OT? "Materials"/metalurgy forum?

Hi,

I'm looking for an *authoritative* answer to some questions about oxidation (of metals). Specifically, if, once started (in earnest), these processes tend to self-perpetuate *or* self-limit IN REAL ENVIRONMENTS. I.e., is it worth while to remove existing oxides before "protecting" (dubious term) the metals or does it not matter?

Not sure if a "scholar" will be able to give me a practical enough solution (non "text book"). I.e., if the environment is already conducive to oxidation...

Thx,

--don

Reply to
Don Y
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There's the rec.crafts.metalworking forum. Some smart guys over there.. if you can ignore all the political talk.

For non-authoritative answer, it depends on the metal. Aluminum 'self seals' iron doesn't, and gold is magic.

George H.

Reply to
George Herold

From what I know it depends not only on the metal but on the alloy (i.e., not all steels are alike), and on the treatment used.

Steels and iron usually need to be cleaned -- but there are some industrial coatings that more or less (more than nothing, less than you'd like) work on rusty steel.

Aluminum -- that depends. I believe that when anodizing (which is basically forcibly creating an oxide layer) they strip off the old before they lay on the new. Other processes -- other things.

(With aluminum it's pretty much impossible to strip the oxide layer off completely if there's any oxygen around -- aluminum Really Likes to be oxidized).

Beyond that -- I dunno. And manufacturers are constantly coming up with new stuff.

Ditto what George said about r.c.m -- it's a fine place to learn very useful bits about metals, and quite a lot about setting up your kill files.

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Tim Wescott 
Wescott Design Services 
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Reply to
Tim Wescott

Your question is not well posed. The answer depends critically on the metal and its reactivity as well as the exact chemical environment. Trace impurities and even the crystal structure can matter.

Like all things in the real world the devil is in the detail. We used to have sample mild steel nails in test tubes with all but one of the requirements for rusting to occur satisfied as a demo.

In the real world they only self limit when the oxide coat becomes thick enough to prevent further reaction. This may be never if the oxide coat expands and flakes off.

Depends whether the oxide layer is responsible for preventing the corrosion progressing or not. Aluminium is hopelessly unstable and will oxidise overnight if you prevent the surface oxide coat forming.

The devil is in the detail. Simple example - some stainless steels can resist insanely aggressive oxidative environments but if there is even a trace of chloride ions in the mix will be destroyed in short order.

You could try asking in sci.chem or sci.engr.metallurgy but you are going to have to be a *lot* more specific about your corrosion problem and metal(s) involved before anyone can give you a sensible answer.

Ask one of the tech reps for a major industrial coatings company to recommend a solution but even then take it with a big pinch of salt. Allegedly the Fourth Rail bridge will not need repainting for 25 years now - I don't know anyone in the industry who actually believes that!

--
Regards, 
Martin Brown
Reply to
Martin Brown

OK, I will take a peek. Thx!

Yup. And, probably the environment, as well. I long ago learned to keep a light coating of machine oil on most of my (steel) tools. Once they show a little sign of "rust", it seems like you can never

*keep* them free of it (I suspect some have fine coatings that you end up removing when you try to remove the "rust").

Initially, I'm worried about copper -- pipe and wire -- exposed to The Elements. I.e., in soil, in air, etc. -- but invariably "outdoors". Does, for example, the oxide formation tend to provide sites for water and salts to adhere (whereas a "clean" metal would be more resistant to this)?

Amazing how "slow" processes can still "become fairly progressed" when you let them stew for a few decades! :>

[I think a neighbor has (had?) an *aluminum* (electric) service. And, it had managed to "degrade" over the decades to the point where it proved an unreliable connection. The panel actually caught fire under load! And, this failure cascaded to the distribution transformer that served his house -- and a few of his neighbors!]
Reply to
Don Y

Yes.

Aluminum (the better alloys), copper alloys (i.e., almost anything with a copper base, including any kind of bronze), stainless steel and others (titanium, etc.) share the property of a resistant oxide (or other corrosion) layer.

