A magical corrosion proof, marine growth proof, strong metal. What was it?

Decorative Chrome has as its major underlying component a first layer of nickel then a very thin coating of chrome. Nickel is cheaper than chrome but has the benefit of lower current density during its application so as to permit an extremely
smooth surface to which a thin layer of chrome is applied. The decorative chrome reflects/mirrors,copies the smoothness of the underlying substrate, and produces a high quality shine. ( I believe that in the 70's there was another coating applied even before the nickel but it was found later to be unnecessary)

You can apply a chrome finish at thin layers without the nickel undercoat and this finish can often be found on high quality tools, Snap On, Mac etc where the tools are polished prior to the application of chrome. So a shiny finish is produced
on top of a polished shiny substrate. A term often referred to this is flash chroming. Of course this process can be applied on an irregular surface for limited corrosion protection and cheaper non prepolished tools would be an example. As the
decorative chrome layer is thin, a file can easily cut through the chrome finish even thought the hardness of the chrome may be in the 64-70 Rockwell C. So a hard thin skin on a softer substrate.

Hard Chrome
I had mentioned in an earlier thread that hard chrome has/most often has some porosity in its deposition and while in most corrosion environments, salt water, wet atmosphere etc this minute amount of porosity
is insignificant as to its purpose. Hardchrome for wear and corrosion resistance is normally applied in thickness up to .020" directly on say a steel substrate. Rebuilding of wear areas for bearings is a common
shaft/hydraulic ram repair option. Machine off the irregularities, ie make it round, plate the area, remachine, polish and you have a fully functional ram. Hard chrome is applied at higher current densities and can produce hardnesses
of around 70 RC

In an acidic environment, H2S, H2SO4, such as in the sour gas/hydrocarbon applications, chrome generally does not stand up very long as the chrome porosity permits the corrosive acid into the substrate.

David Long had made the comment that chrome does not have porosity but the reference information that he supplied makes note of the same porosity and Type 1 Decorative having the nickel coating.

So a summary
Decorative chrome plating as the type that you would see on an automotive bumper is plated on a prior plated nickel coating to provide a high finish and the nickel providing the majority of the corrosion resistance.
Hard chrome plating is applied in greater thickness without the nickel pre coat

 

Nickel or copper is usually used as a filler for imperfections and then polished before chroming in decorative applications.

 

As I found the original question in this thread really fascinating, I decided to ask at the Smithsonian's Air and Space Museum if they may have any technical insight into how those landing gear hydraulic cylinders may have been made. I received a very nice reply copied below. The short answer is that they were likely made of a 4130 steel with a chromium hard coat plating. I don't think the Japanese were using 4130, exactly, but more likely a Japanese equivalent, but no matter, if 4130 is being used, they had to have been using some darned tough steel. I'm wondering if these hydraulic cylinders are corroding from the inside out, and that is simply not visible to divers. Just thoughts...

For what it's worth, I attach below the response.

dj
_____________________________________________________________________________________________________________________
Dear Mr. Jones,


I am one of the restoration specialists in the National Air and Space Museum’s Preservation and Restoration Unit, and your question (copied below) was sent to us from Visitor Services. I don’t have all the answers, but here’s my experience:


The struts on the gear on my airplane (a Pazmany PL-2, an obscure homebuilt airplane) are hard chrome plated over 4130 steel tube. I know this because one of them is getting replated right now. Chrome plating involves toxic materials (chromic acid), so due to environmental regulations it’s harder to find platers than it used to be, but it is still available. I don’t know whether that is what WWII Japanese manufacturers used. It was and is done for functional, not decorative, reasons —struts move a lot during landing and need to not leak, so a very smooth, wear-resistant and corrosion-resistant surface that slides through seals is needed. A marine biologist would know more about whether chrome surfaces would repel or be toxic to marine organisms.


One of my colleagues with Japanese aircraft experience concurred with my remarks, and added that it must have been well plated.


I hope this helps dispel the mystery to some extent. And thank you for your kind remarks; we hope to see you when we reopen.


Restoration Specialist


_______________________

(Mr. Jones original inquiry, copied below)


First let me say, I really love the Air and Space Museum. You folks have an amazing collection and a beautiful museum. I haven't been there in some time, I'm due to go back soon.


My question: I'm wondering if you have a group of knowledgeable folk that may have some insight into the following. There was a question posted on a sailing (as in sailboats) forum that is quite intriguing.

I quote:"Many years ago I taught science on the island of Guam. In the harbor you could dive to a Japanese dive bomber at a depth of about 30 feet. After 40 years of sitting upside down on the bottom of the sea the entire plane was a piece of junk. Marine growth covered it and profound corrosion had taken root. Except one surface, the hydraulic landing struts. Those two hydraulic rams were shiny as new with zero marine growth on them. No rusting, pitting, or discoloration of any kind were evident on those shafts. What magical formula was in that stainless steel? Did they use some toxic metal that industry can no longer use? I have always wondered about this, I see stainless rust all the time but those struts still perplex me after 30 years. Vibranium?"

This is a rather interesting observation. Does anyone know what the hydraulic landing struts on Japanese dive bombers might have been made from? Were they perhaps made with some kind of coating system, like chromium hard coat, or a cadmium based coating system, or .... I just have no idea. Would they even have been made from a stainless steel as suggested by the post above? Or might the base alloy have been one of the more lightweight alloy systems, and then perhaps coated? Any and all insights would be welcome.


If there is someone that may have some knowledge in landing gear, I would love to hear from them. It's quite interesting from several points of view, for me in any case.


My best regards and I hope to hear from you soon.