Passivation of surgical instruments, implants and other medical devices made of stainless steel
To achieve optimum corrosion protection, it is imperative that stainless-steel instruments and implants be subjected to sufficient passivation. This applies in particular after marking by means of etching or laser. There are several methods to restore or improve the formation of a passive layer. Electropolishing, nitric acid and other methods have yielded the desired results in the past, but were also associated with various environmental and safety problems.
The following will describe a method that – underpinned by several scientific experiments – can produce the best possible result. The results of the study by the NMI have confirmed that the effect of this method is not only equal to but superior to other well-known alternatives. This method conforms to the procedures described in the US standard ASTM 967A and required for the passivation of stainless steel.
All grease and oil residues are removed with an alkaline cleaning (pH>11). In addition, the sulphur components are removed from the surface, which could lead to a reduction of the chromium in these areas and thus to pitting. In carbon-containing steels (1.4021, 1.4034 ...) the excess carbon is removed from the surface by an alkaline cleaner, which reduces the risk of grey discolouration. With a temperature of 60 °C and by using ultrasound, the cleaning effect can be significantly heightened. Additional rinse cycles reduce entrainment of cleaner residues in the passivation solution.
In the 10% citric acid, the free iron particles are removed from the surface. The use of ultrasound and a temperature of 55°C optimise the expected outcome. In subsequent rinsing cycles in distilled water and in the drying cycle, a sufficiently thick chromium oxide layer is formed. The chromium–iron ratio and the ratio of chromium oxide to iron oxide should now be five to six times better than before treatment. The resulting chromium oxide layer is three times as thick as in natural passivation. The product should then no longer be subjected to actions that could affect the integrity of the passive layer. Steps such as polishing, grinding, finishing, sharpening and marking must be performed before passivation. Upstream electropolishing optimises the passive layer because the surface is smoothed. Electropolishing does not replace passivation.
Since the parts are completely free of grease and dry after treatment with CitriSurf®, it is absolutely necessary to oil them with instrument milk or oil, especially the box locks and all other moving parts. Otherwise, damage may be caused, particularly in scissors and instruments with box locks.
This method polishes the surface, but does not improve the chromium–iron ratio. The targets for electro-chemical oxidation processes are reduced, and a visually homogeneous surface is created.
The first step before passivation is cleaning with an alkaline cleaner in the range of pH 11.5 or higher at the manufacturer’s recommended temperature of 55 °C. The manufacturer of CitriSurf® developed KleerKleen®4002 specifically for this step. The parts are then washed off in water and rinsed in distilled water.
The use of a passivation solution from Stellar Solutions has proved to be particularly effective. CitriSurf®2250 is supplied as a concentrate and must be diluted. For steels used in surgery, Stellar Solutions, the manufacturer of CitriSurf®, recommends a dilution of 1:5, i.e. one part CitriSurf®2250 and four parts distilled water. The ideal temperature ranges between 45 °C and 55 °C. In a sufficiently large basin, the use of ultrasound is not absolutely necessary, but it saves moving the passivation material around in the solution and allows the passivation solution to get to the otherwise inaccessible parts of the instruments. This effect can also be achieved with circulation pumps or air injection. The time depends on the geometrical shape of the object to be passivated (for laser-marked items, we recommend not exceeding five minutes because the laser marking may otherwise fade). Ten to fifteen minutes are more than adequate for complex parts/instruments.
Rinsing and drying
After treatment with CitriSurf® the parts/instruments must be rinsed with distilled water (preferably twice). Subsequent drying can be carried out in the air or in a heat-drying oven or basin. The drying time will be shorter when the rinsing cycle uses warm water. If ultrasound was used for the passivation process, it should also be used for the rinsing process to ensure that all residues from the passivation solution are removed.