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Seamless tube and pipe
Pitting and crevice corrosion
Stress corrosion cracking
Sandvik SAF 2507 is highly resistant to corrosion by organic acids, e.g. experience less than 0.05 mm/year in 10% formic and 50 % acetic acid where AISI 316L has corrosion rate higher than 0.2 mm/year. Pure formic acid see Figure 4. Also in contaminated acid Sandvik SAF 2507 remains resistant. Figure 5 and Figure 6 show results from tests of Sandvik SAF 2507 and various stainless steels and Ni-base alloys in acetic acid contaminated with chlorides which in practise are frequently present in processes.
Figure 4. Isocorosion diagram in formic acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in stagnant test solution.
Figure 5. Corrosion rate of various alloys in 80% acetic acid with 2000 ppm chloride ions at 90°C.
Figure 6. Corrosion rate of various alloys in concentrated acetic acid with 200 ppm chloride ions.
Practical experience with Sandvik SAF 2507 in organic acids, e.g. in teraphthalic acid plants, has shown that this alloy is highly resistant to this type of environment. The alloy is therefore a competitive alternative to high alloyed austenitics and nickel-base alloys in applications where standard austenitic stainless steels corrode at a high rate.
Resistance to inorganic acids is comparable to, or even better than that of high alloy austenitic stainless steels in certain concentration ranges. Figures 7 to 9 show isocorrosion diagrams for sulphuric acid, sulphuric acid contaminated with 2000 ppm chloride ions, and hydrochloric acid, respectively.
Figure 7. Isocorrosion diagram in naturally aerated sulphuric acid. The curves represent a corrosion rate of 0.1 mm/year (4 mpy) in a stagnant test solution.
Figure 8. Isocorrosion diagram, 0.1 mm/year (4 mpy) in a naturally aerated sulphuric acid containing 2000 ppm chloride ions.
Figure 9. Isocorrosion diagram in a hydrochloric acid. The curves represent acorrosion rate of 0.1 mm/year (4 mpy) in stagnant test solution.
Pitting and crevice corrosion
The pitting and crevice corrosion resistance of stainless steel is primarily determined by the content of chromium, molybdenum and nitrogen. The manufacturing and fabrication practice, e.g. welding, are also of vital importance
for the actual performance in service
A parameter for comparing the resistance to pitting in chloride environments is the PRE number (Pitting Resistance Equivalent).
The PRE is defined as, in weight-%
PRE = %Cr + 3.3 x %Mo + 16 x %N
For duplex stainless steels the pitting corrosion resistance is dependent on the PRE value in both the ferrite phase and the austenite phase, so that the phase with the lowest PRE value will be limiting for the actual pitting corrosion resistance. In Sandvik SAF 2507 the PRE value is equal in both phases, which has been achieved by a careful balance of the elements.
The minimum PRE value for Sandvik SAF 2507 seamless tubes is 42.5. This is significantly higher than e.g. the PRE values for other duplex stainless steels of the 25Cr type which are not super-duplex. As an example UNS S31260 25Cr3Mo0.2N has a minimum PRE-value of 33.
One of the most severe pitting and crevice corrosion tests applied to stainless steel is ASTM G48, i.e. exposure to 6% FeCI
with and without crevices (method A and B respectively). In a modified version of the ASTM G48 A test, the sample is exposed for periods of 24 hours. When pits are detected together with a substantial weight loss (>5 mg), the test is interrupted. Otherwise the temperature is increased by 5 °C (9 °F) and the test is continued with the same sample. Figure 11 shows critical pitting and crevice temperatures (CPT and CCT) from the test.
Potentiostatic tests in solutions with different chloride contents are presented in Figure 11. Figure 12 shows the effect of increased acidity. In both cases the applied potential is 600 mV vs SCE, a very high value compared with that normally associated with natural unchlorinated seawater, thus resulting in lower critical temperatures compared with most practical service conditions.
Figure 10. Critical pitting and crevice temperatures in 6% FeCl
, 24h (similar to ASTM G48).
The scatter band for Sandvik SAF 2507 and 6Mo+N illustrates the fact that both alloys have similar resistance to pitting, and CPT-values are within the range shown in the figure.
Tests were performed in natural seawater to determine the critical crevice corrosion temperature of samples with an applied potential of 150 mV vs SCE. The temperature was raised by 4°C (7
F) steps every 24 hours until crevice corrosion occurred. The results are shown in the table below.
In these tests the propagation rates of initiated crevice corrosion attacks, at
15-50°C (59-122°F) and an applied potential of 150 mV vs SCE were also determined. These were found to be around ten times lower for Sandvik SAF 2507 than for the 6Mo+N alloy.
Figure 11. Critical pitting temperatures (CPT) at varying concentrations of sodium chloride, from 3 to 25% (potentiostatic determination at +600 mV SCE with surface ground with 600 grit paper).
Figure 12. Critical pitting temperatures (CPT) in 3% NaCl with varying pH (potentiostatic determination at +600 mV SCE with surface ground with 600 grit paper).
