Surface Treatment of Surgical Instruments by Chemical Passivation: New Insights from a HAXPES Study
Presentation at the
DGSV - congress 04.10.2022 WFHSS -congress 17.11.2022
Abstract published in
Central Services| Volume 30 | Suppl. Oktober 2022
Authors: Matthias Buhmann, Olga Guseva, Patrik Schmutz, Qun Ren
Title: Surface Treatment of Surgical Instruments by Chemical Passivation: New Insights from a HAXPES Study
Background and aim of the study
Surgical stainless steel instruments are mostly intended for regular reprocessing in cleaning, disinfection and sterilization processes. Although the instruments are made of stainless steel designated as "rust-free", corrosion phenomena such as corrosion-induced discoloration or damage cause high costs. These consist of the time required for internal investigations (is it rust or blood?), repair costs and replacement.
When stainless steel instruments corrode, metallic iron oxidizes to form brownish iron oxides, resulting in rust. One of the causes of corrosion damage is the material used to manufacture the instruments: although there are very corrosion-resistant steel grades (alloys), these are often not suitable for the manufacture of surgical instruments due to their material properties. Especially in the case of less corrosion-resistant material compositions (alloys), the treatment of the instruments in the disposal and reprocessing process plays a decisive role. In particular, long contact times with chloride-containing media such as blood and physiological saline solutions, or halogen-containing disinfectants such as povidone-iodine (PVP-iodine) lead to corrosion. Added to this is mechanical damage caused by improper instrument dropping or instruments being knocked against each other during transport.
Chemical passivation processes can improve the corrosion resistance of surgical stainless steel instruments. The presented work summarizes fundamentals of chemical passivation and investigates the quality and composition of the passive layer using test specimens of typical instrument steel grades after chemical passivation with a typical, citric acid-based, versus an oxidative, nitric and phosphoric acid-based process.
The passive layer of stainless steel instruments
The mechanical requirements of the respective intended use, but also properties which influence the manufacture of the instrument (e.g. hardness, toughness, tensile strength, mechanical machinability), determine the stainless steel alloy used. The composition and processing of the alloy in turn influence the formation of a uniform passive layer and thus the degree of corrosion resistance. The chromium content is particularly decisive here: a chromium content of ≥12% by weight or more enables the formation of an oxide layer a few nanometers thick on the material surface, the so-called "passive layer". At a chromium content of 12%, the formed passive layer offers a certain degree of protection against corrosion, but the resistance of the material to environments that promote corrosion is limited.
The passive layer formed in acidic media consists primarily of chromium oxides/hydroxides and only small proportions of iron and molybdenum oxides/hydroxides. In aqueous environments, the passive layer forms spontaneously and acts as a barrier to atmospheric oxygen and the diffusion of iron atoms to the instrument surface (Wallinder, Pan, Leygraf, & Delblanc-Bauer, 1998). The higher the chromium content, the better the barrier properties of the passive layer and thus the corrosion resistance. For example, a 1.4112 alloy contains a relatively high chromium content of 17-18%, while a 1.4021 alloy contains a rather low amount of chromium at 12%. In addition to chromium oxides, small alloying amounts of molybdenum also reduce the diffusion of atoms through the passive layer and enhance corrosion resistance (Seyeux et al., 2022). Not only the alloy, but also the manufacturing process of the instruments and thus the uniformity of the metal structure play an important role in the formation of a high-performance passive layer. Iron contained in the chromium oxide layer, as well as defects in the material, such as foreign material inclusions, distortions in the metal structure or laser markings can hinder the formation of a continuous chromium oxide layer. Therefore, different corrosion tolerances can be achieved despite identical alloy compositions.
The exact mechanisms of corrosion inhibition by the passive layer of stainless steel have not yet been fully elucidated, but experimental data indicate that the passive layer is built up as a double layer with a surface layer of iron oxides/hydroxides, chromium hydroxides and molybdenum oxides/hydroxides and an underlying chromium oxide layer. This prevents aggressive halide anions (such as chloride from saline solution) from migrating in on the one hand, and iron atoms from migrating out to the surface on the other (McCafferty, 2010).
Attack of the passive layer / de-passivation
The passive layer should not be regarded as a static layer; it is continuously influenced by environmental conditions and is in a dynamic equilibrium between degradation of the layer ("de-passivation") and its reconstruction ("re-passivation") (Schmutz & Landolt, 1999a, 1999b). In daily use, mechanical damage during use, dropping or transport can injure the passive layer. If the passive layer is mechanically injured or degraded only to a limited extent, it is possible that it will "heal" by itself. In the presence of aggressive halide ions (e.g. Cl–, Br–, I–), however, the passive layer is attacked and self-healing is also impaired (Soltis, 2015). However, contact with media containing chloride is common in practice: in the operating room and in the waiting position for reprocessing, instruments are frequently in contact and for prolonged periods with blood and other organic contaminants, physiological saline solution, and disinfectants. Steam sterilization of the reprocessed instruments is another factor to be considered, as the ultrapure water used in the sterilizer also leads to surface attack of the passive layer in the autoclave.