Now, in the case of copper, I'm not really sure how resistant it is -- it's constantly evolving with the atmosphere. Usually, the progression is something like:

- Freshly prepared copper with an atomic-scale oxide (yes, copper oxidizes freely in air; in fact, nanoparticles are pyrophoric, i.e., burn spontaneously!)

- Visible oxidation, usually dull brown (probably mostly Cu2O, may include sulfides?)

- Brown to black corrosion, usually involving sulfides (depends on environment)

- Green or occasionally blue (verdegris), carbonates

Each stage involves transformations and deepening corrosion, so I don't think it really stops, it's just slow. How many ancient bronzes remain (that haven't been melted down or shoved inside museums..)? How good do they look? That would be the real deciding factor.

The nature of those transformations will involve whatever's in the environment (obviously): oxygen, sulfur (usually trace H2S from decomposition, or SO2 from industry), CO2, acids and sunlight. Thin surface oxides form spontaneously in air, as do sulfides (when sulfur is present).

Reaction with SO2 probably involves a redox reaction, where it makes sulfate and sulfide from sulf*ite*. I don't think sulfides can be formed from sulfates (i.e., acid rain -- mostly dilute sulfuric acid), which will play an important role in sloughing off what corrosion is there. Sulfides may be somewhat stable (most metals have a hard-on for oxygen, but certain metals have a special hard-on for sulfur atoms, making some sulfides surprisingly stable), and resistant to mild acidity.

Oxides and carbonates will react readily with acids, either forming insoluble carbonates (from carbonic acid in regular rain) or dissolving as copper sulfate (from acid rain, causing staining and further corrosion).

Copper is also quite sensitive to bases, primarily ammonia, but this isn't very common in the atmosphere. (It is something to keep in mind if you were dealing with chemical apparatus.)

Anyway, enough with copper. That's about what I know about its chemistry, as applied to the environment.

Aluminum and stainless, I think, are pretty stable, but they can be coaxed into forming those nasty, crusty, rusty oxides that fester and grow.

Aluminum's primary enemy is alloy, salt mist and pH. Like copper, acid or base will disrupt the oxide. Electrolysis (salt) is enhanced with particularly inhomogeneous alloys, like 2024 (which contains guess what.. copper!). As I recall, some NASA rocket parts had been left on display out in the Florida sun; within a decade or few, the aluminum parts (which were 2024 alloy) had all but completely turned to rock, in situ! The alloys with less electrolytic mismatch (i.e., magnesium and silicon in the

4, 5 and 6xxx series alloys, and zinc in the 7xxx's) are much better on corrosion resistance.

Aluminum can also be amalgamated with gallium or mercury, which turns it to mush, and then crud, over the course of hours or days. (Terrorist hint: use gallium, or a mercury chloride solution: mercury metal has so much surface tension, it's almost impossible to rub into an aluminum metal surface!)

Stainless steel is part alloy, but mostly preparation. The metal isn't quite homogeneous, so a freshly cut surface will streak with rust. It must be passivated (selectively etched), usually using some nasty hydrofluoric acid bearing paste -- or citric acid something or other (why even bother with the first, right?!). The chrome oxide passivation layer is resistant to most stuff, but it can be stripped at high pH (but the underlying metal doesn't react) or low (where the metal does react).

Even stainless steel will form crusty deposits when left in the presence of hydrochloric acid fumes for a while. I don't know if these deposits continue to grow once the hydrochloric part has been neutralized, but given the porosity of rust, good luck with that. After such history, I would guess it'll continue, but will proceed much slower than mild steel (which will basically turn to dust and ooze in the same environment!).

And that's the main problem with rust, it's porous so it traps whatever agent induced the corrosion, and it expands and flakes off rather than stopping in place. Add moisture and you've got creeping death.

Tim

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Seven Transistor Labs 
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Reply to
Tim Williams

Yes, you can see this (the first three stages) in very short order:

- "buff" a copper pipe to a bright shine

- almost immediately, it assumes the "used penny" coloration

- add heat and it accelerates to a very dark brown (black) Getting to the green patina seems to take a bit more time/effort.

So, the patina doesn't "self-limit" the process.

