Rust marks around Stainless Steel fitting

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ferroboat

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We had Moonshine re decked in the summer of 2007 and all the stainless steel deck fittings put back in situ by a top notch shipwright with no problems. The engineer made us a new stainless steel U bolt with flange for the baby stay, which was not cheap, around £250. Last Saturday I noticed rust stains seeping out from under the flange. Any ideas what could cause this? The new deck is 2 sheets of staggered 12ml marine ply, epoxy bonded with grip fast nails, covered by fiberglass sheeting and scrim, sanded down and painted with Alwgrip. Thanks in anticipation
 
Ah well mate you see it's not stainless steel its actually stain resistant steel. If it is in an oxygen starved environment, the oxide layer doesn't build up and there you go, RUST.
Don't worry Vyv Cox will be along in a moment to give you a proper explanantion.
Cheers (get out a glass of the orthophosphoric acid stuff)
 
Sounds like you have crevice corrosion as saltwater hinted at.

Stainless requires oxygen to form a fine layer of oxide over the surface to stay shiny.

With a lack of oxygen in a crevice for example where water or air are not readily available you get corrosion.

You need to either open it up to air or bry it and seal it with a sealant.

Remove , clean it up, dry it and refit witha a sealant Sika or similar.

keep the water out and you should be ok .
 
As far as I know, true crevice corrosion on its' own doesn't usually give rise to rust staining, which is not to say that crevice corrosion is not present, but there is something else going on as well. Most stainless steels will show signs of rusty coloured stains when in contact with or close to rusty carbon steels. 304 grade stainless will show the same discolouration without the presence of any other sort of steel.
You could try the magnet test - 316 grade is almost non-magnetic, and 304 is similar to ordinary mild steel. You might find the fitting itself is 316, but the bolts and washers are 304.
(Actually there are a lot of variants of these grades, but most folk just refer to these two. When talking about fasteners, 316 is called A4 and 304 is called A2. A metallurgist would pick holes in this explanation, but for simpletons like me it's enough to get by on).
You'd be unlucky if it was crevice corrosion after only a couple of seasons as a deck fitting, and it might be that changing the fixings and bedding the fitting down properly on Sikaflex will cure it.
 
it could also be due to a lack of proper pickling after welding, any grade of stainless stainless will show signs of resting if not pickled.

Standard pickling of stainless steel involves the application of and acid based paste over the welded area, it left to stand then washed off. During welding the irin content is brought to the surface, the acid works in the surface contamination.

Hope this helps.

Avagoodweekend......
 
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316 grade is almost non-magnetic, and 304 is similar to ordinary mild steel

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No both 316 and 304 are considered to be non magnetic. They can become a little magnetic due to mechanical working.
 
As many have said, it sounds very much like it's crevice corrosion. The classic illustration of the problem is a single drop of water lying on a flat surface. Oxygen levels at the periphery of the drop are high, wheras at the centre of the drop they are lower. This difference represents a galvanic cell and electrons are passed between the two areas, partially through the metal.

Now transfer this idea to a crevice in a metal component. It may be a fitting bolted to a surface, a cut thread, a washer under a bolt head, micro cracks resulting from the hard drawing of wire cable, or countless other situations. Deep inside the crevice the water is 'stagnant' and oxygen is not replenished. On the surface there is plenty of oxygen. The oxide film on the metal at the two points has differing potential, thus creating a galvanic cell. The degradation product of the cell is an iron hydroxide, commonly known as rust!

Eliminating the crevice will overcome the problem, easily said but not always easily done. For fittings a good sealant will exclude the water. Polishing cable will help to remove the micro cracks. For threads there is not much that can be done except coating with Lanolin, grease or something similar, which may not be convenient.

The problem occurs in most metals but is particularly annoying in stainless steels that are supposed to be partly decorative.

All 300 series stainless steels are non-magnetic. Heavily worked 304 can acquire some magnetism although the level of working needed is probably more than the vast majority of yacht components ever see. Even shroud cable is given a remarkably small amount of cold work, just a few percent, to increase its strength to the required level.
 
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