Internal corrosion deposits - repair in situ with internal coating?

During routine inspection of the open hazardous drains caisson flame arrestor, it was noted that a significant amount of corrosion had formed within the downstream swan neck which is made from carbon steel pipe. 

The corrosion deposits have the potential to block the flame arrestor which would block the vent line of the caisson.
 

Currently, following options are being considered:

- Replace with 300 series stainless steel, like-for-like;

- Replace with new carbon steel pipe coated internally;

- Carry out in-situ repair and coat the inside of the pipe spool with a form of lining.

- Routine cleaning.

We are wondering whether there is any smart, in-situ solution, which would cover the internals of the current swan neck (i.e. something along the lines of AquaTerra's CureShield for caissons). 

Alternatively, are there any cost-effective options for a like-for-like replacement of this section of pipework with carbon steel but internally coated? For both solutions to be viable, they would need to offer cost / effort savings when compared to 300 series like-for-like replacement.

Thanks in advance.