Article by Dr. Kailasanathan Elayaperumal, Metallurgical Consultant, Chandan Steel Ltd, India
This case study deals with the use of type 316 stainless steel (SS) tubes in an application where a suitable duplex stainless steel (DSS) should have been specified as the appropriate material of choice.
The 45 tubes under discussion meansure 25.4 mm OD x 1.6 mm WT x 6 M long, made of A-213/ Type 316 SS and were cold finished seamless, annealed and white pickled. A shell- and-tube heat exchanger made from these tubes leaked after about three months of service. Seven out of 45 tubes were found to have leaked.
Cracking from the inside
Photographs 1 and 2 below show a sample cut from a leaking tube. Through and through cracks, visible on both outside and inside surfaces and thick black coating on the inside surface can be seen. The crack is wider on the inside than on the outside surface, indicating that crack initiated from the inside.
These features represent a localized corrosion phenomenon and not a general corrosion phenomenon attributable to any quality deficiency of the tubes.
Photographs 3 and 4 show the metallurgical microstructures across the wall of the tube at the cracked position.
The following features are noticeable:
- The general structure is that of properly annealed austenitic stainless steel representative of 316SS.
- There are cracks initiated at multiple locations (not only one location) on the inside surface and propagated towards the outside surface.
- Cracks have propagated in a transgranular manner (cutting across grains and not necessarily through grain boundaries).
- Cracks have propagated in a branched manner (tree-like branches). All the above features are representative of Chloride Stress Corrosion Cracking of Austenitic Stainless Steel (CSCC) initiated on the inside surface of the tubes.