Letter 40059

Intergranular attack on Nimonic 105 alloy  

++++++

We currently etch forged components of Nimonic 105 alloy in the following sequence:
1.Activation for 30 secs in 32% commercial Hydrochloric Acid
2. Etch in a solution of Ferric Chloride Liquor (Fe3+ = 50-150 g/l), Hydrochloric Acid (3.5 - 4 moles H+), Nitric Acid (1.4-2 moles NO3-) and Hydrofluoric Acid (0.8-1.2 moles F-) followed by
3.Desmut in a solution of Copper Chloride Dihydrate (100g/l) and Hydrochloric Acid (175g/l)
On metallurgical examination we are encountering high levels of Intergranular attack at the grain boundaries and I wish to know if anyone has encountered similar problems etching this alloy and if so..what could be the source of my high levels of intergranular attack..fluoride too high, nitrate levels depleting, chloride in desmut too high??
Any help given will be highly appreciated!

Nigel Gill, BSc MIMF MRSC
Aerospace - Glasgow, Scotland

I was once faced with a related problem, pretreatment for plating. I found that ferric chloride was the worst etch. A high level of intergranular attack, some areas not affected and a large amount of smut. Nitric hydrofluoric acid mixtures were better. Adding hydrochloric acid makes a difference but both the nitric and hydrochloric contents must be low because they react with each other giving the solution a short life and maybe damaging the tank. It is safer to avoid chloride altogether. None of my experiments were on Nimonic 105 though.
You did not say why you want to etch the surface. Would you be better off cleaning with alkaline permanganate or polishing with an electroplish?

Nick Clatworthy
- Whtstable, Kent, UK