HIGHLY unlikely that it is Pd plating - the likely hood of that being Pd plating is next to zero - if not zero
aniaxi is spot on here - Pd plating in this application (transformer cores) simply has NO reason &/or purpose &/or function for being there whereas permalloy actually has reason/purpose/function for being there
https://www.electricity-magnetism.org/permalloy/
Quote from that link ------------
" Permalloy’s unique properties make it indispensable in a variety of applications.
Its high permeability and low coercivity have made it a popular choice for the cores of electrical transformers, where it improves efficiency and reduces energy loss. Additionally, its anisotropic magnetoresistance has led to its use in magnetic field sensing applications, such as in read heads of hard disk drives."
And ------------
"
Aside from its extensive use in electrical transformers and hard disk drives, Permalloy is also employed in a range of other technologies. These include telecommunication devices, magnetic amplifiers, and shielding materials to protect sensitive electronic devices from magnetic fields. Because of its high magnetic permeability, it’s frequently used to create a pathway for magnetic fields in various devices, such as radio-frequency transformers and inductors.
The picture you posted with the above comment does not prove anything concerning Pd in solution - it only shows a color change of the solution taking place as copper ions go into solution
A picture of a stannous
along with a picture of a DMG test would be the proper testing method to prove/confirm Pd in solution
Also - when you put a piece of copper in a solution that has Pd dissolved in it - if you do not provide
very aggressive agitation to the solution it will cause the Pd to "plate" to the surface of the copper in such a tight bond on the surface of the copper that it very quickly shuts down the cementing process
In other words - unlike gold or silver wherein the gold or silver falls off the copper allowing the cementation process to continue - that does not happen with Pd unless you provide
very aggressive agitation to the solution to keep the Pd "knocked of" the copper & prevent the Pd from making a tight plating bond on the surface of copper - which happens very quickly shutting down the cementation process
I am sorry but HCl alone will not dissolve Pd plating - it would dissolve the iron under the plating thereby leaving you with Pd foils much the same as gold foils you get if you use nitric to dissolve away the copper from gold plated copper
Even IF (the VERY BIG IF) HCl would dissolve the Pd plating (which it wont) because of the iron under the Pd you would get a redox reaction wherein the iron itself would cement the Pd out - so there would be no need to cement the Pd with copper
On the other hand HCl will most certainly dissolve nickel so if HCl is dissolving both the plating & the iron under the plating that is a very good indication that the plating is permalloy as anarxi has pointed out
Bottom line - in as much as it may be Pd plating (though HIGHLY unlikely) - after 2 pages of posting you have yet to prove it is Pd
A picture of a color change in the solution after adding copper proves nothing
Show us a stannous test
along with a DMG test
Kurt