新不锈钢 Zecor合金腐蚀曲线

%H2SO4
Oxidizing Behavior of Concentrated Sulfuric Acid
• M (s) + 2 H2SO4 (l) MSO4 (s/l) + SO2 (g) +2 H2O (l)
• Note that no hydrogen is released
© ATI Allegheny Ludlum 2009
© ATI Allegheny Ludlum 2009 18
Stainless Steels
• Addition of 5% or more silicon to an austenitic stainless steel greatly improves corrosion resistance to 98% sulfuric acid.
© ATI Allegheny Ludlum 2009
10
Stainless Steel
• The passive film on stainless steel is harder and much more resistant to flow-induced corrosion than is the iron sulfate film that forms on carbon steels. • Use of "corrosion resistant alloys" for components such as valves, inlets, outlets, and wear plates is advised by NACE SP0294-2006.
• S (s) + O2 (g) → SO2 (g) • 2 SO2 (g) + O2 (g) → 2 SO3 (g) (in presence of V2O5) • H2SO4 (l) + SO3 → H2S2O7 (l)
• H2S2O7 (l) + H2O (l) → 2 H2SO4 (l)
© ATI Allegheny Ludlum 2009 3
© ATI Allegheny Ludlum 2009
4
Selection of Materials
• Selection of materials of construction for various concentrations of sulfuric acid is a balance between the features and benefits of a material versus its capital cost. • Corrosion resistance is not the only consideration that determines material selection. • Cost considerations are always important, particularly in capitalintensive industries such as sulfuric acid manufacturing © ATI Allegheny Ludlum 2009 5
7
Carbon Steel - Isocorrosion
70 60
0.51 mm/y oleum
50
degrees C
0.13 mm/y
40 30 20 10 0
70
© ATI Allegheny Ludlum 2009
80
90 % H2SO4
100
110
8
Carbon Steel - Penetration
© ATI Allegheny Ludlum 2009 11
Effect of Velocity on Corrosion Rate 99.3% Sulfuric Acid, 102°C
10 Ductile Iron
corrosion rate, mm/y
S31600 1
N08020
Cast iron
© ATI Allegheny Ludlum 2009 17
High-Silicon Alloys
• Silicon alloys exhibit high resistance to corrosion by hot, concentrated sulfuric acid. • 14.5% silicon cast iron (UNS F47003) is used for sulfuric acid plant equipment. • This cast iron is extremely brittle.
Improving Energy Recovery
• Reaction heat liberated during the manufacture of sulfuric acid is recovered by cooling hot process gases to produce high pressure steam. • This has created a demand for alloys that can reliably withstand corrosion by hot (120-200°C), concentrated (98-100%) sulfuric acid.
Fe
balance
balance
balance
balance
© ATI Allegheny Ludlum 2009
15
Acid Coolers
• The use of high chloride cooling water for acid cooling creates a challenging materials selection problem in sulfuric acid plants. • In addition to handling the sulfuric acid, on the water side the alloy must be able to resist pitting, crevice corrosion and stress corrosion cracking caused by chlorides.
© ATI Allegheny Ludlum 2009 14
Stainless Alloys used for Sulfuric Acid Service (Percents by mass).
UNS C Mn Si Cr Ni Mo Cu Cb S30400 0.07 max 2.00 max 0.75 max 17.5-19.5 8.0-10.5 ------S31008 0.08 max 2.00 max 1.50 max 24.0-26.0 19.0-22.0 0.3-1.5 ----S31603 0.03 max 2.00 max 0.75 max 16.0-18.0 10.0-14.0 2.00-3.00 ----S44627 0.010 max 0.40 max 0.40 max 25.0-27.5 0.50 max 0.75-1.50 0.20 max 0.06-0.20 S44735 0.03 max 1.00 max 1.00 max 28.0-30.0 1.00 max 3.6-4.2 --Ti+Cb 6x(C+N) min, 1.00 max balance N08020 0.07 max 2.00 max 1.00 max 19.0-21.0 32.0-38.0 2.00-3.00 3.0-4.0 8xC min, 1.00 max balance
4.0 3.5 3.0 2.5 49°C 2.0 1.5 1.0 0.5 0.0 84 86 88 90 92 %H2SO4
© ATI Allegheny Ludlum 2009 9
rate, mm/y
66°C
38°C
27°C
94
96
98
100
Stainless Steel
• Stainless steels are more resistant to concentrated sulfuric acid than are carbon steels. • NACE recommends the use of stainless steel for handling acids above 99.5% to 100.5% and 80% to 88% concentrations.
© ATI Allegheny Ludlum 2009
19
High-Silicon Stainless Steels
© ATI Allegheny Ludlum 2009
16
Acid Coolers
• AL 29-4C® alloy, UNS S44735, exhibits the highest resistance to these forms of chloride-induced corrosion as well as exhibiting very low corrosion rates in 98% sulfuric acid. • Its combination of acid and water side corrosion resistance offers optimum durability and performance in cooler designs, and provides the most cost-effective cooling system.
High Alloy Stainless Steels for Concentrated Sulfuric Acid
ATI Allegheny Ludlum September 2009
© ATI Allegheny Ludlum 2009 1
John F. Grubb
Boiling and Freezing Temperatures for Sulfuric Acid
180 160
E-BRITE®
304
310
140 120 100 97 97.5 98 98.5 99
(passive)
99.5
100
13
% H2SO4
© ATI Allegheny Ludlum 2009
Stainless Steel
• Molybdenum does not enhance resistance to corrosion in the highly oxidizing environment of concentrated sulfuric acid. • Molybdenum does improve resistance to somewhat diluted acid (less than 93%). • Nickel does not add much resistance to corrosion in this environment.
Corrosion Potential of Sulfuric Acid (25°C)
0.7 0.6 0.5 0.4
V sce
0.3 0.2 0.1 0 -0.1 -0.2 -0.3 0 10 20 30 40 50 60 70 80 90 100
6
© ATI Allegheny Ludlum 2009
S30400 0.1 0 2 4 6 velocity, m/s
© ATI Allegheny Ludlum 2009 12
8
10
12
Isocorrosion for Three Stainless Steels
240 0.13 mm/y isocorrosion 220
(active)
200
Temperature, C
350 300 250 200
degrees C
150 100 50 0 -50 -100 0 20 40
BOILING
H2SO4 . H2O H2SO4 . 4 H2O
FREEZING
© ATI Allegheny Ludlum 2009
60 % H2SO4
80
100
2
Contact Process