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I don't know if there's a name for this phenomenon, but it's sort of like an invisible Chesterton's fence. "Just make them out of a different steel" feels like a idea that is elegant, but even as someone that's never built a ship, I have to imagine there's some good reason why they're built out the materials they are, and why a simple material change is unlikely to be a solution. Maybe I'm wrong (it's not like organizations haven't done dumb things before despite a having a good simple solution), but it really feels like that's probably not the case here.

Stainless doesn’t really hold up in saltwater, with the exception of a few grades. The material properties (toughness, ductility, brittleness, weldability) can also be significantly different than non stainless.

Corten can work in less aggressive environments, but it has been known to fail to provide the expected protection in places with more acidic rain than was expected. Salt water is really aggressive.

Hot dip galvanizing does work, and is common for small boats, dock fittings, and other smaller items. There are also likely sacrificial anodes on the ship made of zinc.

There are many sacrificial zincs on ships, including in tanks.

I don't know about modern ships -- I suspect a lot of this is classified -- but as an example, the WWII Bismark had a hull which was around a foot thick, as armor.

(1) Rust wouldn't go all the way through. Surface rust isn't a problem.

(2) You want high-quality armor; toughness is key. Steel composition should be primarily chosen for toughness-to-weight

(3) And cost, of course.

Stainless would have been expensive, and given little upside, and potentially a lot of downside in terms of less effective armor plating.

Obviously, modern ships are entirely different (much less armor, actually, instead relying on active defenses), but I assume there are sensible reasons for materials choices.

The exact construction of modern ship hulls is definitely a secret, although perhaps not the most closely guarded one. There are plenty of photos of the Cole with a giant hole blown in it for example.

But modern ships generally don't have any amount of armor except against the smallest of gunfire. The idea being is that if someone gets close enough to shoot at you with a sizeable cannon you're already dead.

Modern missiles with explosive heads and get through anything with can make so there is no point to armor. In the battleship era you could get armor that would give your own guns trouble, but most battleships saw action against bigger battleships and good tactics meant they did okay.

The Navy would love armor that can stand up to modern missiles, but it isn't really possible, and the compromises in costs and inside space mean it isn't worth it. Maybe someday we will find a new material that would work, but for now ships rely on not getting hit.

Peacetime ships are also expected to last (Googling) 36 years in good condition.

I'm sure it was a concern in WWII, and that they took all steps they could to preserve their vessels, but I'm also guessing that as long as they weren't rusting so fast as to become junk in three or five years, they were much more worried about surviving combat than rust.

Why not coat the hulls with gold? As a noble metal, gold doesn’t have sufficient valence electrons to oxidize.

What could go wrong?

Stainless steel is prone to stress cracking corrosion (SCC) in saltwater environments which causes catastrophic sudden failure when under load. You REALLY don’t want that in a ship.

Galvanizing basically works like anodic protection. Already used, but a galvanized coating will disappear (even a very thick one) in just a few years at sea, and can’t be reapplied in the field effectively. Paint works better.

Cor-ten works best where the rusting surface isn’t exposed to wear directly (like a ship hull). Something like a bridge span member or a shipping container side, or a side of a building. It has to rust for it to work.

If exposed to a particularly corrosive environment it still rusts through pretty quickly. A lot of Corten products (like shipping containers) still get painted to slow the process down.

Ships have desired operational lives of 20+ years. Painting and rust removal is the only way that works.

I honestly don't know too much about the maintenance that goes on when a ship is underway, but I do know that stainless releases hexavalent chromium when you're welding on it and requires a number of extra considerations for safety.

So any sailor doing any kind of patching or repair to whatever part of the hull is stainless needs extra education and extra PPE. As well as monitoring for blood levels and other health issues. Nothing insane, but an added inconvenience in an already inconvenient job.

That's a good idea. Everyone knows chromium does not contain any Rust.

Joking aside: There are many ways to prevent rust, though I think environmental concerns are also an issue. Galvanising might leak heavy metals into the sea? And chrome is also a pollutant. Iron is not.

> Use a weathering steel, also known as “COR-TEN” steel which contains up to 21% of alloying elements such as chromium, copper, nickel, and phosphorous. The alloys form a protective rust patina which reduces the corrosion rate with time. COR-TEN steel tends to be cheaper than stainless steel.

If the point is for the ship to look 'not-rusty', Cor-Ten is probably about as far from that goal as one can get

You’re not a marine engineer, are you?

... I'm not.

You can’t just replace a material like that in something as large as a ship.

You need to have the right toughness, ductility, fatigue, etc.

Steel varies a lot. You can buy a 1000 page book that just lists different steels, along with their material properties.

Saying “just use stainless” is kinda like saying “just 3d print it” or “just make it out of carbon fiber”

Stainless steel is pretty expensive, but I suppose it would be no big deal for the US Navy. I thought they used sacrificial anodes to stop rust.

I don't think those anodes are 100% effective though? I've seen many ships that have them and still have signs of rusting.

Is it mechanically a 100% replacement for traditional steel used in naval construction? I recall specialty alloys like Wotan steel being used in WW2 warships. Perhaps such kinds of armor are not in vogue anymore, but corrosion resistance surely isn't the only requirement for naval construction, even if it's an important one. I'm not a metallurgist though, so I have no idea of all the implications of using stainless steel pervasively in naval construction.

I think that the sacrificial anodes work for places where it’s a hull in electrolyte, it wouldn’t necessarily work for salt spray in the superstructure.

So surprising no one involved in a multi billion dollar industry hasn’t tried to consider the solutions you propose

Wood is also corrosion resistant, maybe they could make the ships out of wood. /s

The US Navy literally built some ships out of wood up through 1994. Those were minesweepers where minimizing the magnetic signature was more important than strength or corrosion resistance.