Several people have touched on the subject of dissimilar metal or galvanic corrosion.  The subject can be extremely complicated, but there are several good generalities that can be used as a guide when picking compatible metals.  First - you need a chart or table of the galvanic series.  You really need one that lists the alloys you are contemplating using.  Just saying aluminum or stainless is not enough if you really want good results.  Different alloys behave quite differently.  Understand how the tables work too (see below.)  Two of the better tables I have found are listed below.

Second - pick compatible metals based on how close they are in the galvanic series.  The closer the galvanic potentials are, the less corrosion that will occur. Third - the amount of corrosion is proportional to the ratio of cathode to anode area ratio.  This leads to a number of methods to minimize corrosion.  Make the anodic metal piece much larger than the cathodic one.  The corrosion will be spread out over a larger piece of metal and thus the loss of some atoms from the surface will likely not result in much weakening.  If you have to use a different metal as a fastener, use one more cathodic than the metal in which it is to be used.  Fourth - remember that sometimes the above rules must be bent a little for other reasons, and that corrosion can be far more complex than just galvanic corrosion.  Look at the stainless steels for example.  Most can be both cathodic and anodic to themselves!  Corrosion while immersed in sea water can be quite different than corrosion from exposure to pollutants in the  atmosphere.

So before presenting the table, let's look at the original question - stainless fasteners on aluminum.  First we need to know what alloy of aluminum and stainless we are talking about.  With no other information all we can do is note that aluminum is generally more anodic than any of the stainless steels.  This is good from the standpoint of having a small fastener.  But note that zinc or cadmium plating is likely better as they are closer to aluminum in the galvanic series.  I would probably pick a hot-dip galvanized screw over a cadmium plated one because of the thicker coating, but I would pick a cadmium plated one over a simple zinc plated one (which is likely to be a rather thin plating).  But I would certainly not pick brass, copper, or bronze.

This brings up the point of why I listed two tables.  The shorter one gives a better idea of how far apart the metals tend to be in the series.  The longer list gives many alloys, but if you view

these with their electromotive potentials (not shown), you will find many alloy series differ little in their actual placement in the series.  Thus with aluminum, you might notice little difference between the corrosion with either 304 or 17-7PH stainless steel.

Corrosion can be a nearly exact science if everything you work with is exceptionally pure.  But in the real world, this is not the case and an experienced metallurgist is a wonderful friend to have.  I first learned this in a corrosion class, but it really sank home after I learned it by experience too.

[And on one or two occasions, I have seen my metallurgist friend scratch his head in confusion too!]

When your choice of materials is limited, sometimes you have to accept that some corrosion is inevitable and design accordingly.

Galvanic Series of Certain Metals & Alloys Arranged in Order of Corrosivity:

ANODIC (Least Noble) End Material
Magnesium
Magnesium alloys
Zinc
Aluminum 25
Cadmium
Aluminum 17ST
Steel or iron
Cast iron
Chromium-iron (active)
Ni-Resist
18-8 Chromium-nickel-iron (active)
18-8-3 Chromium-nickel-molybdenum-iron (active)
Lead-tin solders
Lead
Tin
Nickel (active)
Inconel (active)
Hastelloy C (active)
Brasses
Copper
Bronzes
Copper-nickel alloys
Monel
Silver Solder
Nickel (passive)
Inconel (passive)
Chromium-iron (passive)
18-8 Chromium-nickel iron (passive)
18-8-3 Chromium-nickel-molybdenum-iron (passive)
Hastelloy C (passive)
Silver
Carbon and graphite
Platinum
Gold
CATHODIC (Most Noble) End material

Second Table from MIL-STD-889:

Active (Anodic) End
------------------------------------
Magnesium
Mg alloy AZ-31B
Mg alloy HK-31A
Zinc (hot-dip, die cast, or plated)
Beryllium (hot pressed)
Al 7072 clad on 7075
Al 2014-T3
Al 1160-H14
Al 7079-T6
Cadmium (plated)
Uranium
Al 218 (die cast)
Al 5052-0
Al 5052-H12
Al 5456-0, H353
Al 5052-H32
Al 1100-0
Al 3003-H25
Al 6061-T6
Al A360 (die cast)
Al 7075-T6
Al 6061-0
Indium
Al 2014-0
Al 2024-T4
Al 5052-H16
Tin (plated)
Stainless steel 430 (active)
Lead
Steel 1010
Iron (cast)
Copper (plated, cast, or wrought)
Nickel (plated)
Chromium (Plated)
Tantalum
AM350 (active)
Stainless steel 310 (active)
Stainless steel 301 (active)
Stainless steel 304 (active)
Stainless steel 430 (active)
Stainless steel 410 (active)
Stainless steel 17-7PH (active)
Tungsten
Niobium (columbium) 1% Zr
Brass, Yellow, 268
Uranium 8% Mo.
Brass, Naval, 464
Yellow Brass
Muntz Metal 280
Brass (plated)
Nickel-silver (18% Ni)
Stainless steel 316L (active)
Bronze 220
Copper 110
Red Brass
Stainless steel 347 (active)
Molybdenum, Commercial pure
Copper-nickel 715
Admiralty brass
Stainless steel 202 (active)
Bronze, Phosphor 534 (B-1)
Monel 400
Stainless steel 201 (active)
Carpenter 20 (active)
Stainless steel 321 (active)
Stainless steel 316 (active)
Stainless steel 309 (active)
Stainless steel 17-7PH (passive)
Silicone Bronze 655
Stainless steel 304 (passive)
Stainless steel 301 (passive)
Stainless steel 321 (passive)
Stainless steel 201 (passive)
Stainless steel 286 (passive)
Stainless steel 316L (passive)
AM355 (active)
Stainless steel 202 (passive)
Carpenter 20 (passive)
AM355 (passive)
A286 (passive)
Titanium 5A1, 2.5 Sn
Titanium 13V, 11Cr, 3Al (annealed)
Titanium 6Al, 4V (solution treated and aged)
Titanium 6Al, 4V (anneal)
Titanium 8Mn
Titanium 13V, 11Cr 3Al (solution heat treated and aged)
Titanium 75A
AM350 (passive)
Silver
Gold
Graphite
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Passive (Cathodic) End

 Article posted October 17, 1999 by Barry L. Ornitz: [email protected].