What eats away the metal wall of my drain trap?

Question

Under my bathroom sink the drain trap developed holes on it at several points, and some white powdery material appeared on them. Pictures before and after cleaning:

Additional information:

• The white residue you see on the 1st picture is dry, powdery. The holes don't leak water (yet).
• The holes didn't develop suddenly. It took it years to become like this.
• Of course, I didn't use any unusual chemicals in the bathroom. Soap to wash my hands, and occasionally detergents to clean the sink. The drain trap was clogged only two times during these years. I used a drain cleaner solution to unclog it, but these are not supposed to eat away the material of the plumbing.
• I am living in an apartment downtown in Europe. The building was built around 1940, so the water pipes can be very old.
• The tap water is somewhat brownish. It's transparent if you look at it, but after two months visible brown residue precipitates in the shower tray (picture). Maybe this brown thing could interact with the metal wall of the drain trap?
• The stainer, the pipes and the drain trap are from a different set, i.e. could be made of different metals.
• Around 5 years ago I already had these holes and white powder, and I replaced the drain trap.

What causes this, and how to avoid it?

Answer 1

The scale didn't cause the puncture, any more than stalactites cause dripping. There's a leak, water is evaporating and leaving behind minerals, and the scale builds up over time.

So the real question is finding and fixing the leak. Which may not be at the buildup, but above and dripping down into it. Stalactite rather than stalagmite.

Answer 2

That's some sort of metallic corrosion, which has destroyed the chrome plating in that area. I see no "puncture", which implies an actual hole in my local version of English. Unsuitable alloys and unsuitable methods of plating plumbing parts are all too common in the markets these days. The solution is to use better-quality plumbing fixtures.