Stainless steel is not 'rust proof' it is mostly rust retardant, it has a chromium coating that reduces oxidization but does not stop it completely. The more chromium, the better the rust resistant properties.
If you want (much) better rust resistance than that you are going to be into coatings or active protection such as cathodic protection using a sacrificial material.
Coatings work well as long as there is no mechanical abrasion of the coating, cathodic protection works very well until you run out of sacrificial material.
I was not familiar with these grades, very interesting reading and finding out about them. The most common grades of stainless I have encountered in the marine industry is either 316 (best) or 304 (budget, requires regular cleaning to keep rust free). Both have issues with crevice corrosion and are also very expensive compared to zinc-plated equivalents (which I would never use on a boat since they will effectively disintegrate over a few months). Marine grade stainless often suffers from crevice corrosion and hard to detect failures which is sort of terrifying when you really think about it (things look OK until one day they break without clear visible warning).
For bridges or large-scale industrial applications where you dont care how the metal itself appears, I agree that coatings (especially galvanization) is the best bang for the buck. All of my insights are purely anecdotal though, as a hobbyist...
I worked for a sailmaker in the Netherlands who also did rigging. You really get to appreciate the difference if you see saltwater exposed rigging after a year or two from slightly different steel formulations, one still pristine, the other looking as though someone is pulling a bad joke on your with rust colored paint. Also: maintenance made all the difference, people that would immediately fix a small issue were often able to arrest it without further damage. But ignore it for a while and one thing leads to another.
edit: as to your point regarding crevice corrosion: yes, that is very nasty indeed, especially because a visual inspection will typically turn up nothing out of the order, all it takes is a bit of trapped moisture and some time.
most objects like this I have passed in the water seem to be made of some sort of painted steel. I think this is a good combination of durability vs economical. I suspect your electronics will go long before anything corrodes away to nothing, since in my experience its always the electronics that fail first (even when properly sealed for marine use).
Note that the main issue with any barrier coating (assuming there are no collisions) is going to be due to sea life living on/underneath your object and slowly breaking down the material. A copper based metal (read bronze) could be interesting (but expensive) since it tends to stand up fairly well in marine environments and has biocidal properties that prevent fouling/growth. Also very clear to visually inspect since green means good and red means bad which I always thought was very easy to remember.
edit: re-reading your initial comment it looks like you are describing an autonomous vessel of some sort. I used to work at a company that produced such devices and they were made out of composite plastics (which have their own issues that cause them to break down eventually in the water as well).
I'd look at the nickel alloys, Inconel 625 and Hastelloy C22 would be a good start. They should be orders of magnitude more corrosion resistant than stainless steel and still have respectable strength. Of course they are more expensive and harder to fabricate.
"Stainless steel is not 'rust proof' it is mostly rust retardant, it has a chromium coating that reduces oxidization but does not stop it completely."
Just to be clear ... you can indeed coat things with stainless steel.
However, most stainless steel objects (like screws or bolts or tools, etc.) are stainless throughout - you cannot abrade or scratch into a non-stainless inner core.
This is unlike, for instance, galvanized hardware which is merely steel with a coating over the top. Galvanized items can, indeed, be mechanically altered to reveal non-galvanized material underneath.
As for my grandparents question:
You can, indeed, buy stainless steel beams, rebar[1], etc. They have all the fantastic properties you imagine and are, again, not merely coated like (for instance) galvanized rebar. They are also extremely expensive.
So this isn't about 'objects coated with stainless steel' it is about objects made entirely from stainless steel. Have a look at what a typical all through stainless steel terminal on an ocean going ship looks like after a couple of years. Especially when they are made from lower grades of stainless (such as for instance 304) vs better ones. Not all stainless is made equal when it comes to corrosion resistance.
Yes and yes. Stainless is expensive. It's stronger than some types of steel but not as strong as others. It's also difficult to weld. Plus, it's not 100% corrosion resistant - it only offers partial protection.
