Sanicro 28 was originally developed for use in the manufacture of phosphoric acid, especially for heat exchangers in the concentration unit, where corrosive conditions are at their worst.
Phosphoric acid, manufactured by the "wet" method, contains varying concentrations of impurities derived from the raw material, the phosphate rock. The most dangerous of these impurities are chlorides, Cl – , and fluorides in free form, F – . Laboratory tests carried out in wet process phosphoric acid have shown that Sanicro 28 is far more resistant to impurities of this kind than other high alloy materials. Figure 1 shows the corrosion rate in contaminated phosphoric acid at different chloride concentrations.
Temperature is another factor that has a great influence on corrosion. See figure 2.
Laboratory tests at 200°C (390°F) in contaminated 95% super phosphoric acid gave the following corrosion rates after 20 days: Sanicro 28, 0.03 mm/year (1.2 mpy); Alloy 904L, 0.10 mm/year (4.0 mpy); UNS N08020, 0.23 mm/year (9.2 mpy); Alloy G, 0.03 mm/year (1.2 mpy).
Table 1 Chemical compositions of materials tested
Figure 3 is an isocorrosion diagram for Sanicro 28, Alloy 904L and AISI 316L in deaerated sulphuric acid. As can be seen from the figure, Sanicro 28 is more resistant than the other alloys. Naturally aerated sulphuric acid is more corrosive than deaerated acid in the intermediate concentration range. Sanicro 28 exhibits very good corrosion resistance in concentrated acid.
Sulphuric acid is sometimes contaminated with chlorides which increases the corrosivity of the solution. However, Sanicro 28 has good resistance, better than 904L, also in chloride contaminated sulphuric acid, especially at high concentrations. Above about 20% sulphuric acid Sanicro 28 is even more resistant than the super-duplex stainless steel SAF 2507, see iso-corrosion diagram in figure 4.
Sanicro 28 is more resistant to hydrochloric acid than stainless steels with lower chromium and molybdenum contents and can, therefore, be used to advantage in cases where chemical process solutions are contaminated with hydrochloric acid, see iso-corrosion diagram in figure 5.
Sanicro 28 resists hydrofluoric and hydrofluosilicic acid very well and can be used where these acids occur as impurities (see corrosion diagram for hydrofluoric acid, figure 6). Both Sanicro 28 and AISI 316L are completely resistant to pure acetic acid at all temperatures and concentrations at atmospheric pressure. However, at elevated temperatures and pressures, AISI 316L will corrode while Sanicro 28 will remain resistant. Acetic acid is often contaminated with formic acid, which renders it more corrosive. Laboratory tests show that Sanicro 28 is more resistant than AISI 316 and AISI 317L in such solutions.
Sanicro 28 is far more resistant to formic acid than conventional stainless steels of the AISI 316L type and more resistant than 904L, see isocorrosion diagram in figure 7. In nitric acid Sanicro 28 performs also very well. In test according to ASTM A262 Practice C (Huey test, 5x48 h in boiling 65% HNO3) corrosion rates lower than 0.15 mm/year (6 mpy) are obtained.
The high alloying contents of chromium and nickel give Sanicro 28 considerably better resistance to sodium hydroxide than standard stainless steels of the type AISI 304 and AISI 316. At moderate temperatures and concentrations, Sanicro 28 is a suitable alternative to pure nickel, which may be attacked by erosion corrosion.
At high temperatures the general corrosion rate increases. The risk of stresss corrosion cracking (SCC) also increases when chlorides are present. Table 2 and 3 demonstrate the good resistance of Sanicro 28 against general corrosion and SCC in sodium hydroxide contaminated with chlorides.
Table 2. SCC in boiling 43% NaOH + 6.7% NaCl, 142°C (288°F), 500h.
|Alloy 800||Yes, cracks <120μm|
Yes, cracks <150μm
Table 3. General corrosion in NaOH and in NaOH+NaCl, mm/year.
|8% NaCl||6.7% NaCl|
As can be seen, Sanicro 28 is superior to both Alloy 800 and Alloy 904L.
Sanicro 28 can withstand very high temperatures in aggressive environments without being attacked by pitting. Figure 8 shows the critical pitting temperature (CPT) for some alloys in chloride-bearing water with a salinity comparable to that of sea water. The figure shows that Sanicro 28 has a higher critical pitting temperature (CPT) than Alloy 904L and Alloy 825 even in acidic chloride solutions. The curves are displaced at higher temperatures in solutions with lower salinities.
Laboratory tests show that Sanicro 28 has good resistance to crevice corrosion. In tests according to ASTM G-48 method B (6% iron(III)chloride), the material exhibited better resistance than Alloy 825.
Stress corrosion cracking
Ordinary austenitic steels of the AISI 304 and AISI 316 types are susceptible to stress corrosion cracking (SCC) in chloride bearing solutions at temperatures above about 60°C (140°F). This susceptibility declines with increasing nickel content. Chromium contents above 20% can also be beneficial. Sanicro 28, which is alloyed with 27% Cr and 31% Ni, exhibits very good resistance to SCC, both in laboratory tests and in practice. This is demonstrated in figure 9, which shows results of SCC tests in a 40% calcium chloride solution.
Tensile specimens which were spring-loaded to stresses close to the proof strength and tested for SCC in aerated water at temperatures of up to 200–250°C (390–480°F), were not attacked, see figure 10. These tests were performed in autoclaves with an oxygen content in the water of 4.6 to 10 ppm and a pH-value at room temperature of 4.5-7.1. The testing time was 1000 hours. The curve for AISI 316/316L and AISI 304/304L is based on experimental data and practical experience.
Sanicro 28 also displays very good resistance to SCC in environments where hydrogen sulphide is present together with chlorides. This is true for both solution annealed and cold worked material, as well as for welded joints. For further information, see Sandvik R&D lecture S-58-7-ENG.
The TTC diagram, figure 11, shows results of intergranular corrosion testing according to ASTM G-28 (120 hours in boiling iron(III)sulphate and sulphuric acid solution). As the figure illustrates, Sanicro 28 can be kept in the critical interval of 600-700°C (1100-1300°F) for at least 30 minutes without intergranular corrosion occurring in this highly corrosive medium. As can be seen in figure 11, Alloy 904L is more susceptible to intergranular corrosion than Sanicro 28. In normal welding operations, heat input to the parent metal takes place for a much shorter time than 30 minutes. This means that the risk of intergranular attack after welding of Sanicro 28 is minimal, which is also verified by tests on welded specimens.