Structure of the passive layer by chemical passivation
If only an iron-binding (chelating) chemical, such as citric acid, is used for passivation, free iron is effectively removed from the passive layer and "imperfections" in the chromium oxide layer are eliminated, allowing the formation of a more uniform passive layer by spontaneous oxidation. If an oxidizing acid is also used in the chemical passivation, such as sulfuric or nitric acid, oxidation of the metallic chromium can be assisted and the chromium content in the surface can be further increased (Wallinder et al., 1998). As a final step after manufacture, surgical instruments are therefore often chemically passivated - a treatment that can also be carried out in the repeated reprocessing of the instruments.
How can the chemical resistance of a passive layer be determined?
A frequently used criterion for the chemical resistance of a passive layer is the ratio of Cr(III) oxides/hydroxides to Fe(II/III) oxides/hydroxides (Crox/Feox – Verhältnis) was als Maβ für die Stabilität in korrosionsfördernden Umgebungen angesehen werden kann und durch Röntgenphotoelektronenspektroskopie (XPS) abgeschätzt werden kann.
This method provides semiquantitative insights into the composition of the passive layer, but due to the limited penetration depth of common X-ray sources, only the upper layers (maximum approx. 5 nm) of the workpiece can be examined. In addition, with the penetration depth of 5 nm and the only very weak signals that can be measured from the metal, XPS cannot properly investigate the passive layer/metal interface. In contrast, a novel extension of the method (HAXPES, Hard X-ray Photoelectron Spectrometry) allows to chemically analyze surfaces down to a depth of 20 nm by using a high-energy Cr-Kα X-ray source and thus to obtain more detailed insights into the layer structure of the passive layer
In addition, the integrity of the passive layer can be estimated by electrochemical potential measurements. Application-oriented tests investigate corrosion resistance by simulating challenging conditions, such as the spraying of saline solution.
Methods
In the study presented here, polished coupons made of 1.4021 and partly 1.4112 stainless steel were treated with citric acid and a solution based on phosphoric acid and nitric acid. This was characterized in comparison with untreated workpieces via HAXPES as well as electrochemical methods, contact angle measurements and application-related tests.
Results
The comparative study showed that the different chemical passivation methods resulted in significantly different compositions of passive layers. The method using a phosphoric acid and nitric acid based solution resulted in a 5-fold thicker passive layer, which in preliminary data showed better tolerance to aging under atmospheric conditions. Passivation processes were shown to affect not only corrosion resistance, but also other surface properties such as surface energy (i.e., hydrophilic/hydrophobic behavior, adhesion). At present, additional experiments are being conducted to characterize electrochemical properties and to investigate the influence of chromium content in the test specimen (comparison of 1.4021 and 1.4112 steel). The extent to which the altered surface properties have an influence on the adhesion of impurities, such as proteins or bacteria, is being investigated.
Conclusions
Passivation of stainless steel surfaces not only improves corrosion tolerance, but also influences the surface properties, thus potentially the adhesion of contaminants and microorganisms. Chemical passivation can make a significant contribution to maintaining the value of the instrumentation, thus reducing costs and conserving valuable resources, as well as facilitating reprocessing.
References
McCafferty, E. (2010). Passivity. In Introduction to Corrosion Science. New York, NY: Springer.
Schmutz, P., & Landolt, D. (1999a). Electrochemical quartz crystal microbalance study of the transient response of passive Fe-25Cr alloy. Electrochimica Acta, 45(6), 899-911.
Schmutz, P., & Landolt, D. (1999b). In-situ microgravimetric studies of passive alloys: potential sweep and potential step experiments with Fe–25Cr and Fe–17Cr–33Mo in acid and alkaline solution. Corrosion Science, 41(11), 2143-2163.
Seyeux, A., Wang, Z., Zanna, S., Carrière, C., Mercier, D., & Marcus, P. (2022). ToF-SIMS investigation with 18O isotopic tracer of the ion transport mechanisms in surface oxides on nickel-chromium and nickel-chromium-molybdenum alloys. Electrochimica Acta, 426, 140797. Retrieved from https://www.sciencedirect.com/science/article/pii/S0013468622009562
Wallinder, D., Pan, J., Leygraf, C., & Delblanc-Bauer, A. (1998). EIS and XPS study of surface modification of 316LVM stainless steel after passivation. Corrosion Science, 41(2), 275-289.
