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303 Stainless Steel Passivation Problem
An ongoing discussion from 2003 through 2015 . . .(2003)
Q. I have some 303 stainless steel 3/8" pins that will see salt water and fresh water environments. These parts were passivated. I subjected them to 500 hours of salt fog testing and they rusted mostly at the ends where they were machined/cut. I also ran 304 pins at the same time and they did not rust. I was told that 303 should not rust if they are properly passivated. How can I tell if they are properly passivated in the future? Is there a test I can run?Sid Tryzbiak
- Tulsa, Oklahoma
A. Most of the 303 that I saw at two shops was "free cutting" and was normally 303Se. This proved to be much harder to passivate than plain 304. Salt spray of a larger sample would be my approach to telling if it were properly passivated. Copper sulfate testing can be done fairly easily "in house" also. Ferroxyl is a far more discriminating test, but it is more sensitive than most peoples requirements. You might look into doing a citric acid passivate in house with a proprietary product such as Stellar. For the ultimate, in my opinion, have them electropolished.James Watts
- Navarre, Florida
A. Sid, dump the 303. Use 304 or 316, or some other stainless that isn't free machining. The small manganese sulfide inclusions that give 303 its easy machinability mess with the corrosion resistance.
If you're stuck with 303, try passivating it with a Type VIII solution, rather than the type II your vendor is likely using. We've had better luck with VIII.
Challenges with Passivation of 303S(2003)
Q. I have found this forum to be extremely useful over the last several years. I have read numerous posts and the FAQ in this forum related to passivation problems - especially those related to "unexplained" frosting, burning, etching, and discoloration. Problem: 303(S) displays "frosted" (light etch) after nitric/dichromate (QQ-P-35 [link is to free spec at Defense Logistics Agency, dla.mil]) passivation. Extent of frost varies (apparently at random) from piece to piece and load to load. Frosted appearance varies from ~100% to 0% surface coverage. Several loads of other 303(S) parts were successfully processed on the same line on the same date with identical processing (automated line) with no problems.
Some of the parts were process in a smaller "hand" line at another site that display the same problem. Results suggest the same mechanism is at work. However, the parts have less "frost". (Possibly related to the random nature of the problem, possibly to some unknown difference in conditions?)
One of the suggestions on this site referred to soaking the parts in alkaline solution prior to processing. We have tried one hour soaks in both alkaline cleaner and 5% NaOH solutions, both heated to 140 °F, with no visible change in results.
We are a captive shop. We are using many of the "free machining" CRES alloys. At this time, it is not an option to change raw materials. We are ordering several forms of 303 raw stock (hot rolled, drawn, cold rolled, etc.) to see if we can identify a specific form of the material that seems to give us problems.
We recently had a similar, though much more catastrophic episode with some 416 parts. The attack was far more aggressive, but reminiscent of this problem in that the attack appeared to be random, varied from piece to piece, and from load to load. The variance in the attack was more severe, varying from nearly complete destruction of the part to no visible signs of attack.
1. Precisely what is the mechanism of the attack on the sulfides? Is it removal of the sulfides from the surface or a secondary reaction forming (sulfuric or sulfurous?) acid which then attacks the material?
2. What are the accepted methods for prevention?
3. My company works on military hardware and is currently working to AMS QQ-P-35. Citric acid is sometimes quoted as a potential fix but is not approved (according to the way I read the spec). If other shops are using citric acid, are they doing so on military hardware? If so, how do you conform to the spec and still use citric acid?Jeff Walton
Process Engineer - Sherman, Texas
A. Jeff, don't use Type II. Use Type VIII. It's cheaper, it's less hassle, and it works for both 303 and 416. We switched several years ago and haven't had any more etching.
You may have to run some tests so as to have documentation to satisfy some of your customers. Type VIII isn't listed as a recommended bath for 303 or 416 in the back chart of QQ-P-35. Yet if folks get porky about specsmanship issues, you can likely point to the first cancellation notice of QQ-P-35, which sent you to ASTM A967 [link by ed. to spec at TechStreet] . That spec allows various combinations of bath, time, and temp provided you pass the testing. In ASTM nomenclature, I recommend Nitric 4 rather than Nitric 1.
A. I agree with Lee's comments about using nitric Type VIII if you have to use nitric acid for these grades. A number of our customers have had success with this.
