Abstract
Objectives. Cathodic protection (CP) design codes are focused on steel and do not provide the necessary information about how to design with Al. The objective of this work was to investigate CP design criteria for aluminium in seawater, i.e. effect of alloy composition on cathodic current density.
Results. Various 6000 series model alloys, and commercial 6082 and 5083 were studied. Cathodic current density depended on their contents of Cu, Fe and Si. An initial peak in current density after about 40 days exposure with CP correlated strongly with Cu content. A TEM and GDOES investigation showed that a Cu rich film had been formed at the interface between the alloy and a layer of Al2O3. The current density then decreased over a period of about 100 days, and the Cu film was shown to disappear. The current density stabilized at a lower level that correlated with the Fe/Si ratio in the alloy, and a simple mathematical relationship was found. Si is known to reduce the cathodic activity of Fe rich intermetallic particles, which was verified by cathodic polarization of synthesized Fe rich particles with varying Si content. The design current density for CP of aluminium in DNV RP-B-401 is 10 mA/m², while the current density in this work stabilized on values between 0 and 20 mA/m², depending on alloy composition.
Conclusions and significance of the work. The current demand for CP of aluminium alloys depends on the alloy composition, i.e. the contents of Cu, Fe and Si. The current demand for some alloys is higher than the design value in DNV RP-B-401, i.e. non-conservative.
Results. Various 6000 series model alloys, and commercial 6082 and 5083 were studied. Cathodic current density depended on their contents of Cu, Fe and Si. An initial peak in current density after about 40 days exposure with CP correlated strongly with Cu content. A TEM and GDOES investigation showed that a Cu rich film had been formed at the interface between the alloy and a layer of Al2O3. The current density then decreased over a period of about 100 days, and the Cu film was shown to disappear. The current density stabilized at a lower level that correlated with the Fe/Si ratio in the alloy, and a simple mathematical relationship was found. Si is known to reduce the cathodic activity of Fe rich intermetallic particles, which was verified by cathodic polarization of synthesized Fe rich particles with varying Si content. The design current density for CP of aluminium in DNV RP-B-401 is 10 mA/m², while the current density in this work stabilized on values between 0 and 20 mA/m², depending on alloy composition.
Conclusions and significance of the work. The current demand for CP of aluminium alloys depends on the alloy composition, i.e. the contents of Cu, Fe and Si. The current demand for some alloys is higher than the design value in DNV RP-B-401, i.e. non-conservative.