Passivation of surfaces for medical technology
Stainless steel alloys used in medical technology must be resistant to corrosion caused by all ambient media as well as body fluids and tissues. In order to restore the natural passive layer of stainless steel, which is contaminated and damaged during the mechanical manufacturing of implants and medical instruments, we process such parts by passivation processes in our Medical Surface Center.
Passivation – areas of application
Passivation – How it works:
The chromium contained in the stainless steel reacts spontaneously with the atmospheric oxygen on the surface of the part to form a chromium oxide layer that is 3 to 5 nanometers thick and protects the base material against corrosion (passive layer). This passive layer is contaminated and/or damaged during the manufacturing process. The exposed iron then oxidizes, producing surface defects that can encourage corrosion and may result in breakage of the implant and medical instrument. Surface treatment, commonly referred to as passivation, is essential.
Nitric acid also removes the exposed iron and other metal impurities from the surface and encourages chromium oxide to form. If the natural passive layer is extremely contaminated and damaged, pickling prior to passivation is useful for forming a metallically clean surface. Nitric acid should be used for passivation in this case.
If the passive layer is not damaged and there are only foreign metal traces on the surface of the part, these can also be removed with other acids. Citric acid is known to be very effective. It notably removes iron from the surface of the part by forming an iron citrate complex, thereby preventing oxide formation, which is particularly useful in medical technology. However, citric acid is not able to develop a chromium oxide layer. Titanium must not be chemically passivated because it already forms a stable oxide layer spontaneously. In the case of superficial foreign metal impurities, treatment with nitric acid or citric acid can prove useful.
Passivation technology for medical technology
We use an immersion method to passivate stainless steel parts, whereby the parts are immersed in the passivation acid in baskets made from a chemically resistant material. Acid concentration, temperature and passivation period are all in accordance with the nitric process for nitric acid or the citric process for citric acid, as described in the ASTM A967 standard. After passivation, the parts are rinsed thoroughly with high-quality water (osmosis water, demineralized water) and dried immediately. As a rule, the surface treatment processes used, including pre- and post-treatment during passivation, are based on ASTM A380 and ASTM F86.