Galvanic corrosion of Al alloy 2

[Hlengani]

I want to couple 5xxx series and 6xxx serie Al alloys so want to know if there might be a possibility of galvanic corrosion when immersed under seewater/corrosive medium since they are both Al and i assume that they might be close to each other in the series (Galvanic Series.

 

[CoryPad]

5XXX and 6XXX alloys are close to each other galvanically. For full immersion, you won't need to worry about galvanic corrosion - 6XXX series alloys have mass loss and pitting that are twice or three times that of 5XXX, so their general corrosion behavior is the limiting factor.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[strider6]

You can see the electrochemical potential of the Al alloy in this table:

http://esmat.esa.int/Services/bimetallic_data/Bimetaltable/bimetalemf.html

hope this help

regards

S.

http://www.corrosionist.com

 

[Hlengani]

Thanks so much CoryPad, although they are close to each other, i think there should be a maximum recommended pontential difference above which galvanic corrosion could be the limiting factor and i am looking for that info as well.

 

[strider6]

have a look at the table in this page, i think could be what you're looking for:

http://www.corrosion-doctors.org/Definitions/galvanic-series.htm

regards

S.

http://www.corrosionist.com

 

[Bamend]

keep in mind that with galvanic corrosion there is more to consider than just the potential difference. The relative area of the anodic vs cathodic alloy is also important and can have a strong effect on the rate of corrosion. Small anode w/ big cathode = worse than the reverse stituation.

 

[kenvlach]

First, the good news: As seen in the table in strider6's link, Al 5052 is so close galvanically to Al 6061-T6 that, depending on its temper (thermomechanical history), it can be higher or lower in galvanic potential.

Now, the bad news: In many 5000-series alloys, particularly those above 2.5% Mg, poor temperature control during initial processing or any exposure to high temperatures (e.g., storage in summer sunlight), segregation of Mg to the grain boundaries occurs. This Mg (or Mg₂Al₃ given enough time) is anodic to the [α]-Al solid solution matrix. So, intense localized galvanic corrosion can occurs. Commonly called 'cauliflower' as it resembles white florets.

A good idea to anodize or else conversion coat + paint.

 

[strider6]

bamend make a good point, area ratio is fundamnetal in predicting galvanic corrosion.
I'm not an expert on Al materials, try also to look on this website, i think is the best site regarding Al.

http://aluminium.matter.org.uk/content/html/eng/default.asp?catid=&pageid=1

hope this help
regards

Strider

http://www.corrosionist.com