Plastic welding-hdpe

bit off subject not sure if anyone can help. i have a tank made from thick hdpe. it holds water and when i heat it up (uses a element like a kettle) it has a very small leak at a join. it was custom made years ago and it holds some nasty chemicals for cleaning stuff. it looks like it has hard plastic joins on it (i guess its welded) anyone know what i can use to repair it. was going to try silicon but wasnt sure if this would work due to the chemicals inside. looked locally for plastic welders but all i can find is bumper repair places LOL.

rephrase that, it only leaks when its warm, its a real small drip, when its cold its fine. i guess when it expands its causing the problem.