303 Stainless Steel Passivation Problems

A. For starters, there's the usual issues with free machining (high sulfur) grades like 303, they need the proper passivation process, either nitric type II aka Nitric 1, or an alkaline pretreatment prior to citric passivation. Extra surface sulfur could explain the general darker color.

The blotches are probably exactly that, fingerprints and lack of sufficient precleaning. The alkaline cleaner ought to be able to take care of those, or switch to handling the parts with gloves only.

A. Pitted, dark, stainless steel passivation: I passivated some aviation bushings Half came out pitted and blackened the other half pretty shiny finish. I informed the customer and they were upset that I ruined their parts. I never understood what happened but I was confident the the chemicals did not lie, they had two different alloys.
Former Metal Finisher, I crossed over to the "DARK SIDE" now I work for the state's environment department.

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 sulphate ⇦this on eBay or Amazon [affil links] 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 Solutions [a finishing.com supporting advertiser]. For the ultimate, in my opinion, have them electropolished.

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.

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. 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 A967 for passivation of their parts, since the cancellation of AMSQQP35 [canceled]. If it is aerospace, however, you must use either AMSQQP35 [canceled] or AMS2700, which is replacing AMS QQP35. AMS2700 allows the use of nitric or citric acid formulations.

A. Lee is right. Citric will cure your ills. I like the caustic hotter, 160 °F 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.

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 [a finishing.com supporting advertiser]. 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)

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!)

A. Kelly,
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.

adv.
Let me know if I can help.

A. Hi Jean-François,

A couple of pictures would help here, to see exactly what you are talking about.

I have had problems with high sulfur compounds in the past. The hypothesis here is that the sulfur stringers are attacked in the alloy preferentially, which then sets off the chemical reaction that causes etching.

To get over the problem we treated the alloys as if they were high carbon alloys, such as 440C, using a good, strong alkaline cleaning process and using fresh passivation solution. This solved the problem, without us truly getting to the root cause, which is what often happens in a production environment.

A. A 1% each citric acid and sodium nitrate solution is not very likely to actually passivate stainless steel. 4 wt% minimum citric acid is the industry-recognized mixture. A380 code N falls under the heading of "cleaning" rather than "cleaning and passivation".

As far as free machining grades go, yes, the sulfur is a source of various problems. In the old nitric acid passivation days, it was "solved" with the addition of dichromate to the bath. These days we recognize that a good alkaline bath pretreat usually gets enough sulfur out of the surface that it won't be an issue.

 Hi Ray,
I 100% agree with you when it comes to standard operation of Citric tanks, and a good deal of what I know about their unique benefits comes from your posts over the years.
This all started with one particular large batch of parts from a single heat lot of 416 that one day (cut on first shift) was running fine, and the next (cut on second shift) was burning black in the tank we historically use for 416, from a recipe given verbally to my mentor years ago by a technical rep at Carpenter Steel. And it usually works!
We then tried the parts in our standard Code N hot citric 4-6% tank, which we use for a range of alloys, and they literally disintegrated. Like, there was NOTHING left in the baskets but bubbles.
By this point, I had two senior managers and a VP hollering on the phone for parts as I'm trying to squeeze in questions about the run conditions, cutting fluid, deburring op, etc...
That's where we ended up throwing a Hail Mary pass- um, I mean- trying the weak cleaning solution. All our work is tested post-process, and the parts passed 24 hour water immersion, so we continue to use it.
I'd definitely reiterate the importance of the statement 'to pass specified test requirements', and definitely NOT to assume that the method worked without running water immersion on it!
It's just one more weapon in the arsenal against PITA parts :)