The 'Pickling' section of the following internet site may possibly help; with regards to Inconel 625 alloy: http://www.californiametal.com/Inconel_625_Sheet_Plate_Pipe_Tube_Rod_Bar_Tech_Data.htm

It is best to use an SEM analysis, then you do not need to etch. Otherwise, Inconel and cast iron are removed differently in each etchant and you will get differences in the sample surface. Especially the interface of Inconel - cast iron is deeper etched and looks as a crack in optical microscopy.

Hello Sebastian

I think that you need you use two different etchants, or I do not understand how your sample look like; when you say inconel alloy on cast iron, what do you mean? please explaine.

May have luck with electrolytic oxalic acid etch. not sure how the cast will respond to that.

you can use nital solution (5%nitric acid),  it is the better etchant solution for all cast iron and carbon steel

What about the Inconel alloy part and the Interface etching? The interface is the main part, where I need the analysis to be done.

try first nital solution it may appear the structure of the interface

Dear Roshan, you can find appropriate etchants for cast iron here:

http://www.metallographic.com/Etchants/Cast%20Iron%20etchants.htm

maybe the etchants for stainless steel work well for inconel:

http://www.metallographic.com/Etchants/Stainless%20steel%20etchants.htm

Hope this is useful,

Jose

http://www.metallographic.com/Etchants/Cast%20Iron%20etchants.htm

http://www.metallographic.com/Etchants/Stainless%20steel%20etchants.htm

Dear Roshan Sebastian,

Please see the paper in RG.

I think you may also contact Dr.  Hoyeol Kim (or Dr. Weilong Cong or Dr. Zhi-Chao Liu) on RG who is working on this problem of laser engineered net shaping:- https://www.researchgate.net/publication/315694100_Laser_Engineered_Net_Shaping_of_Nickel-Based_Superalloy_Inconel_718_Powders_onto_AISI_4140_Alloy_Steel_Substrates_Interface_Bond_and_Fracture_Failure_Mechanism

Best wishes

Article Laser Engineered Net Shaping of Nickel‐Based Superalloy Inco...

I am not sure whether Kalling's reagent  (both No 1 or No 2) will work, because it is an interface (caution:- please take care of safety before you use etchants)

You may also refer to:- 

(i) APPLIED METALLOGRAPHY, Edited by George F. Vander Voort 

(ii) GUIDE TO ETCHING SPECIALTY ALLOYS, Katharine B. Small, David A. Englehart, and Todd A. Christman, ADVANCED MATERIALS & PROCESSES, February 2008, pp. 32-37.

Since it is an interface between Nickel base Inconel 625 and cast Iron, I am not able to suggest the Kalling's reagent. 

 (i) For cast iron,  there are 3 etchants:- Taken from the link given below:-  "The first is 4% alcoholic nitric acid ("nital") used at room temperature to reveal the ferrite grain boundaries and reveal phases and constituents such as cementite and pearlite. The second is alkaline sodium picrate (25g NaOH, 2g picric acid and 75 mL distilled water) used at 60 to 100°C for up to 30 minutes (1-3 minutes is usually adequate). This is used to color cementite yellow to brown (ferrite is not colored). The third is the standard version of Murakami's reagent (10g KOH, 10g potassium ferricyanide and 100 mL distilled water) used at 50°C for 3 minutes to color iron phosphide dark yellow or brown (cementite and ferrite not colored)". Please see the link if you have access:- http://www.metallography.com/technotes/iron/etching.htm

You may also please see the link if you have access: 

https://www.slideshare.net/usman31/metallography-of-cast-iron

(ii) For Nickel base superalloys, Kalling's reagent is OK.

(iii) But for the interface between Inconel 625 and cast iron , I am afraid to suggest Kalling's reagent.

I think you can take Dr. Vadim Verlotski suggestion regarding SEM analysis. 

Sir, will the Microstructure of the Interface be revealed, without etching, in the SEM?