Comparative Corrosion and Abrasion Resistance of Hastelloy C4, Duplex 2205 and 316 Stainless Steel
Duplex Stainless Steel (DSS) is a combination of ferrite and austenitic stainless steel (hence the name Duplex). Duplex type 2205 wrought steel (UNS # J92205) and its cast version CD3mn (ASTM-A890 (4A)) account for 80% of all duplex steels made. Type 2205 and CD3mn duplex are approximately 22% chrome and 5% nickel. Hastelloy C4 (UNS #N06466) and its cast version CW-2M are nonferrous alloys. Hastelloy C4 is 65% nickel, 16% chrome, 16% molybdenum plus a small percentage of other metals.
According to a study performed for the Steel Founders’ Society of America (SFSA) “Duplex Stainless Steels are being specified for chloride containing environments due to their enhanced pitting and corrosion cracking resistance. They exhibit improved corrosion performance over traditional austenitic stainless steels. Duplex steels can also offer improved strength properties…”
Hastelloy comes in many varieties and most grades show excellent resistance to pitting and corrosion caused by chloride.
Resistance to Corrosion:
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.
2. Hydrogen Sulfate H2S -Toxic and Corrosive. This is commonly known as Sewer Gas. It can be lethal. Per Wikapedia
- At 5 ppm it has a distinctive rotten egg smell.
- At 100–150 ppm the olfactory nerve is paralyzed after a few inhalations, and the sense of smell disappears, often together with awareness of danger.
- 320–530 ppm leads to pulmonary edema with the possibility of death.
- 530–1000 ppm causes strong stimulation of the central nervous system and rapid breathing, leading to loss of breathing.
- 800 ppm is the lethal concentration for 50% of humans for 5 minutes exposure (LC50).
- Concentrations over 1000 ppm cause immediate collapse with loss of breathing, even after inhalation of a single breath.
- It can be extremely corrosive if the PH is < 6.5 and H2S is > 250 ppm. If the part is under stress, H2S can cause Sulfide Stress Cracking (SSC), hydrogen blistering, and hydrogen embrittlement. H2S is found in Sour Crude, Sour Gas, Biogas, and Anaerobic Digestion processes.
- NACE MR0175 establishes standards for sulfide stress corrosion cracking resistant materials for Oil Field Equipment and is widely used worldwide.
- NACE MR0175-2002 establishes environmental limits of 450°F maximum and a maximum H2S partial pressure of 1.5 psia (10 kPa abs) for solution annealed duplex stainless steel with a pitting resistance equivalent number (PREN) between 30 and 40. There are no hardness or environmental limits on the use of Hastelloy C4 (CW-2M) in NACE MR0175-2002.
- When H2S is present the pump is required to pump both gas and liquid. In many pumps this will cause cavitation and subsequent pitting. Our rotary lobe pumps are well suited for pumping liquid containing under 150 PPM of H2S gas. Great care must be taken if H2S is present in toxic levels to prevent it escaping .
3. Caustic Stress Corrosion Cracking Resistance
- Resistance to caustics is generally in direct proportion to nickel content. The nickel content for 316 SS is 10% vs. 5% nickel for 2205/CD3mn vs. 65% for Hastelloy C4/CW-2M.
Resistance to Abrasion
Resistance to abrasion is a combination of hardness and toughness. Brinell is a measure of hardness; Charpy is a measure of toughness. Hardness alone is not sufficient. For example glass is very hard but would be easily broken by hard solids impact. CD3mn from LobePro’s supplier has a minimum Brinell hardness of 227. 316 Stainless has a minimum Brinell hardness of 113. Hastelloy CW-2M has a minimum Brinell hardness of 160.
Toughness of material is often measured by the Charpy V notched Impact test. The test measures the energy absorbed by a 10mm x 10mmx 55 mm sample when struck by a pendulum axe. The sample has a 2 mm notch cut at the back of the point where the pendulum axe strikes. ASTM A923 Method B has a minimum energy absorption acceptance standard of 40 ft-lbs (54J) for the Charpy impact test at -40°C. This tests ductility at low temperature.
Charpy V Notched Test @ -40C
Source of Info
University of New Mexico
316 Stainless Steel
British Stainless Steel Assoc
Ductible Iron Institute
BS4360 Grade 40(E) Steel
New Zealand Engineering News
Nickel Based alloys such as Hastelly are virtually all suitable for use at temperatures down to -46°C and generally much lower. (Note 3)
Note 1: ”Cracking Susceptibility Of Duplex Stainless Steel at An Intermediate Temperature In The Presence Of H2s Containing Environments “ by Julio G. Maldonado, InterCorr International and James W. Skogsberg, ChevronTexaco in CORROSION 2004, March 28 – April 1, 2004 , New Orleans, La
Note 2: “Surface Engineering for Corrosion and Wear Resistance,” ASM, International, Page 20.
Note 3: Brookhaven National Laboratory, A. Hurlich, 1963.