The Causes and Prevention of Smut on Etched AISI 300 Stainless Steels

During the fabrication of stainless steel parts by photochemical machining (PCM), a seemingly random occurrence of a partially-adherent, brown/black deposit, known as smut, can occur. This quality problem has plagued the PCM industry for over 30 years. In the past, the problem has been attributed to “poor etch-quality” stainless steel. However, an extensive investigation by the authors, comprising some 7 person-years of effort, has established that while the metallurgy of the stainless steel has some influence on the formation of smut, by far the most important factor is the condition of the ferric chloride etchant used for etching the parts. The influences of etchant concentration, free hydrochloric acid content, temperature, atmosphere and hydrodynamic flow have now been assessed quantitatively for the first time. The investigation has led to the development of a theory outlining the causes of smut formation, thus providing solutions for its prevention.