Rust prevention: Pickling and passivation of stainless steel surgical instruments

The surgical instrument behaves passively, as is known in everyday applications and the damage to this passivation layer will be 'repaired' naturally.




Stainless steel (SS) is obtained when a minimum of 12% free chromium is present in the lattice. When oxidized at high temperatures and sufficient oxygen, a porous thermal oxide layer is formed as with most other metals. However, when oxidized at low temperatures and sufficient oxygen, an extremely thin "colorless" chromium oxide layer (Cr2O3), also called the passivation layer, is formed. The surgical instrument behaves passively, as is known in everyday applications. Damage to this passivation layer will be 'repaired' naturally (auto passivation).


During stainless steel processing, such as machining and welding, the naturally formed passivation layer is removed. Due to the presence of chromium, a new passivation layer can spontaneously form. Until the layer is intact again (this takes a few minutes to days), corrosion processes with the base material can be initiated. Once such a corrosion process has started, the passivation layer cannot be formed locally and corrosion will continue unhindered. This is because passivation can only take place on a metallic blank surface. The formation of the passivation layer can be hindered by, among others:


  • contamination of the surface by iron (iron contamination), grease and/or dirt.

  • deformation of the surface.

  • weld discoloration.

  • the composition of the atmosphere.


As a result, an active layer is formed instead of a passive one. As a result, the corrosion resistance of the surgical instrument is severely reduced.



Iron Contamination

Iron particles, originating from processing, environment or tools, can settle in the surface of a stainless steel product. These contaminants impede the formation of the natural oxide skin. If these particles are not removed, an electrochemical cell is formed that will eventually oxidize the particle. In many cases, this leads to crevice corrosion or pitting corrosion, a form of very localized deterioration in the form of deep and narrow crevices. To prevent this, the surface of the surgical instrument must be cleaned of these iron particles.



Weld Discoloration

Weld discoloration is caused by differences in the thickness of the oxide skin of the instrument. This can vary from light yellow (estimated at about 300°C) to dark blue (estimated at about 600°C). These colors are visible in the heat-affected zone of the weld. In the temperature range 450 to 850°C brittle phases are formed, which weaken the welded joint and chromium carbides, which consume large amounts of chromium. This results in a shortage of chromium at the surface. The corrosion resistance reduces significantly. The stainless steel becomes susceptible to crevice corrosion, stress corrosion and pitting. With a pickling process, this discoloration can be excellently removed.


The passivation of a heat-treated product consists of four steps:

  • Degreasing

  • Pickling

  • Rinsing

  • Passivation


Degreasing

Grease, oil, and other inorganic components can quickly contaminate the pickling bath. Before pickling, the product must first be cleaned. The cleaning method depends on the type of material.



Beating

Heavy contaminants, iron contamination and mill/cast skins are removed so that a metallic clear surface is obtained. This can serve as a basis for a good passivation layer. When stainless steel is exposed to air at elevated temperatures, such as during rolling or casting processes, welding or heat treatments, oxides are formed on the surface. These heavy oxide skins (such as forging skin, rolling skin, casting skin, and approach colors) are visible as (often) dark gray layers. For good corrosion resistance these layers need to be removed. This can be done mechanically by, for example, blasting, grinding and brushing, but it can also be done chemically by means of a pickling treatment. With pickling, the product is usually immersed (immersion pickling) or sprayed (spray pickling) in a solution of nitric acid (HNO3) and hydrofluoric acid (HF). Other solutions include dilute sulfuric acid (H2SO4), hydrochloric acid (HCl), and ferric sulfate ((Fe2(SO4)3). Pickling pastes (gel) applied manually by brush or sprayer can also be used. The applicable solution depends on the stainless steel type.


The acid (e.g. H2SO4) enters into a chemical reaction with the oxide skin. It also penetrates the cracks and crevices of the oxide layer where it enters into a chemical reaction with the innermost part of the oxide skin and the base material. In the process, hydrogen (H2) is formed, which helps detach the oxide skin from the base material. What is left is a metallic clear surface that has excellent passivation properties. When the removal of the oxide film and corrosion products is the main objective, inhibitors (pickling inhibitors) are often added to limit deterioration of the underlying metal. The surface becomes less rough and offers better protection against pitting.



Rinsing

Surface attack by the pickling liquid is stopped by thoroughly rinsing the product.



Passivation

By passivation, a thick chromium oxide layer is formed quickly and in a controlled manner by artificial means, thus obtaining the greatest possible corrosion resistance, for that particular alloy. Pickling and machining removes the protective passivation layer (Cr2O3). Restoration of the passivation layer by natural means may take several minutes to days. This process can be accelerated by giving the product a passivation treatment. This oxidizes the chromium (present in the surface), forming a protective layer that normally takes a long time to form. Light impurities (e.g. iron particles from other processes (so-called fly rust), tools and cutting fluids) on the surface are removed with this process. The product is immersed in an oxidizing bath (such as nitric acid, dilute sulfuric acid or sodium dichromate). The composition of the bath depends on the alloy. There are several theories about the formation of the passivation layer. One theory is that the oxygen of the liquid at the metal surface causes an insoluble oxide film to form. Another theory states that the film is simply absorbed gas that forms a barrier against the diffusion of metal ions from the substrate. Yet another relies on the theory that passivation is a consequence of electron vacancies in the d-shell of the metal atoms. Which theory is correct, we will leave aside here.



Slot

Stainless steel possesses the property of forming a passive oxide layer (Cr2O3) that protects the material against corrosion. In many cases, a damaged layer can be repaired by natural means (autopassification). This preserves the original corrosion resistance. In specific cases, help is needed to form this protective layer. The pickling and passivation processes create the passive oxide layer by electrochemical means by cleaning the surface and then oxidizing the chromium present at the surface. A thin but consistent protective layer is formed that gives the material its maximum corrosion resistance.



 


Jensen Instrument Technologies is a long-established company distributing precision surgical instrumentation across Australia and New Zealand. Our company specialises in the supply of maintenance of precision mechanical, optical and power surgical instruments for surgical theatres. Learn more about us here.

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