the customer s vessel by using optimized corrosion testing methods at RHTs Technology Center. 

While corrosion testing of metallic materials is the focus here, planning and design of environmental perfoirnance tests for nonmetaUic materials, such as paints, plastics, ceramics, and refractories is similar. Specific corrosion test methods have been developed by ASTM International [70-12], NACE International [73], and the International Organization for Standardization (ISO) [74]. 

A refractory sample exposed to a temperature of 1300°C was mounted on a stainless steel shaft with a section of stainless steel tubing placed around the shaft which in turn served as a high temperature "sacrificial anode. Corrosion tests were conducted at rotation speeds of 60 and 120 rpm, along with a few under-static conditions. 

Crites, M. D. and Schlesinger, M. E., Corrosion Testing of Chrome-Free Refractories for Copper Production Fiu-naces," Proceedings of Copper 99-Cobre 99 International Conference, Volume Vl-Smelting, Technology Department, Process Modeling and Fundamentals, The Minerals, Metals and Materials Society, 1999, pp. 187-194. 

Two refractory groups will be considered in this paper, AZS based and alumina based. For AZS based products, refractories made with fusion-casting, dry pressing, and vibro-casting processes will be considered. The Alumina based products include only fusion-cast and vibro-cast rehactories. Although the bulk of the paper will focus on performance of these materials in a corrosion test, some information on glass defect potential will also be provided. 

In order to better understand the below melt line corrosion behavior, a crucible test was performed next. The crucibles had approximately 3 inside diameter and approximately 2.25" depth of cavity. Therefore, this sample represents a large area and is more representative of coarse-grained refractory products. Figure 6 shows the bonded and fusion-cast AZS samples, cut in half, following a corrosion test at 1427°C for 96h. Although the bonded AZS sample showed more material loss at the metal line compared to fusion-cast AZS sample, the bottom sur ce of both crucibles showed no material loss and similar visual appearance. 

The use of bonded AZS refractories as paving blocks in glass furnace bottoms for the past several years suggests that these refractories can indeed be used, in place of fused-cast AZS, in some furnaces. However, this study clearly showed the inadequacy of common finger corrosion test and the use of below metal line data in accurate comparison of different bonded AZS refractories. Crucible test used in this study could not differentiate between different bonded AZS refractories available commercially. 

Laboratory corrosion testing has been in existence for many years, and yet there is no uniformity in these attempts to evaluate the corrosion resistance of specific refractory for specific conditions. Roughly, corrosion tests may be divided into static and dynamic tests. The term static usually means that the corrosive liquid is stationary, it doesn t move against the tested refractory, and the system corrosive liquid-refractory will tend to reach equilibrium. During a dynamic test, the corrosive liquid moves against the refractory surface and the thermodynamic and chemical equilibria wmi t be reached. 

Finally, the aim of the applied research on corrosion testing is to find suitable and relatively inexpensive and simple procedures to evaluate whether a refractory will have a long service fife in a specific metallurgical process. In real metallurgical processes, too many factOTS affect the corrosion that s the reason for the lack of uniformity in corrosimi testing. The same may be said about standardization Only a few corrosion tests are standard. 

The variation of the cup corrosion test is [183] in using the graphite crucible for electrolyte. The porosity of the graphite crucible may vary. The graphite crucible is placed on a refractory layer. In this type of test, the sodium vapor is taken into account. If the porosity of the graphite crucible is 12-14 %, the result depicts the interaction of refractory with sodium, which moves in graphite. If the porosity of graphite is 28 %, the result depicts the interaction due to the capillary movement of... 

Siljan [187] has found a good correlation between the corrosion resistance in a cup cryolite corrosion test and the depth of cryolite infiltration. A 220-kA reductitm cell was lined with eight different alumina silica refractory materials and switched off for investigation of refractory conditions after 500 days. The results proved the good correlation. However, it appears that it is not a strict correlation from a physicochemical point of view between the interaction during hours and the kinetics of penetration of the melt within many months. 

Gao B, Wang Z, Qiu Z. Corrosion tests and electrical resistivity measurement of SiC-Si3N4 refractory materials. Light Met 2004 133 419-24. 

Allaire C, Pelletier R, Siljan OJ, Tabereaux A. An improved corrosion test for potlining refractories. Light Met. 2001 130 245-9. 

