Aluminum galvanic anodes are used for cathodic protection in an environment containing chloride ions, because in fresh water and soil aluminum passivates. These anodes are used both for the protection of onshore and offshore constructions. They are used to protect such structures as: steel sheet pilings of wharfs, steel piles of reloading posts in ports, steel anti-flood barriers, etc.
They are mounted on offshore oil and gas platforms, where zinc anodes are generally not used – their heavy weight increases the mechanical stress of the platform legs, raising the probability of stress corrosion.
Aluminum anodes are made of alloys containing alloy additives that improve the electrochemical properties of the anode material, i.e. current efficiency and anode potential.
The Al-Zn-In type anodes are used most frequently. According to the DNVGL-RP-B401 and NORSOK M-503 standards, this anode alloy should contain 2.5-5.75% zinc and 0.015-0.040% indium. The EN 12496 standard provides for the amount of these alloying additives at 2.0-6.0% and 0.010-0.030%, respectively.
The current capacity of the Al-Zn-In anodes according to the standards should not be less than 2500 Ah/kg (according to the NORSOK M-503 standard even 2600 Ah/kg). The potential of the anodes during operation should not be more electropositive than -1050 mV vs. Ag/AgCl reference electrode.
The Al-Zn-In-Mg type anodes are used in some applications in the USA, e.g. for the protection of sheet pilings of wharfs. They are characterized by the addition of magnesium in the amount of 0.5-1.0%, low content of zinc in the amount of 1.0-3.0% and the content of indium in the amount of 0.20%. European standards do not consider Mg as an alloying additive for aluminum anodes.
The current capacity of the Al-Zn-In-Mg anodes according to NACE TM0190 should be in the range 2354-2742 Ah/kg. The potential of the anodes during operation should not be more electropositive than -1032 mV vs. saturated calomel electrode, which corresponds to -1078 mV against the Ag/AgCl electrode.
Anode parameters for a given series of types
Series I: Al-Zn-In alloys with regular zinc content
These anodes can be widely used in the sea water environment and in sea mud.
Anodic material meets the requirements of the following standards: DNVGL-RP-B401, NORSOK M-503, EN 12496 regarding the alloy composition and electrochemical properties.
|
Alloy components |
Content |
Impurities |
|
Zn |
3.0 – 4.5% |
Fe – max. 0.040% |
|
In |
0.016 – 0.020% |
Cu – max. 0.002% |
|
Si |
0.08 – 0.12% |
Cd – max 0.002% |
|
Al |
remainder |
other – every component max. 0.02% |
|
other – total max. 0.05% |
|
Electrochemical properties |
|
|
current efficiency (electrochemical capacity) |
minimum 2600 Ah/kg |
|
closed circuit potential |
≤-1050 mV vs Ag/AgCl |
Series II: Al-Zn-In alloys with increased zinc content
These anodes can be used in the sea water environment and sea mud, especially in deep and cold waters. They are characterized by higher current efficiency and more negative working potential than the ones from series I.
Anodic material meets the requirements of the following standards: DNVGL-RP-B401, NORSOK M-503, EN 12496 regarding the alloy composition and electrochemical properties.
|
Alloy components |
Content |
Impurities |
|
Zn |
4.75 – 5.75% |
Fe – max. 0.090% |
|
In |
0.016 – 0.020% |
Cu – max. 0.002% |
|
Si |
0.08 – 0.12% |
Cd – max 0.002% |
|
Al |
remainder |
other – every component max. 0.02% |
|
other – total max. 0.05% |
|
Electrochemical properties |
|
|
current efficiency (electrochemical capacity) |
minimum 2650 Ah/kg |
|
closed circuit potential |
≤-1050 mV vs Ag/AgCl |
Series III: Al-Zn-In-Mg alloys
These anodes can be used in the sea water environment and in sea mud.
They are characterized by the addition of magnesium, a reduced content of zinc and an increased content of indium in comparison to Al-Zn-In type anodes.
The current efficiency of the anodes is lower in comparison to the Al-Zn-In type anodes, but they have a more negative potential during operation.
Anodic material meets the requirements of the NACE TM0190 standard regarding alloy composition and electrochemical properties.
|
Alloy components |
Content |
Impurities |
|
Zn |
1.5 – 3.0% |
Fe – max. 0.1% |
|
Mg |
0.6 -1.0% |
Cu – max. 0.01% |
|
In |
0.020% |
other – every component max. 0.02% |
|
Al |
remainder |
other – total max. 0.05% |
|
Electrochemical properties |
|
|
current efficiency (electrochemical capacity) |
minimum 2400 Ah/kg |
|
closed circuit potential |
≤-1100 mV vs calomel electrode (≤-1146 mV vs Ag/AgCl electrode) |