Toughness test for 700 tons of metal
High-performance material at one of the world’s largest flue gas desulfurization plants

Syncride of Canada, the world’s largest producer of oil extracted from oil sand deposits, is planning to increase annual production from 90 to 130-135 million barrels during the course of this year. ThyssenKrupp VDM supplied the outer layer of the Syncrude absorber for the flue gas desulfurization unit on this project. The unit is one of the largest in the world.

Wet scrubbing, normally in a limestone suspension, is used in flue gas desulfurization units (FGD) to minimize sulfur dioxide emissions which occur during the combustion process. The end product is gypsum, which can be used in the construction industry. Sulfuric acid can form during the scrubbing process, which at high temperatures can expose the construction materials to extremely corrosive conditions. In addition to sulfuric acid corrosion, chlorides create the added risk of pitting. Corrosive stress is highest underneath deposits which contain high concentrations of corrosive substances.

The form of corrosion which occurs in design-related gaps is called crevice corrosion. FGD presents a tough test for the materials used in the design, and this is the case for the nickel-based Nicrofer 5923 hMo alloy 59 from ThyssenKrupp VDM. Syncrude Canada, the world’s largest producer of oil extracted from oil sand, joined the ranks of those companies which have decided in favor of this material when it opted to use alloy 59 in a new FGD unit. ThyssenKrupp supplied 700 tons of the material for the project. The FGD unit is 94.5 meters high, and the absorber sump has a diameter of around 21 meters. The absorber itself is 15 meters in diameter, and the chimney has a diameter of six meters.
The order is one of the largest that the company has ever received for this type of application, and it is by far the largest order for ThyssenKrupp in North America. It is interesting to note that alloy 59 belongs to a family of alloys that were introduced more than 70 years ago. ThyssenKrupp believes that market demand will increase significantly for these high-performance materials in the future. Current estimates indicate that extraction of oil from oil sand will rise from today’s level of about 600,000 barrels a day to around 2.2 million barrels in 2010. Other experts predict a rise to 3.5 million barrels if all the projects which have been announced are realized. One of the special features of the Syncrude project is the use of Marsulex technology for the desulfurization process. Ammonia is used in place of limestone in the scrubber. The end product is ammonium sulfate, which can be used as a fertilizer. The specified life of the alloy 59 absorber is a minimum of 20 years, and the unit must remain in uninterrupted operation for three years at a time before the unit has to be shut down for maintenance and inspection.
C alloys: highly-alloyed and versatile
The so-called C alloys deliver the best performance in FGD units, even in comparison to non-metallic materials. The alloys have a high nickel content (greater than 50 percent) plus 16 to 23 percent chromium and 13 to 16 percent molybdenum. The high amount of chromium enhances corrosion resistance in oxidizing media, and the molybdenum content provides corrosion protection in reducing media. The high nickel content protects against stress corrosion cracking. The first alloy of this type called “C” was introduced in America in 1930. Since then, there have been seven new members of the alloy family. ThyssenKrupp VDM patented its alloy 59 in the early 1990s. Alloy 59 is a special member in this series, because it has the highest purity of the NiCrMo alloys. Whereas the other materials contain 2 to 6 percent iron as well as tungsten or copper in addition to nickel, the iron content of alloy 59 is less than 1.3%. Alloy 59 had to outperform C alloys from four American suppliers in the selection process on the Syncrude project. Tests showed that alloy 59 provided better corrosion resistance and better thermal stability than the other materials.
To perform testing in crevice conditions, the Syncrude project team developed a chemical solution which is similar to the medium which will be generated in the FGD unit.
Rendezvous with “green death”
Another stress test used a hot solution called “green death” which contained 11.4% sulfuric acid, 1.2% hydrochloric acid, 1% copper chloride and 1% iron chloride to test corrosion resistance. The model solution was developed to simulate the corrosive conditions in the FGD unit and to test the pitting and crevice corrosion resistance of the C alloys in particular. Alloy 59 showed the highest resistance under these conditions. In addition to corrosion resistance, the fabrication characteristics of an alloy are important criteria in the selection process. Thermal stability has a significant influence on both properties. For example, welding materials that have very good corrosion resistance must not lead to carbide and intermetallic phase precipitation. This effect is called sensitization, and it can occur during multi-layer welding on large cross-sections such as those used in the Syncrude project, or during any heat forming or heat treatment process.
Standardized tests such as ASTM Test G28-A are used to evaluate these properties. Test results showed that it took about two hours at 870 °C before alloy 59 showed any sensitization effects. This is due to the low iron content and the fact that alloy 59 contains no additives such as tungsten. Alloy 59 is very suitable for reliable welding, hot and cold forming and roll-bonding, and as Nicrofer S 5923 it is the preferred welding additive for all stainless steel and nickel based alloys used in environmental engineering applications. The material is used as solid sheet, roll-bonded sheet and as a welding additive on the Syncrude project.