Influencing Factors and Measuring Methods of Corrosion Resistance
Corrosion resistance refers to the ability of refractories to resist the erosion and erosion of various erosion media at high temperature. These erosive media include a variety of slag (blast furnace, electric furnace, converter, refining furnace, non-ferrous metal smelting furnace, calciner, reaction furnace), fuel, ash, flying dust, iron chips, lime, cement clinker, alumina clinker, garbage, liquid molten metal, glass solution, acid and alkali, electrolyte solution, various gaseous substances (gas, CO, sulfur, zinc and alkali vapor).
Corrosion resistance is a very important index to measure the resistance to chemical erosion and mechanical wear of refractories, which is of great significance for the formulation of correct production process and the rational selection of refractories.
The factors that affect the corrosion resistance of refractories are internal and external. The internal factors mainly include the chemistry of refractories, mineral composition, microstructure and other properties of refractories, and the external factors include the properties of erosive media, operating conditions (temperature, pressure, etc.) and the interaction between erosive media and refractories under service conditions.
Chemistry and mineral composition of refractories. The corrosion resistance of different chemical materials is different. Acid refractories have good corrosion resistance to acid corrosion media, but alkaline refractories have very weak ability to resist acid erosion media erosion.
Refractories are multiphase aggregates, which are composed of main crystal phase and matrix. The refractory degree of the main crystal phase is high, the grain size is large, the grain boundary is less, and the corrosion resistance is relatively good. If the impurity content in the matrix is high, it is easy to form liquid phase, and if the viscosity of the liquid phase is low, it is disadvantageous to the corrosion resistance of the material.
The structure of the refractory material. It mainly refers to the distribution and combination of the materials in the refractory material and the number, size, shape and distribution of the air holes.
Other properties of the refractory. The bulk density, apparent porosity, thermal shock resistance, oxidation resistance and high-temperature volume stability of the refractory material have a great influence on the erosion resistance. The dense material with high bulk density and low porosity has better corrosion resistance; when the material with poor thermal shock resistance receives the thermal shock, cracks or cracking and peeling can occur, so that the erosion medium enters the interior of the material, and the erosion resistance is reduced; the carbon-containing refractory material with poor oxidation resistance, the decarbonization layer is formed after the surface is oxidized, the structure is loose and easy to fall off, the erosion resistance is reduced, and the material with poor high-temperature volume stability can generally make the anti-erosion resistance To get worse.
The influence of erosive media. It mainly refers to the chemical composition, acidity and alkalinity, viscosity, temperature, flow velocity (static or dynamic), pressure and atmosphere (to gaseous erosion medium, oxidation, reducibility, etc.).
The interaction between refractories and erosive media. The reaction of refractories with erosive media to form high melting point or high viscosity phase is beneficial to reduce the erosion of materials.
The influence of the conditions of use. It mainly includes temperature and fluctuation, pressure, atmosphere, contact time and area. High temperature, large fluctuation, large pressure or vacuum, strong atmosphere corrosion, long contact time, large area, the more serious the erosion of the material.
Based on the above analysis, the following measures can be taken to improve the corrosion resistance of refractories:
Improve the purity of raw materials, improve the chemical mineral composition of products, and reduce the content of low melting materials and impurities as much as possible;
Attention should be paid to the selection of refractory materials, as far as possible with the chemical composition of the erosion medium similar refractory materials; in addition, refractory materials in use, it should also be noted that the chemical properties of the materials used should be similar, to prevent or reduce the interface damage reaction between the materials used in high temperature conditions.
Choose the appropriate production method to obtain the product with dense and uniform structure.
Because of the diversity and complexity of erosive media, the test methods for corrosion resistance of refractories should also be different. The test methods of slag resistance, acid resistance, alkali resistance, glass melt corrosion resistance and CO corrosion resistance are introduced in this paper.
resistance to slag
Because all kinds of burden, fuel, ash, fly dust, iron chips, lime, cement clinker, alumina clinker, garbage, liquid molten metal and so on can be broadly called slag, the slag resistance here includes a wide range of contents.
The method for determining the slag resistance of the refractory material is divided into two types: static method and dynamic method.
The static method includes melting cone method, crucible method and immersion method.
The dynamic method includes rotating slag erosion method, rotating immersion method (rotating cylinder method), slag casting method, high temperature slag drop method, slag spraying method and induction furnace method. In addition, there is slag absorption load deformation method and so on.
The national standard GB 8931 / 1988 specifies the test method for the determination of slag resistance of refractories by rotary slag corrosion method. The principle is that the test panel with polygons is composed of sample brick, which is used as the lining of rotary cylinder furnace, heats to the test temperature, and bears the erosion and scour of selected slag according to the specified time. Before and after the test, the thickness of the post-sample brick was measured to compare its slag resistance.
