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51316-7451-The Effect of a Magnetic Field on the Galvanic Corrosion of Ni-Al Bronze in Natural Seawater

Product Number: 51316-7451-SG
ISBN: 7451 2016 CP
Author: Hedda Krogstad
Publication Date: 2016
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Nickel-Aluminium Bronze (NAB) is extensively used in large casted components such as propellers due to its high strength superior resistance to cavitation-damage and fairly good corrosion properties However the alloy is susceptible to selective corrosion due to it’s complex microstructure containing several intermetallic phases with varying corrosion potentials.The alloyis also known to suffer galvanic corrosion when coupled to more noble alloys such as titanium in unfavorable area-ratios.Recent field-experience from a marine application under the influence of a magnetic field indicate that NAB suffers unexpectedly heavy corrosion when galvanically connection to the austenitic stainless steel AISI 316 in a 2:1 area-ratio NAB:AISI 316 respectively. As magnetic fields may influence mass transport in corrosion processes by exerting a force on the charge-carrying and/or paramagnetic species in solution it was hypothesized the magnetic field accelerates the galvanic corrosion.A review of state-of-the-art within the fields of corrosion of NAB corrosion in magnetic fields and effects of biofilms on cathode efficiency has been conducted. Corrosion tests have been performed on both freely exposed and galvanically coupled NAB and AISI 316 in the presence and absence of a magnetic field of 02 Tesla. Natural circulating seawater holding a temperature of 10°C was used as electrolyte. Potentials and galvanic currents were measured during the experimental period ranging from 15 to 40 days. Linear polarization resistance and polarization curves have been recorded for the freely exposed samples and surface characterization by optical microscope and SEM have been performed on all samples. Corrosion products were analyzed by XRD.Testing of NAB in natural seawater shows that when NAB is connected to AISI 316 in a 1:1 area-ratio the mixed corrosion potential of the galvanic couple is very similar to that of freely exposed NAB the first 8 days before a rapid increase of 150 mV is observed by the 11th day. This is caused by an increase in the cathode efficiency as a biofilm is formed and developed on the AISI 316 surface. The rise in the mixed potential is accompanied by an increase in the galvanic current from 0 to 150 mA/m2. The corrosion attack on galvanically coupled NAB is in the form of selective and localized corrosion.No significant effect of the magnetic field was observed on the corrosion of freely exposed or galvanically coupled NAB. However more experimental work will be performed on NAB both coupled to AISI 316 and freely exposed in natural seawater with the presence of a magnetic field. The effect of the different area-ratios will also be investigated i.e. 2:1 1:2 and 1:8 NAB:AISI 316 respectively.
Nickel-Aluminium Bronze (NAB) is extensively used in large casted components such as propellers due to its high strength superior resistance to cavitation-damage and fairly good corrosion properties However the alloy is susceptible to selective corrosion due to it’s complex microstructure containing several intermetallic phases with varying corrosion potentials.The alloyis also known to suffer galvanic corrosion when coupled to more noble alloys such as titanium in unfavorable area-ratios.Recent field-experience from a marine application under the influence of a magnetic field indicate that NAB suffers unexpectedly heavy corrosion when galvanically connection to the austenitic stainless steel AISI 316 in a 2:1 area-ratio NAB:AISI 316 respectively. As magnetic fields may influence mass transport in corrosion processes by exerting a force on the charge-carrying and/or paramagnetic species in solution it was hypothesized the magnetic field accelerates the galvanic corrosion.A review of state-of-the-art within the fields of corrosion of NAB corrosion in magnetic fields and effects of biofilms on cathode efficiency has been conducted. Corrosion tests have been performed on both freely exposed and galvanically coupled NAB and AISI 316 in the presence and absence of a magnetic field of 02 Tesla. Natural circulating seawater holding a temperature of 10°C was used as electrolyte. Potentials and galvanic currents were measured during the experimental period ranging from 15 to 40 days. Linear polarization resistance and polarization curves have been recorded for the freely exposed samples and surface characterization by optical microscope and SEM have been performed on all samples. Corrosion products were analyzed by XRD.Testing of NAB in natural seawater shows that when NAB is connected to AISI 316 in a 1:1 area-ratio the mixed corrosion potential of the galvanic couple is very similar to that of freely exposed NAB the first 8 days before a rapid increase of 150 mV is observed by the 11th day. This is caused by an increase in the cathode efficiency as a biofilm is formed and developed on the AISI 316 surface. The rise in the mixed potential is accompanied by an increase in the galvanic current from 0 to 150 mA/m2. The corrosion attack on galvanically coupled NAB is in the form of selective and localized corrosion.No significant effect of the magnetic field was observed on the corrosion of freely exposed or galvanically coupled NAB. However more experimental work will be performed on NAB both coupled to AISI 316 and freely exposed in natural seawater with the presence of a magnetic field. The effect of the different area-ratios will also be investigated i.e. 2:1 1:2 and 1:8 NAB:AISI 316 respectively.
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