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Corrosion resistance of 303 SS vs 304 SS


Q. I notice etching in a 303 Stainless steel item when submersed in 38% H2SO4 under vacuum. It seems to me that 304 did not exhibit the same problem. Is this correct?

Colin McClennan
electronics mfgr - Providence, Rhode Island, USA


A. Could be, Colin. 303 is the free machining version of 304 -- meaning it has controlled amounts of 'dirt', manganese sulfides, which cause the machining chips to break easily. We, and others, have noticed passivation problems with 303 that we don't see with 304. I have no experience with your particular corrosive, but could easily believe 303 is inferior to 304.

lee gearhart
Lee Gearhart
metallurgist - E. Aurora, New York


A. I agree with Lee that 303 will usually be less corrosion resistant than 304. You may find that soaking the 303 first in hot NaOH solution or similar will help you immensely by removing the sulfides from the surface before passivation.

lee kremer
lee kremer sig
Lee Kremer
Stellar Solutions, Inc.
supporting advertiser
McHenry, Illinois
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Q. None the less, is 303 considered a CRES?

Terry Jamison
- Glendora, California, USA
March 22, 2012

March 22, 2012

A. Hi, TJ. I personally don't know of any document that puts a fine point on the required parameters of CRES.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

March 23, 2012

I ask that question because, over the past 45 years I've been involved in machining, there has always been the occasional snide remark we make when quoting a job that gives the option of 304, 321, 347, 303. Those callouts are usually stuck on a print by an engineer who has never machined any of those materials.

Terry Jamison [returning]
Mfg - Glendora, California, USA

October 24, 2012

Q. Why is 304 so magnetic? And what is the difference between 303 and 303HS

Rogelio Rangel
- harlingen Texas USA

October 24, 2012

A. Hi Rogelio. Here is an extract from www.azom.com/article.aspx?ArticleID=1140

All austenitic grades have very low magnetic permeabilities and hence show almost no response to a magnet when in the annealed condition; the situation is, however, far less clear when these steels have been cold worked by wire drawing, rolling or even centerless grinding, shot blasting or heavy polishing. After substantial cold working Grade 304 may exhibit quite strong response to a magnet, whereas Grades 310 and 316 will in most instances still be almost totally non-responsive.
The change in magnetic response is due to atomic lattice straining and formation of martensite. In general, the higher the nickel to chromium ratio the more stable is the austenitic structure and the less magnetic response that will be induced by cold work.

I don't see much evidence that "303HS" has any accepted meaning. Although HS could stand for high speed, I don't see agreement on that.


Ted Mooney, finishing.com
Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

March 17, 2016

A. In response to the question about the various grades of materials, many engineers will list materials that all exhibit the strength, corrosion resistance and compatibility characteristics they need regardless of machinability.
In some cases, a vendor might not have the original material on the shelf but a similar material they do have will suffice. Then the engineering might be revised to include the less common, though available, materials.

303 would be conventionally accepted as a CRES simply due to the fact that it exhibits similar corrosion resistant properties to other corrosion resistant steels.

While 303 is easier to machine than 304, 304 is usually easy enough to not justify the ~20% increase in material cost.

Brian Herr
Marlen International - Riverside, Kansas, USA

adv.: ResinTech banner

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