Type of Surface Change
- Shiny grayish-black chromium dioxide stains, particularly on the surfaces of chromium steel (not to be mistaken for corrosion caused by phosphoric acid overdosage - see also "surface corrosion").
- In the case of hardenable chromium steels due to neutralizer carried-over into the last rinsing cycle in machine reprocessing and due to other causes in the cleaning process not yet identified causing coatings or passivated films..
- Passive films of chromium oxide can appear transparent to black, epending on composition, density and thickness.
Picture right at the top = Surgical hook with a black discolored hardened CR steel shaft and a non-hardenable CRNI steel handle and blade that has remainded bright/bare.
Picture right below = Left valve: uniform green initial condition. Right valve: irridescent colors after reprocessing
Origin & Causes
Experience has shown that treatment with a basic cleaning agent remains without effect due to drastically increased corrosion resistance.
- Mechanical surface treatment by the manufacturer or repair service may be necessary because - as experience shows - intensive cleaning with an acid detergent is ineffective due to the significantly increased corrosion resistance.
- Titanium and titanium alloys may react to hot humid environments and/or chemicals in conjunction with several reprocessing steps by surface oxidations and, as a result, discoloration.
- Prevent neutralizer carry-over by ensuring precise dosing and sufficient final rinsing.
- With titanium materials his can hardly be prevented because hot humid environments and/or chemicals will always cause surface reactions.
- No corrosion - only aesthetic significance; no hygienic hazards.
- Makes the affected instrument significantly less susceptible to corrosion
- Provided that a possible loss of the coding function on titanium materials does not represent any risk, surfaces discolored due to oxide layers are totally harmless, i.e. non-objectionable. In other words: there are no limitations with regard to toxicity, hygiene, function or service life.