any idea what kind of metal this is

[Yggdrasil]

Additionally, stainless are not corrosion free unless it is clean.
Even a speck of paint or other pollutants may/will start a reaction of corroding.
We worked with stainless equipment for the food industry for years.
Often when we came back to service the equipment it was a sad sight.
Needed to retreat it with "whink/ or similar" and polish it up again.

But water had no effect.

We worked exclusively with the 300 series for this.

 

[kurtak]

Additionally, stainless are not corrosion free unless it is clean.

This is correct

There is why it is called stainless & not stainproof

In other words - it stains (corrodes) less then normal iron or steel --- but that does not make it stain (corrosion) proof

Kurt