Pickling of steel in Hydrochloric Acid

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SUMMARY

The discussion focuses on the effectiveness of 15% Hydrochloric acid for rust removal from mild steel pipes. While initial tests showed successful rust removal and slowed corrosion, subsequent large-scale applications revealed rapid corrosion due to residual chlorine from the acid. This chlorine contamination poses significant challenges, particularly in marine environments and automotive applications. Alternative rust removal methods, such as immersing items in molasses or using commercial rust converters that apply a phosphate coating, are also highlighted as viable options.

PREREQUISITES
  • Understanding of rust chemistry and corrosion processes
  • Familiarity with Hydrochloric acid properties and handling
  • Knowledge of alternative rust removal methods, such as molasses treatment
  • Awareness of commercial rust converters and their applications
NEXT STEPS
  • Research the long-term effects of residual chlorine on steel surfaces
  • Explore the molasses rust removal method in detail
  • Investigate various commercial rust converters and their effectiveness
  • Learn about best practices for rust prevention in marine environments
USEFUL FOR

Metalworkers, corrosion engineers, automotive restorers, and anyone involved in rust removal and prevention strategies.

selseg
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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
 
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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.
 

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