Rotating cylinder method: the cylinder sample is immersed in a lead crucible containing slag, and the erosion test is carried out at a certain high temperature (such as 1600 ≤ 1700 ℃) and rotating velocity (200~500r/min) at a certain time. Then, the sample is raised to tell the rotating sample to get rid of the adhesive slag on the surface of the sample. The corrosion resistance of the material was determined by the size change of the cylinder sample before and after the test. The advantage of this method is that it can simulate the corrosion resistance and scour resistance of different materials under high temperature smelting conditions.
acidoresistance
Acid resistance is the ability of refractories to resist acid erosion. Sulfuric acid is generally used as corrosion agent in the determination of acid resistance of refractory products. The national standard GB/T 17601 / 1998 (equivalent to the international standard ISO 8890 / 1988) specifies the sulfuric acid corrosion resistance test method for dense shaped refractory products. The main points of the test are as follows: the sample prepared by the prescribed method (0.63~0.80mm particles) is etched in boiling sulfuric acid with a concentration of 70% (m ≤ m) for 6 h, and then the mass loss is determined, and the acid resistance is represented by the percentage of the initial mass of the sample (acid corrosion rate). Calculation formulas such as Next:
Ra=(m1-m2)/m1
The acid corrosion rate of Ra= sample is%.
M1 = the initial mass of the sample, g;
M ~ 2 = the mass of the remaining sample after acid corrosion, g.
alkali resistance [fastness]
Alkali resistance is the ability of refractories to resist alkali corrosion at high temperature. In order to determine the alkali resistance of refractories, anhydrous K2CO3 is usually used as erosion medium, there are two methods: mixed erosion method and direct contact melting erosion method.
The national standard GB/T 14983 / 1994 specifies the alkali resistance test method of refractories, which adopts the mixed erosion method. The principle is that at 1100 ℃, K2CO3 reacts with charcoal to form alkali vapor, which erodes refractory samples and forms new alkali metal silicate and carbonate compounds, which changes the properties of refractories. The results were evaluated by naked eye, microstructure test and calculation. Among them, the strength reduction rate and linear change rate are used to express the alkali resistance.
The strength drop rate is calculated according to the following formula:
Pr=(P0-P1)/P0
In the formula, the strength of Pr= decreases by%;
P0 = compressive strength at room temperature before alkali resistance test, MPa;
P1 = compressive strength at room temperature after alkali resistance test, MPa;
The line change rate is calculated according to the following formula:
Lc=(L1-L0)/L0
The linear change rate of the sample after Lc= alkali resistance test is%.
L1 = length of sample after alkali resistance test, mm;
L0 = sample strength before alkali resistance test, mm.
The intensity decline rate is the main evaluation basis, and the line change rate is the auxiliary basis for evaluation.
Resistance to glass melt erosion
Cambali solution erosion is the ability of refractory materials to resist glass melt erosion and scour in glass kilns. The national standard GB/T 10204 / 1988 specifies the test method for static resistance to glass melt erosion of refractories for glass furnaces.
When the refractory is in contact with the glass molten liquid phase, the physical and chemical reactions will be found on the contact surface, thus leaving obvious indentation on the surface of the material. The test method is to measure the depth of the indentation of the sample to express the ability of refractory to resist glass melt erosion under specified conditions.
Anti-CO corrosion
Resistance to CO corrosion refers to the ability to resist cracking or disintegration from refractories in CO atmosphere. When the refractory encounters a strong CO atmosphere at 400 ℃, due to the decomposition of CO, free C will be deposited around the iron point of the material, which will cause the material to crack and damage. Reducing the porosity and iron oxide content of the material can improve its ability to resist CO erosion.
At present, there is no unified test method for the determination of CO corrosion resistance of refractories in the world. Most of them use visual crack and carbon deposition to judge the degree of damage, such as ASTM C288 in the United States and BS 1902 / 3 in the United Kingdom. These methods place the samples to be tested at a certain temperature (the former is 495 ℃, the latter is 450 ℃) in the CO atmosphere (the former CO content is more than 95%, the latter pure CO), content is more than 95%). The corrosion resistance of the material to CO was characterized by the change of the performance of the sample after 200 hours of erosion. Marit Steinmo The expansion measurement method is difficult to evaluate the damage caused by micro-carbon deposition. In addition to observing the failure condition of the sample by visual measurement, Meng Qingmin et al., the damage degree was quantitatively determined according to the changes of compressive strength at room temperature, apparent porosity and mass of the sample before and after CO failure.
In the practical application of blast furnace refractories, the erosion behavior occurs in a mixed atmosphere with hydrogen content of about 7%. In order to simulate the actual situation of CO erosion more accurately and shorten the test time of 200h in the original standard method, some researchers took blast furnace injection as the object to study the resistance of blast furnace injection to CO erosion under the condition of pure CO and 95%CO 5%H2. The results show that the reaction of the same sample in 95%CO 5%H2 gas phase for 24 h is similar to that in pure CO gas phase for 200 h. 95%CO 5%H2 can be used instead of pure DNA. CO is tested for the resistance of refractories to CO erosion to shorten the detection time.