Can I use Muriatic acid for cleaning air conditioning coils?

Yes you can but it has to be done very carefully and precisely.try using trisodium phosphate substitute in warm water, or a tile and grout cleaner containing phosphoric acid. Brush the solution into the stain, let it set, then rinse with water. As a last resort, try diluting muriatic acid with water (about 10 parts water to 1 part acid) and apply it to dampened concrete with a soft-bristled brush. Scrub gently. Be careful with the muriatic acid, which is extremely caustic; always add acid to water to avoid splashing. Wear a respirator, goggles and rubber gloves, and make sure you cover your arms and legs! Be sure the area is well ventilated, and have a neutralizing agent (baking soda or garden lime) on hand in case of spills. Also, you'll want to avoid using any acid before or after trisodium phosphate because the mixture creates a gas. Instead, rinse the area with water, and let it dry before trying another cleaner.