Sanicro® 28

Due to its outstanding corrosion properties, Sanicro® 28 can be used in the most diverse environments. Listed below are a few examples of applications for which this alloy is particularly suitable.

Phosphoric acid: Today, Sanicro® 28 is the most widely used metallic material for evaporator tubes in the manufacture of phosphoric acid by the "wet " method. Several units have now been in service for more than 10 years. The graphite heat exchangers, replaced by Sanicro® 28, often had repeated problems with broken tubes and loss of production.

Sulphuric acid: Sanicro® 28 is a suitable material for piping and heat exchangers, particularly at concentrations of between 40 and 70% of deaerated acid and over 85%. Sanicro® 28 has approximately the same resistance as Alloy C in concentrated acid (98% H₂SO₄).

Oil and gas: Sanicro® 28 is used for production tubing, casing and liners in deep, sour gas wells. The material is also recommended for oil wells with a corrosive environment. For these purposes, tubes are supplied cold rolled with high strength. In the solution annealed condition, Sanicro® 28 is also used as piping for transporting of corrosive oil and gas and for heat exchangers in treatment facilities. Sanicro® 28 wirelines are used for lowering tools and controlling instruments in deep oil and gas wells.

Fluoride-bearing media: Fluoride-bearing off-gases can form during the manufacture of phosphoric acid and mixed fertilizers. These off-gases must be disposed of for environmental reasons. Sanicro® 28 is ideal for this purpose. Tests have shown it to be preferable to higher alloyed CrNiMo grades for the recovery of fluoride-bearing gypsum.

Nuclear power plants: Due to its high resistance to SCC, pitting and crevice corrosion, Sanicro® 28 has been selected for heat exchangers in nuclear power plants.

Seawater and chloride-bearing cooling water: Its high resistance to pitting and crevice corrosion makes Sanicro® 28 a very suitable material for seawater-carrying piping and seawater-cooled heat exchangers. This is confirmed by practical experience. Sanicro® 28 has replaced nickel alloys, CuNi, bimetallic tubes and coated carbon steel tubes, which failed due to corrosion. The performance of Sanicro® 28 has been excellent. In seawater-cooled heat exchangers and heat exchangers that work with chloride-bearing cooling water, Sanicro® 28 offers high corrosion resistance to both the water and the cooled medium. When a seawater-cooled plant is shut down, there is no need to drain the piping system or flush with fresh water, provided that the shutdown period is shorter than one month and the water temperature is lower than 30°C (85°F). The duplex stainless steel Sandvik SAF 2507® is more resistant than Sanicro® 28 in sea water.

Approved by the American Society of Mechanical Engineers (ASME) for use in accordance with:

The excellent formability of Sanicro® 28 permits cold bending to very small bending radii. Annealing is not normally necessary after cold bending.

For operating temperatures over 400°C (800°F), heat treatment should be carried out after bending to ensure good ductility after prolonged service times.

General corrosion: 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 ASTM 316L in deaerated sulfuric acid. As can be seen from the figure, Sanicro® 28 is more resistant than the other alloys. Naturally aerated sulfuric acid is more corrosive than deaerated acid in the intermediate concentration range. Sanicro® 28 exhibits very good corrosion resistance in concentrated acid.

Stress corrosion cracking: Sanicro® 70 has very good resistance to stress corrosion cracking, particularly in chloride-bearing solution.

Sulfuric 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 sulfuric acid, especially at high concentrations. Above about 20% sulfuric acid Sanicro® 28 is even more resistant than the super-duplex stainless steel SAF 2507®, see isocorrosion 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 isocorrosion 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 stress 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.

Table 3. General corrosion in NaOH and in NaOH+NaCl, mm/year.

Pitting corrosion: 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.

Crevice corrosion: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 sulfide 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.

Intergranular corrosion: The TTC diagram, figure 11, shows results of intergranular corrosion testing according to ASTM G-28 (120 hours in boiling iron(III)sulphate and sulfuric 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.

Sanicro® 28 is expanded into tube sheets in the same way as standard austenitic stainless steels.

Tubes are delivered in the heat treated condition. If another heat treatment is needed after further processing, the following is recommended.

Solution annealing: 1100-1160°C (2010-2120°F), 10-30 minutes, quenching in water. Thin walled tubes can also be cooled rapidly in air.

The machining of Sanicro® 28, as with other stainless steels, requires an adjustment of tooling data and machining method, in order to achieve satisfactory results. Compared to Sanmac® 316/316L, the cutting speed must be reduced by approximately 50-55% when turning Sanicro® 28 with coated, cemented carbide tools. Much the same applies to other operations. Feeds should only be reduced slightly and with care.

Detailed recommendations for the choice of tools and cutting data are provided in the brochure S-02909-ENG. Data should be selected as for steel grade Sanmac® 316/316L, while taking into account the provisions above.

Forms of supply:

Seamless tube and pipe is supplied in dimensions up to 260 mm outside diameter in the solution annealed and white-pickled condition, or solution annealed in a bright annealing process. Seamless tubes are available from stock in ANSI pipe and heat exchanger tube sizes. Details of our manufacturing and stock programme are given in catalogue S-110-ENG. Using modern bending equipment, Sandvik can bend tubes to customers' particular requirements and, if required, anneal after bending.

Materials for oil and gas production

Cold worked seamless tube and pipe: For production tubing and casing in oil and gas production. Sanicro® 28 is supplied cold worked with high strength properties (Sanicro® 28-110 and Sanicro® 28-125). Specific Oil and Gas information is available on request.

Other forms of supply

Further details concerning sizes and finishes are available on request.

The weldability of Sanicro® 28 is good. Suitable methods of fusion welding are manual metal-arc welding (MMA/SMAW) and gas-shielded arc welding, with the TIG/GTAW method as first choice.

In common with all fully austenitic stainless steels, Sanicro® 28 has low thermal conductivity and high thermal expansion. Welding plans should therefore be carefully selected in advance, so that distortions of the welded joint are minimized. If residual stresses are a concern, solution annealing can be performed after welding.

For Sanicro® 28, heat-input of <1.0 kJ/mm and interpass temperature of <150°C (300°F) are recommended. A string bead welding technique should be used.

Recommended filler metals:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 27 31 4 Cu L/AWS A5.9 ER383 (e.g. Exaton 27.31.4.LCu)
• MMA/SMAW welding
  • ISO 3581 E 27 31 4 Cu L R/AWS A5.4 E383-16 (e.g. Exaton 27.31.4.LCuR)