all about rusty stainless steel 316

tcm

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i'm having a moderate run-in with a new boat supplier about rusty stainless steel. They said "it's 316" and I said "ah but..." and having explained to theme the rudiments of the problems, they're gong to sort it out.

Essentially, stainless steel is steel with additives, and these aditives are such that a layer of chroium oxide forms on the surface - hence "stainless". Once the surface layer has formed, the steel is known as 316(passive). Otherwise, its 316(active).
Technically, 316 shouldn't rust - if it does, the passive layer has broken down.

Breaking down the passive layer does two things :

firstly, the material is less resistant to rust: so the ends of a gurdaril wire covered in plastic rubs away at the s/s underneath, removes the passive layer and hence rusty ends of guardrail wires, rough/sawn stainless going rusty at the end, a light sprinkling of rust over a whole component, and so on.

Secondly, it's a diferent material, so with an active 316 and a passive 316 touching each other, then together with seawater as electrolyte, there'll be galvanic corrossion - you'll see this as perhaps one stainless steel nut and bolt going rusty when everything else is fine.

the passive layer isd broken down by
1) poor manufacturing- by introducing carbon impurities. drilling holes with high-carbon drills, cleaning with steel wool, using tools that have previously been working on mild steel, especially bending dies and so on. Big problem for any factory producing mild steel and stainless steel components.

2) fretting at the passive layer whilst in use; anything that rubs on the stainless steel may remove that passive layer. So stainless steel conectors/fasteners should always be tight, to avoid movement, and stainless steel chains or chained connectors may not resist rust for long in the presence of seawater. If possible, two pieces of stainless steel should be isolated with plastic washers, but movemnt (eg under o-rings) can remove that passive layer.

3) poor assembly: engineers have tendency to clean all parts before assembly - but should not do so with stainless steel. Once passive, the parts should be greased up and isolated with plastic washers, clingfilm if out of signt, and so on.

4) poor maintenance: of course, if a problem has not occurred as above, then maintenance should be minimal. But any maintenance and aftercare must avoid use of carbon steel: removing rust from stainless steel should be with a plastic or fabric but NOT wire wool or mild steel brushes which will make rust return even faster/worse. Use ceramics whwever possible. If you use a hackssaw to cut stainless steel, expect the sawn araea to be active, and remove steel deposits from the blade by grinding down, polishing as below.

Banishing rust from stainless steel 316:
1. The surface layer of impurities must be heavily ground off, with a polishing/buffing wheel and paste, using a series of new polishing wheel covers and repeated washdowns, returning a high polish to the surface.
2. Diluted nitric acid will dissolve anyremaining carbon impurities and allow the passive layer to re-form.
3. Light oils/wd40 etc will protect the general surface
4. Expect occassional small dings and bashes from tools to remove the passive layer: re-treat the area as above

Please respond if you have usefull additions and experiences
 

Chris_Robb

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Re: Standing Rigging

I had a complete set of standing rigging go rusty within 6 months of installation. Supplier suggested I poilished it to remove the rust!

Advice I had was that although the steel quality was good, manufacturers often ruined their product becasue the wire was not sufficiently polished to make it passive, so when twisted together - rusted.

They finnaly agreed to change it all - after solicitors - no rusting on new stuff.
 

richardandtracy

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Good advice.

Just a couple of things to add.
1) There is a commercially available pickling material for stainless (no need to muck about with neat Nitric Acid). You can get it from most suppliers of welding consumables. The one I can get easily is Polinox-P, but at £24.66 for 2kg I'm going without until I can find a cheaper supplier.
2) Remember grade A2 bolts are 304 type stainless, grade A4 are 316 type.
3) With harmonisation of standards in Europe, 304 may soon be referred to as 1.4301 and 316 as 1.4304 (nice, easily remembered designations?!?)

Regards

Richard
 

tcm

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More info: s/s 316 v 304, & telling them apart

I have found more info from a world expert metallurgist.

Firstly, he says, 316 should always be used in an open marine environment - exposure to liquid chlorides (seawater) would make this a mandatory requirement. You are on safe ground to go potty and demand replacements if you find deck fittings in 304 instead of 316.

Easy test - 304 is magnetic, 316 isn't. Get a little magnet and test. Some cheapy knives and forks i found are magnetic - hence 304. Cleats and guardrails (at least, these guardrails) aren't magnetic - hence likely to be 316.
 

DepSol

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Matt

Just spray the rails with a light coating of T9 to ensure protection against rust. Developed by Boeing and has a money back guarantee.

