Aluminum corrosion

[HardMetal]

I am looking to replace aluminum rivets used in a 6063-T6 aluminum structure. The main reason is to reduce assembly costs. I am thinking of using Monel 400 or 304 stainless steel. Their higher tensile strength will reduce by 1/2 the number of rivets. I am concerned about galvanic corrosion. No surface treatement will be performed to the aluminum structure. The environment is mostly air but may be subjected to atmospheres with some salt content, such as on a ship. Will Monel or 304 stainless be acceptable? Does anyone have any suggestions on altertive materials for rivets used in aluminum?

 

[boo1]

It depends on the stress level in rivet joints, loading and the operating enviroment. We use both aluminium and stainless steel in 6000 series plate. If stress levels allow aluminum is prefered. With SST wet assemble with proseal sealent to isolate the disimular metal. For a design guide check Mil-Hdhk-5.

 

[Metalguy]

You might try this--make a small test panel using the monel rivets (K-monel can be HT to pretty high strength levels), and run a salt-spray test on it. You have a very favorable anode/cathode area relationship, so it may turn out OK.

 

[HardMetal]

Metalguy: We were thinking of doing the salt spray test but we're not sure if the program has the money. The salt spray test would be good to try. I've recommended that as something that should be done, if affordable. We don't want the rivets to be too high in strength otherwise it will be difficult to form/install the rivet.

boo1: What is proseal? Is it like RTV?

 

[boo1]

Often the aluminium blind rivits have monel stems.

 

[JimMetalsCeramics]

If you zinc plate the rivets, you should reduce the galvanic coupling with the aluminum.

 

[AlanD]

You could try the monel rivets with a coating of zinc cromate paint (duralac otherwise known as yellow gunk) in between the aluminium and the rivet. This is the standard product used for (pop) riveting stainless steel fittings, monel rivets and aluminium masts on sailing boats and yachts.

 

[boo1]

Pro-Seal is a sealant, commonly used on aircraft. It is used to separate flaying surfaces. We use Pro-seal 870. In marine enviroments the sealant lasts for 5-8 years.

 

[HardMetal]

Thanks for all your suggestions

 

[JimMetalsCeramics]

http://www.zingacoating.com/

 

[RPstress]

If you really have to use CRES, Monel or titanium rivets in Al alloy (e.g. when joining it to carbon or titanium), then standard practice is to use cadmium coated (or aluminium coated if cad is a no no) rivets and to wet assemble. That's usually adequate. The Al alloy is usually anodised and primed as well.

The Islander aluminium fuse floor is riveted with Monel rivets, and has a very good service record. It must have seen a *lot* of abuse (including many awful and strange fluid spills) over the years, and there're over a thousand (?) of them out there.

 

[boo1]

RP we see a shift away from the Cadmium plating due to the toxisity liability issue.

 

[RPstress]

Moving away from cad is increasingly the case. We have been tending to use zinc instead recently, especially on things like stainless fasteners and bushes in Al alloy fittings. However, we have as yet comparatively little service feedback, though there have been no negative experiences so far. Can anybody advise on a theoretical basis for choosing a replacement for cadmium?? We're mainly intersted in protecting the parent metal from the bush/fastener here, rather than protecting a non-corrosion resisting fastener (cad never seemed that wonderful at the latter, anyway).

-R.

 

[boo1]

Consider Tri-valent Chromium Pretreatment as an alternative
Formulated by Naval Air Warfare Center (NAVAIR), POC’s: Dr. Vinod Agarwala, C. Matzdorf. We have tested the Tri-valent Chromium on aluminium as a primer and a stand alone coating with very good success.

We always wet install fasteners with Proseal 870 sealant. This helps seperate the galvanic couple and seal water penetration.

protective paint for aluminum is Zinga:

http://www.zingacoating.com/

 

[JimMetalsCeramics]

The corrosion of a metal is accentuated by any difference between its inherent electrical potential and that of another conductor that it is placed in contact with. Here is a table of some galvanic potentials in flowing sea water:

Range of Corrosion Potential
MATERIAL (in Volts)

Mg and Mg -1.60 to -1.63
Al Alloys -0.76 to -1.00
Zn -.76
Wrought Iron -0.60 to -0.71
410 Stainless 0.46 to -0.58
316 Stainless -0.43 to -0.54
Aluminum Bronze -0.31 to -0.42
Yellow Brass -0.30 to -0.40
Cadmium -0.40
Tin -0.31 to -0.33
50-50 Pb-Sn Solder -0.28 to -0.37
Aluminum Brass -0.28 to -0.36
Silicon Bronze -0.26 to -0.29
Lead -0.19 to -0.25
Nickel 200 -0.10 to -0.20
Monel 400 -0.04 to -0.14
Ti -0.05 to +0.06
Graphite +0.20 to +0.30


Zn is more compatible with Al than Cd (closer potentials). In addition, Cd can be mildly toxic.

 

[RPstress]

Sort of makes you wonder why we used so much cad in the past. Toxic, not too effective and embrittles titanium... Mind you, it's good for 450 F; we tend to limit Zn plate to 250 F.