Mg alloy etchants works well for Mg-HA composites. It is difficult to obtain grain structure of Mg alloys. Very fine polishing and freshly prepared etchant are the important to obtain good micrographs. Commonly acetic picral is used with composition:- 6 gr picric, 10 ml acetic, 70 ml ethanol, 10 ml water.

A solution containing 100 ml ethanol, 2.5 g picric acid, 25 ml acetic acid and 25 ml water was found to be a suitable

Usually, I can see structural features just with polishing (but must be a really good one), with diamond polishing suspension and ethanol. Yet, when etching is needed, I tend to follow ASTM-E407-07 even for metal-matrix composites. Other options for Mg-based materials are:

· 0.7 mL H3PO4 + 4 g picric acid + 100 mL ethanol (95%), or

· 60 mL H3PO4 + 100 mL ethanol (95%), or

· 1-5 mL HNO3 + 100 mL ethanol (95%), or

· 1 mL HNO3 + 75 mL diethylene glycol + 25 ml water, or

· 1 mL HNO3 + 20 mL acetic acid + 60 mL diethylene glycol + 20 mL water.

I believe the ones with picric give better results. I have used other solutions though (I need to choose according the reagents available in our lab) and it was also fine, but I prefer not using the ones with water.

Moara Marques de Castro yes you are right about the water. I have polished the Mg samples in water suspension and got degraded sample surface after two 0r three days. I think this happened due to the reaction between Mg and H2O.

Do you have any idea what can be the reason behind this?

True Shivani Gupta, my problems are often related to that, especially in samples which present discontinuities such as pores, microcracks, matrix-reinforcement interface, etc, (where liquid can permeate, and which are not easy to dry). It is important to note that Mg easily corrodes in water… so long polishing times and/or long etching times and/or insufficient drying before analysis/storage often compromise good results.

Thus, I prefer using ethanol-based reagents/lubricants, since ethanol evaporates easier than water. I also try polishing my samples quickly (some of them get easily corroded during long preparation). Yet, even when I get a clean well-prepared surface, a few stains could appear after a while around that discontinuities, depending on reagents or lubricants I used (that didn’t evaporate completely from the sample during drying step). So I also try drying the samples very carefully and store them properly to avoid humidity.

Good luck!

The solution which I found suitable for the Mg-HA matrix is elaborated thereunder:

"10 ml deionized H2O, 70 ml ethanol, 4.2 gr picric acid, and 10 ml acetic acid".

You could also see my research article for more detail using the following link:

https://www.sciencedirect.com/science/article/pii/S0272884219328123

However, based on my experience, try to use ethanol rather than water during the polishing process.

Thank you all for the valuable information.

Can I use liquid Picric acid inplace of solid?

How much proportion of liquid Picric acid has to be used?

Nital reagent can be used as an etchant for Mg-HA composites.