Written by: Sandy Williams
ArcelorMittal Lázaro Cárdenas, Mexico recently launched production of specialized slabs for use in pipelines that transport sour gas.
Lázaro Cárdenas is known as a leading producer of high quality, value added steel. In order to increase its share of the growing market for API (American Petroleum Institute) pipes, best technological practices were implemented at the site to be able to produce defect-free slabs with a reduced centre segregation rate.
Because sour gas contains significant quantities of toxic gases such as hydrogen sulfide (H25), carbon dioxide and similar contaminants, the slabs must have high resistance against hydrogen-induced cracking (HIC). The H2S present in the sour gas poses a major challenge: this compound reacts with water creating a corrosion mechanism due to the release of hydrogen in diatomic and monatomic form. The monatomic hydrogen diffuses through the steel and becomes trapped in voids in the metal microstructure, giving rise to brittle areas when these atoms recombine to form hydrogen molecules. This creates pressure mechanisms, leading to the formation of blisters and subsequently micro-cracking which can result in the failure of the metal with potentially disastrous consequences.
The steel’s resistance was tested by a method developed by corrosion engineers from NACE International. The steel is placed in an acid bath for 96 hours to see if cracking occurs. If that is the case, cracks are measured against an index based on two criteria: the crack length ratio (CLR) and the crack sensitivity ratio (CSR). The tolerance level is dictated by the customer, in this case the pipes producer, but average ratios are between 10-15 percent for CLR, and between 2-5 percent for CSR.
After several tests, the internal quality of the slabs exceeded expectations. The customers for this project are tube manufacturer Welspun Corp in India and coils plate producer Ternium in Mexico.
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