304 vs 316 stainless steel

[hpeer]

I had a bad rust spot around where the rudder stock comes through the Steel deck, it had been capped with plywood, duh! I cut an approximate 8” donut with a ~2” hole out of 1/4” 316 plate using a hole saw for the middle and an angle grinder. Came out surprisingly well.

What really shocked me was a needed about 8” of a special diameter tube. I was able to make it out of some (1/8”?) plate by bending and beating and working it. Took an hour or so and in the end I held it with some hose clamps while I welded the seam. I then cut it on an angle to match the rake of the rudder.

I didn’t bother to polish it up so it looks a bit rough, but the darn thing actually fits really well and works far better than the original solution by the “pros.”

Once in a while I get things right. It’s not often so I gotta brag when it happens.

[coopec43]

I haven't read all the posts but I buy all my SS fasteners on eBay and 316 would probably be 20% more than the cost of 304.

The cost of a 304 Hex Bolt M10 50mm - $4.50
The cost of a 316 Hex bolt M10 50mm - $5.40

https://www.ebay.com.au/itm/M10-M12-...aURrZfy78MA94Q

[chasmains]

Originally Posted by artisanmach

I use weld tint remover from the welding supply store. it's a thickened nitric acid paste. simply paint it on with an acid brush, wait 30 min then rinse it off. it completely removes all the rust spots. Wichard also makes a passivation paste, it seems to be a much weaker solution and does not work as well.

Totally agree with this sauce treatment. I use it too when welding 304 or 316. It’s great for small jobs like welds or grinding touch ups. I think the point being missed by many is that embedded iron is NOT taken out easily. No way! It is at the molecular level and above but below the visible level. Any tool that is iron based or has touched iron will cause embedded iron in SS. Includes tubing extenders, benders, SS wire brushes, grinding wheels, threading tools, drill bits, and everything else. Polishing with tools that may have touched iron are to be considered contaminated. This means that any SS part, if not passivated or confirmed to be passivated, will likely show rust spots. So demand passivated parts if you don’t want the problem.

I don’t trust anything from bulk commercial suppliers like Amazon, eBay. I do trust McMaster-Carr if they state that passivation has been performed on the parts you order. (You can email them to get confirmation, too. You can get certifications from them and others if you want to pay for them.) This is always iffy for materials not under control, like salvage parts.

Note that I am not discussing the differences between 304 and 316 grades, which is inherently chemistry related.

[georgelewisray]

Deep/hidden Crevice Corrosion:
Don't know much about surface corrosion/rust-stains of 300 series CRS but mostly I am concerned about deep/hidden Crevice Corrosion . . . . .

I have seen stainless (not 316) fail in places where it is buried and exposed to salt without any oxygen, such as fastenings that pass thru the deck and they are under strain and in a constantly wet place such that the bedding may be compromised and then allow salt water to intrude.

The most dramatic failure I has seen was a West Sail 32 backstay fitting bedded to the stern 'pushpit' that failed and led to a dismasting. The old stainless weldment when the remains were removed looked like swiss cheese in the hidden places were salt water creeped in and sat for long periods. I fabricated the new fitting/weldment from McMaster-Carr 316 plate and TIG welded it with 316 rod.

[captain38]

Originally Posted by a64pilot

304 if it’s well passivated and well polished will do fine.
I’ll give you as an example Kato Marine, who’s davits, Radar Poles etc. are among the more expensive ones, but have a very good reputation, they are also mostly 304, but well passivated and highly polished.

300 series SS is 300 series SS, to see much difference you need to change series. I’m not saying they are identical but I am saying that the difference isn’t as much as many think and passivation and polish as well as how it was tooled has as much to do with it as whether it’s 316 or not, run 316 tubing through steel dies which is how it’s made, don’t passivate it well and polish, and it will rust profusely, from the steel contamination from the die of course, same for wire, maybe even more with wire.

THIS answer is exactly right. Further, it must be noted that the strength characteristics of rigging wire play an important part in the decision about 302/4 vs. 316. For my money, I would not buy 316 for stantions or deck hardware that is not under stress, as the surface corrosion factor is mostly a matter of how the parts are tooled and passivated, as noted above.

But for rigging wire the answer is more complex. Strength is paramount. Rigging wire is in motion, subjecting it to work hardening through vibration. Further it is subjected to intense iron contamination in manufacturing as it is squeezed through dies... and even further, 316 is 15% weaker and somewhat softer than 302/4. The lower strength may require upsizing, while the softness may allow longer use before work hardening destroys the cable integrity.

Tough decisions for a tough job!

[blazing928]

From our commercial real estate perspective, I was told that most rust seen on Ss railings was air born particles. It would require periodic cleaning to ensure no rust stains.
Obviously other posters with more experience have noted iron contamination and passivated as a better solution.

Our rear stern rail is electro polished and I wonder if that also stops rust stains as it’s apparently very smooth?

I coat Ss rails in a nano ( just love that term) polish.

[a64pilot]

Originally Posted by georgelewisray

Deep/hidden Crevice Corrosion:
Don't know much about surface corrosion/rust-stains of 300 series CRS but mostly I am concerned about deep/hidden Crevice Corrosion . . . . .