Dom

I am boating again ;-)
 

BarryH

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Na! what I suggest on tcm's boat, wire brush the lot and coat in hammerite, It'll look the mutts nutts!!!

OK, to hell with it. Unbolt it and we'll use it as an anchor!
 

BarryH

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T9 is available at your nearest Boeing sevice agent/dealer. If you can't find one of them, try Nauticalia in shepperton, they also do mail order. It aint cheap tho, but it does do exactly what it says on the tin.

OK, to hell with it. Unbolt it and we'll use it as an anchor!
 

ean_p

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are you sure that its rust or not just discoloration.....and as to the knives in 304.....well if you want them to maintain an edge you have to accept the price...there is a great deal more to stainless then the simple matter of it being ostensibly rust free......it is an alloy like any other alloy and it's various forms should be chosen with care to suit the use to which it is put.....incidently I spent 6 months as an apprentice in a 'copper smiths' workshop ( they work with non ferrous metals) on a leading petro chemical plant......by and large bollocks about using steel and the rest of it to work or touch stainless....and the same as regards machining it....HSS and the right angles and speeds.....and as to tanks rusting through....try putting the inert gas on the inside aswell while welding.....stainless is great...it just needs to breath..
 
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I have experienced some "rusting" on some stainless on my boat....on the chainplates. I used "Wichinox" made by Wichard of France to re-passivate the stainless steel as well as clean it. It contains the acids - one of them phosphoric, that are needed to re-passivate the stainless steel. Here in the states a container of it costs about $12.00. Sorry I don't how much that is in UK Pounds. It worked quite well. My boat is 27 years old and the chainplates looked less than perfect. Look pretty good now and no problems with the "rust" returning in the 5 months since I used the Wichinox.

Reality is the cause of all stress!!
 

kingfisher

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Cheap manufacturing

After construction or manufacturing, the producer has two polishing option:
Polishing mechanicaly: lasts longer, but takes a lot of manpower (ergo £€$)
Polish Electrolyticaly: Just dump it in a bath and let it sit for a while. less labour intensive, good for mass production, but starts to 'rust' relatively soon.

Just guess what 9 out of ten producers use nowadays ?



Group of people on the pontoon: skipper is the one with the toolbox.
http://sirocco31.tripod.com
 

tcm

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Re: by and large bollocks?

Cheers ean. Unfortunately, totally tripe, but never mind. There are others with far more knowledge of the metallurgical issues. Making cutlery in 304 is perfectly acceptable.
 

tcm

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Re: electropolishing

I understood electropolishing is *better* than mecahncial polishing? Both methods remove material, but electropolishing preferentially removes carbon, iron and nickel impurities, no?
 

vyv_cox

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Not quite all about rusty stainless steel 316

A few misconceptions here. 304 stainless steel is 18 Cr, 8 nickel, balance iron. Its phase structure is austenite, referred to as austenitic. Its general corrosion resistance is provided by the chromium content. 316 stainless is the same composition plus the addition of 2% molybdenum for increased resitance to pitting in chlorides. Both are non-magnetic.

Carbon content needs to be kept low for welded construction in corroding conditions. These alloys are designated 304L and 316L. Otherwise the normal carbon content is perfectly OK.

300 series alloys are good in general corrosion but are susceptible to crevice corrosion. This accounts for staining commonly seen around small gaps, such as threads, under fittings, etc. Where staining occurs on shroud wires and similar components it is caused by very small cracks present in the surface, due to wire drawing with poor or insufficient lubricant. Polishing helps to remove these, so is a "good thing".

Stainless steels passivate in air within a very short period from being made. An active surface is only present when chemical action prevents the formation of the passive film, crevice corrosion being one of these cases.

Use of plastic washers, sealants and the like is useful because it prevents the formation of crevices, for example I fill my shroud swaged fittings with hot grease or Waxoyl to prevent entry of water. This is a prime point for crevice corrosion.
 

kgi

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Well i've been on to some experts as well, it would appear that most prop shafts are 304, a lot of your deck hardware is as well on the grounds of adequate performance at an affordable price, 304 is also15% stronger, so for all you people out there that want 316 as standing rigging the recomendation is to go up one size, it would appear that the best stainless for rigging is Nitronic 50 or aquamet 22 but you will pay through the nose!!!!! From what i can gather use 316 fasteners below the waterline forpintles anode bolts etc, but 304 is acceptable above the waterline, indeed on many production boats its the norm..........keith
 
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