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Alloys 316, 316L, and 317L are more resistant to atmospheric and other mild types of corrosion than the 18-8 stainless steel. In general, media that do not corrode 18-8 stainless steel tube will not attack these molybdenum-containing grades. One known exception is highly oxidizing acids such as nitric acid to which the molybdenum-bearing stainless steel tubing are less resistant.

Alloys 316 and 317L are considerably more resistant than any of the other chromium-nickel types to solutions of sulfuric acid. At temperature as high as 120°F (38°C), both types have excellent resistance to higher concentrations. Service tests are usually desirable as operating conditions and acid contaminants may significantly affect corrosion rate. Where condensation of sulfur-bearing gases occurs, these alloys are much more resistant than other types of stainless steel. In such applications, however, the acid concentration has a marked influence on the rate of attack and should be carefully determined.

The molybdenum-bearing Alloys 316 and 317L stainless steel also provide resistance to a wide variety of other environments. As shown by the laboratory corrosion data below, these alloys offer excellent resistance to boiling 20% phosphoric acid. They are also widely used in handling hot organic and fatty acids. This is a factor in the manufacture and handling of certain food and pharmaceutical products where the molybdenum-containing stainless steel are often required in order to minimize metallic contamination.

Generally, the Alloy 316 and 316L grades can be considered to perform equally well for a given environment. The same is true for Alloy 317L. A notable exception is in environments sufficiently corrosive to cause intergranular corrosion of welds and heat-affected zones on susceptible alloys. In such media, the Alloy 316L and 317L grades are preferred for the welded condition since low carbon levels enhance resistance to intergranular corrosion.



Corrosion Resistance in Boiling Solutions

Boiling Test
Solution

Corrosion Rate in Mils per Year (mm/y) for Cited Alloys
Alloy 316L Alloy 317L
Base
Metal
Welded Base
Metal
Welded
20%
Acetic Acid
0.12
(0.003)
0.12
(0.003)
0.48
(0.012)
0.36
(0.009)
45%
Formic Acid
23.4
(0.594)
20.9
(0.531)
18.3
(0.465)
24.2
(0.615)
1%
Hydrochloric Acid
0.96
(0.024)
63.6
(1.615)
54.2
(1.377)
51.4
(1.306)
10%
Oxalic Acid
48.2
(1.224)
44.5
(1.130)
44.9
(1.140)
43.1
(1.094)
20%
Phosphoric Acid
0.60
(0.15)
1.08
(0.027)
0.72
(0.018)
0.60
(0.015)
10%
Sulfamic Acid
124.2
(3.155)
119.3
(3.030)
94.2
(2.393)
97.9
(2.487)
10%
Sulfuric Acid
635.3
(16.137)
658.2
(16.718)
298.1
(7.571)
356.4
(9.053)
10%
Sodium Bisulfate
71.5
(1.816)
56.2
(1.427)
55.9
(1.420)
66.4
(1.687)
50%
Sodium Hydroxide
77.6
(1.971)
85.4
(2.169)
32.8
(0.833)
31.9
(0.810)

Pitting/Crevice Corrosion
Resistance of austenitic stainless steel tube to pitting and/or crevice corrosion in the presence of chloride or other halide ions is enhanced by higher chromium (Cr), molybdenum (Mo), and nitrogen (N) content. A relative measure of pitting resistance is given by the PREN (Pitting Resistance Equivalent, including Nitrogen) calculation, where PRE = Cr+3.3Mo+16N. The PREN of Alloys 316 and 316L (24.2) is better than that of Alloy 304 (PREN = 19.0), reflecting the better pitting resistance which 316 (or 316L) offers due to its Mo content. Alloy 317L, with 31.% Mo and PREN = 29.7, offers even better resistance to pitting than the 316 alloys.

Alloy 304 stainless steel is considered to resist pitting and crevice corrosion in waters containing up to about 100 ppm chloride. The Mo-bearing Alloy 316 and Alloy 317L on the other hand will handle waters with up to about 2000 and 5000 ppm chloride, respectively. Although these alloys have been used with mixed success in seawater (19,000 ppm chloride), they are not recommended for such use. Alloy 2507 with 4% Mo, 25% Cr, and 7% Ni is designed for use in salt water. The Alloys 316 and 317L are considered to be adequate for some marine environment applications such as boat rails and hardware and facades of buildings near the ocean, which are exposed to salt spray. The Alloys 316 and 317L stainless steels all perform without evidence of corrosion in the 100-hour, 5% salt spray (ASTM B117) test.

