Comparative Corrosion and Abrasion Resistance of Hastelloy C4, Duplex 2205 and 316 Stainless Steel

1. Corrosion Caused by Chlorides: 316 Stainless Steel is suitable if chlorides are < 1000 ppm and the water is neutral PH, oxygenated and at room temperature. In other conditions, a different metal should be used according to the Nickel Development Institute. Type 2205 (CD3mn) Duplex is often selected in the other conditions. Below are more specifics concerning Chloride Corrosion:

• Chloride Stress Corrosion Cracking (CSC or CSCC). CSCC is caused by a combination of tensile stress, heat and chlorides and /or hydroxides. CD3mn has a minimum tensile strength of 90 ksi vs. 70 ksi for 316 SS. Yield strength is minimum 60 ksi for CD3mn vs. 30 for 316 SS. Because of its superior strength, CD3mn is often used in place of 316 SS in applications where CSCC is a concern. However, note the ferrites in duplex, while making them more resistant to cracking, also make them more vulnerable to hydrogen embrittlement than austenitic stainless. Hastelloy C4 (CW-2M) is preferred in applications where hydrogen embrittlement is a concern.
• Ferric Chloride Corrosion Test – ASTM A 923 Method C is a 24 hour pitting corrosion test in 6% ferric chloride. The corrosion rate for Duplex CD3mn, when solution annealed, is less than 1 mdd (mg/dm2 /day) per this test. The maximum acceptance corrosion rate to meet A923(C)’s standard is 10 mmd. Note that CD3mn parts which have been autogenously welded will fail the A 923(C) max corrosion rate standard. Autogeneous welding is a fusion welding process using heat without the addition of filler metal to join two pieces of the same metal. Hastelloy C 276 is ranked #1 in test results when tested with a 10% ferric solution.
• The PREN number (Pitting Resistance Equivalent Number) is a theoretical way of comparing the pitting corrosion resistance of various types of stainless steels, based on their chemical compositions. The most commonly used version of the formula is
  
  • PREN = Cr + 3.3Mo + 16N for Duplex Steel
  • PREN= CR+ 3.3Mo + 30N for Austenitic Stainless Steel
  • The PREN for CD3mn is =>34; The PREN for 316 SS is => 24 and => 19 for 304 SS.
  • The PREN for Hastelloy C4 => 40.
  • “Despite their good resistance to general corrosion, stainless steels are more susceptible to pitting than many other metals. Pitting failures in Hastelloy C are relatively uncommon.” (Note 2)
• Critical Pitting Temperature (CPT) Testing. There are two approaches:
  • ASTM G48 Method C is test in which weighed samples are immersed in 6% ferric chloride for 24 hours at controlled temperatures. At the end of the test the samples are removed, cleaned and the weight loss determined. The highest temperature where the samples showed no weight loss are: 3°C for 316 SS; 40°C for Duplex CD3Mn and 42°C for Hastelloy C4.
  • In the ASTM G-150, the solution temperature is increased from O C at one degree per minute until pitting occurs. When the corrosion current density reaches 100 micro-amps/cm2 the test is stopped, and the temperature noted. At this point CPT has been reached. ASTM G150 reveals a CPT of 20°C for 316 SS, 49°C for 2205 Duplex and 140°C for Hastelloy C4.
• Critical Crevice Corrosion Temperatures (CCCT) . This is tested in according with ASTM G48 Method B6. This test shows that CCCT is also closely correlated with the PREN. The CCCT for 316 SS is -3°C, 22°C for 2205 Duplex, and 50°C for Hastelloy C4.