Oprea G. Corrosion tests on refractories for aluminium electrolytic cells, Proceedings of the 9th symposium on refractories for the alirminum industry, 1999, p. 189-205. 

Na corrosion tests of refractory alloys were carried out for potential use of the alloys for cladding materials of Na-cooled fast breeder reactors. Compatibility tests for Mb [77] and Ta [78] in Na showed that the increase in O level in Na accelerates oxidation corrosion. The O control in sodium is cmcial for keeping good compatibility with these materials. 

In most cases, we cannot avoid simulation methods (corrosion tests in the laboratory) which can guide us in choosing the refractoiy. However, industrial reality is complex. The examination of refractories used on different scales associated with techniques of analysis (chemistry, x-ray diffraction, electronic microscopy) is essential in order to identify the mechanisms and to give directions to the solutions. 

Because sulfuric acid and halogen are very corrosive, selection of the structural materials is an important issue. Screening tests have been carried out using test pieces of commercially available materials at GA [29], JAEA [30,31], etc. As for the gas-phase environment of the H2S04 decomposition step, some refractory alloys that have been used in conventional chemical plants showed good corrosion resistance. Figure 4.13 shows one of the experimental results of Alloys 800 and —600 obtained under gas-phase sulfuric acid decomposition environments at 850°C. Gas compositions in the upstream and downstream... 

Plastic pellets, threat from, 20 231 Plastic pumps, for corrosive liquids, 21 76 Plastic refractories, 21 482 Plastics. See also Styrene plastics adhesion of coatings to, 7 91-92 advanced materials, 1 693 cellulose ester applications, 5 404 chemical exposure tests on, 19 583 citric acid application, 6 648 colloidal suspensions, 7 273t colorants for, 7 358-380 development of, 10 168... 

In spite of the knowledge available through current research tests and studies, the best information can only provide relative performance predictions between various types of refractories. Unit operation variables also affect the service life of refractory linings which contain these corrosive atmospheres. 

Valuable information on the corrosion process is provided by phase diagrams if they are available for the given system. They show whether the substances in question actually react mutually producing a melt, what is the respective saturated concentration (equilibrium melt composition), and if any new products are formed. However, phase diagrams are often not available for multicomponent systems the other characteristics determining the corrosion rate are usually also unknown. The resistance of refractories to corrosion is therefore in practice determined by tests providing useful Complex information which, however, holds exactly only for the test conditions. 

Corrosion of the glass-making melters must be maintained at an absolute minimum to increase the lifespan of the melter. Laboratory-measured corrosion rates indicate that melter lifetimes of several years can be achieved with high chrome oxide or zircon refractories metallic melters may have lifetimes of several months if alloys such as Inconel 690 are used. These conclusions have been reached on the basis of extrapolation of laboratory tests. Long-term tests, particularly with waste glasses in engineering-scale continuous melters, have not yet been made. 

To determine the mechanisms of alkali attack and resulting reaction products, laboratory-prepared powders of alkali and alumino-silicate refractories have been fired at various temperatures. Results obtained from these reaction tests have supported the findings from the field samples. Alkali slag tests are performed by the various manufacturers to determine how their products withstand corrosion by alkalies. 

Although many different types of investigations in the area of alkali corrosion of refractories have been performed, there still appears to be doubt regarding the best method for testing. In addition, there are few comparative studies in the literature showing the differences in alkali corrosion of the different alumino-silica refractories. Studies concerning silicon carbide bricks exposed to alkalies are practically nonexistent. 

In the second phase of his study, Yamaguchi [28] investigated the corrosion of certain burned refractories by sodium carbonate vapor. He suspended the test piece with a platinum wire from the bottom of an alumina crucible placed upside down. The entire assembly was heated at 1200°C for various times. Included in this part of the study was a fireclay refractory composed of mullite and silica minerals. Mullite reacted with NazO to form nepheline and alumina. The nepheline increased in amount as the remaining soda vapor reacted with the newly formed alumina and the preexistent silica. For refractories composed of mullite and corundum, carnegieite solid solution was the major reaction product. The formation Yamaguchi described resulted when Na2C03 vapor reacted with the alumina liberated from mullite and preexistent as corundum, forming... 

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