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Old 11-08-2012, 21:47   #1
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Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

I have some 314 barstock that I want to make some chain plates out of, but they're mil finished. If I have the time I'll polish them if that's unnecessary, but if there's some sound metallurgical reason to polish them I'll do it for sure.

Thanks metal guys.
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Old 11-08-2012, 21:56   #2
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Re: does stainless steel finish matter for corrosion protection or anything else?

Yes!

Polished SS will resist crevice corrosion. Not so many voids for salt to hide in. Also after polishing an acid dip helps to warn off micro corrosion.

Any time SS it processed it needs to be treated against micro corrosion. SS polish compounds usually has enough acid to help polished finishes.
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Old 11-08-2012, 22:22   #3
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Quote:
Originally Posted by delmarrey
Yes!

Polished SS will resist crevice corrosion. Not so many voids for salt to hide in. Also after polishing an acid dip helps to warn off micro corrosion.

Any time SS it processed it needs to be treated against micro corrosion. SS polish compounds usually has enough acid to help polished finishes.
Okay smart guy, here's another one for you:

Why don't people epoxy them regarding the whole corrosion thing. I know it won't adhere to it perfectly but it will stay on there pretty damn good. Hard to imagine they bend enough for the epoxy to crack.
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Old 11-08-2012, 22:48   #4
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Re: does stainless steel finish matter for corrosion protection or anything else?

Stainless needs breath fresh air so to speek. One hole in that barrier coating and your chainplates are history..
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Old 11-08-2012, 22:52   #5
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Originally Posted by TeddyDiver
Stainless needs breath fresh air so to speek. One hole in that barrier coating and your chainplates are history..
Bedding compound gets put on stainless all the time. Hell tons of stainless bolts and screws are saturated in bedding compound and left for years with no problem. I don't see how epoxy would some how be any different. The top of a screw head "breathing" for the rest of the screw seems a little zen.
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Old 12-08-2012, 00:12   #6
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Re: does stainless steel finish matter for corrosion protection or anything else?

Yes but when the bedding compound breaks down, you have the perfect environment for crevise corrosion. Most SS chain plates fail in the space where they pass through the deck. When the goo loses it's adhesion to the SS, for whatever reason, salt water gets in and does it's thing in the absence of oxygen. SS requires oxygen to form a film that protects the metal underneath. When you don't have oxygen, the metal exfoliates much like regular steel produces pitts and eventually results in cracks and failure.
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Old 12-08-2012, 00:46   #7
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Re: does stainless steel finish matter for corrosion protection or anything else?

If you want to wade through the technical end of Stainless Steel corrosion especially in the salt water environment, see: Corrosion, stainless steel and: http://www.corrosionclinic.com/types...n_cracking.htm

What it amounts to in real life is that stainless steel in the presence of salt water and absence of oxygen develops intragranular corrosion where the bond between grains of the metal lose adhesion to each other. This is quite common anywhere that the stainless steel is mechanically (e.g. paint, caulk, adhesives, fiberglass, etc.) shielded from getting oxygen from the air but can allow stagnant water to enter.

Specifically, as mentioned by others, where the chain plates pass through the deck and the caulking breaks down or where the stainless bolts and nuts are exposed to sea water but not fresh air. It is typical that stainless steel bolts will fail in the portion of threading inside the nut or at the bolt head to shank area. Examination o the failed bolt will show corrosion eating through the bolt shank until not enough metal integrity is left to withstand the forces imposed on it.

I have removed and personally seen chain plates where you can stab a screw driver through the 1/4" thick plate that has advanced intragranular corrosion. You can also do the "superman" routine and snap the bar in half with your bare hands.

This is why if you do need to install stainless steel "whatever" in an application where part of the surface will be prevented from fresh air access, that you make sure that the metal is surgically/chemically clean and the adhesive or bedding compound is afforded a very good chance of adhering and staying adhered to the stainless steel. Keeping sea water (without the presence of fresh air/oxygen) from reaching the surface of the metal is the key to a long service life.

None of these problems happen "overnight" but instead over years. Just like steel chain or bolts that rust, they work fine until the corrosion advances to the point that insufficient metal strength exists, then the part fails.
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Old 12-08-2012, 07:56   #8
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Okay, thanks for all of that smart metal guys. I was telling my wife last night "you know, there aren't a lot of places where I can ask a question about the corrosion properties of stainless as defined by it's level of finish polishing". She shook her head and said, "normal people don't talk about things like that."

