Splotches on Hypotubes

A. There could be oxide deposits on the surface, but I would think that unlikely unless the tubes have been handled improperly. I have seen "splotches" due to surface finish variations that one would have sworn to be surface deposits. A good analysis should be able to absolutely identify the condition and cause.

A. I know that it is a long time since you posted the problem but I have just come across it now. We make millions of such Hypotubes and I think we have encountered nearly every problem! The splotches are probably oxides but they could just be residuals of processing oils. If the tube is being used as a catheter shaft it is very unlikely that patient safety is being compromised. There is, however, no need to have such markings and, if they are visible to the naked eye they would normally be considered unacceptable. If they are oxides they may indicate inadequate rinsing after passivation. Note, however that all 304 stainless steel has oxides at the surface. You just cannot see them.

Best regards,

A. Tony,
My only advice to you is to look carefully at the "stains" under magnification. In my experience, the majority of stains blamed on residue turn out to be etch marks, which is why no amount of cleaning or rinsing will remove them.

Q. Thank you for your responses.

The tubes are 304 stainless passivates with Nitric.
Our Passivation process is as follows:
Degrease-rinse-rinse-pickle-rinse-nitric-rinse-hot rinse.

What is the function of sodium bicarbonate post Nitric and would it beneficial in reducing staining.
We hold the tubes in a circular gripper I.e. 1000 tubes together. I know this goes against the principles of correct rinsing but capacity dictates at the moment. Would ultrasonics in the final hot rinse help clean the tube OD's when they are bunched together like this.

A. Tony,
I'm still not really sure what your stains are or what exactly is producing them, so any talk on what will remove or prevent them is mere speculation.

An alkaline bath is often used after nitric passivation to ensure that any residual acid is neutralized and cannot cause any etch damage to the surface. Sufficient water rinsing really should be enough on its own, but the "AAA" or Alkaline-Acid-Alkaline process is traditional for nitric passivation.

I would say that ultrasonics certainly isn't going to hurt anything when it comes to making sure your chemical solution or rinsewater is getting access between stacked surfaces.

I would also be interested in seeing if your problem goes away if you were to passivate using a citric acid based system rather than the nitric.

A. Hi Joshua. We probably need some clues. The parts are 304 stainless? The passivating solution is nitric acid or citric acid or something else?

Regards,

Q. My question was a general follow-up to Ray Kremer's response regarding staining following passivation. We run nitric (AMS2700 Type 6) and nitric/dichromate (Type 2) passivation lines, and when we've had staining issues, we have usually approached troubleshooting from the standpoint of rinsing, so I'd be interested to hear if there could be liabilities within the process itself that could be causing these issues.

Thank you,

A. Joshua,
The core of the issue is that you are assaulting the surface of the stainless steel with an acid, and hoping that it's going to attack and remove lots of iron but very little of the chromium, nickel, and other stuff that makes up the alloy. How much that's going to be true depends on the grade of steel, the composition of the passivation bath, and other process parameters such as time and temperature. It's pretty easy to ruin parts, or worse, if you mix a grade of stainless with a nitric type bath that isn't meant for it. As a broad generalization, you're more likely to get surface etching by leaving parts in the bath too long if you are using nitric acid than you are if you are using citric acid . Nitric acid residue that isn't properly rinsed or neutralized counts as "too long" but that isn't the only way to end up with undesired marks on the surface.

Given that you are having issues, I would run some trials with adjusting some of your bath parameters (concentration, time, temperature) and see what affects the outcome. You also might try out a citric acid based process and see if you are happy with the results from that.

Also: any time surface etching comes up regarding a 400 series grade of stainless, the first question out of my mouth is "Was it heat treated to full hardness?" If the answer is no, there's your problem.