The corrosion resistance of Sandvik SAF 2507 in oxidising chloride solutions is illustrated by critical pitting temperatures (CPT) determined in a "Green death" -solution (1% FeCI
+ 1% CuCl
+ 1.2% HCI) and in a "Yellow death" -solution (0.1 % Fe
+ 4% NaCl + 0.01 M HCI). The table below shows CPT-values for different alloys in these solutions. It is clear that the values for Sandvik SAF 2507 are on the same level as those for the nickel-base alloy UNS N06625. The tests demonstrate a good correlation with the ranking of alloys for use as reheater tubes in flue gas desulphurisation systems.
Critical pitting temperature (CPT) determined in different test solutions.
Critical pitting temperature (CPT), °C
Sandvik SAF 2507
Stress corrosion cracking
Sandvik SAF 2507 has excellent resistance to chloride induced stress corrosion cracking (SCC).
The SCC resistance of Sandvik SAF 2507 in chloride solutions at high temperatures is illustrated in Figure 13. There were no signs of SCC up to 1000 ppm Cl
/300°C and 10000 ppm Cl
Sandvik SAF 2507 U-bend specimens exposed for 1000 hours in hot brine (108°C, 226°F, 25% NaCl) showed no cracking.
The threshold stress for Sandvik SAF 2507 in 40% CaCl
at 100 °C (210 °F) and pH = 6.5 is above 90% of the tensile strength for both parent metal and welded joints (TIG-welded with Sandvik 25.10.4.L or MMA-welded with Sandvik 25.10.4.LR).
Figure 14 shows the result of testing in 40% CaCl
at 100 °C (210 °F) acidified to pH = 1.5. Acidifying of the standard test solution to pH = 1.5 lowers the threshold stress for Sandvik SAF 2205, but not for Sandvik SAF 2507. This applies to both parent metal and welded joints.
The threshold stress for both parent metal and welded joints of Sandvik SAF 2507 in boiling 45% MgCl
, 155°C (311°F) (ASTM G36), is approximately 50% of the proof strength.
Figure 13. SCC resistance in oxygen-bearing (abt. 8 ppm) neutral chloride solutions. Testing time 1000 hours. Applied stress equal to proof strength at testing temperature.
Figure 14. Results of SCC tests with constant load in 40% CaCl
, pH=1.5, at 100 °C (210°F) with aerated test solution.
Figure 15. Constant load SCC tests in NACE solution at room temperature (NACE TM 0177).
Figure 15 shows the results of SCC tests at room temperature in NACE TM0177 Test solution A (5% sodium chloride and 0.5% acetic acid saturated with hydrogen sulphide). No cracking occurred on Sandvik SAF 2507, irrespective of the applied stress.
In aqueous solutions containing hydrogen sulphide and chlorides, stress corrosion cracking can also occur on stainless steels at temperatures below 60 °C (140 °F). The corrosivity of such solutions is affected by acidity and chloride content. In direct contrast to the case with ordinary chloride-induced stress corrosion cracking, ferritic stainless steels are more sensitive to this type of stress corrosion cracking than austenitic steels.
In accordance with ISO 15156/NACE MR 0175 solution annealed and liquid quenched wrought Sandvik SAF 2507 is suitable for use at temperatures up to 450 °F (232 °C) in sour environments in oil and gas production, if the partial pressure of hydrogen sulphide does not exceed 3 psi (0.20 bar). Sandvik SAF 2507, with a maximum hardness of 32 HRC, solution annealed and rapidly cooled, according to NACE MR0103, is suitable for use in sour petroleum refining.
Sandvik SAF 2507 is a member of the family of modern duplex stainless steels whose chemical composition is balanced to give quick reformation of austenite in the high temperature heat affected zone of a weld. This results in a microstructure that provides the material with good resistance to intergranular corrosion. Sandvik SAF 2507 passes testing to ASTM A262 Practice E (Strauss test) without reservation.
The mechanical properties combined with corrosion resistance give Sandvik SAF 2507 a good resistance to erosion corrosion. Testing in sand containing media has shown that Sandvik SAF 2507 has an erosion corrosion resistance better than corresponding austenitic stainless steels. Figure 16 below shows the relative mass loss rate of the duplex Sandvik SAF 2507, Sandvik SAF 2205 and an austenitic 6Mo+N type steel after exposure to synthetic seawater (ASTM D-1141) containing 0.025-0.25% silica sand at a velocity of 8.9-29.3 m/s (average of all tests is shown).
Figure 16. Relative mass loss rate after testing of the resistance aginst erosion corrosion.
Duplex stainless steels which have a high tensile strength usually have a high fatigue limit and high resistance to both fatigue and corrosion fatigue. The high fatigue strength of Sandvik SAF 2507 can be explained by its good mechanical properties, while its high resistance to corrosion fatigue has been proven by fatigue testing in corrosive media.
A document from the Sandvik Materials Technology web-site.