Pickling of steel in Hydrochloric Acid

[selseg]

I have some pieces of rusted mild steel pipes that i intend to remove rust from.I inserted a test sample in 15% Hydrochloric acid for 4 hours after which all the rust was removed from the surface and corrosion has slowed dramatically after washing off the acid.The subsequent pieces of steel pipe I tried on a large scale showed rapid corrosion soon after the pipes have been removed from the acid .What could be the cause of this

 

[Baluncore]

Chlorine from the HCl remains on the steel surface and causes corrosion to return. Once an iron surface is poisoned with chlorine it is close to impossible to remove. It is a real problem in marine situations and causes car bodies to rust where salt is used on the road to prevent ice.

A slower process, that is often preferred by tool and machinery re-builders, is to immerse a rusty item in molasses for a week.
http://www.practicalmachinist.com/vb/general-archive/rust-removal-using-molasses-90452/

There are commercial rust converters that protect the surface with a phosphate.