SEM images of an etched SS410 (20% ferrite) reveal triangular-shaped marks on the ferrite phase. Do you know the cause? The samples are polished with standard procedure (final polishing with 1µm diamond paste and OPS-A +ultrasonic 2 min) and etched with Vilella 7s at 25°C. Do you think the picric acid in the Vilella can cause this form of pitting corrosion?
Regards
If I wouldn't know it is SS4 I would guess that you have a certain amount of porosity and these are grains which are not grown together. It might be also that precipitates have been removed and you see now the places where they were before? You have also more cube-shaped holes . Do they have the same origin?
In order to answer your answer regarding the acid: I don't believe that you can generate such a very special etching in a steel. In semiconductor materials it happens because of the selective etching at dislocations but this doe not happen in steel since the dislocation density is much higher so that such a selective etching I would expect
As Gert mensioned, it looks like grains which did not grown together, so when their crystallization fronts met, a further grow in those directions stopped. HRTEM near those 'triangles' should explain (or deny) this.
I agree with Gert because there is exceptional relief where the ferrite phase is not present. In this second phase there are a variety of etch features.
I found an example of the same feature in the book "Iron and Steel in Ancient Times" by Vagn Fabritius Buchwakd, where those triangle based features are identified as "etch pits" in ferrite.
one of the best way you can distinguish the porosity caused by etching is UT testing before metallugraphy examination. you can do the UT test on original and as- recieved material. if you didnt see these defects in the UT test so they have formed by etch solution.
this is the best way and we do it in our factory permanently.
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