The most accepted theory about what is happening is that the sulfides bloom to the surface through the grain boundaries. This causes a number of problems and reactions with the nitric acid solution, especially after you have done a number of parts in the solution and pulled some sulfide into the solution.
If you read the manuals on this most everyone agrees that you need to pretreat with alkaline before passivation of these grades. This is not only to prevent the frosting, but also to get better passivation and prevent post passivation discoloration from the sulfides. Talk to the steel companies.
Military has approved the use of ASTM A-967 for passivation of their parts, since the cancellation of AMS-QQ-P-35 [link by ed. to spec at TechStreet]. If it is aerospace, however, you must use either AMS QQ-P-35 or AMS2700 [link by ed. to spec at TechStreet], which is replacing AMS QQ-P-35. AMS 2700 allows the use of nitric or citric acid formulations.
Stellar Solutions, Inc.
A. Lee is right. Citric will cure your ills. I like the caustic hotter 160 min. If someone has their undies in a bundle over the spec we'll passivate with citric first and finish with the Type II which we must cert to. Silly but meets customer specs and gives them what they want, passivated parts.Jon Quirt
- Minneapolis, Minnesota
Q. We are a small CNC machine shop, running mainly 316ss,17-4PH, and XM-27. As a service to one of our larger customers, we run a small amount of 303ss. Because of the fact that most of our parts are passivated in-house to QQ-P-35 [link is to free spec at Defense Logistics Agency, dla.mil] either type II or Type VI, we are a little apprehensive of passivating 303 after reading some of your articles on the problems that may be encountered, with thta alloy.
What we are wondering is there any test we can run on parts made from 303ss to check for sulphur, which we believe may be the variable that interferes with passivation?
We do run a sample part through prior to running the entire batch, and as a precaution, all parts to be passivated are ultrasonically vapor degreased prior to passivation attempting to remove any sulphur contaning oils, but we are concerned about sulphur in the metal itself.Robert Prichard
heat treat / passivation mgr - Chadwicks, New York, USA
A. Good for you you are being pro-active rather than reactive. Yes be concerned with 303. More so if is nicely machined as this will smear the sulfur over the surface. You can try the AAA process, Alkaline/Acid/Alkaline, NaOH 5% by wt. 160 Min 60 minutes/Type II/ Again alkaline. Or make life simple and use Citric passivate from Lee at Stellar Solutions. If you must spec to the obsolete QQ-P-35 then pre-passivate with a good long citric. It will complex the sulfur. (Lee will most likely answer also, Hello Lee)Jon Quirt
- MInneapolis, Minnesota
A. Jon is correct. If you pretreat the 303 with hot alkaline solution you should have no problems. We have many customers doing this with complete success. There is a considerable variation lot to lot with 303 and sometimes you can get by with minimal treatment, but you need to treat like Jon says or pretest every lot. (Hi, Jon!)
Stellar Solutions, Inc.
Passivate 303 free machining stainless per ASTM A380February 9, 2015 -- this entry appended to this thread by editor in lieu of spawning a duplicative thread
Q. I have a part that calls for passivate IAW ASTM A380, Code A.
The material is free machinining 303 stainless per ASTM A582.
Code A (Table A1.1) of ASTM A380-13 seems to be for pickling (descaling, i.e.heat treat scale or whatever). Our part is not heat treated, it is 100% machined surfaces, nice and clean.
Question: Code A doesn't seem like the best choice for 'passivate'. Would a more appropriate Code per ASTM A380-13 be J, K or L (Table A2.1)?
Quality Engineer - Portland, Oregon, USA
^- Privately contact this inquirer -^
February 10, 2015
Passivation in A380 refers to the processes in Table A2.1 part II, which is codes F-M. Code A is in Table A1.1.
You are correct, Code J/K/L is stated for "200, 300, and 400 Series free-machining alloys", which 303 falls into. Although in my opinion, codes K and L do not make sense for what they are supposed to be doing and should not be used for anything.
The 2013 revision also opens the door to using passivation per A967 to meet the requirements, which in my opinion is much better than anything from Table A2.1, especially if you choose to use citric acid passivation with alkaline precleaning rather than nitric acid with added sodium dichromate. Your process would be much less hazardous in that case.
Let me know if I can help.
Stellar Solutions, Inc.
Ed. note Feb. 2015: Readers may also be interested in letter no. 639, "Passivation of 303 stainless steel".