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topic 584

Using 7075 fasteners with 5052 panels


(1996)

I am looking for information regarding the use of 7075-T6 Alodined fasteners with 5052 aluminum panels.

Is there a possibility of galvanic corrosion over time between these two types of aluminum? The application is an electronics enclosure that will NOT see a particularly harsh environment.

Anyone with galvanic corrosion experience, please contact me with any ideas or opinions.

Thanks,
PK

Pat Kelly
Senior Applications Engr


Galvanic Corrosion

I don't know exactly how 7075 lines up against 5052 in the electrochemical series, and I unfortunately don't know of an activity chart which puts that fine a point on it to distinguish between different alloys.

Alodine is a proprietary name for a Mil-C-5541 [link by ed. to spec at TechStreet] chromate conversion coating; it will certainly retard corrosion rather than accelerate it.

I can't imagine galvanic corrosion between these two alloys being a problem in an electronics enclosure which will not see a harsh environment.

Ted Mooney, finishing.com Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey


(1996)

Pat:
I am no expert by far, BUT the level and rate of anode-cathode contamination and i.e., build-up is dependent on the potential difference, the relative size of the contact areas, and the conductivity strength of the corroding material. It is safe to assume that GALVANIC corrosion will be minimal if the electrical potential is very low.

I hope this was some help. I am hunting around for a GC chart for metals to help me select materials better.

Doug Dina
instruments - Duluth, Georgia


(1998)

You responded to question in the Jan issue of Metal Finishing about galvanic corrosion of Al alloys. If you are still interested, you can find the wrought and casting galvanic potentials in "Aluminum Properties and Physical Metallurgy" [link is to info about book on Amazon] by John E Hatch. In my edition (1984) they may be found on pages 257 and 258.

We have had extensive problems with galvanic corrosion of Al, but with the T73 temper of 7075. Depending on storage and other conditions all alloys and tempers we use seem to have degrees of problems.

Tony Naylor



(1998)

Pat:

I predict that you will not have a galvanic problem between 5052 and 7075-T6. Consider the following explanation:

While, Mil-STD-889 [link is to free spec at Defense Logistics Agency, dla.mil] ("Dissimilar Metals") Table II shows that 7075 is more active than any heat treat of 5052 the potential difference between the aluminum alloys must be considered to be very minute. Mil-F-14072 [link by ed. to spec at TechStreet] ("Finishes For Ground Electronic Equipment") Table VI shows these materials to be permissible couples. The fact the at least one member (i.e., 7075-T6) has been Alodined will help. (Actually it would be better if both aluminum parts were Alodined or conversion coated per MIL-C-5541, Class 3.) Other steps which could be taken to minimize galvanic potential would be to limit the size of the cathodic member (i.e., 5052) in contact with the anodic member (i.e., 7075-T6) of the couple. The larger the relative anode area is, the lower the galvanic current density on the anode, and the lesser is the attack.

The potential for galvanic corrosion to occur with this couple is relatively small (especially in an electronic enclosure). If you do observe a corrosion problem, then I would suspect that you will have far greater functional problems with your electronic components than you will with the corrosion of these two aluminum metals because the inside of your enclosure must be getting wet.

Hope this helps.

Regards,

Blair Smith
aerospace - Windsor Locks, Connecticut


(1998)

Believe it or not but we actually have that information.

On a silver reference scale the initial potentials for 5052(H-34) and 7075(T-6) are -0.75 and -0.77 respectively. This would be great news for you, unfortunately, in our wealth of information we also include potentials on service aged materials. In that case the potentials of old aluminum 5052 and 7075 change to -.96 and -.74. The potential difference of .22 volts is acceptable to all but the most conservative.

In addition, the Alodine will provide some protection.

T E Collins
materials & processes engineering - aerospace


(2002)

I have had receive a spec. one part as follow: "Undercoat: Alodine per mil-c-5541, class 3, (yellow) which material is 5052-H32 alum 0.050 thick. Is it a coating such as nickel plated for battery contact plate? Or it is a powder coating? Can anyone help me?

Thanks,

Alex Tsang
- Hong Kong


(2002)

Alodine is a brand name for a chromate conversion coating on aluminum. Mil-C-5541 is the U.S. military specification for this process. It is not an electroplating, it is an immersion process that leaves a conductive and fairly corrosion resistant finish. If you've seen the goldish color external fuel tanks on the space shuttle you've seen chromate conversion coating.

Ted Mooney, finishing.com
Teds signature
Ted Mooney, P.E. RET
finishing.com
Pine Beach, New Jersey



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