120 100 80 60 40 20 0 |
6 Moly |
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Table 2 : Pitting Resistance Equivalent Number ( PREN )
Common grade name
6 moly 43
Alloy 926 46
N08367 47
904L 37
Alloy 28 39
Duplex 35
Super Duplex 42
316L 27
304L 19
PREN Max
PREN = % Cr + 3.3 × % Mo + 16 × % N Note : PREN are average values , but in general values could be bit lower . This calculation is only for the purpose of ranking the grades .
passive layer of Chrome oxide on surface . Similarly , a high % of Molybdenum helps in pitting resistance as it helps lowering the pit growth rate . Higher Nitrogen content in steel is useful to fight pitting corrosion as it helps in neutralizing the acidic corrosive solution . PREN , or pitting resistance equivalent , is used as a rule-of-thumb for material selection in these conditions , which is % Cr + 3.3 % Mo + 16 % N ( in case of austenitic , duplex , and super austenitic steels ). As can be seen in Table 2 , super austenitic stainless steels have PREN more than 27 , which is higher than standard austenitic grades and some of these grades have PREN higher than duplex ( 35 ) and super duplex steels ( 42 ). Critical pitting temperature ( CPT ) is calculated by ASTM G 48 Method A corrosion test , which involves the exposure of the materials in a 6 wt % ferrite chloride solution for test durations of 72 hours ( typically 24 hours ). This is temperature at which pitting corrosion may start . The CPT of materials which is a direct proportion of their PREN is good indication of how good these materials are against pitting corrosion . When it comes to chloride induced pitting corrosion , super austenitic performs much better than standard austenitic steels , and equally or better than duplex steels .
CPT ( Degree Centigrade ) & PREN
CPT
PREN
Duplex
Super Duplex
316L
304L
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Stress corrosion cracking Stress corrosion cracking ( SCC ) is the cracking induced from the combined influence of tensile stress and a corrosive environment . The impact of SCC on a material usually falls between dry cracking and the fatigue threshold of that material . The required tensile stresses may be in the form of directly applied stresses or in the form of residual stresses . Chloride stress-corrosion cracking ( CSCC ) is one of the most serious forms of localized corrosion . Higher temperatures and reduced pH will increase the probability of CSCC . It has been determined that alloys become more resistant to SCC as the nickel content increases above 12 %, and the molybdenum content rises above 3 %. SSC of super austenitic stainless steel is superior to the standard 300 series austenitic stainless steels , and some duplex stainless steels . The alloys 6 Moly , Alloy 926 , and N08367 , exhibit a good resistance to chloride induced stress corrosion cracking , as seen in Graph 2 . It improves from 6 moly to Alloy 926 to Alloy 08367 . They are immune to SCC up to boiling temperatures when in the presence of chlorides . The grade N08926 has more than 20 % Nickel , and more than 2 % Molybdenum , which leads to improved SCC as compared to standard austenitic steels .
Reducing and oxidizing acids Most of super austenitic stainless steels are copper alloyed , which gives it good resistance to reducing and nonoxidizing acids , such as sulfuric and phosphoric acids . It is also seen that high Chromium % gives a strong passive layer , and improves corrosion resistance to sulfuric and phosphoric acids . Alloy 28 has corrosion rates less than 0.1 mm per year in high concentration sulfuric acids , up to a temperature of 50 degrees Celsius . This grade has also performed well in phosphoric acid heaters with under conditions of phosphate rocks contaminated with high concentration of chlorides and fluorides . 904L has also been used extensively in phosphoric acid industry .
Applications Conventional and bio refineries : Several exchangers work as condensers and coolers . For overhead condensers , surface condensers , coolers and interstage coolers using water on tube side , often can face pitting corrosion , the extent of which depending on water quality , % of chlorides , and temperatures . Due to high pitting resistance equivalent number of super austenitic , this can be used in such conditions . Higher temperature limits of super austenite ’ s as compared to duplex grades also reduces the chance of tube failures , due to operational upsets , resulting in increase in surface temperature conditions . The fouling factor should also be considered , which increases surface temperature leading to pre-mature failure of duplex grades . Lean and rich amine coolers have similar conditions and can be potential application for super austenitic grade . There are already references of 6 Moly and Alloy 28 used for such applications in refineries . Other potential applications in refineries include sour water strippers and sulfur condensers . Additionally , high Moly % on super austenitic grades make it suitable for resisting naphthenic acid corrosion .
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