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TP 304L

Corrosion Resistance of 304L Stainless Steel






General Corrosion The Alloys 304, 304L, and 304H austenitic stainless steel provide useful resistance to corrosion on a wide range of moderately oxidizing to moderately reducing environments. The alloys are used widely in equipment and utensils for processing and handling of food, beverages, and dairy products. Heat exchanger, piping, tanks, and other process equipment in contact with fresh water also utilize these alloys.

The 18 to 19 percent of chromium which these alloys contain provides resistance to oxidizing environment such as dilute nitric acid, as illustrated by data for Alloy 304 below.

Alloys 304, 304L, and 304H are also resistant to moderately aggressive organic acids such as acetic and reducing acids such as phosphoric. The 9 to 11 percent of nickel contained by these 18-8 alloys assists in providing resistance to moderately reducing environments. The more highly reducing environments such as boiling dilute hydrochloric and sulfuric acids are shown to be too aggressive for these materials. Boiling 50 percent caustic is likewise too aggressive.

In some cases, the low carbon Alloy 304L may show a lower corrosion rate than the higher carbon Alloy 304. The data for formic acid, sulfamic acid, and sodium hydroxide illustrate this. Otherwise, the Alloys 304, 304L, and 304H may be considered to perform equally in most corrosive environments. A notable exception is in environments sufficiently corrosive to cause intergranular corrosion of welds and heat-affected zones on susceptible alloys. The Alloy 304L stainless steel pipe is preferred for use in such media in the welded condition since the low carbon level enhances resistance to intergranular corrosion.

Intergranular Corrosion in the 800°F to 1500°F (427°C to 816°C) range may cause precipitation of chromium carbides in grain boundaries. Such steels are "sensitized" and subject to intergranular corrosion when exposed to aggressive environments. The carbon content of Alloy 304 may allow sensitization to occur from thermal conditions experienced by autogenous welds and heat-affected zones of welds. For this reason, the low carbon Alloy 304L stainless steel tube preferred for applications in which the material is put into service in the as-welded condition. Low carbon content extends the time necessary to precipitate a harmful level of chromium carbides but does not eliminate the precipitation reaction for material held for long times in the precipitation temperature range.

% Nitric Acid Temperature
° F (°C)
Corrosion Rate
Mils/Yr (mm/a)
10 300 (149) 5.0 (0.13)
20 300 (149) 10.1 (0.25)
30 300 (149) 17.0 (0.43)

Intergranular Corrosion Tests
ASTM A262 Evaluation Test Corrosion Rate, Mils/Yr (mm/a)
304 304L
Practice E
   Base Metal
   Welded
No Fissures on Bend
Some Fissures on Weld
(unacceptable)
No Fissures
No Fissures
Practice A
   Base Metal
   Welded
Step Structure
Ditched
(unacceptable)
Step Structure
Step Structure

Stress Corrosion Cracking
The Alloys 304, 304L, and 304H are the most susceptible of the austenitic stainless steel to stress corrosion cracking (SCC) in halides because of their relatively low nickel content. Conditions which cause SCC are: (1) presence of halide ions (generally chloride), (2) residual tensile stresses, and (3) temperatures in excess of about 120°F (49°C). Stresses may result from cold deformation of the alloy during forming or by roller expanding tubes into tube sheets or by welding operations which produce stresses from the thermal cycles used. Stress levels may be reduced by annealing or stress relieving heat treatments following cold deformation, thereby reducing sensitivity to halide SCC. The low carbon Alloy 304L material is the better choice 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
304
33% Lithium
Chloride, Boiling
Base
Metal

Welded
Cracked, 14 to 96 hours


Cracked, 18 to 90 hours

26% Sodium
Chloride, Boiling
Base
Metal

Welded
Cracked, 142 to 1004 hours


Cracked, 300 to 500 hours
40% Calcium
Chloride, Boiling
Base
Metal
Cracked, 144 hours

--
Ambient Temperature Seacoast Exposure Base
Metal

Welded
No Cracking


No Cracking

Pitting Corrosion/Crevice Corrosion
The 18-8 alloys have been used very successfully in fresh waters containing low levels of chloride ion. Generally, 100 ppm chloride is considered to be the limit for the 18-8 alloys, particularly if crevices are present. Higher levels of chloride might cause crevice corrosion and pitting. For the more severe conditions of higher chloride levels, lower pH, and/or higher temperatures, alloys with higher molybdenum content such as Alloy 316 should be considered. The 18-8 alloys are not recommended for exposure to marine environments.

General Properties
Chemical Composition
Resistance to Corrosion
Physical Properties
Mechanical Properties
Welding
Heat Treatment
Cleaning
304/304L/304LN/304H Tubing and Pipe

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