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Wednesday, March 17, 2010: 10:00 AM
210 A/B (Henry B. Gonzales Convention Center)
CuNi alloys have been successfully applied as sheathing material for corrosion protection of hot risers in the spray and splash zone of offshore installations for oil and gas poduction. However, so far data comparing the performance of the relevant alloys CuNi 90/10 (C70600), CuNi 70/30 (C71500) and NiCu 70/30 (Alloy 400, N04400) for this application are scarce. This paper reports on experimental results obtained with these alloys under heat transfer conditions closely imaging the conditions at hot risers in the spray and splash zone.
Thus, tubes of these alloys were heated internally to an inside temperature of 100 and 120 °C, respectively, and exposed vertically for 1000 h to i) a spray of substitute ocean water (ASTM D1141) performed in a modified salt spray chamber kept at 50°C and ii) substitute ocean water thermostated at 50°C. In the last case the tubes were arranged vertically in a way that part of the tubes was above the water line forming a metal/water/air interface. At these sites salt crusts were formed on the outer surface of the tubes creating conditions favouring crevice corrosion. Corrosion loss was quantified by topographic scanning.
In the spray test each type of alloy experienced only uniform metal losses in the order of 2 to 4 µm regardless of the temperature inside the tubes. Only under the salt crusts which formed predominantly on the spray side of the tubes, very small needle-like pits were found with depths of only 5 to 10 µm. The deepest pits amounted to 20 µm (CuNi70/30).
In the test runs with half submerged tube samples the formation of a protecting thin Ca containing scale was observed below the water line. After scale removal only some micropits with max. depths of 10 µm were visible. The highest uniform mass loss was found under the salt crusts with max. penetrations of 40 µm (NiCu 7030), 30 µm (CuNi 70/30) and 10 µm (CuNi 90/10), respectively. The maximum local pit penetration depths were 80 µm (NiCu 70/30), 45µm (CuNi 70/30) and 40 µm (CuNi 90/10) respectively. Below the salt crusts NiCu 70/30 showed clear tendency to denickelification, both at 100 and 120 °C. This effect diminishes with increasing copper content in the alloy. Although, all CuNi alloys tested can be recommended as effective sheating material for protection of hot risers in the splash zone, CuNi 90/10 exhibited the best overall performance.
Thus, tubes of these alloys were heated internally to an inside temperature of 100 and 120 °C, respectively, and exposed vertically for 1000 h to i) a spray of substitute ocean water (ASTM D1141) performed in a modified salt spray chamber kept at 50°C and ii) substitute ocean water thermostated at 50°C. In the last case the tubes were arranged vertically in a way that part of the tubes was above the water line forming a metal/water/air interface. At these sites salt crusts were formed on the outer surface of the tubes creating conditions favouring crevice corrosion. Corrosion loss was quantified by topographic scanning.
In the spray test each type of alloy experienced only uniform metal losses in the order of 2 to 4 µm regardless of the temperature inside the tubes. Only under the salt crusts which formed predominantly on the spray side of the tubes, very small needle-like pits were found with depths of only 5 to 10 µm. The deepest pits amounted to 20 µm (CuNi70/30).
In the test runs with half submerged tube samples the formation of a protecting thin Ca containing scale was observed below the water line. After scale removal only some micropits with max. depths of 10 µm were visible. The highest uniform mass loss was found under the salt crusts with max. penetrations of 40 µm (NiCu 7030), 30 µm (CuNi 70/30) and 10 µm (CuNi 90/10), respectively. The maximum local pit penetration depths were 80 µm (NiCu 70/30), 45µm (CuNi 70/30) and 40 µm (CuNi 90/10) respectively. Below the salt crusts NiCu 70/30 showed clear tendency to denickelification, both at 100 and 120 °C. This effect diminishes with increasing copper content in the alloy. Although, all CuNi alloys tested can be recommended as effective sheating material for protection of hot risers in the splash zone, CuNi 90/10 exhibited the best overall performance.