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Old 08-05-2015, 22:22   #1
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Preventing Stainless Steel from Pitting

So I'm new to salt water sail boat maintenance. I find that most places where I have close fitting stainless to stainless there is a rust film and/or pitting between the two. For example between the steering wheel and its shaft, between the control levers and their shafts, between any stainless clamped to stainless etc etc.
I've searched the forums and don't see any fix besides replacement. What I can't find is any way to prevent it from reoccurring in the replaced parts or getting worse in the existing.
Any suggestions?
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Old 08-05-2015, 22:46   #2
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Re: Preventing Stainless Steel from Pitting

You could fill the interface with a corrosion inhibiter. Lanocote, Tefgel, copper grease, spray lubricant, even vasoline.
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Old 08-05-2015, 23:01   #3
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Re: Preventing Stainless Steel from Pitting

For mounting clamps to pulpits, cover the rail with just enough vinyl tape for the width of the clamp before mounting.

For everything else, Tefgel. Especially dissimilar metals.
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Old 09-05-2015, 01:21   #4
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Re: Preventing Stainless Steel from Pitting

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Originally Posted by bcboomer View Post

[...]

I've searched the forums and don't see any fix besides replacement. What I can't find is any way to prevent it from reoccurring in the replaced parts or getting worse in the existing.
Any suggestions?
Have a look at This :

http://en.m.wikipedia.org/wiki/Passivation_(chemistry)

I suspect a passivation problem.
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Old 09-05-2015, 09:44   #5
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Re: Preventing Stainless Steel from Pitting

To Tufgel, copper grease, I would add nickel anti-sieze.
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Old 09-05-2015, 09:53   #6
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Re: Preventing Stainless Steel from Pitting

First you should think of stainless steel as "stain less", not stain free.

If the rust and pitting is between two stainless parts that are close fitting but will still allow water to enter then most likely crevice corrosion.

Stainless is stainless because of the chrome content of the SS alloy. The chrome reacts with oxygen in the air or water around it to form a hard surface of chromium oxide, Cr2O3. This layer is what protects the iron in the SS alloy and prevents rust.

When SS is in a situation where there is no fresh oxygen available to keep the oxide coating then the coating deteriorates and you get rust and pitting. In your case, between the shaft and hub of the wheel for example, water can get in but just sits and new water doesn't flush through that crevice. So the oxygen in the water is depleted, the oxide coating breaks down and corrosion happens.

You get a similar problem when you grind, weld or scratch SS. You can fix that by passivation, which removes free iron (which is what rusts) from the surface of the SS part and leaves the chromium oxide coating and your SS is again stainless.

So to fix the problem, as others suggested, fill the crevice with an appropriate goop.
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Old 09-05-2015, 10:18   #7
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Re: Preventing Stainless Steel from Pitting

You have to maintain it often for sure. There are a lot of products but I ended up just keeping one on the boat for fiberglass and stainless.....3M Finesse-It. The problem is it's hard to maintain places that are trapped or have crevices. For the steering wheel shaft you mention just remove the wheel (should come off easy on most) remove the rust with Finesse It or your favorite product.
When cruising, your lifeline stancions will rust often. Just clean them often with your product. For crevices etc put it on a rag and pull it thru back and forth like a rope.
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Old 09-05-2015, 15:45   #8
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Re: Preventing Stainless Steel from Pitting

Just redid chain plates and had them electropolished/passivated. Iron removed from surface of the metal, rust and pitting stops at least for a very long time.....30 years ish. They use this process in nuclear plants to keep radioactive particulates from sticking to stainless. No iron in contact so no rust.
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Old 09-05-2015, 17:35   #9
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Re: Preventing Stainless Steel from Pitting

for passivation and rust removal, i've tried oxalic acid and phosphoric acid...phosphoric acid is way faster/better...

but have never tried nitric acid yet!!! i'll keep that one for my next acid buzzzzz!!!!!
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Old 09-05-2015, 17:40   #10
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Re: Preventing Stainless Steel from Pitting

Quote:
Originally Posted by skipmac View Post

Stainless is stainless because of the chrome content of the SS alloy. The chrome reacts with oxygen in the air or water around it to form a hard surface of chromium oxide, Cr2O3. This layer is what protects the iron in the SS alloy and prevents rust.
I always was told it was Nickel and iron that made stainless?
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Old 09-05-2015, 17:59   #11
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Re: Preventing Stainless Steel from Pitting

A relatively new product is called Spotless Stainless. It's gotten good reviews, although I am sure responses here will point out its chemical composition and suggest alternatives.

I also use a cleaner waxer called Prism.

I also wax the pulpits when waxing the hull.

Cleaning, clean it, do whatever you choose, there are millions of products out there.
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Old 09-05-2015, 21:25   #12
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Re: Preventing Stainless Steel from Pitting

Phosphoric acid is the primary ingredient in diet coke. You might try that. Shines pennies right up.


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Old 09-05-2015, 21:35   #13
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Re: Preventing Stainless Steel from Pitting

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Originally Posted by capngeo View Post
I always was told it was Nickel and iron that made stainless?
Chromium is roughly 10-20% depending on the specific SS alloy. For example SS 316
Nominal Composition in Percentage:

Ni 12.00
Cr 17.00
Mo 2.50
Fe Balance
Si 1.00
Mn 2.00
C 0.080
P 0.045
S 0.030

Here's a link to a list of common SS alloys.

Stainless Steel Specification and Composition Chart - Engineers Edge
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Old 11-05-2015, 05:01   #14
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Re: Preventing Stainless Steel from Pitting

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Originally Posted by skipmac View Post

You get a similar problem when you grind, weld or scratch Stainless Steel.
IIUC, a part made of welded Stainless Steel must get protected by a passivator immediately. Old welded parts, such as U-Bolts, that were not passivated accordingly, may have already developed crevice corrosion before the new owner gets to passivate them, right?

OTOH, "working" (ie. grinding and scratching) parts must be re-passivated continuously, right?
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Old 11-05-2015, 05:26   #15
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Re: Preventing Stainless Steel from Pitting

So am I screwing up my old stainless steel fresh water tanks by sanding them down to shiny again, and fiberglassing over the pitting that runs along all the welds? I've been filling the little holes with JB Weld Marine, and then glassing a layer of cloth over the seams. The welds are getting brittle and eaten up.
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