Steel vs. aluminum

[Skyflyer]

I am a new RV8 builder close to completing the empennage and have taken delivery of the QB wings and fuselage. This question is really water-over-the-dam at this point but I thought I would throw it out there anyway. I have been touch-up priming the powder coated steel parts after drilling, by masking and spraying SW 988 into the holes. I have no illusions about the effectiveness of this procedure. When assembling it may be necessary to use a small drift to align parts and I know the primer likely gets worn off such that aluminum and steel are closely apposed. Is anyone worried about eventual dissimilar metal corrosion at these important joints? If so how are you dealing with it? I will put all good suggestions in my "Next time I'm gonna do..." box.

By the way, I am looking for TC's in the Sugar Land, TX area willing to have a look at things so far.


Chuck McCluggage

[Ironflight]

Hello Chuck,

I think you have already answered your own question - the fasteners in the holes are going to wear off what you sprayed in. But you haven't done any harm, and it shows that you are trying to be pretty meticulous, which is a good attitude.

Have you attended any of the Houston RV Builder's Group get-togethers yet? It's a good group of folks that get together once a month - tomorrow it will be down in Wharton. You can check the thread on the South Texas part of these forums. It pays to have lots of local contacts to have others look at your work, and maybe borrow a few specialty tools now and again!

I'm a TC, I live over in the Clear Lake area - we might be able to swing a visit sometime - give me a PM if you're interested.

Paul

[cytoxin]

i have painted them after assy as well, to prevent the intrusion of water.i do use a top coat (glossy) paint for this though over primer.

[nucleus]

I think a little Boeshield could go a long way here. I too am very concerned about corrosion because I intend to fly to tropical areas with a lot of salt in the air. I am using either dielectric grease or the red goo for battery terminals on all my wiring, and boeshield on all my hardware after it has been torqued down.

Hans

[Robert M]

My first concern about disimilar metals came while attaching the trim tab parts with a steel rod through aluminum hinge haves. When I questioned Van's, they said don't worry about it. Aren't we supposed to be worried about corrosion?

This is a particular area of the plane that WILL be exposed to the open weather on a raining day. I don't want to sound as though I'm not concerned about corrosion because I am but there can come a point when we are using overkill and actually waisting time priming every little thing. Unfortunately, I don't know where that point is so my $.02 is worth even less. I plan to put a layer of primer between the surfaces of any two metals - disimilar or not.

[az_gila]

Originally Posted by Skyflyer

....
I have no illusions about the effectiveness of this procedure. When assembling it may be necessary to use a small drift to align parts and I know the primer likely gets worn off such that aluminum and steel are closely apposed. Is anyone worried about eventual dissimilar metal corrosion at these important joints?
......
Chuck McCluggage

The cadmium plating on the AN steel hardware helps here....

Some good details here...

http://www.eaa1000.av.org/technicl/c...n/galvanic.htm

and a simple chart from Boeing...



Shows cad plated hardware in the same group as aluminium...

Full article here...

http://www.boeing.com/commercial/aer...7/corrosn.html

gil in Tucson

[AlexPeterson]

Originally Posted by Skyflyer

[font=Times New Roman][size=3]SNIP Is anyone worried about eventual dissimilar metal corrosion at these important joints? SNIP

Just to be clear, dissimilar metal corrosion will only happen if their galvanic potentials are different (the chart in an earlier post is great) AND if moisture is present. If you keep water out, there won't be a problem. It is difficult for water to get into a bolted joint typically, unless is is submerged. Perhaps star style and other lockwashers could allow it?

[rzbill]

I think the most common error I see in industry is the use of brass threaded NPT pipe fittings into aluminum parts. I have seen many fluid manifolds completely eaten away from the inside, due to the brass/aluminum couple.

It has been a struggle sometimes to change the "we've always done it that way" paradigm from brass over to Zinc/Cad plated steel fittings.

[Skyflyer]

Lots of good info, the beauty of this site. I acknowledge the protection of cad plating and do indeed prime all mating surfaces. It is that small interface at the hole edge where unprotected steel and aluminum come in contact that I was thinking about. I will make attempts to seal those areas from moisture incursion and of course check at each inspection, every critical plate and bracket for signs of loosening. Now to roll some leading edges. This is turning out to be harder than I thought.

Chuck

[GAHco]

I know this does not exactly adress the initial post, but people need to know this, it may make a difference in the life of their aircraft.

In aircraft there are a lot of places where cad plated steel meets aluminum.

The electrolysis happens when an electrolytic substance enters the joint, such as salty water or similar. Even out it the desert there is alkaline dust in the air. add a little dew and deterioration can start happening. This can compromise the corrosion resistance of the structure.

What others have said about corrosion inhibitors is still good, and still valid.
This may help some more.

When the fastener is not subject to high tensions stresses (most of them are not, but some of them are), it may be acceptable to exchange the cad plated washer with an anodized aluminum washer, NAS1149D(xxxx)K or a Chemically coated washer ( Like alodine ) NAS1149D(xxxx)J. What this does it move the point of electrolysis of of the structure, (now aluminum washer its touching aluminum structure), to between the head or nut of the fastener. The aluminum washer is inexpensive compared to the skin or other structure.

The washers are easier and less expensive to replace than structure and it typically will be visible on the outside edges of the washer if deteriorations starts.

This only works with Steel Cad fastener and Aluminum Structure or Skin.

For tech page go to
http://www.gen-aircraft-hardware.com...minum-2024.pdf

To purchase go to
http://www.gen-aircraft-hardware.com...239~239&id=249

If you are not in a corrosive area or environment cad plated steel and aluminum get along pretty well.