RISK WARNING IF USING HCl. written by an industrial chemist who has a wealth of knowledge in the chemical/ metal corrosion field.
HCl warning: The use of uninhibited hydrochloric acid (HCl) on ANY metal surface would not be recommended. Any HCl purchased "off the shelf" would not have an inhibitor system added to it - this means that the acid will not stop acting when it reaches "clean metal" but will continue to etch into the metal surface. Effectively, uninhibited hydrochloric acid continues to etch into the metal surface at the same rate that it removes scale or rust. If you have a motor which has varying depths of scale - as most do - this means that the acid will be eating through scale in some parts of the motor, but will be eating through the base metal where the scale is thin. Further, HCl will cause pitting of the surface as it will follow a corrosion pathway in areas of the metal that may be heat stressed - possibly leading to breakthrough. Further, residual chloride ion will continue to promote corrosion on ferrous parts even after the acid has been flushed out.
Finally, HCl will "de-zinc" brass and bronze leading to "sponginess" in the metal and loss of mechanical strength in yellow metal components.
SX50, on the other hand, has a number of advantages:
It's a "soft" acid - effective at removing scale, but weak at attacking base metals.
It has a "three way" inhibitor system - with specific inhibitors for ferrous metals, aluminum & "yellow metals". This means that it will quickly etch to the metal surface and then stop reacting once it hits base metal. Further, these inhibitors, in combination with the film forming protective agents present in SX50, protect the metal from ongoing corrosion.
Being chloride free, SX50 will not cause "chloride corrosion" as seen with HCl - in fact the exact opposite is true – SX50 displaces chloride ion at the metal surface and protects the metal against salt attack.
Finally, the film forming agents within SX50 have a lubricating effect on seals and bushes.