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#115108 - 10/04/04 11:09 AM aluminum vs. galvanized steel
janderscu Offline
first timer

Registered: 10/04/04
Posts: 11
Loc: Greenville, SC, USA
Which is better to use for dryer vent ducts, aluminum or galvanized steel? I need to use rigid pipes for my installation as well make some turns with elbows in the crawlspace. I've noticed that aluminum seems to be used in the dryer vent kits, but I can't find the specialty elbows I need in aluminum, only galvanized steel. Does it make a difference?

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#115109 - 10/04/04 11:45 AM Re: aluminum vs. galvanized steel
JasonB Offline
Sharp -Shooter
Carpal Tunnel

Registered: 04/27/04
Posts: 14198
Loc: Cape Spencer, New Brunswick, C...
Either should work fine. Aluminum might last longer (rust). Just don't mix steel and aluminum!

J
_________________________
er, somethin'....

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#115110 - 10/04/04 11:54 AM Re: aluminum vs. galvanized steel
Nestor_Kelebay Offline
Carpal Tunnel

Registered: 09/13/03
Posts: 8552
I'm with Jason on this one. It doesn't make a difference. How many elbows, how straight your piping and how smooth the duct you use is far more important than whether you're using aluminum or galvanized. Also, officially you're not supposed to use any screws to hold dryer ducts together. The screws tend to catch lint, thereby causing more of it to accumulate in the duct, and increasing the liklihood of a duct fire if your dryer overheats. Use duct tape instead.

Jason: For galvanic corrosion to occur, not only do you need dissimilar metals in electrical contact (which we have if he mixes aluminum and steel), but we also have to have an electrolyte between those dissimilar metals to complete the circuit (which we don't have). Any condensation that forms in the ducting will be fresh water and therefore a good insulator (and therefore a lousy electrolyte). I'm not saying it's a good idea to mix aluminum and steel. I'm just saying it's not a problem to (so far as I know, and I've been wrong many many times in my life).

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#115111 - 10/04/04 11:58 AM Re: aluminum vs. galvanized steel
JasonB Offline
Sharp -Shooter
Carpal Tunnel

Registered: 04/27/04
Posts: 14198
Loc: Cape Spencer, New Brunswick, C...
No arguement on galvanic corrosion's requirements! I'm a petroleum system designer, we work with cathodic protection systems all the time. I just like to give consistent advice, especially on things confusing to many lay persons like mixed metals. The next person might think it's OK to mix them in an application where salts etc. are present!

Good call on the screws!

J
_________________________
er, somethin'....

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#115112 - 10/04/04 11:59 AM Re: aluminum vs. galvanized steel
MTeator Offline
"I am the Ipeman"

Registered: 05/05/03
Posts: 7622
In reply to:

Any condensation that forms in the ducting will be fresh water and therefore a good insulator (and therefore a lousy electrolyte).




Ducts are full of dust and all other kinds of fun contaminates to unpurify that water.

I wouldn't mix them.
_________________________
so long and thanks for all the fish.

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#115113 - 10/04/04 01:47 PM Re: aluminum vs. galvanized steel
kframe19 Offline
Carpal Tunnel

Registered: 03/05/03
Posts: 8536
Loc: Virginia
And the dust is likely to have about as much electric potential as a T-shirt.


_________________________
THAT'S RACIST!

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#115114 - 10/04/04 06:01 PM Re: aluminum vs. galvanized steel
JimTheTinkerer Offline
" Humming Bulb "
Carpal Tunnel

Registered: 03/24/03
Posts: 20773
Loc: US
A spot of good luck!
Aluminum and zinc are close in terms
of eV potential. What does that mean? Not much galvanic
action. In other words, if you stacked galv. and aluminum as
a battery, it would only squeek out .1-.2 V

_________________________
"Pay no attention to the man behind the curtain!"
--The Wizard of Oz

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#115115 - 10/04/04 09:10 PM Re: aluminum vs. galvanized steel
Nestor_Kelebay Offline
Carpal Tunnel

Registered: 09/13/03
Posts: 8552
Jason: You said: "we work with cathodic protection systems all the time"

I'm wondering if you can shed some light on some corrosion stuff if you happen to know.

