Two sections of 304 stainless steel pipe were received for analysis of the source of leaks. Additional materials were also received, including insulation, strapping, two process fluids and a water sample from the DI system used to mix the fluids.
One pipe showed minor pitting and the other contained a network of fine cracks. The cracks appear to have originated on the outside of the pipe. Both pipes were covered with debris.
Drops of the process fluids and the DI water were dried on aluminum stubs.
The DI water contained numerous sodium chloride deposits. Ion chromatography (IC) testing confirmed that the DI water sample contained 9 ppm of chlorides. This is the likely source of chlorides that caused the corrosion.
The other dried process fluids, debris, pieces of the supplied insulation and strap material were analyzed using scanning electron microscope with energy dispersive spectroscopy (SEM/EDS) equipped with a light element detector. None of these materials contained any chlorine.
The debris on the surfaces of the tubes was from the silica-based insulation and contained sodium and chlorine, likely from trapping moisture against the pipes. 300 series stainless steels are susceptible to stress corrosion cracking (SCC) in moist chloride environments.
IMAGE CAPTIONS (LEFT TO RIGHT)
- As received sample after cleaning
- Failure analysis of cracks in the pipe section – as polished
- SEM image of surface pitting on pipe sample
- Pitting attack on the inner surface – as received