There are 8 official 420 chemistries that have UNS designations.
S42010, S42025, and S42035 all have slightly higher Cr would would very slightly increase corrosion resistance.
S42025 is an aerospace grade (AMS 5925) that also has Mo in it for better corrosion resistance. This is a very special alloy.
S42035 has Ti in it to provide stabilization when welding.
You need to find out which alloy (and specifications) they are talking about.
But if you need more than just a very small improvement in corrosion resistance you will need to look at higher Cr alloys.

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P.E. Metallurgy, consulting work welcomed

@EdStainless

Thanks for the feedback. Looks like I need to figure out what type I need.

Any idea on which standards I will need to purchase that will help control the material chosen? I'm thinking a UNS designation with an ASTM standard. Is there a standard that AISI 420 (modified) is modified per?

The UNS book lists the various applicable specifications for the different 420 chemistries. You can then look up the names of those specifications to determine which ones apply to the product you are dealing with.

What product form will you be buying?
Bar? Rod?, Plate?
That will dictate which ASTM applies.

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P.E. Metallurgy, consulting work welcomed