[TheForge] Spring steel (and other) breaking in mild acid

[Clyde Wynia]

I make part of a piece of sculpture that serves as a back for a cat. I take
a piece of 3/8" rod about 18" long and bend it into not quite a semi circle.
I then take a piece of garage door spring about 2" diameter and slip it over
the U shaped rod. This is just a very mild bend around the rod. I do not
weld the spring in any place, only the rod is welded to the legs.

As I do with hundreds of other pieces, I put the piece, not hot, into a
mild, 10 to 1, bath of water and muriatic acid to remove the mill scale,
rust etc. The solution by this time of year is very weak. However, when I
remove the piece the spring is broken into 1 to 4 pieces.
 
This also happens with bearing races, 5" to 7" diameter that have been put
into the acid. The cross crack does not occur near a weld and is not put in
the solution hot.

So what is happening to cause these breaks in this very mild acid solution?

Clyde Wynia