Corrosion resistance of duplex stainless steels
The duplex steels offer an excellent range of corrosion resistance in the diverse media. Following is the brief explanation of their resistance in the variety of application conditions.
Uniform corrosion is featured by even attack on the steel surface that comes in contact of the attacking media. The prevention of corrosion is assumed to be suitable if the attack rate is lower than 0.1 mm per year. Duplex steel grades contain large magnitude of chromium that enables them to offer good corrosion resistance in the different environments.
Following diagram shows the corrosion resistance in sulfuric acid. It describes duplex stainless steel 2205 offers significantly better resistance than SS 904l.
Stainless steel grades offer controlled service in hydrochloric acid application due to uniform and local corrosion attack. On the other hand Duplex stainless steel 2507 and 2205 provide adequate service in the aq. HCl. Pitting is generally not an issue however crevice attack should be prevented.
In the powerful oxidizing chemicals including nitric acid, commonly used molybdenum-free steels offer better corrosion resistance than molybdenum based steels.
Pitting and crevice attack
The prevention of pitting and crevice corrosion improves with increasing magnitude of chromium (Cr), molybdenum (Mo) and Nitrogen (N) in the steel grades. It is normally described by pitting resistance equivalent number for a specific material. This value is significant for an estimation of performance of the various materials. Critical pitting temperature (CPT) is also found to be more useful to rank the materials.
While considering the corrosion resistance by crevice, it is usual to determine the critical temperature limit at which the attack begins in a specific solution. The general critical crevice corrosion temperature determined in 6% ferric chloride (FeCl3) +1% Hydrochloric acid (HCl) shows that duplex grades 2205 and 2507 need higher temperature to be attacked as compare to traditional steels. Hence the latter is more sensitive to attack.
Stress corrosion cracking
Stainless steels are damaged by stress corrosion cracking in conditions containing chlorides at the high temperatures. Traditional austenitic steels are found to be more susceptible to such attack however duplex grades are less prone to it.
Various techniques are utilized to grade the steel types considering their resistance to SCC and the outcomes vary on the base of test method and condition used. Following table shows the comparative resistance offered by traditional steels and austenitic-ferritic duplex steels.
|Steel grade||45% MgCl at 155oC||40% CaCl2 at 100oC||40% CaCl2 at 100oC, 0.9 x Rp0.2||25% NaCl, pH 1.5, 106oC||25% NaCl, 106oC||1500 ppm Cl- at 100oC|
|2201||Probable||Not probable||Not probable||Not probable||Not probable||Not probable|
|2304||Probable||Not probable||Not probable||Not probable||Not probable||Not probable|
|2404||Probable||Possible||Not probable||Not probable||Not probable||Not probable|
|2205||Probable||Not probable||Not probable||Not probable||Not probable||Possible|
|2507||Probable||Not probable||Not probable||Not probable||Not probable||Not probable|
|254 SMO||Probable||Not probable||Not probable||Not probable||Not probable||Not probable|
It states that duplex steel grades are resistant to vigorous conditions including boiling concentrated magnesium chloride (MgCl2), they resist stress corrosion cracking in the diverse media where traditional steel grades usually damage.
Sulfide based stress corrosion cracking
The availability of hydrogen sulfide and chlorides cause the SCC attack at the minor temperature limits. This type of corrosion may occur such as in boreholes for oil and gas plants. The recommended alloys for use are duplex grades 2205 and 2507 that offer suitable prevention of such attack however steels containing 13% chromium are prone to SCC. Care should be taken while considering the environments with large partial pressure of H2S and when the steel is exposed to large internal stress.
According to NACE MR0175/ ISO 1515, duplex stainless steel grades 2205 & 2507 are proven for use in the petrochemical and natural gas refining units. They are fit for use in the conditions containing H2S in the oil and gas production wells.
The pair of large mechanical strength and excellent resistance to corrosion offers duplex grades good corrosion fatigue strength. The corrosion fatigue strength of steel 2205 is larger than grade 4404.
The duplex microstructure and small carbon magnitude make duplex steel grades to offer prevention against intergranular attack. The chemistry of steel confirms that austenite is reproduced in the heat affected region subsequent to welding. The risk of unnecessary precipitation of carbides and nitrides in the grain limits is hence reduced.
Stainless steel normally provides good prevention of erosion attack. The duplex grades are resistant to their combination of high surface toughness and offer large corrosion resistance. Instances of applications where this is advantageous are systems that are exposed to hard wear such as pipe systems comprising of water with sand or salt crystals.
Galvanic attack may take place when two unlike materials are joined. The noblest material is secured while the less ideal material is more vigorously attacked. When the duplex steel grades are passive, they are in generally conditions more ideal than other metallic construction materials, stating that this steel is secured while the attack rate of such as carbon steel is accelerated. The galvanic attack doesn’t happen between two unlike grades of steels when the both types are passive.
Characteristics of Duplex steel grades
- Excellent prevention of uniform corrosion, pitting and crevice attack
- Prevents stress corrosion cracking and corrosion fatigue
- Large mechanical strength
- Suitable abrasion and erosion prevention
- Nominal thermal expansion
- High weldability
Paper and pulp processing, desalination, flue gas cleaning, transportation tanks and piping in chemical tankers, seawater processing, firewalls and blast walls on offshore units, bridges, parts for structural units, storage containers, pressure vessels, heat exchanger, water heaters, rotors, impellers and shafts and strengthening bars for concrete structures.