Summary for nitinol electropolishing corrosion resistance Discussion
Nitinol thickness removal after electropolishing is discussed in terms of how much material must be removed before corrosion resistance stabilizes. The question asks whether there is literature linking removal depth to corrosion resistance, beyond studies on oxide thickness. In the answer, the respondent says that if more than 3 microns are removed during electropolishing, no significant corrosion-resistance change is seen in relation to removal rate. However, when less than 3 microns are removed, corrosion resistance depends on how fast material is removed because the outer thin layer has a different Ni-Ti composition and should be removed. The answer also notes that the first few seconds of electropolishing can be uneven because the original oxide layer varies across the surface, and longer processing makes the surface more uniform. Additional passivation can further improve corrosion resistance by increasing oxide thickness and uniformity. The respondent adds that these comments apply to acid-free electropolishing, and says they do not have comparable data for methanol-based niti…
Q. Hi! Can anyone help me to understand the correlation between the amount of material removed (during the electropolishing) from Nitinol and the corrosion resistance of Nitinol? Is there any literature out there that reports in this topic? There are a couple of articles that correlate oxide thickness and corrosion resistance of Nitinol, but I cannot find anything on the topic I posted yet. Any information will be greatly appreciated. Many Thanks!!
Boon
Boon Ng
– Sylmar, California, USA
September 5, 2012
A. If more than 3 microns are removed during electropolishing – then no significant effects on corrosion resistance are seen in correlation with material removal rate. Also corrosion resistance increases by additional passivation treatment that increase thickness and uniformity of the oxide film.
When less than 3 microns are removed, the corrosion resistance depends on rate of material removed, since the outer thin layer has different Ni-Ti alloy composition, and it is important to remove this layer.
Moreover during the first few seconds of electropolishing, the process of the metal dissolving goes very unevenly, since the initial oxide layer has uneven thickness and composition along the metal surface. As a result on some areas the old oxide layer still can be seen, on another areas – the new formatted oxide film appears. By increasing duration of electropolishing the whole metal surface becomes uniform. It is clearly seen if you examine the electropolished surface after 1,3,10, 30 and 100 seconds.
The above observations are true for acid-free electropolishing method. The newly developed Ultrapolishing does not require additional passivation procedure, and the uniform metal surface is created in less than 20 seconds. We do not have such data for methanol-based nitinol electropolishing, since we do not use this method.
Anna Berkovich
Russamer Lab – Pittsburgh, Pennsylvania
(ed. update July, 2025: sorry, Anna has retired)
September 6, 2012