In my case, a buried, unprotected copper pipe, having assumed that patina, shouldn't be "reassuring". Rather, I should consider buffing the pipe back to its original sheen then "taping" it before reburying it.

Similarly, the (copper) wire connections that were not protected, originally, should be trimmed back (it seems like corrosion works its way *under* the insulation, a bit), reconnected and then "protected".

Goal is not to try for an eternity but, rather, greatly increase the expected lifetime of each (wire, pipe).

E.g., the wire connections that I "protected" prior to BURIAL 20 years ago appear unaffected by the moisture, salts, etc. in the soil over that time. OTOH, those that were NOT protected and only exposed to air/elements have fared poorly.

Think: soil.

[other metals elided -- thx!]

Exactly. And, since water tends to be associated with and around pipes, its a foregone conclusions -- not to mention water soaking through the soil.

It seems the safest course of action is to clean things up, *then* protect them. I.e., AS IF this had been done from the beginning...

Reply to
Don Y

Ahh... burial. Galvanizing will help (e.g., if you can put it in a conduit), at least until it wears out. (You can get zinc anodes -- normally for marine use -- and bury new ones every, er, 5-10 years or so.)

To some extent, underground isn't too oxidizing, in fact it can be reducing -- depends on soil conditions. I think bogs are acidic but reducing (lots of organic material, anoxic environment). I want to say iron fairs relatively well under those conditions -- not much to rust it, despite the relative acidity. Interesting artifacts (and meteorites!) have been recovered from such areas. See also sunken treasure (cannons rusting undersea, etc.).

Well-aerated soil will oxidize fine though...

FWIW, in my childhood (this was the 90s), I once dug a penny out of the ground. 1889 Indian Head! Green as all getout, but the features were well preserved nonetheless. It was buried about two feet under standard Wisconsin loam.

Yeah, I've definitely seen that. I've had some, err, accelerated experiments before -- one summer, I ran a chlorate electrolysis cell, which gives off plenty of chlorine and salt spray fumes -- I soldered connections where possible, but everything copper turned green within days, and anything that didn't get covered with a mist of dried salt got soaked with the spray. Some of the wires I stripped back for a foot and still saw brown stuff (that doesn't tin worth a damn, by the way).

Under rather more ordinary conditions, I've heard of, actually, supposedly the cable TV at my parents' house corroded so bad that several feet were just nasty -- the core was just gone, which explained the bad DTV reception and terrible internet connection! They replaced that and all was well again. That was after probably ten years after the original installation, which, one would hope, simply wasn't done right (nick in the outer jacket? improper crimp terminal?).

Besides galvanizing, plastic and (old school!) lead conduit are good options too. You could even bury your own not-teflon-but-nearly raceway, but I'm guessing that's a bit much :)

Lead has the added benefit that its carbonate AND sulfate are highly insoluble, so, the surface turns white (from either), and, sits that way. For centuries. (Supposedly there are lead pipe water mains, in USA and Europe, still in use? Not at all a hazard to water quality, as long as what's dissolved in the water doesn't change over time -- lots of CO2 and carbonates, awesome -- organic acids, not so awesome! (lead acetate, for example, is "sugar of lead"..)).

Yeah, exactly. Anything you can do to keep moisture away -- e.g., museum pieces will last millenia in still air, but as soon as you get a bit of dust in the air, or moisture (condensation or seepage), or rainfall (especially acid rain), you've reduced the survival by orders of magnitude.

Water, in and of itself, isn't bad -- like, err, someone else in this thread mentioned, test tubes with all-but-one of the rusting ingredients included will not proceed in the absence of that last one. You can have steel water pipes that are decades, even centuries old -- they'll give you some orange water every once in a while (in my current, aging apartment, this is an infrequent occurance), but as long as the water doesn't have access to air, it's not going to do anything. The worst that can happen is, the little bit of chlorine and dissolved oxygen reacts, and after that, well, it just sits there forever, until you turn on the tap.

