OT? "Materials"/metalurgy forum?

[Don Y]

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

 

[George Herold]

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

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.

 

[Martin Brown]

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.

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.

I.e., is it worth while to
remove existing oxides before "protecting" (dubious term)
the metals or does it not matter?

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.

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

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!

 

[Don Y]

Hi George,

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

OK, I will take a peek. Thx!

For non-authoritative answer, it depends on the metal.

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").

Aluminum 'self seals' iron doesn't, and gold is magic.

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!]

 

[Tim Williams]

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)?

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

 

[Don Y]

Hi Tim,

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

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.

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.

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.

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.)

Think: soil.

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

[other metals elided -- thx!]

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.

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...

 

[Tim Williams]

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.

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.

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".

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?).

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

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"..)).

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.

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.

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

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

 

[Robert Baer]

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

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.

 

[Martin Brown]

Hi Tim,



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.

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.

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

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

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.

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.

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.

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.

Think: soil.

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.

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

[other metals elided -- thx!]

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.

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...

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

http://www.northtowncompany.com/polyc.html

 

[Don Y]

Hi Tim,

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.)

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!

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...

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).

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).

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).

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?).

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...]

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

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...

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.

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!

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.

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

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.

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

 

[Don Y]

Hi Martin,

On 17/10/2013 05:15, Don Y wrote:

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.

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.

In alkaline or acid soil? How much rainfall and salts in the ground?

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]

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.

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

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

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

 

[Don Y]

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...

[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!)