[ Fertiliser ]
[ Fertiliser ]
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UNS S32750 : 25Cr-7Ni-4Mo super duplex stainless steel .
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UNS S32760 : 25Cr-7Ni-3.5Mo-0.75W super duplex stainless steel .
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UREMIUM29 ( UNS S32906 ): 29Cr-7Ni- 2Mo super duplex stainless steel .
The immersion tests were conducted in a five-litre Zr autoclave . The test solution contained a mixture of urea , ammonia and water . Temperature and pressure for the experiments were set in the upper level of the typical ranges in a stripper . The solution was de-aerated before starting the tests . Test duration was 15 days . ASTM G31 ( Standard Practice for Laboratory Immersion Corrosion Testing of Metals ) standard indications were followed for specimen preparation and corrosion rate measurement using the gravimetric method . The corrosion rate and measured depth of attack for each studied steel are summarised in Table 1 .
Lowest corrosion rate Figure 2 depicts the cross-sections of the test specimens after 15 days of immersion in the autoclave . Considering the measured corrosion rates and the metallurgical investigation , the following aspects can be highlighted :
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UNS S31050 : with the highest corrosion rate , experiences a uniform attack . The high corrosion rate is related to the absence of oxygen , which avoids attaining stable passivation in this austenitic grade . UNS S31050 is currently established as a successful material for urea , as its corrosion rate when oxygen is present is very low .
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UNS S31803 , UNS S32750 and UNS S32760 : the corrosion rate of these duplex stainless steels is less than half the corrosion rate of the austenitic stainless steel ( UNS a ) b )
c ) d )
S31050 ). The microstructure of these duplex stainless steels reveals a preferential attack of the austenite phase , although when chromium content is lower , an attack of the ferrite phase is also observed . The depth of attack increases as chromium content in the matrix decreases .
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UREMIUM29 ( UNS S32906 ): this super duplex shows the lowest corrosion rate with the lowest depth of attack . In this case , a selective attack of
e )
Figure 2 . Cross sections of the tested specimens : a ) S31050 , b ) S31803 , c ) S32750 , d ) S32760 and e ) UREMIUM29 .
the austenite was also detected . Chromium content is high enough to guarantee the passivity of ferrite under the conditions inside the autoclave . Austenite suffers a slight attack as no oxygen is present . This preferential attack of the austenite has been reported to disappear at longer exposure times , as anodic protection of the austenite can occur when the ferrite / austenite area has become large enough as a result of corrosion of austenite .
Strength ( MPa )
900 800 700 600 500 400 300 200 100
0
316L - UG
25-22-2 - WT ≤ 6 mm
25-22-2 - WT > 6 mm
Uremium29 - WT ≤ 10 mm
Uremium29 - WT > 10 mm
Steel grade Min . Rp0,2 % Min . Rm Min . Elongation
Figure 3 . Mechanical properties of UREMIUM29 at room and high temperature .
40 35 30 25 20 15 10 5 0
Elongation (%)
Strength ( MPa )
900 800 700 600 500 400 300
20 |
100 |
200 |
300 |
T (° C ) |
Min . Rp0,2 % - WT ≤ 10 mm |
Min . Rp0,2 % - WT > 10 mm |
Min . Rm - WT ≤ 10 mm |
Min . Rm - WT > 10 mm |
24 Stainless Steel World September 2023 www . stainless-steel-world . net