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#059 Stress Corrosion Fracture

Category : Corrosion - Corrosion Protection
October15, 2010

As seen in metal tensile tests, a metal specimen pulled at two ends will eventually result in a fracture. This is called Tensile Strength. In certain environments, metals can fracture at less than rated tensile loads. The phenomenon is called Stress Corrosion Fracture. The stress that cause Stress Corrosion cracking is in tensile, including stresses during the usage and internal stress applied during machining.

[Fig.1] is a schematic illustration of Stress Corrosion cracking. Corrosion points occurring on the surface gradually progress. Either internal or external tensile stress is applied on this material and the corrosion and fracture tips will become sharp, and the direction of the advancing fractures will zigzag. The fractures may be crystal grain boundary type or intra crystal type.


Stress Corrosion cracking occurs in some specific combination of metals and environments, as shown in [Table 1]. As shown, the corrosion occurs only when the specific corrosion environment exists, even when tensile stress exists. It can be said that this is a type corrosion in rather limited environments.
A classical example of Stress Corrosion cracking known is the corrosion of steam locomotive boiler rivet areas. This was caused by a combination of carbon steel material and hot water with NaOH additive used as a corrosion inhibitor.
Also, well known is the seasonal fracture of brass material used for ammunition cartridges where numerous cartridge brass fracture was seen during the Monsoon season. The cartridge brass cold-process manufactured with a combination of brass and NH3 experienced Stress Corrosion cracking affected by humidity, oxygen, sulfur dioxide gas, and ammonia in the air.
Austenitic stainless steel is likely to experience Stress Corrosion cracking in chloride solutions and sea water, therefore, post forming stress relieved by heat treating is applied, as well as substituted with duplex grade material with austenite and ferrite characteristics.

[Table 1] Stress Corrosion cracking Causing Metals and Environments
MetalCausative MaterialEnvironment Example
Carbon SteelNO3-High Temp. NaNO3Solution
OH-High Temp./Concentration NaOH Solution
High Tensile SteelH2SH2SSolution
Austenitic Stainless SteelCL-High Temp. Sea Water
OH-High Temp./Concentration NaOH Solution
Polythionic AcidExposed to hygrophitic environment after sulfurating
High Temp. WaterPlumbing for boiling water type nuclear reactors
BrassNH3Atmosphere containing NH3
AminesAmine Solution
High Strength Aluminum AlloyCL-Sea Water
High Strength Titanium AlloyCL-Sea water,high temp NaCL