The drain mystery

I recently moved to a new house. As expected, there was a lot of cleaning up to do. One of the tasks was the cleaning of the washbasin. Usually, the drain is a circular part with  5 to 6 holes for the water to flow out. What I saw was this -

I have not had a chance to analyse the material of the drain. However, a quick search tells me that this is most probably stainless steel. The water that this drain is exposed to is the bore water. Thus, the drain has encountered a lot of chlorides. There is general as well as localized corrosion.

The damage started off as a simple process of pitting. Pitting due to chlorides is one of the most common headaches for poor stainless steel. They break the passive oxide film, and reach the underlying fresh iron. This iron then reacts with the usual suspects (ions, oxygen, water) and forms what we see as the rust.

As can be seen in the picture, the thin sections of the drain between the holes have disappeared in three places. This may have happened because the pits formed continued to grow through the thickness of the material, which finally gave way and fell down the drain. The shape of the circle to the top left is distorted and we can observe a small nick in the circle. There is also a variation in the widths of each of the sections between the circles - all because of corrosion.

Then, there is the green color. This is the corrosion product ferric chloride. A quick look shows the green formation around the part between two holes at the top right. General corrosion is visible, and there may be pitting going on underneath the corrosion products.

We will have to wait and see if that small section is the next to break off.

UPDATE: It did break off.