Same goes for copper pipe -- although I'll add one thing -- I've cut into semi-aged modern plumbing before -- I found the copper pipe output of a hot water heater was lined with a distinctive translucent gel -- probably a mixture of lime and silica gel deposits. I'd have to guess, a combination of hard water (dolomite bedrock around here!) plus however silica got into it (from incoming water, or maybe from the glass lining of the water heater dissolving..?). The inner copper surface itself was blue, but didn't look unusually corroded -- the inner diameter was reduced only by the deposits.

Tim

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Seven Transistor Labs 
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Reply to
Tim Williams

Well, in iron,just removing what you see ain't enough, as cracks and microcracks in the iron (caused by the rust) will still be there after even reasonable aggressive cleaning. So, as you say, these processes tend to self-perpetuating. Process is electrochemical in nature,hence use of sacrificial anodes.

Reply to
Robert Baer

It is a slow reaction of atmospheric CO2 and water to mixed carbonate hydroxide sulphide surface that is somewhat inert once fully formed. Your enemy for corrosion with copper or to a lesser extent lead is any SO2, NO2 in the air or chloride ions from sea spray.

Not if there is chemistry available to form soluble copper salts and take them away in water.

It is probably good enough and maybe better to just remove any obvious crystals or signs of local of corrosion and then seal it with a foam sleeve around to keep groundwater and soil away from the pipe.

As would be expected. UK tried using aluminium for phone connections at one time but it is coming home to roost now as places with them cannot get sensible speed ASDL as the oxide coatings rectify the RF signal.

In alkaline or acid soil? How much rainfall and salts in the ground? The latter will see off bare copper pipe pretty quickly. Alkaline isn't brilliant either I know they take measures to protect pipes in concrete.

Basically if there is any chloride or sulphur available and a damp environment the oxygen in the air will allow soluble copper salts to form. You see runs of copper salts off any copper roof. It is a slow process but so long as the dissolved material escapes more will follow.

Not necessarily. Cleaning it back to a smooth mixed copper carbonate/hydroxide patina should be ok if you are going to plastic coat it. Eroding the surface just decreases the thickness of the pipe.

In the UK at least all copper pipes intended for burial in the ground are supplied with a PVC foam? plastic outer sleeve to keep the soil or concrete from being able to attack the copper. You can buy retro fit polywrap but I have no idea how good or bad it is. eg

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Regards, 
Martin Brown
Reply to
Martin Brown

Here, water mains are run in a vinyl "sleeve" prior to burial. It's just a flimsy, loose fitting jacket but *seems* to keep the pipe reasonably intact (wrt oxidation).

A plumber hired many years ago to move the entry point of the main into the house added a short length of pipe *without* said jacket. I'm presently "fixing" some of the things he did poorly and, in the process, this pipe is exposed/accessible. Noticing the omitted sleeve and the extent of oxidization, I am wondering if I should "clean" the pipe before wrapping it or just wrap it "as is".

By way of comparison, some galvanized pipe located in essentially the same area (feeding the irrigation system; the copper pipe was feeding the domestic potable water supply -- same water, though) was badly corroded in a shorter time period. Many of the steel fittings (nipples, etc.) had lost much of their integrity!

The "native" soil is heavy in clay. I.e., you can literally *cut* "bricks" out of it in some places (fire them and build your own home :> ). In other places where the soil has been disturbed (e.g., trenching to run pipes), its more porous. And, in areas where it has been *amended*, very *rich* (in organics).

Ouch! That's not encouraging! I've been planning on sacrificing just a few *inches*. Much more than that will necessitate adding cable to replace what I remove (meaning another splice).

Here, they use a special coax for direct burial. OTOH, they often "forget" to bury it (e.g., new service gets left *on* the soil for a seeming eternity -- they want to get you as a *paying* customer and worry about the niceties of the installation some MONTHS later!

[CATV company seems to be pretty sloppy. E.g., cables run across the roadway are usually only 1" under the surface of the road! Literally! So, any pot holes in that strip of asphalt and the wire is exposed...]

I was planning on wrapping with pipe tape (for the pipe; see below for wire connections). The sleeving used here is probably only

10 mils thick and seems to do the trick (assuming the pipe is suitably shaded to keep the sleeve intact).

