Titanium reaction with Sodium Percarbonate solution

I've been reading about a process for darkening Titanium using Sodium Percarbonate that consists of the following steps:


1. Prepare Sodium Percarbonate solution 1:5 with boiling water
2. Soak titanium piece in solution


I have not been able to find any technical readings on what exactly is going on with this chemical process.




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These guides have also found that **Titanium Grade affects the darkening** of the Titanium:



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> An oxidation on Grade 1 surface is rather aggressive, not even and it's brown not grey... oxidation on Grade 5 is neither that aggressive nor that brown.
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And that **heat affects the reaction** as well:



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> The solution is doing the job only when its warm, so I have repeated the process 3 times.
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> Add some hot water from time to time and stir...
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Links:


* <https://www.rwg.bz/board/index.php?/topic/161199-how-to-force-a-patina-on-a-raw-titanium/>
* <https://www.bladeforums.com/threads/darken-titanium-with-oxi-clean.1019026/>




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What exactly is going on in this reaction with the Titanium? What is the material of this new darkened layer?




The effect is similar to that of [anodizing titanium](https://www.besttechnologyinc.com/surface-finishing/titanium-anodizing-equipment/) (or aluminum) to build up a thin surface oxide coating. There are a few causes for the color change.


* A thin, transparent, layer of oxide causes optical interference due to the reflections within the oxide layer surface and the underlying metal. This can produce a [whole spectrum of colors in the titanium workpiece](https://en.wikipedia.org/wiki/Anodizing#Titanium).
* Rather than having a shiny, metallic reflective surface, a thick oxide layer is rough, at the wavelength of light, causing absorption destructive interference and therefore darkening the part.
* Impurities in the metal such as silicon may cause patchy, uneven anodizing, and others, such as iron, form colored salts that fill the pores of the oxide.


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