[ Duplex ] digester and impregnation vessel digester awaiting assembly .
Common species affecting the redox potential There is a wide range of oxidising and reducing chemicals , but many are rarely found in service environments . Even when present , they need to be at sufficient concentration to have an effect , and the chemical environment must be one where reduction of the oxidising species is possible . In some processes there is an overpressure of oxygen , which clearly creates oxidising conditions . Some other oxidisers that have been observed to cause SCC are cupric and ferric irons and chromium and vanadium 1 , all of which are more likely to be present in significant quantities in mineral processing operations . Sulphide is one of the most common reducing agents present in significant quantities in industrial environments .
Alternative alloys Where lower alloy duplex stainless steels may be susceptible to SCC , the use of a higher alloy duplex may be sufficient to prevent failures . In oxidising environments , titanium and its alloys offer an alternative to stainless steels , albeit at an increased cost , provided that there are no fluorides present . It is important to select the correct grade , as there have been SCC failures of grade 5 titanium in hot , high chloride , low pH oxidising slurries in the mineral processing industry . The high Ni-Cr-Mo alloys ( such as UNS N06625 and N10276 ) are not suitable in highly oxidising environments because in the transpassive region molybdenum salts are soluble and high rates of dissolution occur , and also pitting if chloride is present .
Table 3 . Effect of temperature and chloride on the corrosion of S32760 super duplex stainless steel in a simulated mineral processing fluid .
TEMPERATURE (° C ) CHLORIDE ( g / L ) CORROSION
Table 4 . Examples of the uses of duplex stainless steels in refineries and petrochemical plants 7 .
AREA APPLICATION ALLOY
Refinery
Steam Cracker
Aromatics
Polymers
Intermediates and other fluids
Crude desalters Amine towers lean / rich H / Ex Filter drums ( at risk of SCC ) Fresh and brackish water coolers
Freshwater cooling towers / coolers Dilution steam generators Coolers for acidic process condensate
Feed fraction tower airfins / piping / vessels Aromatics / HC separation units , reboilers , H / Ex tubing
Polypropylene piping , drums , dust collectors , dryers and rotating equipment Elastomer slurry tanks , vessels , piping and rotating equipment LLDPE extruder feed shafts LDPE co-polymer piping Elastomer extruder barrels and screws
Corrosive service with organic acids , sulphuric acid , nitric acid , phosphoric acid , caustic , carbonyls etc at high temperature and pressure Intermediate storage tanks
A Ni-Cr-Mo alloy that could be considered is alloy 33 ( see Table 1 ). Its high chromium content gives it an extended passive range , while the high nickel content gives it good resistance
C . R . ( mm / y ) Pitting SCC 177 90 1.07 No Yes 177 1 0.80 No No 135 1 0.60 No No 100 20 0.27 Yes No 100 5 < 0.01 No No
2205 & super duplex 2205 2205 2205
2101 / 2102 / 2205 2205 2205 & super duplex
2205 2205 2205
2205
2205 2205 2205
2205
2101 / 2102
to SCC . The low molybdenum content means that it will not suffer excessive dissolution under transpassive conditions . Because of the limited corrosion data for this alloy , it is
1,000,000 1,000,000
Chloride ( mg / L ) |
100,000
10,000
|
No Cracks |
Cracks |
Chloride ( mg / L ) |
100,000
10,000
|
No Cracks |
Cracks |
1,000 |
1,000 |
1 |
10 |
100 |
1,000 |
1 |
10 |
100 |
|
|
H 2
S Partial Pressure ( kPa )
|
|
|
|
H 2 S Partial Pressure ( kPa ) |
( A ) |
pH = 3.5 |
pH = 4.5 |
ISO 15156 |
( B ) |
pH = 3.5 |
pH = 4.5 |
pH = 5.5 |
ISO 15156 |
Figure 6 . Effect of pH and chloride on the SCC resistance of duplex stainless steels 7 . [ A = 2205 ; B = super duplex ]
1,000
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