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Corrosion Difference Between 304 and 316

06/28/2012 8:13 PM

Hi all


I have a device for attaching cafe tables and umbrellas, often they can be close to the sea, maybe 100m and I am told that using 304 will not detract from the strength, but will pit and look unsightly.


I am proposing to use 304 for the parts not seen in coastal environments and 316 for the rest, as 304 is considerably cheaper and it's a competitive industry. I would use 304 in other non coastal applications (we don't have road salt).

What has been the readers experience?

Quoting from ssina publication:http://www.ssina.com/download_a_file/corrosionfinal.pdf
"Grade 304 (UNS 30400), the basic "18-8" alloy(18% chromium, 8%nickel), is the most common of the 300 series and has excellent corrosion resistance in most applications. Grade 316 (UNS 31600),has an addition of at least 2% molybdenum, which significantly increases the metal's resistance to "salt"corrosion."

Thanks Tony

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#1

Re: Corrosion difference between 304 and 316

06/28/2012 8:48 PM

That sounds like a good compromise. If welding is involved on the SS316 parts, lower-carbon SS316-L is advised.

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#2
In reply to #1

Re: Corrosion difference between 304 and 316

06/28/2012 9:43 PM

And don't forget to passivate, if you weld.

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#3

Re: Corrosion difference between 304 and 316

06/28/2012 9:51 PM

Hi, what is the part of the table to which your device attaches to made of: aluminum, wood, plastic? Before upgrading to more "sophisticated" types of SS, first thing I would do is to rule out galvanic currents . Good look in your project.

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#4
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Re: Corrosion difference between 304 and 316

06/28/2012 10:54 PM

Thank you
These are for the base of the table or umbrella and would be SS bolted to concrete and be attached to a stainless tube. In non coastal applications they may be attached to powder coated, zinc coated MS.
Tony

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#5

Re: Corrosion Difference Between 304 and 316

06/29/2012 1:39 AM

Tony,

I'd suspect that 304 would be satisfactory. There are two ways you can chek this where you are.

Firstly, what grade do local boatbuilders use.

Secondly, go to the Patawalonga end of Anzac Parade and look at the rails and fittings on the walkways and such near the bars there (And maybe do some research on the local ales.) then ask what they are made of.

We are using 304 for cabinets and such mounted on wharves and ocean frontage and the only issue has been where internal welding of struts was not properly passivated. Our cabinets are out there 24/7 including some areas with high salt spray from ocean.

At 7 years exposure the result would be described as "tarnish" rather than pitting.

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#6
In reply to #5

Re: Corrosion Difference Between 304 and 316

06/29/2012 1:49 AM

Thanks

The boat builders use 316.

Where are your high exposure cabinets, I'd like to have a look.

Please explain more about the passivating process.

Tony

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#7
In reply to #6

Re: Corrosion Difference Between 304 and 316

06/29/2012 2:01 AM

Here's a good explanation:

www.delstar.com/passivating.html

Hope this helps.

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#9
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Re: Corrosion Difference Between 304 and 316

06/29/2012 3:11 PM

Tony,

The boat builders use 316 for things like rails and such that will last until the warranty is gone unless the operator spends a lot of time with upkeep.

The boat builders that make T-Tops, and substantial rigging use aluminum. I have 2 boats with T-Tops that are constantly at sea. The aluminum has never rusted, pitted, and looks like it did years ago.

Use aluminum, you won't be sorry.

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#11
In reply to #9

Re: Corrosion Difference Between 304 and 316

07/01/2012 1:46 AM

You have to use a special quality Aluminum,and ofcourse passivated....

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#12
In reply to #6

Re: Corrosion Difference Between 304 and 316

07/02/2012 1:25 AM

Have a look for "pickling and passivation" on the web. We used to use Avesta products and I see that they have pastes and gels for the purpose on their website. Your SS supplier should be able to give you some good leads if not supply the product(s).

Also look at what ASSDA (Australian SS Development Assoc) has to say and AS 1554.6 'Welding Stainless Steel for Structural Purposes' (AS = Australian Standard for non - Aussies.

Regarding Just an Engineer's comment about tarnish; this might be what's known as "tea staining". Again, Googling "stainless steel tea staining" will unearth plenty on this. I've always found ASSDA informative and they say:

"Tea staining is discolouration of the surface of stainless steel by corrosion. It is a cosmetic issue that does not affect the structural integrity or the lifetime of the material. Tea staining occurs most commonly within about five kilometres of the surf and becomes progressively worse closer to the marine source..."

"CONDITIONS REDUCING THE RISK OF TEA STAINING

1. Absence of corrosives - especially salt.
2. Atmospheric conditions - lower temperatures and low relative humidity (RH) are better.
3. Surface orientation and design - free drainage and avoidance of traps which can concentrate corrosives. This includes open exposure to allow rain washing.
4. Surface roughness - smoother is better.
5. Chemical cleanliness or passivation of the surface improves the corrosion resistance.
6. Appropriate grade for exposure conditions - increasing PRE increases corrosion resistance.
7. Maintenance - or corrosives will accumulate."

There's plenty of information out there.

Good luck with your project.

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#8

Re: Corrosion Difference Between 304 and 316

06/29/2012 10:50 AM

In my country we say "the chain gets broken at the weakest link" ... and here comes my question: have you already considered the spec´s of the canvas you will you use for the umbrellas? Unless you have in mind selling replacement canvases, the risk is that the mean life of the metal structure surpasses by 10 or more times those of the canvases... and that ain´t good for you!

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#10

Re: Corrosion Difference Between 304 and 316

06/29/2012 11:55 PM

If your umbrellas are above the high tide mark split the difference and go with 308.

Having 304 and 316 variants may end up being a bit of a cluster cuss if they get mixed up on deliveries. All savings lost in an instant. Settling on a standard material may be the way to go.

That's my zack's worth. Google for more info.

Cheers.

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