The attached Picture of 316L SS sample cross section of SLM fabricated piece is after etching for 120 sec in an equal amount solution of HCL, HNO3 and Distilled water. the melt pool and grain structure and identifiable at high magnification, however there are alot of unwanted spots on the surface as can be seen in following picture, i wonder what could be the reason , is it some etching issue or impurity or other issues. Also another question is on the same surface some areas are over etched and some are as shown in the attached picture, what could have caused this.
The spots might be etching pits that are formed after the process. It can be indicative of the thermal and mechanical behaviour that the steel has gone through and might help answer some microstructure related analysis questions.
As for the over-etching, it is very common as that particular surface area is attacked by the chemical first and has the longest time.
After the etching process, if you clean the surface well then the etching effect should remain the same over a period of time. However, if minor traces of the etchant remain, then the surface will still be attacked and might give some differences from when you first see it but this difference is only in very very small quantities.
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