Stainless steel laser etch and passivation

A. The laser-etched area has iron oxides on the surface. These oxides makes the laser-etched surface to be visible to the optic devices. It is important to leave the laser-etched surface not impacted with passivation procedure. Both citric, nitric and hydrogen peroxide solutions, if staying longer than 20-30 seconds, will begin to clear the laser-etched surface.

Passivation instructions:

Degreasing and rinsing.
Prepare the modified passivation solution:
5-10g/l (85% Phosphoric acid
0.5 g/l NaCl (kosher salt)
1% Hydrogen peroxide solution.

Use plastic mesh to submerge single-layer parts into room temperature passivating solution. Hold for 20 seconds, remove, rinse with tap water. Submerge into solution of 5% sodium carbonate, rinse again. Submerge into rinsing tank again and check that pH is neutral. Submerge into the tank with deionized water. Dry. If parts are in form of tubes, make sure all water is dried out from the tubes.

Be sure that after passivation the SS parts are rinsed in deionized water and dried completely. Humidity destroys 420SS during extended storage time. Make sure that the storage area is dehumidified.
Also take special precautions when packing that the parts do not touch each other, in order to prevent damage to passivated film.

A. I would say that in general, "do we need to" questions are a matter between you and your customers. ASTM A967 for example is not written with medical applications in mind, though other medical devices standards do refer to it.

If it were up to me, I wouldn't worry to much about "validating" the passivation of a laser mark on stainless. Run it in a citric acid process and cut the time short if need be to prevent the laser mark from being ruined. If you really want to run a test, in my experience the best one for laser marks in the boiling distilled water immersion test (listed in A967 and ASTM F1089).

I'm not sure what you mean about chromium % unless you're looking at the copper sulphate [on eBay or Amazon]test method. copper sulphate is very prone to false failures on laser marks, mainly because you can never really passivate a laser mark to the same degree as the unmarked steel.

A. Hi Mike,

Laser marked colors are generally made of inorganic chemicals that are soluble in acid solutions. As a consequence, the colors will disappear during etching. Furthermore, some of the dissolved color components are able to attack metals.

In general, laser marking operation follows surface finishing. You should be able to get rid of this problem by changing the process.

Ling

A. adv.
We have had great success on this application with our CitriSurf 2250, a citric acid proprietary blend that provides much improved corrosion resistance and lower reject rates for many surgical instrument manufacturers and orthopedic manufacturers. It is a much easier process, yet produces improved results.

However, we have also found that any laser etched surface is going to be the weakest link on the surface, and a potential corrosion site. We have recommended to some manufacturers that they etch a little deeper with the laser and then completely remove the black oxide. This produces an "engraved" surface that looks excellent and has maximum corrosion resistance long term. This can all be done easily.

Please contact me for more details or if you want tests run.

A. We mark medical and aerospace components daily, and have not heard of this problem with regards to the laser marking we provide. It is possible that your supplier is putting a very fast pass with the laser to achieve price points required. If you allow the beam to dwell longer by slowing down speeds or putting multiple passes on product the mark will be blacker and will not go away.

A. If you are getting rust more in your laser mark, than on the rest of the part, I would say that you should passivate after the marking. I don't believe that you remove much material during a proper passivation. But at any rate, you should be able to easily test some scrap parts before going to full production with the new process. -tom

Thanks for the suggestion Tom, I will give it try!

A. adv.
Gary, the surgical instrument industry has attained dramatic reduction in corrosion and rejects due to laser etching with the use of CitriSurf products. They passivate AFTER laser etching, and depending on how you do it (and the grade of stainless) you can either leave the dark etch mark or remove it to get an "engraved" look.

Either way, you will improve your corrosion and rusting resistance of the laser etch by passivation. Unless you remove most of the dark material, it will always be the weakest link in the part, but passivation with CitriSurf will give you considerably improved results. Lee

A. Anthony:

This is a typical problem. The laser etched area is the weakest area on the parts and must be passivated to prevent rusting in the etched area. However, it can be very difficult to get the etched area passive without removing ALL of the darkened color used for identification.

adv.
We have had great success with our citric acid formulations in producing excellent corrosion resistance in the laser etched area without removing the color. Many surgical instrument companies have gone to this system because the amount of problems and rusting are significantly reduced compared to nitric acid passivation.

Let us know if we can help you.

