Sanicro® 35

Due to its extremely good pitting and crevice corrosion properties, Sanicro® 35 is particularly suitable for applications where seawater is used for cooling or heating. Sanicro® 35 also has a high resistance to general corrosion in acid environments, making it suitable for a variety of applications.

Approvals:

Intermetallic phases are precipitated at temperatures above 600°C (1110°F). Therefore, the steel should not be exposed to these temperatures for prolonged periods.

The force needed for bending Sanicro® 35 is higher than that for standard austenitic stainless steels which is a natural consequence of the higher yield strength. The excellent formability of the grade permits cold bending to tight bending radii.

General corrosion: Sanicro® 35 has good resistance to hydrochloric acid compared to stainless steels with a lower chromium and molybdenum content and can, therefore, be useful in environments where hydrochloric acid is present. See Figure 1.

Sanicro® 35 has a high resistance to sulfuric acid and nitric acid. Isocorrosion diagrams can be seen in Figure 2 and Figure 3.

Sanicro® 35 also performs well in mixtures of formic acid and acetic acid, see Table 1.

Table 1. Corrosion rate of Sanicro® 35 in mixtures of acetic acid (CH COOH) and formic acid (HCOOH) at boiling conditions.

Sanicro® 35 performs well also in alkaline conditions showing high corrosion resistance in caustic solutions, see Table 2.

Table 2. Corrosion rate of Sanicro® 35 in sodium hydroxide (NaOH) at various concentrations and temperatures.

Pitting corrosion: One of the main advantages of Sanicro® 35 is that it has excellent resistance to pitting corrosion. The pitting resistance comes from the high contents of chromium, molybdenum, and nitrogen. The PREN-number can be used to compare and rank alloys with respect to the chemical composition and possibility to resist pitting.

The PRE is defined as, in weight-%

PRE = %Cr + 3.3 x %Mo + 16 x %N

The nominal PREN value for Sanicro® 35 is ~52, comparable to the nickel alloy Sanicro 625 (Alloy 625). This is significantly higher than e.g. the PREN values for super duplex and 6 Mo austenitic grades which are commonly used in seawater applications. For reference, Sandvik SAF 2507 and Sandvik 254 SMO have a minimum PRENvalue of 42.5.

The critical pitting temperature (CPT) has been determined in 6% FeCl according to ASTM G48 practice C. The CPT has also been determined in a potentiostatic test in a 3M MgCl solution. The test was performed in a modified ASTM G150 test where the solution was changed from 1M NaCl to allow the CPT-measurement of highly alloyed materials. The measured CPT-values can be seen in Table 3.

Table 3. CPT-values for Sanicro® 35 compared to Sandvik 254 SMO. The CPT was measured on coupons with P120 surface for the ASTM G48 test and P600 surface for the G150 mod. test.

Crevice corrosion resistance is equally as important as pitting resistance since crevices can rarely be totally avoided. Sanicro® 35 has excellent crevice corrosion resistance in chloride environments. The critical crevice temperature (CCT) has been determined by potentiostatic tests in 1M NaCl according to standard ASTM G150 and by immersion tests in 6% FeCl test solution acidified with HCl according to ASTM G48, see Table 4.

Table 4. CCT-values for Sanicro® 35 compared to some alloys according to various test methods. The applied potential was 700 mV vs. SCE as per ASTM G150. Flat coupons were tested with surfaces wet ground with P600 grit paper for the ASTM G150 test and with P120 for the ASTM G48 tests.

Testing in seawater: Accelerated laboratory tests are very good for ranking different alloys, however, real application environment tests are also valuable. Materials are often used in a seawater environment which is very corrosive for many alloys. Sanicro® 35 has been tested for 90 days in natural seawater at 30°C where a biofilm is active and also in 0.5 ppm chlorinated seawater at elevated temperatures.

Table 5. Flat specimens with surfaces ground with P120 grit paper tested in real seawater.

Stress corrosion cracking: Ordinary austenitic steels of the ASTM 316 type are susceptible to chloride-induced 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® 35 has excellent resistance to SCC. This is demonstrated in Table 6, which shows the results of SCC tests in a 40% calcium chloride solution. The grade showed no cracking or corrosion after 500 hours of constant load testing, corresponding to 90% of the actual ultimate tensile strength at 100°C. It should be noted that the high loading of 90% of UTS naturally causes plastic deformation of the specimens.

Table 6. Result of stress corrosion cracking test of different alloys in aerated 40% CaCl , at 100°C (210°F), pH 6.5.

Sanicro® 35 does not suffer from SCC in a NACE MR 0175 / ISO 15156 Test Level VI environment. Slow strain rate testing (SSRT) was performed on cold worked Sanicro® 35 material (140 ksi and 180 ksi), according to NACE TM0198. The environment had partial pressures of 500 psia H S and 500 psi CO . 20 wt-% sodium chloride was used as test solution and the temperature was 175°C ± 3°C. For both the 140 ksi and 180 ksi materials, two specimens were tested in the corrosive environment and one in nitrogen. All tests were carried out at the same baseline temperature. Both materials show ductile fractures with ratios of ≥92% for time to failure, elongation to failure, plastic strain to failure and reduction in area compared to inert environment, which indicates no SCC.

Hydrogen embrittlement: Sanicro® 35 shows as expected excellent resistant to hydrogen embrittlement since it has high austenitic phase stability. Sanicro® 35 is not a precipitation-hardened grade which the latter may experience hydrogen embrittlement.

Sanicro® 35 solution annealed material experienced no cracking in constant load testing at 4°C in 3% NaCl at -1050 mV at two different loads present in Table 7. This indicates that the alloy is not prone to hydrogen embrittlement and is a viable option for subsea applications.

Table 7. Sanicro® 35 results from constant load testing at 4°C in 3% NaCl at -1050 mV SCE

Sanicro® 35 can be expanded into tube sheets in the same way as standard austenitic stainless steels.

Tubes are delivered in solution annealed condition. If additional heat treatment is needed after further processing, please contact Sandvik.

Forms of supply:

Sanicro® 35 can be supplied as seamless tube and pipe.

The weldability of Sanicro® 35 is good and a suitable method for fusion welding is TIG welding (GTAW).

Welding should be undertaken with low heat input, maximum 1.2 kJ/mm, and interpass temperature 100°C maximum. A stringer welding technique should be used. Preheating and post-weld heat treatment are not necessary. To maintain full corrosion resistance of the welded joint, welding must be followed by thorough cleaning to ensure the removal of all oxides and heat tint. Ar+2 %N₂ is recommended as shielding gas and backing gas with TIG welding to achieve the best combination of mechanical properties and corrosion resistance of the welded joints.

Welding of fully austenitic stainless steels and nickel-base alloys often involves the risk of hot cracking in the welded joints if the weldment is under constrain. Sanicro® 35, however, possesses very high purity, and is thereby less prone to hot cracking than most of the nickel-base alloys.

Nickel alloy UNS N06059 (ERNiCrMo-13, NiCr23Mo16) wire or rod is recommended as filler material for gas shielded arc welding. Welding without filler material should be avoided in the as-welded condition.