Intergranular Corrosion
Both Alloys 316 and 317L are susceptible to precipitation of chromium carbides in grain boundaries when exposed to temperatures in the 800 to 1500°F (427 to 816°C) range. Such "sensitized" steels are subject to intergranular corrosion when exposed to aggressive environments. Where short periods of exposure are encountered, however, such as in welding, Alloy 317L with its higher chromium and molybdenum content, is more resistant to intergranular attack than Alloy 316 for applications where light gauge material is to be welded. Heavier cross sections over 7/16 inch (11.1 mm) usually require annealing even when Alloy 317L is used.

For applications where heavy cross sections cannot be annealed after welding or where low temperature stress relieving treatments are desired, the low carbon Alloys 316L and 317L are available to avoid the hazard of intergranular corrosion. This provides resistance to intergranular attack with any thickness in the as-welded condition or with short periods of exposure in the 800 to 1500°F (427 to 826°C) temperature range. Where vessels require stress-relieving treatment, short treatments falling within these limits can be employed without affecting the normal excellent corrosion resistance of the metal. Accelerated cooling from higher temperatures for the "L" grades is not needed when very heavy or bulky sections have been annealed.

Alloys 316L and 317L possess the same desirable corrosion resistance and mechanical properties as the corresponding higher carbon alloys and offer an additional advantage in highly corrosive applications where intergranular corrosion is a hazard. Although the short duration heating encountered during welding or stress relieving does not produce susceptibility to intergranular corrosion, it should be noted that continuous or prolonged exposure at 800 to 1500°F (427 to 826°C) can be harmful from this standpoint with Alloys 316L and 317L. Also stress relieving between 1100 to 1500°F (593 to 816°C) may cause some slight embrittlement of these types.

Intergranular Corrosion Tests

ASTM A262 Evaluation
Test

Corrosion Rate, Mils/Yr (mm/a)
Alloy 316 Alloy 316L Alloy 317L
Practice B
Base Metal
Welded

36 (0.9)
41 (1.0)

26 (0.7)
23 (0.6)

21 (0.5)
24 (0.6)
Practice E
Base Metal

Welded

No Fissures
on Bend
Some Fissures
on Weld (unacceptable)

No Fissures
No Fissures

No Fissures
No Fissures
Practice A
Base Metal

Welded

Step Structure

Ditched (unacceptable)

Step Structure
Step Structure

Step Structure
Step Structure

Stress Corrosion Cracking
Austenitic stainless steels are susceptible to stress corrosion cracking (SCC) in halide environments. Although the Alloys 316 and 317L are somewhat more resistant to SCC than the 18 Cr-8 Ni alloys because of their molybdenum content, they still are quire susceptible. Conditions which produce SSC are: (1) presence of halide ion (generally chloride), (2) residual tensile stresses, and (3) temperatures in excess of about 120°F (49°C).

Stresses result from cold deformation or thermal cycles during welding. Annealing or stress relieving heat treatments may be effective in reducing stresses, thereby reducing sensitivity to halide SCC. Although the low carbon "L" grades offer no advantage as regards SCC resistance, they are better choices for service in the stress-relieved condition in environments which might cause intergranular corrosion.

Halide (Chloride) Stress Corrosion Tests

Test

U-Bend (Highly Stressed) Samples
Alloy 316 Alloy 316L Alloy 317L
42% Magnesium Chloride, Boiling Cracked,
4-24 hours
Cracked,
21-45 hours
Cracked,
72 hours
33% Lithium Chloride, Boiling Cracked,
48-569 hours
Cracked,
21-333 hours
Cracked,
22-72 hours
26% Sodium Chloride, Boiling Cracked,
530-940 hours
No Cracking,
1002 hours
Cracked,
1000 hours
40% Calcium Chloride, Boiling Cracked,
144-1000 hours
-- --
Seacoast Exposure, Ambient Temperature No Cracking No Cracking No Cracking

Back to 316L

General Properties
Chemical Composition
Resistance to Corrosion
Physical Properties
Mechanical Properties
Oxidation Resistance
Heat Treatment
Fabrication
Corrosion of Piping
Corrosion Process
Surface Coatings for Corrosion
Corrosion Resistant Material
Bi- Metallic Corrosion.Galvanic Corrosion
Comparison Grade 316 316L 316Ti