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Metal / Stains - Oxidation (Black Discoloration)

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.

Treatment Recommendations        

  • 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.

Preventive Measures

  • 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.

Risk Assessment        

  • 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.