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00634 QUANTITATIVE EFFECT OF TRANSIENT POTENTIALS AND TEMPERATURE ON CREVICE CORROSION OF NAVAL CAST Cu-Ni-Al-VALVE BRONZE IN NATURAL SEAWATER

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Product Number: 51300-00634-SG
ISBN: 00634 2000 CP
Author: Hans Hoffmeister, Jens Ullrich
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Cast Cu-AI-Ni- bronze valve components for naval seawater supply facilities have frequently been subjected to crevice corrosion, for example, in contact with PTFE. The extent of such local degradation leading to leakage depends on environmental conditions including seawater chemistry as well as local potentials, temperature and flow velocities. The present investigation is based on an actual failure case of a cast Cu-9% Al-5% Ni-5% Fe (UNS C 95800) alloy valve exhibiting crevice corrosion at the contact areas to the PTFE sealing. For identification of incubation times and extent of crevice corrosion at various free mixed potentials and temperatures of the pipe-valve system a "Remote Crevice Assembly,(RCA)"- mock up was engaged including the cast material and PTFE as a small local anodic crevice location and the corresponding wrought 90% Cu-10% Ni pipe material (UNS C70600) as a large cathode. With aerated and stirred Baltic Sea water as a fluid the effect of fluctuating free potentials and anodic as well as cathodic temperatures was investigated. As a result, increasing potentials at the start of the 24 h test runs are providing increased corrosion net currents and reduced incubation times for start of crevice corrosion. A maximum mixed potential o f -1 0 0 mV(/Ag/AgCl ) for avoidance of crevice corrosion at anodic temperatures up to 45°C for the investigated assembly is established. Cathodic protection at -850mV efficiently avoided crevice corrosion. Increasing cathodic temperatures are initially providing accelerated anodic net currents followed by a subsequent net current reverse due to start of pitting corrosion at the pipe material at its respective pitting temperature. The failure mode of the cast anode material is characterized by early grain boundary attack on secondary phases followed by general dissolution of the grains after the marked current increase at crevice corrosion start. As a conclusion, the application of cast Cu-A1-Ni-Fe valves in the present composition and microstructure is to be questioned, in particular, at higher ambient temperatures as prevailing in southern areas. Keywords: Crevice Corrosion, Cu-AI-Ni-Fe-Bronze, Remote Crevice Assembly, Corrosion Currents, Incubation Times, Seawater piping systems.
Cast Cu-AI-Ni- bronze valve components for naval seawater supply facilities have frequently been subjected to crevice corrosion, for example, in contact with PTFE. The extent of such local degradation leading to leakage depends on environmental conditions including seawater chemistry as well as local potentials, temperature and flow velocities. The present investigation is based on an actual failure case of a cast Cu-9% Al-5% Ni-5% Fe (UNS C 95800) alloy valve exhibiting crevice corrosion at the contact areas to the PTFE sealing. For identification of incubation times and extent of crevice corrosion at various free mixed potentials and temperatures of the pipe-valve system a "Remote Crevice Assembly,(RCA)"- mock up was engaged including the cast material and PTFE as a small local anodic crevice location and the corresponding wrought 90% Cu-10% Ni pipe material (UNS C70600) as a large cathode. With aerated and stirred Baltic Sea water as a fluid the effect of fluctuating free potentials and anodic as well as cathodic temperatures was investigated. As a result, increasing potentials at the start of the 24 h test runs are providing increased corrosion net currents and reduced incubation times for start of crevice corrosion. A maximum mixed potential o f -1 0 0 mV(/Ag/AgCl ) for avoidance of crevice corrosion at anodic temperatures up to 45°C for the investigated assembly is established. Cathodic protection at -850mV efficiently avoided crevice corrosion. Increasing cathodic temperatures are initially providing accelerated anodic net currents followed by a subsequent net current reverse due to start of pitting corrosion at the pipe material at its respective pitting temperature. The failure mode of the cast anode material is characterized by early grain boundary attack on secondary phases followed by general dissolution of the grains after the marked current increase at crevice corrosion start. As a conclusion, the application of cast Cu-A1-Ni-Fe valves in the present composition and microstructure is to be questioned, in particular, at higher ambient temperatures as prevailing in southern areas. Keywords: Crevice Corrosion, Cu-AI-Ni-Fe-Bronze, Remote Crevice Assembly, Corrosion Currents, Incubation Times, Seawater piping systems.
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