Materials
Sulfuric and phosphoric acid industry : Some of super austenitic steels like Alloy 28 , 904L , and Alloy 926 , which can also be referred to as Nickel- Chromium-Molybdenum-Copper alloyed steels , have good resistance to these acids . Sites producing sulfuric and phosphoric acid , or industries using these acids for other refining and treatment purposes are applicable , such as Copper refining . Applicable exchangers include sulfuric acid coolers , phosphoric acid heaters ( used in phosphoric acid production by wet process ), and acid reheaters . Flue gas condensers are also subjected to corrosion on tubes , due to condensing sulfur vapours , forming sulfuric acid in combination with moisture . Super austenitic steels are good for pipes carrying diluted sulfuric acid and phosphoric acid even if they are contaminated . LNG / cryogenics : Most of super austenitic steel grades have negative 175 º C as the lower end of operating temperatures . Therefore , it has a dual advantage of being able to work in cryogenic atmosphere , and at the same time , have good resistance to pitting corrosion . Standard austenitic steels like 316L can also be used in cryogenic conditions , but they have much less resistance to chloride pitting as compared to super austenite ’ s .
Conclusion Super austenitic steels have superior corrosion resistance properties , both in terms of pitting and stress corrosion , compared to standard austenitic steels . Additionally , it is equally or better placed compared to duplex steels . Although the application in high concentration and polluted acids would be limited based on temperature , it has been widely used in reducing and oxidizing acids at all concentrations . It is more stable on price and availability compared to high Nickel alloyed steels , and can be looked at as an option accordingly , depending on process conditions and the end application . It is also easy to weld and fabricate . These materials are available in various forms including sheets , plates , pipes , tubes , fittings , and more .
Reference 1 . Alloy selection in wet-process phosphoric acid , Nickel Institute , 2020
Temperature Deg C Temperature
° C |
|
° F |
200 |
Boiling point curve |
400 |
150
100
300 250 200 150 100
50
0
50 0
316L
Duplex 904L 6 Moly C-276
Alloy 2205 Alloy 317LM
Alloy
Alloy 825 Alloy 20 Alloy 904
316 L Stainless
Alloy 28
Alloy 625
Alloy G-3 Alloy 904hMo
317L
0 20 40 60 80 100 120 Phosphoric acid concentration ( wt %)
300
200
100
About the Authors
M Venkat Ramesh , Global Product Manager , Heat Exchanger tubes , Tubacex , has worked more than 17 years in stainless seamless tube and pipe industry in different roles including Sales , Business development , Key Account , Technical marketing and as Product Manager . He has worked closely with some of the refineries and chemical companies involving suggestions and resolving corrosion issues , material testing , material upgrade suggestions etc . He has published technical articles in Leading Industrial Magazines and presented a paper at NACE in the past .
Shabareesh Nair , Technical Marketing Director-Asia , Tubacex , has a Bachelor ’ s in Mechanical Engineering , and a Post Graduate Diploma in Marketing Management . Having a total industry exposure of 15 years , the last 12 years have been exclusively with the Stainless Steel industry and has been associated with a similar industry before joining Tubacex . At Tubacex , responsibilities include developing hydrogen value proposition in Asia , Technical Marketing support to the regional offices in Asia and Identifying and developing new business applications / opportunities .
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