Surface Treatments In Harsh Environment

[OrthoTech]

Does anyone have suggestions for how to treat / coat the surface of a stainless steel product(probably 304 plate) to be used in an orthopaedic environment? The treatment of the surface would need to be autoclavable and able to withstand normal heat/time cycles and any possible chemical cleaning methods used.

 

[kenvlach]

Yes, electropolish per ASTM B912-02 ‘Standard Specification for Passivation of Stainless Steels Using Electropolishing.’ Electropolishing gives a cleaner, more easily sterilized surface that is also passivated for corrosion resistance.

http://www.astm.org/cgi-bin/SoftCar.../htdocs/astm.org/DATABASE.CART/PAGES/B912.htm

I also recommend getting the booklet BioDur® and Other Specialty Alloys for Medical Applications from Carpenter Technology (

http://www.cartech.com).

It has a chart for selecting the right alloy for various medical industry applications.

 

[Ety]

Is 304 biocompatible enough? why not use 316L?

 

[kenvlach]

It's true 316L is better than 304 for biocompatability due to saline nature of blood (if implanted, & there are also better alloys than 316L), but
"autoclavable and able to withstand normal heat/time cycles"
suggests an external application (medical tools?).

 

[Ety]

and if it was to stay in the body for a limited period of time (hours)? would it still have to get surface treatment?

 

[kenvlach]

For an internal medical device, even short-term, it's even more important to electropolish & passivate than for an external device. The aim is to not contaminate the patient with biological agents, cleaning chemicals or traces of Ni dissolving from the surface.

Depending upon how the stainless steel is machined and mechanically, it may have a microscopically smeared surface that with pockets of trapped cutting oil, embedded polishing compound, etc. Conventional cleaning cannot remove these impurities. Electropolishing removes this messy surface layer and its impurities and places for 'bugs' and other foreign substances to hide. Further, the electropolishing and passivation gives a corrosion resistant chromium oxide/hydroxide surface layer. This avoids leaching of Ni into the body.

ASTM F86-01 Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants appears to require at minimum the passivation; it references
A380 Practice for Cleaning and Descaling Stainless Steel Parts, Equipment, and Systems
A967 Specification for Chemical Passivation Treatments for Stainless Steel Parts
B600 Guide for Descaling and Cleaning Titanium and Titanium Alloy Surfaces
I think that the next revision will include ASTM B912-02 (see earlier post) and ISO/DIS 15730 Metallic and Other Inorganic Coatings-Electropolishing as a Means of Smoothing and Passivating Stainless Steels.

 

[Ety]

I'm familiar with ASTM F86-01, but it mainly talks about "metallic surgical implants" meaning I guess long time internal use. The hard thing is to know where to draw the line and be practical
You could always do less surface treatments and do biocompatibility tests to prove your point but it takes lots of time and money of course.
But I guess this is our role as engineers