Resistance to Corrosion
Alloys 317L and 317LMN stainless steel are more resistant to atmospheric and other mild types of corrosion than conventional chromium-nickel stainless steel. In general, environments that are not corrosive to 18Cr-8Ni steel will not attack alloys containing molybdenum, with the exception of highly oxidizing acids such as nitric acid.
Alloys 317LMN and 317L stainless steel pipe are considerably more resistant than conventional chromium-nickel types to solutions of sulfuric acid. Resistance increases with alloy molybdenum content. These alloys are resistant to sulfuric acid concentrations up to 5 percent at temperature as high as 120°F (49°C). At temperatures under 100°F (38°C) these alloys have excellent resistance to solutions of higher concentration.
However, service tests are recommended to account for the affects of specific operating conditions that may affect corrosion behavior. In processes where condensation of sulfur-bearing gases occurs, these alloys are much more resistant to attack at the point of condensation than conventional Alloy 316. The acid concentration has a marked influence on the rate of attack in such environments and should be carefully determined by service tests.
The table below compares the corrosion resistance of annealing samples of 317LMN and 317L stainless steelin a variety of solution related to the process industries as well as standard ASTM test. Data on 316L and Alloy 276 are for comparison.
| Corrosion Resistance in Boiling Solutions and ASTM Tests |
Test
Solution |
Corrosion Rate in Mils per Year (mm/y)
for Cited Alloys |
Alloy
316L |
Alloy
317L |
Alloy
317LMN |
Alloy
276 |
20%
Acetic Acid |
0.12
(<0.01) |
0.48
(0.01) |
0.12
(<0.01) |
0.48
(0.01) |
45%
Formic Acid |
23.41
(0.60) |
18.37
(0.47) |
11.76
(0.30) |
2.76
(0.07) |
10%
Oxalic Acid |
48.03
(1.23) |
44.90
(1.14) |
35.76
(0.91) |
11.24
(0.28) |
20%
Phosphoric
Acid |
0.06
(0.02) |
0.72
(0.02) |
0.24
(<0.01) |
0.36
(0.01) |
10%
Sulfuric Acid |
635.7
(16.15) |
298.28
(7.58) |
157.80
(4.01) |
13.93
(0.35) |
10%
Sodium
Bisulfate |
71.57
(1.82) |
55.76
(1.42) |
15.60
(0.40) |
2.64
(0.07) |
50%
Sodium
Hydroxide |
77.69
(1.92) |
32.78
(0.83) |
85.68
(2.18) |
17.77
(0.45) |
ASTM A262
Practice B
(FeSO4H2SO4) |
26.04
(0.66) |
20.76
(0.53) |
17.28
(0.44) |
264.5
(6.72) |
ASTM A262
Practice C
(65% HNO3) |
22.31
(0.56) |
19.68
(0.50) |
16.32
(0.42) |
908.0
(23.06) |
ASTM A262
Practice E
(Cu•CUSO4•H2SO4) |
Pass |
Pass |
Pass |
Pass |
The low carbon (less than 0.03%) of these alloys effectively prevents sensitization to intergranular corrosion during thermal processes such as welding or forging. The higher chromium contents of 317LMN and Alloy 317L stainless steels also provide superior resistance to intergranular attack. It should be noted that prolonged exposure in the range 800 to 1400°F (427-816°C) can be detrimental to intergranular corrosion resistance and may also cause embrittlement due to precipitation of sigma phase. The higher nitrogen content of the 317LMN alloy retards the precipitation of sigma phase as well as carbides.
| Pitting Resistance Equivalents |
| Alloy |
PRE |
Alloy 316
|
25 |
| Alloy 317L |
30 |
| Alloy 317LMN |
38 |
| Alloy 625 |
52 |
| Alloy C276 |
69 |
High molybdenum and nitrogen contents can significantly improve pitting resistance as illustrated in the preceding table of Pitting Resistance Equivalents (PRE). The PRE is based on the results of corrosion tests in which it was found that nitrogen was 30 times more effective than chromium and approximately 9 times more effective than molybdenum in enhancing chloride pitting resistance.
The temperature of the onset of crevice corrosion as determined in a modified AST G-48B test is a useful means of ranking the relative resistance of stainless and nickel-base alloys. The Critical Crevice Corrosion Temperatures table that follows demonstrates that crevice corrosion resistance for austenitic stainless steel tubing increases with the alloy's molybdenum and nitrogen content.
Crevice Corrosion in a Simulated
FGD System Environment |
| Alloy |
Weight Loss (g/cm2)
for Tests* at Cited Temperatures |
24°C
(75°F) |
50°C
(122°F) |
70°C
(158°F) |
Alloy 317L
|
0.0007
|
0.0377
|
0.0500 |
| Alloy 317LMN |
0.0000 |
0.0129 |
0.0462 |
| Alloy 625 |
0.0000 |
0.0000 |
0.0149 |
| Alloy C276 |
0.0000 |
0.0001 |
0.0004 |
*72-hour exposure based on ASTM G-48B procedure using the following solution:
7 vol.%H2SO4, 3 vol%HCI, 1 wt% CuCl2, 1 wt%FeCl3
Back to 317L
General Properties
Chemical Composition
Resistance to Corrosion
Physical Properties
Mechanical Properties
Oxidation Resistance
Heat Treatment
Fabrication
Welding
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