Stainless steel?

[Laundryman]

I am not an engineer so when I buy something for the boat and it says, stainless steel, I don't expect it to rust.
My current gripe is the fixed outboard bracket I fitted last year has left streaks of rust. The nuts and bolts I bought from the chandlery have also rusted and I'm sure they said stainless when I bought them. I took the bracket off to replace the plywood pad which has delaminated but I'm now thinking of replacing the whole lot. How do I make sure the replacement doesn't go the same way please? Thanks. Alan

 

[vyv_cox]

Stainless steel for this type of application, seeing constant exposure to seawater, should ideally be 316, also known as A4. Bolts are usually marked on the head but nuts are not. Your bracket may only be 400 series, which has lesser corrosion properties. 300 series is not attracted by a magnet, whereas 400 series is.

Rust streaks may be a consequence of crevice corrosion in surface roughness, exposed threads, joints between washers, nuts and bolted surfaces. Roughness can be polished out but the others are inevitable with these alloys unless the crevices are sealed to exclude water.

 

[TonyBuckley]

I had to find all this out few months back.

Wiki has some very good entries on stainless steel grades; seems a big subject.

But as above, 316 is the only one that has Marine usage in the description and products using this seem to like to brag about it. My rule is if it doesn't say, then it is not 316 but a cheaper type.

 

[JClarkes]

My rule is if it doesn't say, then it is not 316 but a cheaper type.

You are 100% correct

I sell a lot of stainless steel products in mainly 316 stainless as I aim at the marine sector. There are certain fasteners that simply do not come in 316 but 304. As for the reason above I always state what grade it is because I am not out to earn a quick pound in hope that the customer forgets who he bought from when it starts to rust but I try and aim my customer service for the repeat customers. Okay it may be slightly more expensive compared to 304 stainless items but your buying a better grade.

As Tony said, if it does not state 316 stainless then its not 316 stainless.

I have a short blog on stainless steel here which you might find interesting

http://www.jclarkemarine.com/blogs/news/6802954-stainless-steel

 

[pcatterall]

I see references here to 316, 304, 300 series and 400 series I don't understand the link between the 'exact' numbers and the series concept ( I would have thought that 304 belonged in the 300 series for example)

 

[William_H]

I would advocate OP attack the bolts and rust stain with oxalic acid or a rust converter type acid. This may be all that is necessary to remove those slight surface stains that seem to occur on SS anyway. After that he can decide oif he reallyw ants to replace all the bolts good luck olewill

 

[Talulah]

I had one chandler a little bit miffed when I took a magnet in to check the grade of stainless.
The nuts/bolts etc had been bagged up with the chandlery's own label so I checked the grade first.
Grades were mixed so worth taking in the magnet.

 

[vyv_cox]

I see references here to 316, 304, 300 series and 400 series I don't understand the link between the 'exact' numbers and the series concept ( I would have thought that 304 belonged in the 300 series for example)

Just as simple as you suggest, 300 series alloys are austenitic, i.e. will have at least 8 parts of nickel and usually 18 of chromium, and will have a number between 301 and 399. In fact there are not quite that many. 400 series are ferritic or martensitic, i.e. will have less than 8 parts of nickel and usually less than 18 parts of chromium. Common ones are 410 and 413 but again there are many more. There are several other types, e.g. precipitation hardening and duplex.

 

[Graham_Wright]

Bolts are usually marked on the head but nuts are not.

Never thought I'd see the day but!

Whether they are marked or not depends on the screw or nut form. For instance, on the MastaClimba the M10 nut (inside) is marked A4 whereas the nyloc M6 nut on the strap is not marked (it also is A4) as there is insufficient room due to the insert. Likewise the pan head screw, which is also A4 is not marked as it would spoil the cosmetics. Hex head bolts in my experience have always been marked and have always been available in A4 and A2. All the plain nuts, from M4 to M30, on my boat are marked. I have never been able to source A4 woodscrews (although I have never tried very hard). They have always been A2.

I also supply Morgan cars with components and they specify 304 and will not accept 303 as it is susceptible to rust especially in salty environments.

The magnetic test is not completely reliable. Even 316 can be made (slightly) magnetic by hammering (try it!).

If the material is strongly magnetic it is probably ferrous. If it is slightly magnetic it could be 303. If it is very slightly magnetic it could be 304. If it is apparently non magnetic, it is probably 316. (Your kitchen sink, if stainless, is probably 304 - try it with a magnet for comparison).

Rust stains on stainless can be caused by contamination from ferrous materials during manufacture (saw blades, grinders and the like). They can also be caused subsequent to fitting by contact from ferrous materials.

The best guarantee is probably to use a reliable source for materials and fasteners. The 316 sheet and tube I buy is always laser marked with the grade (and batch number etc.).

 

[vyv_cox]

The magnetic test is not completely reliable. Even 316 can be made (slightly) magnetic by hammering (try it!).

If the material is strongly magnetic it is probably ferrous. If it is slightly magnetic it could be 303. If it is very slightly magnetic it could be 304. If it is apparently non magnetic, it is probably 316. (Your kitchen sink, if stainless, is probably 304 - try it with a magnet for comparison).

Rust stains on stainless can be caused by contamination from ferrous materials during manufacture (saw blades, grinders and the like). They can also be caused subsequent to fitting by contact from ferrous materials.

The best guarantee is probably to use a reliable source for materials and fasteners. The 316 sheet and tube I buy is always laser marked with the grade (and batch number etc.).

