Metal Finishing

[Alan Wheeler]

The entire reason SST is SST is that a Chromium oxide is created in the presence of oxygen. The Oxide stops any further oxygen from reaching the metal and thus stops any further corrosion. However, there are other metals in SST and Iron is the vast majority. If the oxide for some reason does not form over the iron, the iron will rust. This can then work the opposite direction by sealing off the Chromium from the air and the SST will then rust. Chrome is very hard. Much much harder than Iron. So polishing and I mean mirror polishing here, by either compounds or chemicals is all about removing as much iron from the surface and leaving the chromium there to do the job of protection.
Often an amature can get the polishing to 99% but the 1% left can still cause rust to occur. So after you have finished the job, go place it in salt water and leave it for a few days. Remove and let it rust. Once it starts rusting, apply Phosphoric acid to it. This removes those iron contaminations. If you can hit it with Hydrachloric acid or Nitric acid, even better. But becareful of those. This will greatly reduce the chance of rust retunring. But regular maintenance may be required once a year just to ensure it remains rust free.

[Charlie]

Didn't know it was so complicated. Maybe I should see if I can get it done. So here's a question for you. I am creating a mounting bracket for an attachment point for a Jordan Series drogue. I'm using 3/8 x 3 SST about two feet long. I have a backing plate made of the same material. I will need to attach it to the hull. It is like a big chain plate attached horizontally off the back of the boat. Of course all the penetrations will need to be sealed. I assume that will also keep all the Oxygen out. Will this cause the plate to rust?

[Alan Wheeler]

No because is the thing is sealed, I suspect no water will get in either. No water, not rust. Use a polyurathane sealant adhesive product. It moves and will adhere well to the SST and the hull.

[Wotname]

Note that Wheels said "no water, nor rust" rather than "no O2, not rust". Keep it completely dry and you are OK. If you can't keep it completely dry, make sure O2 can get to it, 'cause SS will get crevice corrosion in a damp, O2 deprived enviroment.

Please confirm if I am holding the right end of the cat here, Mr Wheels?

[Alan Wheeler]

Yep. Although crevice corrosion can be a two headed monster. It can also be an issue with dissimilar metals. Like poorly welded or even the wrong choice of electrode. But you still have to have water.

[cooper]

make sure O2 can get to it, 'cause SS will get crevice corrosion in a damp, O2 deprived enviroment.

and that is the important bit. Fully submerged in normal sea water should provide enough oxygen to keep corrosion at bay. SS shafts that dont get a continous flow of water past (like some older style stuffing boxes have water and no supply of oxygen) . SS is not a perfect solution to corrosion. A perfectly polished item with a crack at the weld or deck interface is prone to corrosion (crevis).

Sandvik Materials Technology - High technology stainless steel and special alloy materials
Go to technical centre and then to corrosion data. You will find the answers to every type of stainless steel corrosion there.

Another interesting type of corrosion is "pit" corrosion. A surface breach then allows a small pocket that in itself stops oxygen and the pit grows inwards and sideways. A similar thing to crevice. But dont under estimate the power of "galvanic" corrosion. Regardless of what type of boat it can cause problems much faster than any other form.
(did you know that timber ..yes timber..is prone to a galvanic reaction when in the presence of metals ?) it can cause the timber to go spongy ! Just what you need around say bronze through hulls.

[Wotname]

Originally Posted by cooper

...

Another interesting type of corrosion is "pit" corrosion. A surface breach then allows a small pocket that in itself stops oxygen and the pit grows inwards and sideways. A similar thing to crevice. But dont under estimate the power of "galvanic" corrosion. Regardless of what type of boat it can cause problems much faster than any other form.
(did you know that timber ..yes timber..is prone to a galvanic reaction when in the presence of metals ?) it can cause the timber to go spongy ! Just what you need around say bronze through hulls.

Gee Coop, just when I thought I had my act together, you come along differentiate between pit corrosion and crevice corrosion .

I always thought they were the same thing, now I know better - thanks.

Maybe going off topic (again) but that wood example you mention is interesting. As I understand it, the dissimilar metals can be quite some distance apart but providing they are "connected" by damp wood and under water, the galvanic circuit is metal, damp wood (conductor), dissimilar metal, with sea water becoming the other conductor. The electrons leave the wood at the annode end of the cell and thus the wood becomes spongy. This is why timber boats (not the epoxy encapsulated ones) should have all the metal through hulls well bonded to each other inside boat so the bonding wire takes the current rather than the damp wood. FWIW.

[Wotname]

Originally Posted by cooper

Another interesting type of corrosion is "pit" corrosion. .

An example, my 30 yr old cap shroud chain plates were full of pit corrosion on the side where they bolted against the timber bulkhead (very very slightly damp) and quite shiny on the "outside" surface.

[Alan Wheeler]

wire takes the current rather than the damp wood.

What actually happens is the wood produces and akaline which in turn breaks it down and makes it go soft.
SST in wet wood becomes deprived of oxygen and the wet timber produces a slight acid which then rusts the SST as if it was just plain steel.
Treated timber has Arsenic and Borate and other conductive metals. Placed against SST and more dangerously Aluminium, can cause the metal to be eaten away very quickly.

[hooked on water]

Wow talk about learning something new every day
Thanks for this info on Stainless, I never knew that depriving oxygen to it was possibly bad depending on the placement.
Will watch that very carefully with everything that I fit
I will also try my hand at polishing my fittings when I get to that
Cheers
Hooked