[mcaswell]

I have some notes on anodizing titanium (found in a dusty corner of my hard drive)
Acidrain's statement that the knife is titanium would certainly give the colors shown.


I have experience with anodizing small parts in titanium (comercially pure grade 1 and
Ti 6Al 4V) and niobium, but nothing as large as your part. The current densities that
I use vary from about 5 amps per square inch to less than an amp per square inch. I
suggest that you start out on the lower range to avoid accidentally etching the piece. I use
a solution of 3-5% by weight trisodium phosphate in water. Keep the solution below 80
degrees farenheight. I get more vivid colors on the 6-4 than the cp titanium, probably
because it seems to take a higher finish easier. If you do not already have a suitable tank,
you might try anodizing the piece with a brush saturated with the solution connected to the
positive lead and the piece grounded to complete the circuit. You could use a painter's brush
with the metal ferrule wired to the lead and then insulate the handle for safety. Artists use this
technique to paint on sheets of Ti.

Ti6-4 surgical instruments are routinely anodized for color coding. The first blue color appears
around 28V. The exact relationship between color and voltage will vary with each setup.
You will have to experiment. If you are using titanium clips to hold the work, the anodized
layer on the clip will need to be removed mechanically or in a dilute (5%) HF solution
(handle with care) between each run.

Ken,

For the anodizing of titanium we use the following bath.

80 vol% phosphoric acid
10 vol% sulphuric acid
10 vol% water at room temperature.

The color of the anodic layer depends on the anodizing voltage.

Anodizing of titanium can also be done in 3 - 5 w% trisodiumphosphate.

Titanium anodizing is a unique coating which partially penetrates into the base metal,
improving fatigue strength, making it stronger and increasing its resistance to galling,
with minimal dimensional change.

With titanium in a bath of TriSodium Phosphate you get the most beautiful color of blue
imaginable. All you need to do is to immerse the part, turn on the current and voil‡ ,
bright blue. Not gray but close??

A true grey from TSP is not probable.

Brown, blue, silver, yellow, purple and green are possible at voltages up to 200. There are several different shades possible for each. Precise color requires precise voltage control. It anodizes nearly instantly. Surface prep makes a significant difference in the luster or look of the product. Does not need any further protection after anodize.

From 10% to 3% solutions are recommended.
The color red is not possible with titanium anodizing nor is black. Apparently neither exists in the spectrum created by the oxide layer formed on the surface. The color exists only as reflected light.

A bath that can be used for anodizing titanium is

phosphoric acid 80 w% Room temperature
sulphuric acid 10 w%
water 10 w%

The bath is controlled, as mentioned by James Watt, by the voltage. The voltage is critical.

I have been coloring Ti for decorative use (on a small scale ) using Coca - Cola
and stainless steel cathodes, powered from my anodising rectifier at + - 60volts .
The trick is in how long you dip it , the longer , the darker . A couple of seconds is
normally enough . It's a great party trick . By the way , the color is dependent on the lighting ,
and finish of the part , it is caused by difraction of the light waves.

Oxalic Acid, Phosphoric Acid (Coke Cola) Sulfate of Ammonium (Fertilizer), TSP
(All Purpose Cleaner)

All of the above mentioned chemicals will anodize refractory metals.

I have been anodizing refactory metals for the past 17 years and have not added
sulfuric acids to my solution. Keep it simple and friendly.


Color anodizing titanium for appearance (not functional) can be achieved in most any
sulfuric anodizing or hard anodizing bath by first cleaning the titanium in a nitric/hydrofluoric
bath (25-35% nitric with 2-4 oz/gal hydrofluoric) for about 10-30 seconds. You then anodize
the parts by slowly raising the voltage from the starting voltage. Color changes occur about
ever 2-4 VDC. Color changes occur much like octaves on a piano - as you increase voltages
the color changes will proceed thru several octave ranges. When you achieve the color you
are seeking, record the voltage and anodize subsequent lots at that voltage - no need to raise
the voltage slowly once it has been determined.If you don't like the coating, strip in the
nitric/HF bath and try again.

I use a trisodium phosphate solution (3% to 5% by weight) to anodize titanium and niobium.
This solution is quite safe and gives excellent colors in a small scale operation. The quality
of the color achieved depends on the surface finish of the part and its cleanliness, (free of
dirt and all oils). Current should be low: I use about 2 amps on small parts. The bath should
be kept below 80 degrees farenheight. If the bath is too hot or the current too strong, the part
will be etched and turn grey. The part must then be polished and anodized again. The voltage
is what determines the color. I use a range of 20 to 75 volts. If I remeber right, yellow occurs
at about 42 volts. Also, I have read that it is best to use a cathode of the same material that
you are anodizing.

Unfortunately, 1 or 2 volts makes a difference in the exact shade that you get,
so you will have to take what ever is near.