Question on ship-building and rust

Presumably with the new UK CVF’s having an in service life of 50 years, mitigating corrosion must be one of the the most important aspects of the design

this problems doesn’t apply only to ships but also to bridge building or port facilities building, therefore there are a lot of techniques to address the problem.

For ship building the most common technique would be sandblasting of steel surfaces after the finish of all welds and application of coating. It is of high importance to achieve the best quality of sandblasting (according to ISO 8501 it is called Sa 3 if I m not mistaken – the swedish were the first ones to introduce such standards) and to paint directly before the steel starts to rust again. Port facilities and bridges might have parts zinc plated (galvanised) however this technique dosn’t have application in ship building, at least with parts that come in contact with water.

However ships are not made all from steel, therefore you could find in the internal parts from aluminum or stainless steel, like consoles etc. All those parts are plated like chromium plated (you could hear the expression “they have Alodine”) from the market name of a solution that is used for plating.

However, whatever measures you can take rust will be the winner, therefore it is necessary to make routine inspections and repair rusty parts.

Rust is a general problem, but “stainless” doesn’t exist. Stainless steel is made by using an acid on the outside layer of the metal. Then only the outside atoms are ionised and can’t oxide/corrode. Once you submit a stainless piece of steel to acids or cause a slight crack/damage/streak in the outside layer of the stainless steel, it becomes useless and it corrodes much faster on the inside than normal steel.
As mentioned by Daniel, in the marine environment it is rough, the salinity and temperature differences constantly attack the skin of the steel construction, if this would be stainless steel, it would get damaged equally fast and cost much more.
Another point Daniel mentioned is also right, if you want to weld such, it is very hard not to damage that outside layer and your welds themselves will corrode causing a severe danger for cracking.

As for strength, there are compounds that are much much much stronger than steel, yet the creation of steel makes it cheap as there is plenty of that around and it’s easy to manufacture. If you look at those compounds you get a totally different story. For now the only intended application for such compounds is in some of the submarine sonar domes. Also in Compressed Natural Gas carriers. If they wanted to use steel in such carriers, it would require a hold wall of 1m concrete steel to reach their required safety level. Needless to say they would have little cargo and a hugh empty weight. Therefore they want to use compounds for that application. But even then the safety requirements are too high and the idea is just rising with a few (4 or 5) designs on the drawing boards.
I expect that this will become reality in the near future as Natural Gas has become increasingly important. Once those compounds are used, I think they will start using them in warship construction too. Submarines would then be easily capable of diving much deeper, although it should also be mentioned that some compounds have some serious weaknesses too. Shafing resistance, acids, chemicals, temperature differences are some of them depending on which compound is used.

As for structural strength, they are calculated for a hull life. The hull is of sufficient thickness to withstand all those years of constant corrosion. They X-ray a hull to know the thickness every once in a while.
One of the drawbacks of the strenger steel versions, like HY-80 and other high tensile strength sorts of steel, is that you can achieve the same strength with less material. For example a hull of 5cm thick HT Steel can be equally strong as 10cm thick “normal steel”. This is good as the weight of the HT Steel will be much lower, so there is more stuff to add. BUT, here is the tricky thing, they’ll last much less long. Corrosion on both sorts of steel goes equally fast, this means that after one year, the HT Steel example only has 4cm left while the other one still has 9cm left. This means that the first, strong, hull would have lost 20% of its thickness and of course a proportional equal part of strength compared to only 10% for the second, weaker hull. Hence the downgrading of the stronger steel versions is higher.
The “grating” you often see in ships is also caused by this thinner hull. The wind dents these hulls so you can see the construction frames inside. They actually dent and tear easier than regular steel too. But, as said, it saves a LOT of weight.

Something I’ve always wondered is you see ships under construction with rust all over them and ships at sea that have rust on them. Why would you want to make ships out of an alloy that is going to corrode (weaken and ultimately fail)? Obviously a stainless steel CVN is going to cost more than a few bucks but is the rust you see on ships pretty much trivial as far as structural concerns go?

Yes, with proper maintenance a bit of surface rust is pretty trivial. Added to that most of your standard “stainless” products have a tough time in the marine environment. Those that don’t cost even more, not just in basic accquisition costs buts also in penalties to do with issues of workability, welding etc. This is definitely IIRC stuff since its more than ten years since I my materials science subjects at uni and I don’t work in the engineering field 🙂

Daniel