I deliberately did not mention quasi-magnetism in 300 series as it confuses people. The effect is extremely weak (and yes, I have tried it many times) barely detectable even with a strong magnet.

Most domestic cutlery and many sinks are made from 400 series and will be magnetic. The sink on my boat is not magnetic but it was a particularly expensive one. My Victorinox knife, marked 'stainless' is magnetic because 400 series is stronger and holds an edge better.

I have a large box of stainless nuts and bolts, all bought from a reliable source. I agree that pan head and countersunk set screws are not marked but hex bolts all are. None of the nuts is marked.

 

[savageseadog]

I think there is a world wide issue with lower grade stainless and other materials being uprated fraudulently

 

[Amulet]

You are 100% correct

I sell a lot of stainless steel products in mainly 316 stainless as I aim at the marine sector. There are certain fasteners that simply do not come in 316 but 304. As for the reason above I always state what grade it is because I am not out to earn a quick pound in hope that the customer forgets who he bought from when it starts to rust but I try and aim my customer service for the repeat customers. Okay it may be slightly more expensive compared to 304 stainless items but your buying a better grade.

As Tony said, if it does not state 316 stainless then its not 316 stainless.

I have a short blog on stainless steel here which you might find interesting

http://www.jclarkemarine.com/blogs/news/6802954-stainless-steel

Thank you for that blog - it clarifies much stuff slopping around in my head. Many suppliers quote A2 and A4. I have been behaving as if A2 is 304 and A4 is 316. Is that correct? I find that A2 is fine in a marine environment above the waterline - my bodger observation, not systematic data. A4 below the waterline. I've managed to break A4 coach screws but not A2. Is that just coincidence or is it more brittle?

 

[vyv_cox]

Thank you for that blog - it clarifies much stuff slopping around in my head. Many suppliers quote A2 and A4. I have been behaving as if A2 is 304 and A4 is 316. Is that correct? I find that A2 is fine in a marine environment above the waterline - my bodger observation, not systematic data. A4 below the waterline. I've managed to break A4 coach screws but not A2. Is that just coincidence or is it more brittle?

The more common 300 series alloys are remarkably ductile, with a yield stress only about half of the UTS. It is easy to overload and break them but not because they are brittle. The only difference between A2 and A4 (304/316) is that the latter has 2% molybdenum added to increase pitting resistance. There is a minor change in mechanical properties but nothing significant.

 

[Amulet]

The more common 300 series alloys are remarkably ductile, with a yield stress only about half of the UTS. It is easy to overload and break them but not because they are brittle. The only difference between A2 and A4 (304/316) is that the latter has 2% molybdenum added to increase pitting resistance. There is a minor change in mechanical properties but nothing significant.

Thanks for the clarification and your web site. While we have your expertise engaged...

Does this mean that the yachtie insistence on A4 over A2 is justified due to the benefits of the molybdenum?

Does the frequently-encountered lore that A2 is ok for marine but non-immersed, while A4 is required for immersed applications have any validity?

(I have scads of above-the-waterline A2 fastened stuff that seems fine after 10 years.)

 

[JClarkes]

Thanks for the clarification and your web site. While we have your expertise engaged...

Does this mean that the yachtie insistence on A4 over A2 is justified due to the benefits of the molybdenum?

Does the frequently-encountered lore that A2 is ok for marine but non-immersed, while A4 is required for immersed applications have any validity?

(I have scads of above-the-waterline A2 fastened stuff that seems fine after 10 years.)

I am no expert in SS. Just put a small blog up as I am often asked about the differences.

To answer your question I would say the addition of molybdenum and increase in nickel are the main benefits. Personally I would try and fit 316 anywhere on your boat when possible.

Salt is not just found on marine environments, but there is a trace in rain water, costal spray, de-icing salt on the roads.

Just to add confusion to the thread, canopy fasteners are most commonly made from brass and are nickel plated. There are a few 304 stainless fasteners but I do not beleive there are any 316. When I say fasteners I refer to Turnbuttons, Lift the Dots, Snap fasteners, Tenax. Most canopy designers will use the genuine DOT brand which are mostly manufactured in BNP.

 

[Halo]

Just to add to the very well informed contributions so far on this thread
1. Stainless is only stainless because a thin protective layer of chromic oxide naturally forms at the surface
2. If rust (iron oxide) gets onto the surface of stainless it will break the protective layer and cause rust, it can act like a wick to draw iron from within the metal (all stainless is mainly iron)
3. There can be iron oxide particles within stainless so the best products are chemicaly "passivated" to remove any surface rust and give a really good chromic oxide layer but all this can be undone by iron contamination - grinding dust from neighbouring boats being a prime source
4. if stainless is separated form the atmosphere and has no oxygen to help restore the chromic oxide layer it can rust badly - especially in crevices etc
5. Stainless on a boat (even 316) will benefit greatly from an occasional polish as the build up of muck from the sky, sea spray and other boats can start the rusting process
Hope this helps
Martin
is within tof any rust put onto the surface of stainbl

 

[vyv_cox]

Does this mean that the yachtie insistence on A4 over A2 is justified due to the benefits of the molybdenum?

Molybdenum is added to reduce pitting, which means it also reduces crevice corrosion that occurs by the same mechanism. There is some information about the mechanism on my website under Metallurgy. Pitting is unsightly but more importantly it reduces the cross sectional area of the component, sometimes to a remarkable extent. Strength can be severely compromised when pitting takes hold. 316 is used for the majority of drive shafts, where it successfully resists corrosion for extended periods. In examples like this, where crevices are almost absent, it does seem to demonstrate the good properties of this alloy sub-sea.