- The following figure represents a laser welded stainless steel 316l sample, and it is electrolytic etched by oxalic acid. i would like to know if this micro-structure is over etched or not. if it is not over etched, what is the black particle phase?
- if it is over etched, what would be the phase of the optimized sample?
Is the image made by a optical or electron microscope?
It appears to me that the black objects are on top of the edged surface and may not be a part of the original microstructure (contamination?)
It is unlikely to be a different phase. You can confirm this by changing the focus slightly and seeing if the black spot and the remaining microstructure focus at different lengths. If so, as pointed out by Erik, it is not part of the original microstructure and could be over-etched (if a pit) or external contamination (if a peak). External contamination is common if the bath is old; oxides settle at the bottom each time you etch, sometimes these deposit on the specimen while stirring etc.
it is an optical microscope image
however this structure is similar in the other samples
I am also supporting the opinion of the Erik & Aashranth. It may be porosities formed in welding process! Conform it by focus & defocus in optical microscopy.
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