I used a drain cleaner containing sodium hydroxide (NaOH) to unclog it. I followed the drain cleaner's instructions to the letter:
- pour the NaOH into the pipe entrance.
- pour hot water.
As soon as I started pouring hot water, it started pooling. No water would pass through. The sink was completely clogged now, because the NaOH formed a solid "bathtub plug" inside the pipe! (I could see solid white NaOH)
Pouring more water had no effect, so I carefully threw citric acid grains (a weak acid, contained in lemons), hoping the acid would react with the base (NaOH). Initially, the acid reacted with the NaOH (I could see bubbles coming out), but then it stopped completely.
WARNING: Acid and NaOH reactions can be very violent. In my case there was enough water to act as a heat-sink preventing boiling and splashing, and I was adding acid slowly. I can't emphasize enough how dangerous NaOH is. Use full face protection and gloves, otherwise you could go blind or get severe skin burns.
I don't want to open the pipes (if flow is restored it could splash NaOH solution on me).
- Is there any other way to safely unclog the NaOH?
- What exactly happened? Why did NaOH block the pipe completely, and why did the acid become ineffective after a while?
Sidenote on why I did it:
My partially clogged kitchen sink would allow small amounts of water to pass through the pipe. I've been using a plunger when it was getting clogged (snake was ineffective).
A plumber had told me in the past, that the problem is deep inside the wall (about 3 meters into the pipe) and is very hard to fix. He also suggested using NaOH a couple of times a year to keep it clean.
Top view and side view respectively: