Masterclass: Anodising titanium with Mooro Cycles
Welcome to Masterclass, a series where we take you behind the scenes with renowned artisans to share the effort that goes into their craft. In some cases, there will be tips and tricks to help you practise at home; other times, we’ll simply provide an advanced understanding of how something is created.
In this edition of Masterclass, we look at how titanium frames are anodised with Chris Morgan from Mooro Cycles.
Titanium has long been revered as an exotic material, helped in part, no doubt, by its exorbitant cost. Discovered over 200 years ago, the metal rose to prominence soon after World War II as it was embraced by the defence and aerospace industries. Offering the strength of steel at a fraction of its weight with little risk of corrosion, it was a wondrous material that ignited imaginations before the rise of carbon fibre composites.
The bike industry first started working with titanium during the early ‘70s. However, it wasn’t until the late ‘80s that framebuilders were able to start capitalising on the best of what the material had to offer. The development of oversized butted tubing was a key step in this evolution in terms of performance, but it did nothing to ease the burden of working with the somewhat stubborn metal. Titanium has always required a high level of expertise and exacting processes that make it inherently ill-suited for mass production.
Nevertheless, titanium has retained a foothold in the industry as a niche material thanks to the devotion of custom framebuilders. Most favour leaving the material unfinished to showcase its distinctive lustre, and the understated aesthetic has become a hallmark for these frames. Eye-catching colour is rarely associated with titanium frames unless it is painted, but that has been changing with anodisation.
Now, a variety of titanium frame builders are using the technique to decorate their frames, including Chris Morgan at Mooro Cycles. It was something that he started experimenting with soon after he built his first titanium frame, and now he often uses it to decorate Mooro’s frames.
“Compared to paint, we can get some unique colours and effects without adding any extra weight, plus it’s more durable and less prone to scratching. We use it for our logos and it allows us to provide personalised graphics for our customers.”
Most cyclists will be familiar with the vibrant colours of anodised aluminium, which are produced by dyeing the anodised layer on the surface of the component. At face value, anodised titanium appears much the same, but there is no need for any dye, because the colours that are perceived by the eye are produced by a trick of the light.
Easy to learn, difficult to master
The whole notion of anodised titanium is nothing new. Researchers were exploring the benefits of the the surface treatment as far back as the ‘50s when titanium use was in its infancy. Soon after, it became clear that a range of colours could be produced, depending upon the voltage applied during the anodising process.
Since then, anodising has become invaluable for improving the physical properties of titanium (such as corrosion resistance) as well providing a durable coloured finish that could be used to judge wear or colour-code specific components (e.g. bone screws). Jewellers have also been anodising titanium for many years, so it’s a little surprising that it has taken so long for the bike industry to embrace it as well.
For Morgan, the reason is clear: “It’s a tricky process and there’s a lot to learn before you can consistently produce all of the various colours. Some of it is in the equipment, some of it is in the chemicals, and some of it is in frame preparation. The basics are pretty easy to pick up, but after that, it takes a lot of experimentation to improve the quality of the finish.”
Firefly Bicycles, the Boston-based custom framebuilder, has perhaps the greatest experience with anodising titanium frames. The company debuted the process on some of its earliest titanium frames in 2011, and it has been working with it ever since to produce some truly stunning results. In fact, it was Firefly’s work that inspired Morgan to start experimenting with anodisation, and he was soon hooked on mastering the process.
“I did some reading, then I started with a solution of Borax and a collection of 9V batteries on my kitchen table,” Morgan said. “Each time I added another battery to the series, I could create a different colour, but it was pretty crude work. I also discovered that there was a limit to the number of colours I could produce.”
Over the next two years, Morgan would go on to explore every aspect of the process, and much of what he learned was gained through trial-and-error.
“Perth is quite an isolated city and I couldn’t find anybody to show me what to do. So I had to do a lot of reading, but I was fortunate to have a friend that understands chemistry. He gave me a lot of help with the chemicals I needed, and from there, I could see my work improving.”
Oxide layers are created by anodisation
Most metals can react with oxygen to form an oxide layer at the surface of the material. In some instances, this can happen spontaneously, while others require the addition of heat to get the reaction started. Titanium is extremely prone to oxidation and a micro-layer of oxide will form within moments of exposure to the atmosphere. While this may seem at odds with the titanium’s inert nature, it is actually this oxide layer (titanium dioxide) that protects the metal from further corrosion.
Increasing the amount of oxide on the surface of a metal can add to this kind of protection, which is where anodisation becomes important. It is an electrochemical process that oxidises the surface of a metal as it serves as the anode in an electrical circuit. The size of the charge, and its duration, determines how much oxidation takes place, and therefore, the amount of oxide that develops on the surface of the metal.
For aluminium, the thickness of the anodised layer is closer to that of paint, and normally measured in micrometers (i.e. one-thousandth of a millimetre). The aluminium oxides in that surface layer are also quite porous, and filling those spaces with dye is what produces those vibrant results. An additional sealant is required to stop the dye from escaping the oxide layer, particularly when exposed to UV light over long periods of time.
Titanium dioxide layers are much thinner and measured in nanometers (i.e. one-millionth of a millimetre), and it is this thinness that is the key to the process of adding colour to the material. Whereas anodised aluminium gains its colour from added dyes, anodised titanium works by selectively filtering specific wavelengths of light through a process called refraction. Depending on the thickness of the titanium oxide layer, certain wavelengths are enhanced, while others are canceled out. Thus, anodised titanium gains its vibrance not by the addition of colour, but by the selective elimination of it.
The magic of interference colours
Soap bubbles and oil on a wet road: in the right kind of conditions, both can produce a spectrum of colours through the interference of light. It’s a passive effect that takes place as waves of light are refracted and reflected by the material. The final outcome is a matter of probability as the various wavelengths that compose white light recombine at the surface, as shown in Figure 1.
In the case of titanium, the outcome depends entirely on the thickness of the oxide layer. The spontaneous oxide layer is too thin to have an effect on the light, which is why the natural colour of the metal predominates. A small voltage of 5V is all that is needed to increase the thickness of the oxide layer to around 20nm, at which point a gold colour is produced. Higher voltages and further thickening of the oxide layer provides a progression of colours from bronze to purple to blue and light blue. Second-order colours — deep gold, orange, pink, another blue, and finally, green — are produced by even higher voltages (70V or more), but beyond that, the effect is lost with prolonged anodisation.
So long as the voltage is carefully regulated, a uniform coat of any colour in this series can be formed on the surface of titanium. The reaction is self-limiting because as the oxide layer increases, so too, does its resistance to current, which eventually puts an end to the oxidation. With that said, the lower-order colours (e.g. gold) can be changed into a higher-order colour (e.g. blue) by re-applying a higher voltage to add to the thickness of the oxide layer. In this way, different colours can be blended together on the surface of a frame with some stunning effects.
Anodising titanium in the kitchen
There are four important components in the electrochemical circuit required for anodisation (see Figure 3). The first is an anode, the second is a cathode, the third is an electrolytic solution, and the fourth is a DC power source. Any part that is to be anodised acts as the anode in this circuit, and as it comes into contact with the electrolyte, the circuit is completed. Once a voltage is applied, electrolysis powers the reaction at the surface of the anode to create an oxide layer.
If the part is immersed in a bath (Figure 3A), then the entire surface will be coated with a uniform finish, which is ideal when anodising screws, bolts and other small parts. Alternatively, spot anodisation (Figure 3B) can be used to coat discrete patches of the part, and this is the approach that Morgan uses to anodise Mooro’s titanium frames.
Rather than fill a tank with electrolyte, Morgan simply uses a cotton ball soaked in the stuff attached to the negative terminal of his power supply. The positive lead is connected to the frame, and once the power is turned on, anodisation will start to take place wherever the cotton ball comes into contact with the frame.
While the prospect of handling an electrolyte in close vicinity to an electrical circuit might seem foolhardy, the solution is quite dilute and the current low, thanks to the poor conductivity of titanium. As a result, Morgan can comfortably anodise a frame with the first-order colours in his kitchen (although gloves and eye protection are still essential). Second-order colours, by contrast, require stronger oxidants and extra precautions such as a well-ventilated workspace and a respirator.
Anodisation takes place quickly as Morgan smears the surface of the frame with his chosen electrolyte.
“I like to gradually build up the colour until I’m happy with it. It’s a bit like painting, really, so I go back and forth over the area, sometimes making fine adjustments to the voltage until I get the colour I’m after.”
The final colour is not revealed until a wet cloth is used to wipe away any residue from the electrolyte. Morgan takes a moment in between each “coat” of oxide to do this, and the effect is much like applying polish. Dull hues suddenly become brighter as they are wiped down, and Morgan can decide if the area needs any more work.
“A good DC power pack makes a huge difference to this work, because you really need fine control over the voltage. A small change of just a couple of volts can change the colour quite a lot, so it’s a good idea to take your time.”
Voltage is just one of many things to pay attention to when anodising titanium. To start with, surface preparation is critical. Morgan takes care to remove all scratches and imperfections from the frame, then it must be carefully degreased and washed before it is ready for anodisation. Similarly, laboratory-grade chemicals are used in conjunction with distilled water to prepare the electrolyte solution, because impurities in either can harm the quality of the result.
The parallels with painting are many, including the risk of drips. The electrolyte solution can drip from the cotton ball at any time, and anodisation will occur if it comes into contact with the frame. This is the kind of thing that can go unnoticed, so Morgan takes care to mask off the areas he wants to anodise, and once he has completed a patch, he will cover it so that drips cannot spoil the finish.
Working with low voltages and the first-order colours is much easier than higher voltages and second-order colours. The biggest risk comes from the extra heat generated by the higher voltages, which can mar the oxide layer. Morgan has found that it is important to keep the surface of the frame moist until anodisation is complete, which can get tricky when working over larger areas. This is where using a bath might be a better choice, but the size and shape of a frame rarely lends itself to dipping.
Rendering logos and artwork
When a frame is decorated with paint, a series of masks is normally used to create the letters, logos, and details in a step-wise fashion until the work is complete. While this approach can also be used when anodising a titanium frame, it is not nearly as effective because the electrolyte is very runny and can easily seep under a mask to ruin a sharp edge.
Morgan (along with other titanium frame builders who regularly use anodised logos) has found that he can get a much better result by applying a mask after anodisation and removing areas of unwanted colour by sandblasting to form letters, logos and shapes. The method allows him to achieve sharp lines and crisp shapes and is carried out as part of the final finish for the frame.
“I use an adhesive vinyl material that resists sandblasting. I design each mask on my computer and a vinyl cutter does the rest of the work. I apply them just like a sticker and then blast away the colour around them. The edges of the mask can lift while I’m sandblasting, so I have to take care to keep a sharp edge. Large masks always work better so I try to avoid tiny letters and shapes.”
The final unmasking is always immensely satisfying, which Morgan likens to unwrapping a present. It always marks the last step of production for the frame, and as the masks are peeled away, the decorations come to life. Morgan has found that when the surface of the frame is highly polished, the colours will pop in low light, but they are much less vibrant in direct sunlight, so he prefers to work with a brushed finish. The colours are brighter in the sun without losing too much lustre in low light.
Once anodised, a titanium frame doesn’t need much attention in order to look after the colours. The oxide is quite hard-wearing, and generally more durable than paint, though the finish can still be marred by strikes or abrasion. In this situation, it’s not possible to touch up an anodised colour, but the entire decoration can buffed away and a fresh layer of anodisation applied to restore the finish if needed.
If there is one drawback associated with anodising titanium, it is the brittleness of titanium dioxide. If it is forced to flex a lot, then it may crack, giving rise to stress risers for the material underneath. This may be enough to have an effect on the fatigue resistance of the frame, which is why Seven Cycles refuses to use the technique.
For Morgan, this kind of risk is largely theoretical, because the anodised layer is only marginally thicker than the oxide layer that spontaneously forms on the surface of titanium. Judging by the number of other frame builders that are using the technique, he is not alone in this thinking. Nevertheless, he sees it as a good argument against anodising weld areas or the entire frame. He doesn’t recommend either, but for those that would rather avoid the risk, he is happy to create sandblasted logos or offer paint as alternative finishes for his frames.