Nickel passivation

[cmarinelli]

Hi everyone,

I read recently that nickel alloys formed or machined with steel tools can have ferrous material smeared or impregnated into the surface. This material, of course, can oxidize when exposed to high temperatures. Is there a common surface conditioning or 'passivation' spec that is employed on Hastelloy X and other nickel alloys? For all of our CRES components, we routinely passivate per AMS-QQ-P-35. This application is a thin (.020&quot Hastelloy X sheet metal part that is spot welded together and goes into the LPT section of a turbine engine at approx 1575F. What can we do to remove the potentially troublesome ferrous material on the surface of the part. If there is such a nickel pickle (sorry), should it be done before the parts are spot welded together?

Thanks in advance for any help you can provide.

Chris Marinelli
Dynatech Engineering

 

[NickE]

I think you might be able to use the standard nitric acid pickle (passivation), I'm not sure if it will etch the high nickel alloy thou. It seems to me that the passivation for SS has the same intent, to dissolve the free iron from the surface.

nick

 

[israelkk]

NickE:

The passivation for stainless steel thickens the natural oxide the makes the stainless ateel resist corrosion. The process to remove the potentially troublesome ferrous material on the surface of the part is called pickling and is done before the passivation process.

 

[unclesyd]

Pickling Acid can get very agressive vs some Nickel alloys. I don't have my tables anymore but I think Hastalloy X is ok. I definitely would run some test pieces.

 

[TVP]

Haynes International, the producer of Hastelloy X, has a very informative brochure on their website called "Fabrication of HAYNES® and HASTELLOY® Solid-Solution-Strengthened High-Temperature Alloys". The section on descaling and pickling begins on page 41. Please use the following link to access it:

http://www.haynesintl.com/pdf/h3159.pdf

 

[NickE]

Israelkk-

from ASTM A967-01:

"3.1.1.1 In this specification, passivation, unless otherwise specified, is defined as the chemical treatment of a stainless steel with a mild oxidant, such as nitric acid solution, for the purpose of the removal of free iron or other foreign matter, but which is generally not effective in removal of heat tint or oxide scale."

The process known as pickling is the removal of heat tint and oxide scale. Pickling is much more aggressive to the surface of the material.

nick

 

[kenvlach]

cmarinelli,
Nickel-based alloys are not CRES (Corrosion REsistant Steel, e.g., stainless steel), and the passivation procedures of QQ-P35C and ASTM A967 should not be used without prior knowledge, e.g., supplier info.

Passivation solutions, both nitric acid and warm citric acid, remove nickel as well as iron. Passivation is intended to produce a chromium oxide surface. Hence, Haynes (TVP’s link) specifies pickling to remove surface oxide and any embedded iron from machining, but not passivation. Their nickel-based alloys naturally passivate after pickling upon rinsing and exposure to air.

My guess is that unclesyd is correct in that passivation may be OK for a 21% Cr alloy such as Hastelloy X, but is dangerous in general for nickel-based alloys.

I note that concentrated nitric acid can be used to strip nickel plating from aluminum, copper, steel, etc.

A passivation solution sometimes used for nickel plated parts is 1 wt% chromic acid in warm water (120^oF). This definitely improves salt spray test results, and I expect it should therefore also work (thicken the natural oxide) on nickel-based alloys. I am unaware of its general use, but have used it for a few aerospace parts when customers requested passivation following pickling (descaling). Anyone have a specification for passivating nickel-based alloys?