Corrosion Resistance of 304H 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 environments 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 acid and reducing acids such as phosphoric acid. 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 acid 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, sulfuric 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
Exposure of the 18-8 austenitic stainless steels to temperatures 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.

Stress Corrosion Cracking SCC
The Alloys 304, 304L, and 304H are the most susceptible of the austenitic stainless steels 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.

Pitting/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.

• Resistance to corrosion in oxidizing environments is a result from the 18 to 19% chromium that the 304 alloys contain
• Resistance to moderately aggressive organic acids is a result from the 9 to 11% nickel that the 304 alloys contain
• At times, Alloy 304L may show a lower corrosion rate than the higher carbon Alloy 304; otherwise, the 304, 304L, and 304H alloys may be considered to perform uniformly in most corrosive environments.