What is the pale green solution from dissolving sodium hydroxide in a stainless steel flask?

I have previously used drain/sink cleaner gel based on sodium hydroxide but noticed that caustic soda crystals were a lot cheaper. I decided to experiment with making my own cleaner.


I bought a 1 litre stainless steel flask and placed 500 ml of tap water in it. Then I added 500 g of caustic soda (from the hardware store) over maybe a 3 hour period. I stirred it with a new, unpainted wooden spoon. The solution got very hot! Then I poured it into a HDPE milk bottle, there was no free solid left and it had all dissolved. However the solution was pale green! When we used sodium hydroxide solution years ago in the lab, it was colourless. Also the end of the spoon which was in contact with the alkali was discoloured to a dark brown.


1. **What is the source of this green?** Is it a chromium compound, having stripped this out of the stainless steel?
2. The stainless steel flash looks normal, and has been thoroughly rinsed. **Is it safe to use for holding tea/coffee?**


