The authoritative public forum
  for Metal Finishing since 1989
  

  -----

Black oxided steel / anodising aluminium: bimetal corrosion?

 

Hi,

My company and I would like to build two parts and put them together (with a screw) with clearance between the two parts. One of these parts will be in powder metal finished by a black oxide. The other one will be in aluminium finished with an anodising.

Both of them will work outside with rain...

Do you think there will have a risk of bimetal corrosion with these parts and these specific surface treatments?

Thanks to answer me as precisely as you can.

Have a good day.
Christophe Petit
- Chicago, Illinois, USA

---

If the two parts are not touching, perhaps you can use insulating bushings and washers, or plastic screws? If they don't touch and there is no metallic path, you can't get galvanic corrosion. If there is a metallic path from one to the other through the screw, you have all the ingredients for galvanic corrosion; and unless the area around the bolt stays dry (which would be a surprise) I would expect corrosion to start in the aluminum around the bolt hole. If the area around the bolt can somehow stay dry, and if the area of the aluminum piece is large compared to the powder metal piece, it is possible that the anodic current is spread widely enough to not cause significant corrosion. Although the anodized coating is an insulator, I wouldn't count on it playing the part of insulating bushings and washers.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

---

Thank you Ted to answer me. I will explain better. In fact the two parts are like 2 coins (25c) and they touch each other. So these two parts (one in powder metal + black oxide and the other in Aluminium + anodising) are in contact. I just would like to know if it's possible to have bimetal corrosion between these two parts (forget the screw).

Thanks in advance.

Christophe Petit [returning]
- Chicago, Illinois

---

Yes. All you need for galvanic corrosion is dissimilar metals, a metallic path between them, and an ionic path between them. You've described the dissimilar metals; the metallic path is by virtue of the parts touching each other; and the outdoor environment provides the ionic path.

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
 

I will be more precise in a question I posted a week ago. I have two parts like coins (25c), one is upon the other. One is in powder metal (black oxided) and the other is in aluminium (anodised). I would like to know if there are some bimetal corrosion risks between these two parts with such surface treatments? These two parts will work outside, with rain,... Thanks to answer me as precisely as possible.

Christophe Petit [returning]
- Chicago, Illinois, USA