Regards,

A. Passivation of stainless steel that has been laser etched must be done with a different solution of Nitric acid. If you have found this problem to be intermittent, it is due to the variation of the chromium content in the SS. For the laser etch to hold the chrome content must be above 16%. 17-4 stainless rides right on this line. Any washed out laser etch issues will have lower chrome then this. If you are experiencing these problems, you are probably using a nitric 2 bath. The solution is to use a nitric 1 bath which is a lesser concentration of acid or to use citric acid .

A. Each company in this industry has different requirements, but usually they want the discoloration of the laser etch to at least partially remain in order to be more visible for doctors and nurses in hospitals, etc. This can still be attained and get good passivation with the correct citric acid formulations. If you are using straight citric acid it is probably not possible.

Significant reductions in corrosion of the laser etch is possible with the correct formulations without removing the mark. This is being done on millions of parts.

adv.
Contact us if you need further information.

A. Gene, I've been laser marking Orthopedic instruments made from 300,400 and 630 series of stainless steels for 15 years. The majority of these parts were passivated in 30% by volume Nitric acid at room temperature for thirty minutes. There have been few problems with this process. However, I have had problems recently with citric acid passivation. Details of laser marking parameters versus passivation too lengthy for this reply.

A. Re: laser marking on SS. What type of laser? Yag or diode pumped? In general, the darker the laser mark the more likely it will rust. A good question to ask is what gives laser mark its dark appearance? A nice rich layer of iron oxide.
I once upon a time helped with a study on surgical tools (17-4). In a humidity cabinet I would see the nice dark laser mark drip red rust in a matter of hours. We played around with means and methods and concluded the lighter laser mark to be more corrosion resistant. I had some idea of electroetching letters but was trumped by a company called Thermark. They have or had a ceramic glaze that was brushed on prior to laser mark. The laser fritted the glass and the resulting laser mark was damn near corrosion proof.

A. Strange that someone has success with nitric acid and not with citric, but maybe the conditions are different.

adv.
We have many customers who have switched to citric passivation totally BECAUSE of vast improvements in corrosion of the laser mark compared to what they used to get with nitric acid.

If the right laser and enough power is used you can get an excellent mark that will passivate very well without being removed by the passivation or rusting in the autoclave.

A. I have been etching all kinds of surgical instruments for almost a year due to the coming UDI by the FDA. We provide total instrument tracking solutions to hospitals.
We have three YAG lasers (Rofin 1064 nm) and this is my experience with Citric passivation and marking:
You can engrave, etch or aniline any material, coming to stainless steel this are my findings --

1 We have tried Citric passivation with good results
2 We designed an ultra fast sensitive 2D camera so we can make really small marks without stressing or removing much material of the instrument
3 We usually run tests with Hydrogen peroxide, which is more aggressive, to see the results of passivation; citric does a good job.
Rule of thumb, marks must have a good contrast black to be easily read by scanner. I have been seeing in SPD (Sterile Processing) people getting frustrated because they cannot read marks.
4 Laser set up is fundamental; power, frequency, focus play the ultimate results.
Really not much literature online to learn this but I have been investing a few months perfecting the DataMatrix marks.

A. I think in general, going for a deeper etch gives something that you can be sure will hold up over the long term. However, most laser marking on stainless is in the medical industry for surgical instruments, and I am told that in the dimly lit surgery room, the very dark black high-contrast mark is key. There are tricks to this, but it always comes down to some trial and error as well.

I think for something like medical ID bracelets, for the most part something with a deep etch would be okay even after the black starts to wear off.

A. Arda,
Passivation is done with an acid, generally a dip bath or a paste version, so I'm not sure why you say an uneven surface is causing a problem for you.

You do need to passivate after laser marking on stainless, though some trial and error is often needed to determine an ideal process time that does not damage the mark itself.

A. Arda,

Passivation of stainless steel after laser marking can sometimes be challenging and there is a different method. In some cases you would want to passivate before laser marking and do a citric gel passivation after laser marking; I have also found that nitric with sodium dichromate passivation works well on 300 series stainless steel to keep the laser marking from lifting.

A. Ak,
Post-laser mark passivation is often done using citric acid formulations. There is still the issue of removing or otherwise ruining the laser mark, but it happens more slowly than with nitric acid, so with some trial and error you can work out when to remove the parts from the citric bath to have better corrosion resistance on the marks.