Sandvik 2RK65™: Premium Austenitic Stainless Steel for Extreme Corrosion Resistance

Typical applications for Sandvik 2RK65™ are found in oil refineries and within the chemical and petrochemical industry. Sandvik 2RK65™ is also used within the pulp and paper industry, the mineral and metallurgical industry, the food industry, in seawater cooling and in many other fields.

The grade is an excellent alternative to standard austenitic stainless steels in heat exchangers using high-temperature water with chloride contamination.

Approvals:

At high temperatures: The steel should not be exposed to temperatures above about 550°C (1020°F) for prolonged periods, since this leads to precipitation of intermetallic phases, which can have an adverse effect on both the mechanical properties and the corrosion resistance of the steel.

The good ductility of Sandvik 2RK65™ permits bending in the cold state to the smallest bending radii attainable with modern methods and machines. Annealing is not necessary after cold bending. If, however, the tubes have been heavily cold-worked and are to be used under conditions where stress corrosion cracking (SCC) is liable to occur, solution annealing is recommended (see under 'Heat treatment').

For pressure vessel applications in Germany, heat treatment may be required after cold deformation in accordance with VdTÜV-Wb 421. Heat treatment should be carried out by solution annealing.

General corrosion: The steel was originally developed for use in sulfuric acid. Its good resistance is achieved by virtue of a high molybdenum content and alloying with copper. Figure 1 is an isocorrosion diagram for Sandvik 2RK65™, Sanicro® 28 and ASTM 316L in deaerated sulfuric acid.

Technical phosphoric acid manufactured by means of the 'wet' method contains varying amounts of impurities from the starting material, the phosphate rock. The most dangerous of these impurities are chlorides, Cl-, and fluorides in free form, F-. Sandvik 2RK65 has been used with success in many applications in phosphoric acid plants and for the handling of technical acid. However, for the severest corrosion conditions, Sanicro® 28, which was developed especially for phosphoric acid applications, provides superior corrosion resistance.

In pure acetic acid, both Sandvik 2RK65™ and ASTM 316L are completely resistant at all temperatures and concentrations at atmospheric pressure. At elevated temperatures and pressures, however, ASTM 316L will corrode while Sandvik 2RK65™ will remain resistant. Experience from acetic acid production has shown that acetic acid contaminated with formic acid is always corrosive. In acid of this kind, Sandvik 2RK65 is far more resistant than ASTM 316L, see table 1 below. Practical operating experience has confirmed the superiority of Sandvik 2RK65™ to ASTM 317L as well.

In formic acid, high-alloy Sandvik 2RK65™ shows better resistance than conventional steels of the ASTM 316L type, see Figure 3. In oxalic acid Sandvik 2RK65™ shows better performance than ASTM 316L, see Figure 4. 2RK65 is resistant (corrosion rate <0.1 mm/year) in lactic acid at all concentrations at temperatures up to or slightly below the boiling point at atmospheric pressure. This means a corrosion resistance similar to or slightly better than of ASTM 316L in lactic acid. Due to its molybdenum content, Sandvik 2RK65™ is less resistant to nitric acid than steels of the ASTM 304L and ASTM 310L types, which are commonly used in these environments.

High molybdenum content is an advantage in hydrochloric acid, and Sandvik 2RK65™, with its 4.5% Mo is consequently far more resistant than, for example, ASTM 316L. Sandvik 2RK65 is therefore suitable for use in chemical process solutions containing small amounts of hydrochloric acid. The isocorrosion diagram is presented in Figure 5. The risk of pitting should, however, be kept in mind. Also in hydrofluoric acid Sandvik 2RK65™ benefits from its high molybdenum content, although hydrofluoric acid is an even more aggressive acid compared to hydrochloric acid, see isocorrosion diagram in Figure 6.

Table 1. results of laboratory tests lasting 1+3+3 days in boiling mixtures of acetic and formic acid.

Due to its high chromium and nickel contents, Sandvik 2RK65™ possesses much better resistance in sodium hydroxide than ASTM 304 and ASTM 316, see Figure 7.

As can be seen, the risk of stress corrosion cracking (SCC) increases at high temperatures. This risk is enhanced if chlorides are present. The alloy Sanicro® 28 provides better resistance against stress corrosion cracking and also general corrosion than is the case for Sandvik 2RK65™.