I have seen stainless (not 316) fail in places where it is buried and exposed to salt without any oxygen, such as fastenings that pass thru the deck and they are under strain and in a constantly wet place such that the bedding may be compromised and then allow salt water to intrude.

The most dramatic failure I has seen was a West Sail 32 backstay fitting bedded to the stern 'pushpit' that failed and led to a dismasting. The old stainless weldment when the remains were removed looked like swiss cheese in the hidden places were salt water creeped in and sat for long periods. I fabricated the new fitting/weldment from McMaster-Carr 316 plate and TIG welded it with 316 rod.

I would argue that most if not all “stainless” steels are not suited for the purpose you speak of, Titanium is not that much more expensive, not really. I think my Grade 5 titanium chain plates (8 of them) were about $200 more than they would have been if made from 316L.
I put stainless in parenthesis as I think stainless is a name like plastic, it means different things to different people.
Aircraft industry doesn’t recognize the name stainless, instead we had corrosion resistant steels, many of which of course were stainless, but not all.

[a64pilot]

Originally Posted by blazing928

From our commercial real estate perspective, I was told that most rust seen on Ss railings was air born particles. It would require periodic cleaning to ensure no rust stains.
Obviously other posters with more experience have noted iron contamination and passivated as a better solution.

Our rear stern rail is electro polished and I wonder if that also stops rust stains as it’s apparently very smooth?

I coat Ss rails in a nano ( just love that term) polish.

Much of stainless is contaminated when being cleaned.
Remember the Delorean car? Well it went through many, many complaints of it rusting and the reason was determined that the idiot dealers were telling their customers they could clean the car with a Brillo pad, so of course they were imbedding the brushed stainless steel body panels with itty bitty bits of carbon steel.
Same thing happens a lot with stainless steel handrails.

The Delorean was 304, Musk won’t tell us what the Cybertruck is, he apparently say’s it’s ultra hard 30X stainless. Maybe that X is a 4?

[Cheechako]

Regarding iron, both 304 and 316 are ~ 70% iron. If it's not treated properly in fabrication, it will rust. What matters is a super smooth surface and the grain orientation. Either become magnetic when the grain orientation changes from being cold worked or welded etc. Solution heat treating/annealing puts all the grains back in place. Or you super polish the surface so rust/corrosion cant take hold in the grain boundaries.

[thinwater]

Originally Posted by Red Sky

I replaced the original curved traveler on a ‘85 Catalina 30 with a straight Harken traveler track mounted on 6’ x 1 1/2” sq. 304 s.s. with 1/4” plate of the same forming the legs and feet. I sanded the raw material after cutting with a belt sander progressively up to 600 grit. Cutting the 1/4” x 4” plate with a 12” metal chop saw was an unnerving experience. It was then welded and taken to the polisher. I used Wichard Wichinox to passivate after installation. It was usually the shiniest thing on the boat.

I don't think Wichinox can be classified as a passivizing treatment. It is just a cleaner.



Only Nitric acid and citric acid have protecols for passivating stainless. I contains only sulfonic and phosphoric acids, plus fine abrasive--a polish and cleaner.

[thinwater]

Originally Posted by captain38

...316 is 15% weaker and somewhat softer than 302/4....

I've heard this, but I'm not seeing it. I've looked up specs from several sources, and 316 is generally the same or stronger than 304. Please post the data.





AISI Type 304 Stainless Steel
Physical PropertiesMetricEnglishTensile Strength, Ultimate505 MPa73200 psiTensile Strength, Yield215 MPa31200 psiElongation at Break70 %70 %Modulus of Elasticity193 - 200 GPa28000 - 29000 ksi
AISI Type 316 Stainless Steel, annealed sheet
Physical PropertiesMetricEnglishTensile Strength, Ultimate580 MPa84100 psiTensile Strength, Yield290 MPa42100 psiElongation at Break50 %50 %Modulus of Elasticity193 GPa28000 ksi

[Red Sky]

Originally Posted by thinwater

I don't think Wichinox can be classified as a passivizing treatment. It is just a cleaner.



Only Nitric acid and citric acid have protecols for passivating stainless. I contains only sulfonic and phosphoric acids, plus fine abrasive--a polish and cleaner.

You should probably let Wichard know the phosphoric acid in their product that claims to “Clean & Passivate”, doesn’t passivate after all.

[MartinR]

On my boat basically everything is made from 316, except the rudder shaft, which is 304. There are no problems with either. All original stainless steel is in perfect condition.

But, some later additions tend to develop rust. Probably because they were not properly passivised, or the steel was of inferior quality.

We had a lot of problems at work with Chinese 316 steel, where impurities led to interior oxidation and ultimately breaking of components. I guess you get what you pay for. For me lesson learned, avoid Chinese stainless steel altogether.

[Icarus]

Originally Posted by captaingregger

Agreed. I’m certain on using 316 but since he said he’s had clients use 304, I thought I’d ask and see what’s up.

What I can’t understand is the price difference being so massive. Probably a typo in his WhatsApp text.

I don't know where you buy your SS from. The cost of manufacture of 316L is slightely higer then 304. Certainly not double.
A 4m lenght of 25mm x 1.6mm anealed tube for railings is about 100 AUD.
For making fittings I usually buy a halve sheet of 4mm plate which cost about twice as much as steel.