Almost all metals oxidize. You could build something out of gold or maybe platinum and avoid oxidation, but it would obviously be expensive. The U.S. navy does have some very large aluminum ships such as the Independence Class[1]. These ships are apparently having some teething issues, but it is quite the feat to have a 414 ft long aluminum ship that can go upwards of 50 knots.
> The U.S. navy does have some very large aluminum ships
Unfortunately aluminum also rusts, it just happens to be a chemically stable and resistant rusting - but not in a stable enough manner to be mechanically/abrasive resistant, that's why you don't see them deployed in widespread use at stressing conditions like internal combustion engine blocks or fast ship hulls.
From said cited Wikipedia article:
> In February 2020 it was announced that the Navy plans to retire the first four LCS ships. On 20 June 2020, the US Navy announced that all four would be taken out of commission in March 2021, and will be placed in inactive reserve, because it would be too expensive to upgrade them to match the later ships in the class.
Very interesting - I’ve always been confused by why aluminium is often stated as being rust-proof, but doesn’t appear to be as useful in the real world.
Aren’t there quite a few aluminium engine blocks in cars though? For example, the LS small block is now aluminium.
Or do you mean used in marine applications, where rust is an issue? That would probably make more sense (and may actually be implied by the context).
Those large aluminum ships crack because aluminum has poor fatigue performance when subjected to repeated cycling. Small aluminum boat? You're probably fine for a lifetime. 400ft aluminum boat? Good luck with that.
In addition to other comments, it's also difficult to weld. Stainless steel retains heat, so prolong welding can warp it. Also unless careful, welding can destroy the Chromium coating, so the area are the weld will rust quicker than the bulk material.
When choosing a metal material, it's important to consider not just its innate properties, but it properties during join. Welding can melt the metal surrounding the weld, which can undo many heat treatment or mechanical processes previously used to get the material to the desired specs.
There is a steel called Corten where the rust develops a protective layer similar to aluminum and won't rust all the way through. The only issue is it isn't good when exposed to constant water as the rust will wash off and not protect.
The USX Tower ("The Steel Building") in Pittsburgh is made entirely of Corten, designed to showcase the material when it was first developed. Other than turning all the sidewalks black when it was first weathering it seems to work great, as long as you're into the industrial aesthetic.
IIRC stainless isn't actually rust proof, especially in applications where there is a lot of interaction with odd chemicals and things like sea water, so bridges may not be an ideal use for the product.
I have heard the same. Actually making rusting proof stuff is very hard. Specially if there is any chance of things like acids being involved. Coatings could help, but even then why not just paint regular steel.
Now other thing I wonder is how structurally sound some of the stainless alloys are? Do they have similar characteristics to steels now used?
IIRC, Coated rebar can actually fail faster or in less predictable and therefore worse ways. The coating is often damaged during install, but even perfectly coated bar forms cracks eventually. The entire system's galvanic potential is focused on these small exposed areas, causing extremely fast rusting. The coating tends to fail in higher stress areas, which means that not only does the bar fail, it fails in the worst possible locations.
Don't confuse stainless steel with galvanized steel. The latter is just normal steel dipped in melted zinc, once you're past that thin coating it'll rust just like normal steel does.
Stainless steel is an is an alloy, you can saw apart a beam made of stainless 316 and the inside will be just as stainless as the outside.
According to http://www.kastenmarine.com/metalboats.htm, mostly cost plus other drawbacks that sound like they would not be blockers if the cost were lower. That author thinks monel would be even better than stainless, but it's even more expensive.
Cost, primarily. I'm not certain but I believe joins and welds also need further attention to ensure the corrosion protection isn't impaired.
Anyway, O&M is the problem for the guy trying to get the next decade's budget approved so if the upkeep is more expensive, hey at least I got this thing built.
Yes, this is called electrochemical cleaning. If you don't do that then the weld will be more vulnerable to oxidization because the protective layer that forms on stainless steel will be damaged and upset by the welding process leaving some of the iron in the steel directly exposed to the environment. This then can cause pitting of the surface, which is the beginning of the end.