Pitting corrosion: The high chromium and molybdenum contents of this steel make it very resistant to pitting. This has been verified by extensive practical experience of service involving chloride-bearing process solutions and seawater cooling.

As can be seen in Figure 8, the mean critical pitting temperature (CPT) for Sandvik 2RK65™ is around 75°C (165°F) at a potential of 400 mV SCE in a neutral solution (pH = 6) with the same chloride content as seawater. This value is 50°C (120°F) higher than for ASTM 316 and 20°C (68°F) higher than for Alloy 825 (21Cr42Ni3Mo).

Stress corrosion cracking (SCC): Ordinary austenitic steels of the ASTM 304 and ASTM 316 types are susceptible to stress corrosion cracking (SCC) in chloride-bearing solutions at temperatures above about 60°C (140oF). At high temperatures, above about 100°C (212°F), chloride contents as low as in the ppm-range (10-4 %) are sufficient to cause stress corrosion cracking in these steels. A nickel content of 25% is sufficient to provide very good resistance under practical conditions. Laboratory tests in calcium chloride confirm the superiority of Sandvik 2RK65™ in resisting stress corrosion cracking compared to ASTM 304 and ASTM 316. As is shown by figure 9, the threshold stress (the stress necessary to induce fracture within the maximum testing time) is considerably higher for Sandvik 2RK65™ than for ASTM 304 and ASTM 316. Sandvik 2RK65™ is resistant up to at least 0.9 times the tensile strength. Autoclave tests at different chloride contents and temperatures provide valuable data for material selection. Also this type of testing demonstrates the good SCC-resistance of Sandvik 2RK65™, far better than ASTM 304 and ASTM 316 types of steels, see Figure 10. It is important to be aware of the fact that the residual stresses around a weld that has not been heat treated often equal the proof strength of the material. These stresses correspond to applied stress/tensile strength ratios of only 0.3–0.5, which is sufficient to exceed the threshold stress and thereby cause stress corrosion cracking in ASTM 304 and ASTM 316.

Crevice corrosion: Both laboratory tests and practical experience have shown that Sandvik 2RK65™ is substantially more resistant to crevice corrosion than ASTM 316L. This is illustrated in Table 2. Crevices should nevertheless be avoided as far as possible, especially in chloride-bearing solutions. Table 2. Results of crevice corrosion tests in aerated stagnant NaCl solution (1.8% Cl⁻) pH = 6, test period 58 days. The area ratio between creviced and non-creviced surface on the specimen is 1/12.

Metric units

Imperial units

The tubes are delivered in heat treated condition. If additional heat treatment is needed after further processing the following is recommended.

Solution annealing: 1080–1150°C (1975–2100°F), 5–30 minutes, rapid quenching in air or water.

Forms of supply:

Seamless tube and pipe are supplied in dimension up to 230 mm (9.1 in.) outside diameter in the solution annealed and white-pickled condition or in the bright-annealed condition. Tubes can be bent according to customer drawings and, on request, annealed after bending.

Fittings: 90 deg. bends are manufactured as standard in Sandvik 2RK65™ according to ANSI B16.9 and, where applicable, ASTM A403. Flanges are made as standard to ANSI B16.5 in the form of slip-on flanges (class 150) and weld neck flanges (class 300), and to relevant sections of ASTM A182. Fittings can be manufactured to other standards by agreement. Other types of fittings such as reducers, tees and couplings can also be supplied on request.

Other forms of supply:

The weldability of Sandvik 2RK65™ is good. Welding must be carried out without preheating, and normally there is no need for any subsequent heat treatment. 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, Sandvik 2RK65™ 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 Sandvik 2RK65™, heat-input of <1.0 kJ/mm and interpass temperature of <100°C (210°F) are recommended. A string bead welding technique should be used.

Recommended filler metals for temperature:

• TIG/GTAW or MIG/GMAW welding
  • ISO 14343 S 20 25 5 Cu L/ AWS A5.9 ER385 (e.g. Exaton 20.25.5.LCu)
• MMA/SMAW welding
  • ISO 3581 E 20 25 5 Cu N L R/ AWS A5.4 E385-16 (e.g. Exaton 20.25.5.LCuR)
  • ISO 14343 S 20 25 5 Cu L wire or strip electrodes are recommended for overlay welding of tube sheets and high-pressure vessels in cases where corrosion resistance, equal to that of Sandvik 2RK65™, is required.