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Corrosive Engineering

Comparative Corrosion and Abrasion Resistance of Duplex 2205 vs. 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 approx. 22% chrome (Cr) and 5% nickel (Ni).

According to a study performed for the Steel Founders’ Society of America (SFSA) "Duplex Stainless Steels are being specifi ed 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…"

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
  • Ferric Chloride Corrosion Test - ASTM A 923 Method C is a 24 hour pitting corrosion test in 6% ferric chloride. The corrosion rate for 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.
  • 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.
  • 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. Solution Annealed CD3mn samples showed no pitting at temperatures less than 40C.
    • In the ASTM G-150, or SEMI F77 test, 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 20C for 316 SS and 49C for 2205 Duplex. This closely correlates to their respective PRENs .
  • 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 -2C and 22C for 2205 Duplex.

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 Sulfi de Stress Cracking (SSC), hydrogen blistering, and hydrogen embrittlement. H2S is found in Sour Crude, Sour Gas, Biogas, and Anaerobic Digestion processes. NACE MR01751 establishes a maximum H2S partial pressure of 1.5 psia (10 kPa abs) for solutionannealed duplex stainless steel with a pitting resistance equivalent number (PREN) between 30 and 40. In a 2004 test (Note 1) Duplex Type 2205 successfully prevented sulfi de stress cracking up to a partial pressure of H2S of 5 psia (34 kPa). The study concludes " These results suggest a possible extension on the applicability of the material in terms of the maximum allowed H2S partial pressure in the NACE MR0175 document."
  • When H2S is present the pump is required to pump both gas and liquid. In many pumps this will cause cavitation and subsequent pitting. Rotary Lobe Pumps are well suited for pumping liquid containing a low percent 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 CD3mn

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 suffi cient. 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.

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 -40C.

Metal Charpy V Notched Test @ -40C Source of Info
CD3mn 52 ft-lbs SFSA
4140 Steel 55 ft-lbs University of New Mexico
316 Stainless Steel 53 ft-lbs British Stainless Steel Assoc
Ductile Iron 5 ft-lbs Ductible Iron Institute
BS4360 Grade 40(E) Steel 27 ft-lbs New Zealand Engineering News

 

CD3mn has a good combination of moderate hardness and high toughness.

Reference: "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

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