Replacement for Picral?

[metalman8357]

Someone mentioned on a recent post that their employer does not allow them to have picric acid, and unfortunately I am in the same boat. I work in the failure lab and deal mainly with plain carbon steels. Does anyone have a good recommendation for an etchant that can act similarly to picral? I understand that their is no substitute, but I would like to experiment with alternatives if possible.

 

[metengr]

Please utilize the search feature on E-T and internet.

thread330-148964

http://www.leco.com/resources/application_notes/pdf/met_tips/met_tip18.pdf

http://www.georgevandervoort.com/met_papers/IronandSteel/EtchingIsothermallyTreatedSteels.pdf

and others

 

[mrfailure]

I had to look into this once. The real problem is solid picric acid is explosive if it dries out. Mixed solutions of picral and Villelas are, by definition, not dangerous because they are aqueous. New solutions should be mixed up if you ever see crystalization of picric acid but even in that circumstance they are perfectly safe. I would recommend either having a lab that can manage dry picric (I used a university lab at the time) prepare your reagents for you or buying the smallest quantity of picric acid you can get to make your reagents and safely discarding the picric crystals after you have made the etchants.

I would thus recommend you clarifying the real dangers associated with picric acid to your employer so you can safely use picral.

Aaron Tanzer

http://www.lehightesting.com

 

[Guest102023]

I concur that dilute picric acid in something like Villela's reagent is relatively safe. However I have seen minute amounts of yellow crystal around the mouth of containers. Good practice is to wipe the cap and container mouth with a wet paper towel or cloth before resealing. It is grinding of dry picric acid powder between cap and bottle upon closing that can cause explosions.

 

[Ron]

Why not use Nital instead of Picral?
Also consider mixing only enough Picral for each use.

 

[TVP]

Ron,

Nital is the usual etchant for carbon and alloy steels, but it is essentially limited to revealing ferrite grain boundaries and general etching of martensite. Picral attacks the interfaces between ferrite and carbides, so it is useful for etching pearlite, bainite, and other carbide-containing microstructures. It is quite useful for etching dual-phase and multiphase steels, as well as tool steels. Another good application is for resolving weld fusion in steel tubes manufactured by the ERW process.