So what I'll probably do then is pull the plates in question and just see how bad they are. If they're pitted or showing signs of corrosion I'll replace them with the unfinished mil stainless and rock those for a few months until I can get properly finished bar stock later.

I have an angle grinder so if there are wheels I can buy that will do the polishing maybe advice me on some of those, but I don't have a full shop to work with and I'm a little pressed for time. In the fall I can work with a real metal fabricator.
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Old 12-08-2012, 08:15   #9
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

Or you could pull those plates and just polish them. Rebel Heart, send me a PM.
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Old 12-08-2012, 09:07   #10
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

RH,

You could counter your wife's derogatory opinion on CF nerds with, "ornithilogical specimens of identical plumage invariably congregate in the closest proximity"
Jus sayin.

Those blue discs with the overlapping segments are a good start on mill finish stainless, then go to the sophisticated polishes, IMHO.
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Old 12-08-2012, 09:08   #11
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

I know there are shops in S.D. that do polish work. If you supply the metal already cut and drilled the polishing would be a reasonable price. To buy chainplates made by a shop will cost ya.

The problem being is not all chainplates are the same or even typical. You would need to pull 2 or 3 plates just to get the dimensions and compare to each other.

SS is not an EZ metal to work with! Been doing it for over 40 years and still don't like it.
If you get it too hot it work hardens. You have to keep a coolant on it for long cuts.
If you don't have the right grade of tooling and the power to push them then it work hardens.
It'll ruin a hand file in no time.
You can only bend it once in one direction.
And it'll crack/brake if worked too hard.

I would prefer titanium, but it's tough to machine also and not available in different size/shapes as other materials.
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Old 12-08-2012, 09:41   #12
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Quote:
Originally Posted by delmarrey
I know there are shops in S.D. that do polish work. If you supply the metal already cut and drilled the polishing would be a reasonable price. To buy chainplates made by a shop will cost ya.

The problem being is not all chainplates are the same or even typical. You would need to pull 2 or 3 plates just to get the dimensions and compare to each other.

SS is not an EZ metal to work with! Been doing it for over 40 years and still don't like it.
If you get it too hot it work hardens. You have to keep a coolant on it for long cuts.
If you don't have the right grade of tooling and the power to push them then it work hardens.
It'll ruin a hand file in no time.
You can only bend it once in one direction.
And it'll crack/brake if worked too hard.

I would prefer titanium, but it's tough to machine also and not available in different size/shapes as other materials.
I have one of those flap wheels so I'll give it a shot regarding stepping up from the mill finish.

My plan is just to pull them one at a time and "fabricate" right on the dock. They're just straight flat bars with a tiny bit of bend at the top so I'm not stepping into something all that crazy. I've got:

- an angle grinder
- 1/2" drill
- various cobalt bits up to the size of the existing cp bolts
- hand files
- a healthy blend of stupid and determination

I think in the fall I'll replace them with silicon bronze and new stainless hardware. Just more project than I can take on at the minute. If the existing ones look like garbage even I replace them with unfinished stainless it will still be a big improvement, just not a permanent or proper one.
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Old 12-08-2012, 09:56   #13
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

+1

It does.

I believe I can see the well polished SS remain ss longest. The murky finish seems to last well too (is it etched or what?).

Both, when scratched, begin to rust where scratched.

b.
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Old 12-08-2012, 10:27   #14
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

The stock that has a ground finish seems to be the worst for rusting/corrosion. You can tell from the linear grinding lines on the finish... or in the case of tubing, somtimes the grinding marks go circumferential around the tubing. Suprisingly, the kind of rough finish that comes from the mill on hot rolled bar stock, doesnt seem to do too bad for rusting. Not sure about crevice corrosion over time though. Polishing to a very fine finish is best. Wow..... I wouldnt do it twice! IF you want bronze, just buy bronze bar stock and use as is! You are going to find that polishing those chainplates is not going to be easy at all. if not done well you might make them worse! Maybe there is a shop that's equipped to do it around there?
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Old 12-08-2012, 10:55   #15
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Re: Does Stainless Steel Finish Matter For Corrosion Protection Or Anything Else ?

Quote:
Originally Posted by Cheechako View Post
The stock that has a ground finish seems to be the worst for rusting/corrosion.
Anytime one uses a ferrous material/tool on SS it will absorb some of the iron and it has to be acid treated to get it out!
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