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.

A Galvanic Series of Certain Metals and 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

----------------------------------

Passive (Cathodic) End

 

 Article posted October 17, 1999 by Barry L. Ornitz WA4VZQ on rec.crafts.metalworking. He can be reached via the ARRL email forwardingWA4VZQ@arrl.net.