In my hot water heating system, I use potassium nitrite as a corrosion inhibitor. This is what both Drew Chemical and the guy who I go to (he's a chemist who works as a consultant in industrial water treatment) recommend for hot water heating systems.

And, to determine if the concentration of corrosion inhibitor in the water is sufficient to be effective, the test basically involves "titration" (I think it's called). I just add a pH indicator solution to a 45 ml sample of my heating water (which makes it turn red) and then count how many drops of acid I need to remove that red color from it. The more drops of acid I have to add, the more alkaline it was to begin with.

I guess what I'm wondering is this: Why does making my water alkaline help to prevent corrosion? It seems to me that an electrolyte can be EITHER acidic or alkaline, and I get that notion from my understanding (correct or not) that alkaline batteries are so named because they use an alkaline material between dissimilar metals.

I guess I'm wondering why a high pH solution wouldn't be as good an electrolyte as a low pH solution.

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#115116 - 10/05/04 06:56 AM Re: aluminum vs. galvanized steel
JasonB Offline
Sharp -Shooter
Carpal Tunnel

Registered: 04/27/04
Posts: 14198
Loc: Cape Spencer, New Brunswick, C...
Hey Nestor,

This type of CP is a bit outta my usual scope, we deal with cathodically protecting buried tanks and pipelines primarily, but the principles involved are the same.

Is there a sacrificial anode in your application?

Basically, CP works by providing oxidizing agents in the water with an easier source of electrons than is found in the metal you want to protect. With tanks and pipelines, underground, we electrically connect them to a zinc or magnesium anode. The zinc or magnesium is much more readily oxidized than the steel we're trying to protect, so as long as the zinc or magnesium is not all used up, it'll absorb the bulk of the oxidizers in the groundwater. Periodic testing and replacemet of the anodes is required.

We test them with something called a half cell. It's actually half a battery, built of a plastic tube with a permiable bottom, a copper piece inside (one electrode) and a copper sulphate solution inside, with the copper immersed in it. When we put the half cell on the ground, the permiable bottom allows there to be an ion path thru the moist earth to the tank. The tank and sacrificial anode is the other electrode in the battery (the other "half cell"). We measure the voltage this setup generates, and a voltage above a certain level (actually 850mV) indicates adequate protection for the tank. Basically, so long as the potential is above .85v, the anode is the primary battery anode, but when the voltage falls below that, it's indicating that the tank is now the anode, and is therefor rusting.

Your boiler water is an electrolyte, certainly. The chemical in it, and it's ph is simply matched to ensure the correct parts corrode first. The potassium nitrite may be providing excess electrons (being basic, it's gonna make OH- ions, and therefore it's got an excess electron to share) to head off some other undesirable reaction. Like the galvanic corrosion of your pumps, valves and pipes.

That's the best I can do for now!

J

_________________________
er, somethin'....

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#115117 - 10/05/04 11:13 AM Re: aluminum vs. galvanized steel
Nestor_Kelebay Offline
Carpal Tunnel

Registered: 09/13/03
Posts: 8552
Jason:

Thanks for the response. I guess I had the two things mixed up. What you're describing in your post is what I always through of as "using a sacrificial annode". I always understood cathodic protection is where you actually apply a low voltage to the pipeline, bridge, storage tank, whatever which would provide the electrons instead of the metal.

My understanding was always that corrosion of ANY metal is ALWAYS associated with the loss of electrons from that metal's atoms, (and often the resulting metal ions then become soluble in water ((cuz water is polar)), resulting in the removal of metal atoms from the surface that's corroding). And I always thought that you could prevent that corrosion by:

a) providing a metal that looses electrons more easily (like zinc or magnesium), or

b) pumping electrons into that metal so that you replace any electrons you loose, thereby preventing corrosion of that metal.

And I always thought the second way was called "cathodic protection".

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