Materials for H2S environments – cracking the problem

Mintek, South Africa’s national mineral technology institute, has developed a unique laboratory facility for testing the susceptibility of alloys to corrosion and hydrogen-induced cracking under the conditions known as “sour service”.

“This type of cracking has been responsible for numerous equipment failures worldwide, resulting in serious health and safety consequences as well as environmental incidents, damage to equipment and downtime,” said Deon Slabbert, co-ordinator of Mintek’s Metals Technology Centre (MTC).

Sour service, or wet hydrogen sulphide (H2S), environments are commonly encountered in the petrochemical industry. The corrosion of steel under these conditions generates hydrogen, which when absorbed by the steel can lead to crack development and eventual failure in pipelines, pressure vessels, and related plant equipment.

“The presence of H2S can cause metallic materials to fail by several mechanisms, and testing of materials to determine their suitability for sour service is critical for both producers and users,” explained Slabbert. “South Africa’s petrochemical refiners – Chevron, SAPREF, Engen, NATREF and SASOL – all specify the use of materials resistant to hydrogen-induced cracking to avoid in-service failures. Seamless pipes are also produced locally for oilfield applications, and these products need to be certified as resistant to hydrogen-induced cracking and in many instances to sulphide stress cracking as well.

“There are several international standards and material recommendations aimed at preventing failures in sour service, and working groups and research projects are active internationally to extend our knowledge in this area. However, owing to the extreme conditions employed in these tests, which involve the exposure of specimens in an acidified brine solution saturated with H2S for up to 30 days, few laboratories are equipped to conduct such investigations”.

The MTC at Mintek is the only laboratory in South Africa that provides this service to industry. The purpose-built facility is constructed to world-class standards, incorporating dedicated stainless-steel equipment and additional safety features beyond those that are required in the standard procedures.

Samples are prepared for metallographic examination using automated equipment, and a specialised spreadsheet has been designed to calculate the critical parameters that must be within specified limits.

“Since commissioning of the facility in 2005, we have tested more than 1 400 samples without incident, owing to the dedication and attention to detail of the staff,” said Slabbert. “As a result, most materials used in South Africa, and also those exported, specifically for sour service, are now accompanied by a Mintek report certifying that the material has been tested and is resistant to hydrogen-induced and sulphide stress cracking.”

Mintek’s MTC also offers other corrosion tests and a wide range of investigations to solve materials-related problems in industry.

For more information about Mintek’s MTC and its services, contact Melanie Smit at Tel: +27 11 709 4501 or e-mail melanies@mintek.co.za