Sour gas-resistant at minus 60 degrees Celsius

Extreme environmental influences call for extremely resistant materials. Salzgitter Flachstahl, in cooperation with Salzgitter Mannesmann Line Pipe GmbH, has developed a material that is suitable for low temperatures while maintaining sour gas resistance to -60° Celsius.

In the future, natural gas will be even more important as an alternative to nuclear power than it is today. Very long transport lines will be needed in order to deliver the gas to consumers. The world's reserves are limited, however. More and more of the easily exploited reservoirs that were particularly clean, large or easily accessible for drilling units are becoming depleted. This is forcing the large energy companies to investigate alternatives.
Natural gas is a mixture of methane and various other gases, such as carbon dioxide, nitrogen, higher hydrocarbons and, in some fields, also hydrogen sulfide. Gas with a hydrogen sulfide content of less than one percent is called "slightly sour" gas, while gas with a higher content is called "sour gas". Gas containing no hydrogen sulfide is called "sweet gas".

As the easily accessible stores of sweet gas have been depleted, it is increasingly the sour natural gasses that have to extracted, transported and stored.

Unlike materials for handling sweet gas, sour gas-resistant materials exhibit a very low sulfur content of no more than 0.001% while also displaying low contents of manganese and carbon. This perforce limits the strengths that can be obtained, so that frequently only materials up to API 5L X65MS with minimum tensile strengths of 535 MPa are available on the market. Work is already underway on X70MS materials with strengths of at least 570 MPa. In work with the sister company Salzgitter Mannesmann Linepipe GmbH, several analyses for sour gas pipes have been coordinated and delivered on a trial basis. Special Salzgitter Flachstahl steel mill processing technology is used to produce this highly demanding material. Particularly comprehensive examinations are conducted in the framework of the release tests for sour gas grades. These are the so-called SSC and HIC tests, in which the steel samples are exposed to a sour atmosphere at a specified pH level for a defined length of time. The samples are then evaluated in metallographic specimens or by visual inspection according to internationally established criteria.

The samples delivered to Salzgitter Mannesmann Linepipe GmbH passed the demanding test program without exception. The next step will be to transfer sour gas resistance to further grades. The development of cryogenic steels without sour gas resistance is being pursued at the same time.