Observations Related to Etching 316 Stainless Steel and 304 Stainless Steel in Partially Consumed Ferric Chloride as a Function of Free Acid Content
Earlier work done by Dr. David Allen of the Cranfield
Institute of Technology on the etching of AISI 304 stainless steel
showed that unusual effects were achieved in etched rates and
surface textures as a function of free hydrochloric acid content.
Carl Whinery reported at our recent PCMI meeting in Rochester
good results in etching 304 S/S, copper and most other alloys
but very poor results on 316 SIS when run concurrently in a
partially consumed bath of ferric chloride.
This author had seen similar results with 316 S/S at Chemart
Company. It was always our experience to add additions of
hydrochloric acid when poor qualitative etching results were
obtained. This generally resolved the problem. The poor results
were manifested by erratically etched areas in which unetched
areas were scattered throughout as though some type of scum
retarded the etch; or alloy segregation had taken place within
the grain boundaries that appeared to cause a different rate of
etch, or in some cases would not etch at all.
Richard Beaupre, Chemart Company