Wear in the oilsands
Metal matrix composite coatings can reduce erosion-corrosion damage in the oilsands industry. Juan Flores and Professor Anne Neville from the School of Mechanical Engineering at the University of Leeds, UK, investigate their performance.
Oilsand slurries are one of the most complex industrial multiphase flows and consist of bitumen, sand particles, warm water and hydrocarbons, among other species. The ore is a mixture of clay and sand grains coated with water and bitumen which occupies the pore spaces between the grains.
Oil production in open pit operations is divided as follows. Bitumen-saturated ore is dug and transported by trucks to crushers and extraction facilities. The oilsand is then combined with warm water and caustic soda to create a slurry and big rocks are separated by vibrating screens. The slurry is transported to the separation vessels where the bitumen floats and the sand and remaining water is pumped to tailings ponds through pipelines.
During slurry hydrotransport the combination of sand particles flowing in a corrosive carrier fluid severely deteriorates the materials and the equipment’s performance, particularly due to erosion-corrosion. Metal matrix composite (MMC) coatings can increase service life and their application in the oilsands industry is being investigated at the University of Leeds, UK, together with Syncrude Canada Ltd, Edmonton Research Centre, Canada.
Centrifugal pumps move the slurry through pipelines to the separation vessels and tailing ponds. The pump impellers are manufactured of high-chromium white iron and the casings made of cast ductile iron and sometimes coated with chromium carbide overlays, are subjected to erosion-corrosion effects facing an accelerated degradation process. The sand particles impact the impellers’ surface, forming and removing material debris that can also be dissolved by corrosion. In erosion-corrosion, the material loss mechanisms are linked to mechanical erosion, corrosion and their interaction – referred to as synergy.
The pipelines used for the transportation of the oilsands slurry are made of API X70 steel and martensitic stainless steel. The pipeline walls and flow measurement devices are also affected by erosion-corrosion. If the slurry is flowing homogenously, the inner walls are subjected to particle impact and wall thinning, increasing the leaking risk. On the other hand, if the solids are settled like in a sliding bed, abrasive processes deteriorate the bottom of the pipelines.
In the mining operation process where the ore is dug and transported by trucks to crushers and extraction facilities, abrasive processes are the main degradation mechanisms; very hard materials, such as chromium carbides overlays and hardened carbon steel can be used to withstand wear. Problems arise in the slurry transportation process where combined degradation processes such as solid particle erosion and erosion-corrosion are present. In erosion-corrosion, materials such as hardened carbon steel cannot resist the corrosive environment and stainless steel also suffers severe degradation due to erosion.
When the crushers fragment the rocks and the extracted material, the hard materials used on the surface of the crushers to withstand the grooving and penetration effects can resist wear. However, during the ore emptying process the surface is subjected to erosion, and therefore the materials used should also show certain ductility to be able to absorb the impact energy.
In the MMCs used in this project, tungsten carbide (WC) makes up the hard reinforcing phase and the corrosion resitant and ductile matrix materials are nickel and iron-based alloys.
The erosion and corrosion resistance of MMCs can be varied by increasing the volume fraction and distribution of the reinforcing phase, or by using a more corrosion resistant matrix phase.
The UK-Canadian research includes a project devoted to the development of MMCs suitable for use in the oilsands and the study of their performance. The coatings are obtained by plasma transferred arc welding technique (see image below, left). An inert gas is introduced in a welding torch to produce a high temperature plasma column. The powder alloy and reinforcing phase are fed into the plasma and melted on the substrate, forming a metallurgical bonding between the coating and substrate to improve impact resistance.
Due to the high temperatures in the deposition process, dissolution of a certain amount of the WC phase leads to the formation of additional W-rich intermetallics in the matrix phase of the MMCs. The image (top, right) shows the microstructural constituents of a UNS S41000 MMCs, the presence of the WC grains and small blocky-shape intermetallics can be observed to be embedded in the matrix phase. The presence of the small intermetallics plays a crucial role in the erosion-corrosion behaviour of the MMCs.
Under erosion-corrosion conditions, every phase in the microstructure helps to withstand degradation. For instance, the reinforcing phase will resist abrasive wear due to its extremely high microhardness (>2,000 HV) and, together with the small intermetallics, protects the matrix phase. In an erosive process, the matrix absorbs part of the particle impact energy by deforming elastically or plastically, providing support to the hard phases.
The performance of the UNS S41000 MMC was compared with conventional X-65 steel and wrought UNS S41000 alloy. The materials were exposed under an impinging jet for 90 minutes in aggressive slurry consisting of 10g/l of sand concentration in a solution that simulated the slurry composition in the tailings line at 65°C. The X-65 steel showed the highest mass loss value, the UNS S41000 alloy displayed a slightly better behaviour due to its improved corrosion resistance, and the MMC was more than 300% better compared to the two alloys (see graph above).
Metal matrix composites can protect the surface of equipment and devices used in oilsands extraction. The maintenance procedure will be changed from the total replacement of failed equipment to the resurfacing of damaged areas. Though the initial cost of the coating may be higher, the mid-term benefits significantly reduce production cost, increase process safety and reliability, and assure the long-term viability of the sector.
Further information: Juan F Flores and Professor Anne Neville