Laser marking for passivation and corrosion resistance

Many stainless steel medical tools and devices require passivation to help withstand corrosion and handle numerous autoclave cycles. These devices also require laser marking for identification, tracking, and traceability in accordance with unique device identification (UDI) requirements. Unfortunately, passivation and autoclaving tend to remove laser marks, and several medical device steel and surface finishes interfere with maintaining a passivation-resistant mark process.

Amada Miyachi America has developed solutions – ranging from standard fiber laser markers to a picosecond laser marking system – for reliable dark marks on medical devices of various materials and surface finishes.

Passivation for autoclaving

The most commonly used medical device materials are stainless steel 304 and 17-4. These materials have a natural passive corrosion-resistant layer, consisting of chromium oxide, which resists repeated sterilization and has an inert surface that will not react inside the body.

During the manufacturing process, multiple machining steps can remove or degrade this passive surface by embedding iron chips and particles into the surface. The material must then be put through passivation to rebuild the passive layer – removing iron from the part’s surface, which also removes potential corrosion sites.

The style of mark required by the medical device industry is called a dark or annealed mark. This mark does not remove any material from the part, avoiding any potential for any contamination to collect. When handling the part, the dark or annealed mark must not be able to be felt on the surface of the material. The heat input needed to build up this oxide layer tends to degrade the passive layer on the marked surface and can cause local migration of alloying elements.

After marking, if the part’s surface layer has an imbalance of chromium, it will be etched away by the acid used in the passivation process, causing the mark to fade or disappear. The level of imbalance determines whether the mark fades over repeated sterilization procedures, and if so, how quickly.

Several passivation processes use citric or nitric acid at or above room temperature for a period of time. The customer may select the passivation method following the ASTM A967 standard. Manufacturers have been moving to citric acid due to its reduced chemical footprint compared to nitric acid. In laser marking, citric acid is preferred as it tends to be less aggressive to the mark. Using nitric acid can degrade the mark’s lifetime in autoclave cleaning cycles.

There are really no alternatives to permanent laser marking for devices that require passivation. For example, an ink-jet mark is non-permanent and contains chemicals that are not FDA-approved. Chemical etch is not practical for marks that require incremental serial number and lot codes as well as data matrix codes. Mechanical pin-stamp machines provide poor mark resolution and create indentations in the surface that cannot be cleaned properly.

Laser selection

Manufacturers struggle with making passivation-resistant and anti-corrosion marks on stainless steels. They either use the right laser with incorrect parameter settings or simply select the wrong laser for the mark. Difficulties increase as materials change.

Even after selecting a suitable laser source for the application, it is important to establish the correct heat balance process window for each part and each mark on that part. The size of the letters, the number of letters, and the thermal mass of the part area are all significant factors.

So many parameters can be adjusted on the laser and the scan head that the starting point may seem daunting. It is essential to get instant feedback from a passivation test immediately adjacent to the machine. The problem is finding the starting point from which marks can be developed based on controlling the heat balance, because there can be false summits that do not provide a stable process.

The limited amount of published work on this type of marking suggests that minimizing heat input is preferred, which is reflected in the laser selection. However, the dark mark by its nature requires significant heat input to the part. Other factors include using a uniform heat source on the part and controlling the rate of heating and cooling. This foundational work provides a starting point to fine tune a specific mark (see Figure 1, pages 28-29).

However, under higher magnification, mark (a) shows surface cracking of the oxide layer compared to the mark that does not pass, which seems counter-intuitive. The picosecond laser (c) mark surface appears completely different, with what appear to be micro features when compared to the base metal. The details of these microstructures are unclear. In addition, an energy-dispersive X-ray spectroscopy (EDX) surface element analysis shows the amount of chromium present in the mark area that passed passivation was comparable to that of the base metal’s surface. Marks that did not pass had about 15% less chromium.

Producing passivation-resistant marks

The laser source needed to produce passivation-resistant marks is directly related to the material, surface finish, and speed of the required mark. This ranges from a standard fiber laser marker, such as a benchtop fiber laser workstation, up to a picosecond laser.

Producing a dark mark for passivation resistance in 304, 316, 17-4, and even 17-7 stainless steels can be achieved using a variety of laser sources, including nanosecond fiber, nanosecond UV, and IR picosecond. However, to achieve such a mark, one must clearly understand laser parameters, with specific reference to uniform heat input. A universal marking solution for every part and each mark does not exist, but one can establish a starting point to develop each mark on each part. For example, the fiber laser marker can be used on 300 series stainless, and in some cases 17-4 steels; however, as the material and surface finish increase in difficulty and cycle times, one must consider nanosecond UV and IR picosecond solutions. (See Table 1 on page 29.)

Selecting the correct laser source for the part and the required mark will lead to a process that is stable in everyday production. This is critical, because substantial costs can be associated with an unstable mark from inconsistent yields, re-working parts, missing delivery schedules, and constant process tinkering. Aside from saving labor costs, the process provides a reliable, planned, and consistent machine output.

Amada Miyachi America

www.amadamiyachi.com

Laser Micro

www.laser-micro.com

About the authors: Geoff Shannon, manager of advanced technology at Amada Miyachi America can be reached at 626.930.8448 or geoff.shannon@amadamiyachi.com. Gary Firment, president of Laser Micro can be reached at gary@laser-micro.com.