Reactions between metals?

[JAZ]

I know that electrolysis or some other reactions can happen when two dissimilar metals are conjoined, but don't know which react. Is there a good textbook out there that covers that sort of info and some of the million other questions I have about fabricating with metals?
I have mechanically assembled pieces that combine mild steel with aluminum and mild with stainless, but without knowing ahead of time whether they will react. What would happen if I bolted a copper piece onto a steel one? Etc.

[sculptor]

... What would happen if I bolted a copper piece onto a steel one? Etc.

The rule in plumbing is-----copper to brass to steel.....I have dismantled brass unions from such connections years after their installation, and seen no negative effect.

Contact with aluminum causes steel to rust faster

The plumbers used to call it a dielectric effect, but I'm not certain they used the word properly.

The 2 steel guys I know are pureist---eg weld stainless with stainless and use a trigas mix(I do not remember the 3 gasses).....

[fritchie]

JAZ - As you might expect, chemists have an exact way of measuring and tabulating this sort of thing, but often pragmatism is better for artists. A very bright experimental artist I know who used to live here but who moved away some years ago told me, when I first began using bronze, that the best electrolytic match for bronze (such as with anchoring bolts) is stainless steel. Over time, there should be little or no corrosion of the stainless (more active) by the bronze (less active). But, or course, there is stainless and stainless, and that’s the reason for pragmatism.

Chemists describe the tendency to oxidize (to lose electrons, or form corrosive salts) as an oxidation potential, or a standard oxidation potential, where solid metals are involved. It also is described loosely as EMF (electromotive force), electrolytic potential, and so on. Sculptor is right with the activity order “copper to brass to steel”, and bronze probably is between copper and brass.

Aluminum is by far the mot active metal for corrosion, and the main reason it isn’t ordinarily destroyed very quickly is a quirk of its oxidation product, aluminum oxide, Al2O3. Al2O3 adheres with extraordinary tenacity to pure aluminum, so if nothing else is present, an aluminum object will last nearly forever. In contact with other metals, it can be destroyed quickly. About a decade or so ago, I can across a collection of European coins I had from about 1958, and all the aluminum coins were heavily corroded from contact with the brass, nickel and other alloys in the same bag.

To continue with your question, in order of decreasing activity, you probably would have magnesium - aluminum alloy (e.g., lawnmower metal), aluminum, mild steel, brass, copper, stainless, and bronze. I’m just guessing about the latter two; traditional bronze, an alloy of copper and tin, might be more active that pure copper. And of course, the popular bronze in sculpture today is silicon bronze, an alloy of about 95% copper, 4% silicon and 1% either iron or nickel (added in editing, following day - I think the alternative to iron actually is manganese and not nickel. These are two patented alloys). I’m really not sure where it would stand.

[sculptorsam]

To continue with your question, in order of decreasing activity, you probably would have magnesium - aluminum alloy (e.g., lawnmower metal), aluminum, mild steel, brass, copper, stainless, and bronze. I’m just guessing about the latter two; traditional bronze, an alloy of copper and tin, might be more active that pure copper. And of course, the popular bronze in sculpture today is silicon bronze, an alloy of about 95% copper, 4% silicon and 1% either iron or nickel. I’m really not sure where it would stand.

Could you flesh this out some more? Are you saying that putting metals of high activity together (ie. mild steel and aluminum) increases the activity while putting a highly active metal with a low active (aluminum with stainless) leads to very little activity? I'm trying to wrap my head around this...

Sam

[Araich]

The higher activity (faster oxidising) metal takes on an increased rate of corrosion by contact with a lower activity one.

[fritchie]

The higher activity (faster oxidising) metal takes on an increased rate of corrosion by contact with a lower activity one.

Araich has this right. In the list of activity I posted above, lawnmower metal (magnesium - aluminum alloy) or aluminum would be the most easily corroded material in the list, if it is in contact with anything farther down the list, and bronze or stainless would be basically uncorroded by materials farther up the list. The greater the separation, the worse the likely corrosion. Materials close together in the list might not be corroded at all in a reasonable time.

And, the more humid the environment, the faster the likely corrosion.

[JAZ]

Well, so much for pragmatism. About two years ago I sold a small sculpture to a guy in Georgia (hot and humid) that was made of mild steel elements with a band of aluminum bolted in between them. The steel had a coat of Penetrol on it. I wonder what the sculpture looks like now. I wish I knew these things before I let stuff drift out of my studio. I need a good textbook or manual that covers all of this sort of very important info.
It would be great if I knew what I was doing some of the time.
JAZ

[Araich]

It would be great if I knew what I was doing some of the time.

LOL! I would not worry too much, the aluminium will be fine...

[jwebb]

I've done the same with aluminum and mild steel, and it has no noticeable effect so far. Life is short and Art is long, as they say. I heard a story about a "conceptual" piece, which consists of an ingot of lead and an ingot of some other metal, stacked on one another. Atom by atom, the two will fuse into some singular alloy, in a million years or so - and a work of Art will have occurred. This was all a very hard lesson for ship builders to learn, though, when rivets literally dissolved due to electrolitic action, and steel plated hulls came apart at sea.

[JAZ]

I've done the same with aluminum and mild steel, and it has no noticeable effect so far. Life is short and Art is long, as they say. I heard a story about a "conceptual" piece, which consists of an ingot of lead and an ingot of some other metal, stacked on one another. Atom by atom, the two will fuse into some singular alloy, in a million years or so - and a work of Art will have occurred. This was all a very hard lesson for ship builders to learn, though, when rivets literally dissolved due to electrolitic action, and steel plated hulls came apart at sea.

I'm always amazed by people who come up with things like that conceptual piece, heavy on the concept, light on the effort. Lots of us could stand a bit of that in our careers, right?
Yikes about the rivets. I should think that the salt water played a major role there.