The Hendrix Group Reporter ©
February 1, 2004 (Vol. XIII No. 1)
Chloride Stress Corrosion Cracking of Types AISI 304SS and 316SS
We have had inquiries regarding the relative resistance of types 304SS and 316SS to chloride stress corrosion cracking (Cl SCC). Due to the increased resistance of type 316SS to pitting corrosion, as compared with 304SS, some may think that 316SS is also a useful upgrade to resist chloride SCC when 304SS fails. We do not recommend 316SS as an upgrade to 304SS when environmental conditions have caused failure of 304SS. First, a brief introduction to chloride stress corrosion cracking of the austenitic stainless steels.
Stress - corrosion cracking is a generic term describing the initiation and propagation of cracks in a metal or alloy under the combined action of tensile stresses (applied and/or residual) and a corrosive environment. Temperature has a very important influence on the chloride cracking of austenitic stainless steels. Experience has shown that cracking does not occur at room temperature. Chloride cracking does occur at temperatures above ambient, generally at temperatures above 60 °C (140 °F). As the temperature increases above this limit, the time to fracture decreases, other factors being constant. Normally, chloride cracking is transgranular; however, they can crack intergranularly, if sensitized.
Factors that influence the rate and severity of cracking include chloride content, oxygen content, temperature, stress level, and pH value of an aqueous solution. It has been established that oxygen is required for chloride cracking to occur. The severity of cracking increases with temperature.
The stresses required to produce cracking can be assumed to be always present. Residual stresses from forming, bending, or joining operations are sufficient for cracks to form. Thermal stress relief treatments at 870 °C (1600 °F) can effectively prevent cracking if done correctly and without the necessity of subsequent cold working (to correct distortion, for example).
In alkaline solutions, the likelihood of chloride SCC is greatly reduced. Consequently, austenitic stainless steels are frequently used for equipment exposed to amine solutions in gas-treating and sulfur recovery units. A survey of plant experiences has shown no reported instances of cracking despite the fact that chloride contents as high as 1000 ppm were measured in the circulating amine solution. In theory, one would need only a single chloride ion in water, with sufficient oxygen and residual stresses present, to cause cracking . In practice, however, the permissible limits on chloride ion content are higher. In researching the relative susceptibility of 304SS and 316SS to chloride SCC, it becomes clear that results from different tests sometimes contradict one another. For instance, Figure 1, to the left, suggests that 316SS exhibits greater resistance to chloride SCC in the 42% boiling MgCl 2 test.
One reason cited for the perceived increased resistance of 316SS to chloride SCC vs. 304SS is its demonstrated better resistance to pitting corrosion. There is some controversy whether chloride SCC cracks typically initiate from the bottom of pits. It has been our experience that detectable pitting is not required for chloride SCC to occur. Our experience also suggests that any laboratory-based results showing a difference in the decreased susceptibility of 316SS vs. 304SS cannot be safely be transferred to field conditions. This opinion is supported by the following sources:
The Sandvik Handbook of Stainless Steels shows the results of testing different alloys in oxygen bearing neutral chloride solutions (Figure 2). The figure shows no difference in the performance between 304SS and 316SS. Testing was done using constant load tests in an autoclave at different chloride contents and temperatures. The sources states "This method gives very reliable results and is often used as a guideline when material selection is made."
Volume 13, Corrosion of the ASM Handbook, 9 th Ed., in a paragraph discussing stress corrosion cracking states that "…all austenitic stainless steels, especially AISI types 304SS and 316SS, are susceptible to some degree."
MTI manual No. 1, Guidelines for Control of Stress Corrosion Cracking of Nickel-Bearing Stainless Steels and Nickel-Base Alloys, in discussing the 300-series stainless steels, states that with respect to 304SS: "Molybdenum varying variants having greater general corrosion resistance in certain environments (although usually no more resistant to SCC) are Types 316and 316L…" Corrosion/99, Paper No. 385, Experience Survey of Chloride Resistant Alloys in Process Plants, cites, " As reported elsewhere, criterion curves of 300's SS for SCC occurrence in T – CL diagrams were proposed for heat exchanger cooling water systems. In the present study, such a tendency was not found. SCC occurrences were widely scattered in T – Cl- diagrams for both 304SS and 316 SS and no significant difference between the two materials was observed ."
In summary, we caution the industrial community when considering an upgrade alloy to combat chloride SCC. As we have previously stated in private communications to a client, " The references share the opinion of this writer in that upgrading to 316SS when a 304SS component has failed due to chloride SCC would invite the real risk of additional, similar failures."