Evaluation of Anticorrosion Performance of New Materials for Alternative Superheater Tubes in Biomass Power Plants

Material design

Because austenite stainless steel has good performance for its face-centered cubic of austenite structure [5–7], Cr–Ni series steel has been taken into account for new materials. In order to strengthen anticorrosion performance, chromium was mainly considered by improving its concentration. At the same time, austenite structure was also a must. Table 1 lists chemical compositions of newly designed material. In the column of “sample series,” “1xx” indicates the first kind of Cr–Ni materials, and “4xx” means the fourth kind of Cr–Ni materials. These four materials were manufactured by smelting method. TP316 is used as a reference sample for performance comparison of material.

Samples and arrangement

Material samples were fabricated into sheet specimens of 50 × 25 × 2 mm. The surface of each sample was treated in turn by the metallurgical sandpaper, ethanol and acetone solution. Then each sample was treated with the sandpaper and placed in a drying box at 150°C for 2 h. Weighing and measuring the length/width and thickness, microstructure characterization was carried out before the experiment.

Material samples were placed in a box-type resistance furnace for testing. With the help of a fine ceramic tube, as shown in Figure 1, metal samples were suspended in the top of the alumina crucible with size of Φ200 × 150 mm. Before the start of each experiment, KCl electrolyte was put in the bottom of the crucible and can form a certain thickness of the salt bed. When samples were heated to high temperature, solid KCl was heated and transformed into KCl vapor. The former samples would only endure KCl vapor. The latter samples would endure both KCl vapors and high-temperature KCl electrolyte.

Figure 1:

Schematic view for sample arrangement of high-temperature corrosion experiment.Note: 1, crucible; 2, crucible groove; 3, suspended sample; 4, fine ceramic tube; 5, KCl.

Corrosion experiments

The experimental temperatures were set at 600°C, 650°C and 700°C. In accordance with “simple static oxidation test” [8], weight method was used for evaluation of high-temperature corrosion kinetics. The exposure time was set to 30 h. During experimental process, samples would be taken out in different times, such as 1, 4, 7, 10, 20 and 30 h. Each sample would be checked and measured for observation of their morphology change and massive change. The corrosion kinetic curves and oxidation kinetic curves can be drawn according to weight changes data from experimental time. Different material would present completely different kinetic law. During the analytical process, Origin 9.0 and Matlab 2013b were employed for data analysis and modeling.

High-temperature experiments for different material

Figure 2 shows weight changes during the corrosion process in KCl for four test materials and reference TP316 material in 600°C. It can be seen there was a small quantity of weight increase for all materials. During the first 15 h, TP316 had higher corrosion increases than other materials. In the final experimental stage, the largest corrosion increase was not higher than 1.5 mg/cm². Figure 3 shows weight changes during the corrosion process in KCl for four test materials and reference TP316 material in 650°C. TP316 showed negative weight change during the corrosion process, which indicated that layer materials have poor adhesion property in the surface of the material matrix due to spalled oxide flakes found in the furnace after the exposure. Tested material, from 1xx to 4xx, showed relatively low weight increase. The largest corrosion increase was no higher than 4.0 mg/cm².

Figure 2:

Weight changes during the corrosion process in KCl for four test materials and reference TP316 at 600°C.

Figure 3:

Weight changes during the corrosion process in KCl for four test materials and reference TP316 at 650°C.

Figure 4 shows weight changes during the corrosion process in KCl for four test materials and reference TP316 material in 700°C. It can be seen that 4xx material cannot endure such condition and showed obvious negative weight change during the corrosion process. The tendency was even worse than that of TP316 material. In the final experimental stage, 3xx material showed the tendency of the weight decrease. The quality of the layer on the surface of 3xx material was inferior to 2xx, 1xx material. Figure 5 shows weight changes of high-temperature oxidation processes in air environment for four test materials and reference TP316 material in 700°C. We can see that TP316 and 1xx material showed relatively higher weight increase during the corrosion process. The largest value of the weight increase was not higher than 0.35 mg/cm², which showed that the oxidation process was relatively stable and the oxidation layer was of denser materials with possible protective ability. All materials have no phenomenon of negative weight change, which displayed that the oxidation layer was in a good state, especially in condition of 700°C.

Figure 4:

Weight changes during the corrosion process in KCl for four test materials and reference TP316 at 700°C.

Figure 5:

Weight changes for four test materials and reference TP316 at 700°C in air.

Experimental results in different temperatures

Because 4xx presented inferior anticorrosion ability in Figure 4 and 1xx presented inferior antioxidation ability in Figure 5, 2xx and 3xx materials were mainly analyzed in different temperatures. Figure 6 shows weight changes of 2xx material in different temperatures. Except 700°C air conditions, other experiments were all in KCl medium condition. We can note that the high-temperature oxidation process displayed the smallest corrosion increase. In KCl condition, with increase of temperature, the corrosion increase increased quickly and obviously. Figure 7 shows weight changes of 3xx material in different temperatures. Except 700°C air conditions, other experiments were all in KCl medium condition. We can see that the tendency of 3xx material was similar to that of 2xx material. In 650°C KCl conditions, 3xx materials displayed highest weight increase in all four experiments. In 700°C KCl conditions, there was the appearance of negative weight change, which may result in nonprotective layer with poor adhesion property to the material surface.

Figure 6:

Weight changes during the corrosion process in different temperatures for 2xx material.

Figure 7:

Weight changes during the corrosion process in different temperatures for 3xx material.

Comparison analyses of anticorrosion ability

According to Figure 2, in 600°C conditions with saturated KCl environment, TP316 showed longer anticorrosion lifetime in relatively low temperature condition. However, Figures 3 and 4 indicate that TP316 presented the obvious disadvantages to 650°C and 700°C conditions. In comparison, 2xx and 3xx materials had better oxide layer on the surface of tested samples in such conditions because of relatively lower weight changes during corrosion process.

Table 2 displays analytic data of correlation coefficient for five materials in 700°C, KCl condition. Spearman correlation analysis was employed due to non-normal distribution data of the corrosion process. It can be seen that 1xx and 4xx materials had very high negative correlation with r=−0.96429. There was also a high correlation between 2xx and 3xx materials with r=0.92857. TP316 had great similarity in anticorrosion performance with 1xx and 4xx materials, while it had no correlation with 2xx and 3xx materials.

Fitting function models of corrosion kinetic processes for four newly designed materials

Table 3 shows the comparison of fitting function of five different materials in 700°C, KCl condition. Much corrosion data were treated with Matlab 2013b. Fitting accuracy was not allowed to be less than 0.99 for R² of fitting function. Based on these fitting functions, weight increase quantity can be predicted for longer corrosion time, such as 50 h. Table 3 indicates that predicted values for 50 h were relatively better for 3xx material.

Anticorrosion lifetime predictions of newly designed materials in biomass power plant

Table 4 shows the comparison of fitting function of three temperatures for 3xx material in KCl condition. According to these fitting functions, anticorrosion lifetimes can be estimated for a special weight increase of 50 mg/cm². Table 4 indicates that 3xx material can endure 37.6 h in 700°C KCl conditions when weight increase is 50 mg/cm². However, TP316 and 4xx materials displayed very early spalling according to Figure 4.