Should I use Stainless or Galvanized chimney pipe?

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.
Status
Not open for further replies.
Have you ever seen a quality (say Simpson) galvanized Class A pipe that's enclosed rust away?

(Real Life Disclosure - I work for a large construction company)...

If you want the shortest possible answer: Simpson uses hot-dip galvanization, which means it cannot possibly rust unless you puncture the pipe.

Oh boy. This opinion I think comes from a misunderstanding of metalurgy. The galvanized pipe is less likely to rust out than stainless. It's entirely counterintuitive. Galvanization works by dipping the steel in zinc, which chemically bonds it to the steel. The metals are hot enough there's actually a layer of steel-zinc co-mingled. Stainless steel is dipped in chrome (yes the same stuff on your car), but because of the properties of chrome its simply on there and not chemically bonded. What happens is light dings and scratches don't cause the chrome layer to be hurt, but when you get under the chrome layer is when the steel can rust and bubble up the chrome layer. This is very similar to the bubbling on a chromed car bumper. Normal wear and tear is called galling. For a chimney this chrome isn't pure chrome, it's usually chrome alloy of some kind. It's entirely serviceable. The empire state building, for example, is stainless. The portland arch is aslo stainless. They have to polish the empire state building because the chrome alloy doesn't deal with acid rain too well, but the portland arch they just let hang out. Like galvanization, big chimney pipe companies know wood smoke tends to be acidic and make their stainless steel to deal with it.

But, you're talking a process that takes 20 years. This is aggrevated in a low oxygen environment where the chrome is forced to give up oxygen it's absorbed and so in a hot, dry, airless environment (a lot like a chimney, actually) you can force the chrome to burn off or change it's PH enough to attack the steel. Short of a chimney fire, this isn't going to happen, but this is why chimney fires are so bad for chimneys. Inside the chimney doesn't matter as much because it's expected to be reasonably dry (no surface water). The outside of the chimney, not so much. This is why chimney pipe erosion usually starts at the top.

Galvanization is where things get weird. Galvanized chimneys from decent brands are almost all hot dip, which is what co-mingles the metals. You can actually electroplate it, and I think this is where the really cheap galvanized stuff got a bad name. Electroplating is worse than chroming because a piece might be galvanized, but the layer is so thin it's a joke. Furthermore you can subject metal to blackening where galvanization is actually semi porous and it's filled with oil. A little oil on a chimeny pipe is OK, it prevents it from getting dull in shipping. If it almost slips out of your hands or it has a sheen from the oil, it's too much and this is a sign that something was blackened rather than hot dipped. If you want an example of this, a lot of construction materials are actually blackened so that when you screw something together, the oil eventually wears off and the parts anneal themselves together from contact.

Now, knowing all that happy horse crap, put a 1ft section of pipe at the top of your chimney system and keep an eye on it. When the chimney starts to erode, just replace the 1ft pipe piece to keep the smoke away from the much more expensive 4ft ones. ;)

EDIT: Before someone asks the obvious: Why is the inside of a chimney stainless rather than galvanized - The stainless surface is a lot, lot smoother which makes it easier to clean. The chrome process is expensive, which is why chrome pipes cost more. However, the chrome inside of a chimney isn't likely to die any time soon because there's nothing in there which could damage the chrome unless you're using a really, really aggressive brush.
 
Tiber -

Thanks alot for the metalurgy education!
Also, a great tip by using a 1 foot scrificial piece at the last section.
 
I'm surprised to hear that stainless is dipped in chrome. I always assumed the chrome was just part of the alloy mix.

pen
 
tiber said:
(Real Life Disclosure - I work for a large construction company)...
I've worked in construction and heard more than my share of bull. I have to call BS on this one.

There are of course, many different grades of stainless. Try holding a magnet to some stainless steel. On some grades, you will feel a bit of pull. On others you won't.
 
LLigetfa said:
tiber said:
(Real Life Disclosure - I work for a large construction company)...
I've worked in construction and heard more than my share of bull. I have to call BS on this one.

There are of course, many different grades of stainless. Try holding a magnet to some stainless steel. On some grades, you will feel a bit of pull. On others you won't.

That's due to what's been galvanized or chromed. Chrome itself is nonferrous. You can chrome dip iron bars if you want.

Anyway don't take my word for it: http://www.simpsonmfg.com/docs/AnnualReport2006.pdf

Simpson sells their "Laser cut hot dip galvanizing" in the 2006 report.
 