For such a process to be effective it has to be done immediately after welding. You can use it to try to repair something that is already rust damaged but in my experience the gain from that is mostly a stay of execution, not a perfect solution.
Anyway, O&M is the problem for the guy trying to get
the next decade's budget approved
This is why democracy, while far better than the alternatives, still absolutely sucks.
It's a system explicitly designed to be short-sighted. There is massive disincentive to produce systems and infrastructure that will actually work some distance into the future. The only incentive for politicians is either (a) merely look like they're doing something (b) produce the fastest, cheapest possible thing that they can take credit for when they're up for re-election.
Democracy would only really thrive if the public valued the future, and had some reliable way of judging how our politicians' solutions actually benefit the future. (ex: I value the future, but if we build a bridge today I have no way of judging if it's built to last for 5 years or 500 years)
> ...democracy... It's a system explicitly designed to be short-sighted.
It's not explicitly designed for that. It's designed to avoid concentration and abuse of power, peaceful transition of power, and to create some level of fairness.
Democracy wasn't designed to produce the best society or the most wealthy society.
Democracy was designed to avoid dictators/kings and other really bad things :)
That said, we often seem to think that we can optimize for something beyond the short term.
It's a seductive thought. But experience with communism/central-planning, suggests that maybe it's best to optimize for the short term. At least that works, and produces results in the meantime.
It's the same with waterfall software development vs agile software development. Optimizing for the short term and iterating is usually better than to try and plan the future top-down.
It's a system explicitly designed to be short-sighted.
I'm not sure what else we'd call a system where elected officials have zero incentive to do anything other than look good for their re-election campaign in a few years.
Clearly, many politicians have gone above and beyond that and accomplished useful things. But there is zero incentive baked into the system for them to do so.
Democracy was designed to avoid dictators/kings and other really bad things :)
It's good at that when implemented well, though most aren't.
Generally it seems you wind up with oligarchs/corporations effectively owning politicians unless there is an extreme level of vigilance, etc.
But experience with communism/central-planning, suggests
that maybe it's best to optimize for the short term. At
least that works, and produces results in the meantime.
This is a false dichotomy. Clearly there are things that benefit from a short-term, MVP-style, iterative approach.
There are also clearly things that benefit from a longer view: climate change, infrastructure, etc.
> I'm not sure what else we'd call a system where elected officials have zero incentive to do anything other than look good for their re-election campaign in a few years.
We call it a side effect.
Democracy certainly wasn't designed with short sightedness in mind. That's a side effect of the far more important design goals around preventing abuse of power.
Democracy may be short-sighted, but not by design, rather as a side effect of the design.
Assuming we buy the idea that democracy is indeed short sighted.
Every considered that as long as democracy avoids establishment of dictators/kings, repression and the unavoidable civil wars that follow, then given time free market forces will produce good results.
One could reasonable argue democracy is just very very long sighted.
> Generally it seems you wind up with oligarchs/corporations effectively owning politicians
This isn't new, and yet we have built infrastructure before. Fought world wars. Irradiated deceases. Walked on the moon.
On the scales of history large corporations tend to slide into irrelevance over time.
The tech giants of today, will be rubber barons of tomorrow. Sure we might seem them around in the future. But a hundred years from now they might not seem so big.
For a plane you want to reduce weight as much as possible, and steel isn’t particularly lightweight.
A lot of commercial aircraft have switch to composite materials to reduce weight even further than would be possible with just metals, though that approach has its own drawbacks.
https://www.pennstainless.com/resources/product-information/...
Is pretty good stuff and
https://www.cralloys.com/alloys/17-chrome/
is possibly better still.
If you want (much) better rust resistance than that you are going to be into coatings or active protection such as cathodic protection using a sacrificial material.
https://en.wikipedia.org/wiki/Cathodic_protection
Coatings work well as long as there is no mechanical abrasion of the coating, cathodic protection works very well until you run out of sacrificial material.