aluminium cages?

Aluminum could be dangerous if it were to receive impact, or excessive pressure.

 

Could also be a toxicity issue. There's the no aluminum in deodorant crowd as an example.

 

Cages and their fasteners tend to be made from 300 series stainless steels (Sometimes known as 18-8 stainless, they are typically 302, 304, 305, and 316). They are non-magnetic, and have a high resistance to both acids and alkalis.

Aluminum will react with a lot of different chemicals, including skin oils, detergents, and many chemicals commonly found in cleaning products. It will leave dark oxidation marks on your skin.

Even aluminum that has been anodized (covered with a layer of protective finish) can react, and anodize dyes have been known to leach out in some conditions.

Titanium is also non-reactive, but is 5 to 10 times as expensive as stainless steel, and is more difficult to work with. This drives up the cost, which explains why one rarely sees them on the market except as wire style cages (eg, Badass Workroom). Steelworks Extreme makes some beautiful bespoke titanium cages, machined from bar stock. They don't list prices, but are out of range for most folks.

 

If there was any movement between the cage and base ring the aluminium would wear away here leaving a black messy deposit. Aluminium does not slide well on aluminium. I have made a belt from aluminium once, but only to get the sizing correct and act as a template before cutting stainless steel. And after a couple of days wear, the lock at the front between the belt and crotch section was a mess from the slight movement.

 

As I understand, aluminium is quite a reactive metal. The reason it resists corrosion in normal circumstances is that it forms an oxide coating on the surface. That protects metal underneath. Movement removes the oxide coating, which is immediately replaced by further oxidisation of the metal. That produces the deposit referred to by @castmenow. If there is damp and another metal present, e.g. a brass lock, galvanic action might also cause corrosion.

 

If you get a cut from aluminium you can guarantee that it will get infected and will take quite a while to heal. I learned this lesson the hard way, working in a machine shop.

 

I made an aluminium belt. It left black lines on skin, so probably should have been powder coated. Also it is easier to remove forcibly than steel and I hate to think what effect urine would have had on it.

 

The modern plastics are getting pretty good now.