LLigetfa said:
tiber said:
(Real Life Disclosure - I work for a large construction company)...
I've worked in construction and heard more than my share of bull. I have to call BS on this one.

There are of course, many different grades of stainless. Try holding a magnet to some stainless steel. On some grades, you will feel a bit of pull. On others you won't.
+1 I worked in a meat processing plant for 29 years and no way is glav. in any way shape or form better than SS!
 
Wow, neat thread guys.

Here's what I found out about the two materials:

From http://chemistry.about.com/cs/metalsandalloys/a/aa071201a.htm

What Is Stainless Steel and Why Is it Stainless?

In 1913, English metallurgist Harry Brearly, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance. In addition to iron, carbon, and chromium, modern stainless steel may also contain other elements, such as nickel, niobium, molybdenum, and titanium. Nickel, molybdenum, niobium, and chromium enhance the corrosion resistance of stainless steel. It is the addition of a minimum of 12% chromium to the steel that makes it resist rust, or stain 'less' than other types of steel. The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film. The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of the metal, forming a stable layer only a few atoms thick. If the metal is cut or scratched and the passive film is disrupted, more oxide will quickly form and recover the exposed surface, protecting it from oxidative corrosion. (Iron, on the other hand, rusts quickly because atomic iron is much smaller than its oxide, so the oxide forms a loose rather than tightly-packed layer and flakes away.) The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistance in low-oxygen and poor circulation environments. In seawater, chlorides from the salt will attack and destroy the passive film more quickly than it can be repaired in a low oxygen environment.

So according to this source, the stainless quality goes all the way through, and the material self-heals from scratches, which is my experience.

Now, on to galvanized steel:

From Wikipedia, http://en.wikipedia.org/wiki/Galvanization :

Metal protection

In current use, the term refers to the coating of steel or iron with zinc. This is done to prevent galvanic corrosion (specifically rusting) of the ferrous item. The value of galvanizing stems from the relative corrosion resistance of zinc, which, under most service conditions, is considerably less than those of iron and steel. The effect of this is that the zinc is consumed first as a sacrificial anode, so that it cathodically protects exposed steel. This means that in case of scratches through the zinc coating, the exposed steel will be cathodically protected by the surrounding zinc coating, unlike an item which is painted with no prior galvanizing, where a scratched surface would rust. Furthermore, galvanizing for protection of iron and steel is favored because of its low cost, the ease of application, and the extended maintenance-free service that it provides.

So, as we all knew, galvanization is a surface coating of zinc, and more vulnerable, but the zinc will protect the steel from corroding if the coating is breached.

And another little tidbit--From http://www.finishing.com/108/38.shtml :

Can you please let me know if there is any statistical information as to the durability & life span or structural astuteness of G60 galvanized steel.

Thanks,

Tonya Booker
trusses & panels. - Markham, Illinois

June 1, 2009

Hi, Tonya. G60 means .60 ounces of zinc per square foot, but includes both sides of the sheet; so the weight is .30 ounces per square foot per side. If we estimate the density of the coating as the density of zinc, 7.14 g/cc or 4.127 oz/in3, this would be 0.0727 in3/ft2, or a thickness of approximately 0.005 inches thick.

In letter 7666, it is suggested that G90 (which would be 1.5X as thick) should be the minimum used for outdoors exposure. Patient application of the search engine will reveal many threads where industry experts consider G60 to be thin and (by implication) questionable. I don't think the galvanizing thickness has any effect on the structural astuteness.

Now we know what the "G90" spec for the Simpson galvanized pipe means! :coolsmirk:

So it looks like galvanized steel is not as durable as stainless, but durable in exposed applications. Since I'm installing mine in a chase, I feel pretty good so far about going with galvanized, and spending my money on quality elsewhere.
 
pen said:
I'm surprised to hear that stainless is dipped in chrome. I always assumed the chrome was just part of the alloy mix.

pen
I am not sure where the idea of it being dipped came from but SS in not dipped.
 
I don't want anything to do with this SS and galvanized-which is better- issue, but just thought I'd add this:

I got galvanized (stainless interior) pipe and painted it black with a $12 can of BBQ paint, which will be enough for years to come should I need it.
After one year, the paint looks as good as the day it went on.
 
Status
Not open for further replies.