Tape is sold in 10, 20 and 40 mil thicknesses. The 20 is the easiest to work with (40 is *really* thick; 10 stretches too much). The copper line that I installed to replace the corroded galvanized looks like it will fare well.

So, I just have to wrap the old, "oxidized" portion of the water main that the plumber neglected...

For the wires that I buried, I opted for a "cheap" scheme to protect each splice/connection:

- punch a (crude) hole in the bottom of a "pill bottle" (i.e., the sorts of little plastic bottles in which you get Rx from pharmacy)

- punch a similar hole in cap

- thread two wires through these two holes (in the obvious manner)

- solder the connection

- tape or shrink wrap (to protect from shorting to the other conductor)

- pull wire exiting bottom of bottle to draw connection into bottle

- overfill bottle with silicone rubber

- mash cap onto bottle

- bury

I cut one of these open the other day to see how the splice fared (the process isn't intended to be reversible so this is a real PITA!) and was very happy with the results!

Thats fine for the *inside* of the pipes. But, doesn't apply to the

*outside* of the pipes! :>

That (uncorroded) seems to reflect what I see in the pipes, here. But, their outsides seems to fare much worse! Though those pipes exposed but NOT buried still retain the light brown "tarnish" despite being in place for decades! E.g., the water heater lines look like "2 week old" pipe.

I'll see if I can tackle the buried pipe tomorrow. Of course, it will be a chore trying to wrap it and *not* end up with all sorts of soil and debris getting caught on the tape's adhesive given that it's *in* the soil. :-(

Thx,

--don

Reply to
Don Y

It'll be easier (physically) to just wrap some steel wool around the pipe and "stroke vigorously". The underside of the pipe wouldn't readily be visible for inspection.

Foam (rubber) probably won't last. The lengths that I've previously applied (and wrapped in foil tape) have fared rather poorly. I'll stick with PVC tape instead.

I think the *native* soil is alkaline. But, how much it has been disturbed and amended in these areas varies. I.e., you have to disturb the soil to bury a pipe! :> (at least when these house were built... nowadays you don't really have to disturb it at all!)

Very little rainfall (~11 in/yr). But, a fair bit of supplemental irrigation. In the case of this pipe, it feeds the house and, out of convenience, has a hose bibb immediately above it! (so, this patch of soil is probably the wettest "unirrigated" portion of the yard as each time someone draws water from that bibb, the ground beneath it ends up wet)

[Our domestic water is ground sourced so has very high mineral content]

"Slow" means "100%" when you think in terms of decades.

I've used a 20 mil, adhesive backed PVC tape to coat other pipes. The original plumbing (elsewhere below grade) is encased in a vinyl "sleeve" intended for this purpose (it provides little/no mechanical protection but does act as a "soil/moisture barrier"). The plumber, here, was just negligent in "coating/taping" this section of ADDED pipe -- thereby creating a two *day* project to replace the 2 *minutes* he saved :-/

Thx,

--don

Reply to
Don Y

[from elsewhere in thread -- copper pipe]

Well, I opted to "clean" any heavy encrustations off the pipe (considerably more the further below grade -- no doubt more salts, retained moisture, etc. vs. surface layers) and "buff" any fine oxide off.

Then, trying to take care to keep oils from hands off the pipes, spiral wrapped the pipe with 20mil vinyl tape (40 mil seemed excessive and a 50% overlap of 20 mil gives that same level of protection). Shaded the hole/trench and buried it all.

Did the same for above grade, "exposed" portions. Then, wrapped in pipe insulation (which was then covered with a layer of foil tape for UV protection).

Also took a couple of spare lengths of pipe, capped the ends (so nothing could attack from the inside) and treated them similarly. I figure I can dig one of them up "in a few years" and reexpose the pipe to see how it has fared and assume the *real* pipe is faring similarly -- at least in terms of attacks on the outer surfaces (can't do anything about the insides as I don;t control what flows through it!).

Based on those observations, decide when (or *if*) I ever dig up the second sample (hopefully the NEXT homeowner has that task!)

Reply to
Don Y

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