mnewnham
 I'm on the road at the moment so I can't check my library, but I would look first in Corrosion of Stainless steel by Sedriks. Then, in the Handbook of Stainless Steel by Peckner and Berstein. I'll get that information by Friday if you haven't gotten it by then.

600-700C is within the recommended range for the use of
347.The maximum "recommended"(Handbook of Stainless Steel) temperature is 800C in intermittant use and 860C in continous service with htis amount of water. The water is quite significant and increases the weight loss by a factor of ten, but only reduces the maximum service temperature by 65C. The oxygen is less important since the oxidation rate is governed by cation transport through the scale. The net result is that under these conditions you should expect a loss of base metal of about 0.001 g/cm2.
This can be improved within the permissable composition range of the alloy by maximizing chromium and silicon content and minimizing sulfur content.

P.S. Thanks for being the first member of this forum!

Do make sure that check the corrosion data to see if it is for continious exposure or cyclic, and that the exposure matches your application.
I would guess that cyclic oxidation at this temp causes a lot of damage (I am on hte road also).  347 is no more resistant to oxidation damage than is 304.

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Corrosion never sleeps, but it can be managed.
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Thanks everyone for the help.
I have obtained a little published data on the oxidation of Type 347 stainless steel, in air and in steam. I have also assumed that it behaves like alloys 321 and 348, which are chemically very similar, which helps beef up the data set.

The data is given as a weight gain in mg/cm2 over time at a given temperature. However I am more interested in metal loss in microns over time, because I am dealing with thin x-section. Do you know how I might do a conversion? And how might I use the data to get to a general equation so that I could make predictions of metal loss at different temperatures and times?

If the data is presented as the de-scaled weight loss per unit time then the section loss, assuming a plane with oxidation on one side only, is the weight loss in gm/cm squared divided by the density in grams per cc.

If the data is presented as a weight gain,and then you assume that the oxide is Cr2O3, then the weight gain is equal to a loss of metal to the oxide of approximately 104/152, two chromium atomic weights divided by two atomic weights of chromium plus three of oxygen. This is only approximate because there's some iron in the scale and some internal oxidation. Once you have calculated the weight loss to the oxide, the thinning is determined as in the first case.

The weight loss often reported in test data is simply due to the spalling of the oxide.  If weight loss is reported for descaled samples then this give you a better idea of the actuall average depth of attack.
What you really need is samples that have been examined in cross section after testing in order to determine the maximum depth of attack.

I was just looking at some data and it shows 321 being significantly worse than 347.  My guess is that 321 suffers more internal attack because of the Ti.  I will look for other data to see if this is correct.

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Corrosion never sleeps, but it can be managed.
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EdStainless
 It appears that oxidation data is among the least available data for stainless. Would you please cite your sources so our new book can have a good starting point.
 In fact, since you clearly have a broad knowledge of stainless, I would always appreciate your passing along good references.

My favorite source for this is "High-Temperature Corrosion of Engineering Alloys", George Lai (who is the worlds formost authority on weld overlay for high temp), ASM, ISBN 0-87170-411-0

There is a figure for long term oxidation (10x1,000hrs) at 1500F.
There is also a figure on cyclic oxidation (15min hot, 5min cool) at 1800F.

There is a lot of discussion on the mechanics of the corrosion and the original references are cited for everything.

McG, when I get a chance I will make a list of my 20 favorite sources and send it to you.

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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

Thanks very much. I really appreciate it. It will help everyone in the forum.