I think it was less the soap and just time that eventually caused a lime buildup in the coil.
Steel coils will have 'black water' on startup after sitting for a very short time. Simply this is rust and a natural byproduct of the water contacting the coil material. Over time, hard water deposits will form a lime scale on the surface of the pipe minimizing the amount and severity of 'black water'.
In high mineral areas (20+ grains of hardness) this can happen quite quickly, areas usually served by water well. In areas with soft water this may never happen, areas served by river water. We have been known to run a coil in our facility to lime it up for a customer to prevent what you describe.
You must be careful with injecting soaps into a hot coil. Many soaps have a high solids composition that will precipitate out in the coil. This will plug the coil and since soap is essentially alkaline will neutralize any attempts to acid clean the coil. There is no easy way to clean a coil plugged with soap, that is why we have gone away from high pressure soap application in the units we build (still offered as an option).
Some manufacturers gave extended coil warranties if you only used their soap. It is quite possible that it contained a coil conditioner to accomplish this.
Hope this helps? Cheers.