Nanoscale Titanium Polishing

Titanium polishing is a process to reduce the roughness, and thereby increase the brightness, of a metal surface made of titanium or titanium alloy. A patented electropolishing method can produce polished titanium pieces with surface smoothness down to the nanometer level, with the additional benefit of lower power consumption compared to other available electrochemical methods.

Unlike mechanical polishing (such as the use of abrasive powders of decreasing particle size, fine machining, honing, etc.), chemical and electrochemical methods make use of a specially formulated chemical bath to dissolve metal from the surface of the sample. The metal part is immersed in an electrolyte (chemical bath) and subjected to direct current. The part is made anodic (it becomes positively charged), and under certain conditions a controlled dissolution of the metal is achieved. Depending on the working tension or current density, this dissolution reduces the roughness of the metal surface.

The polishing process is usually linked to the formation of a viscous layer near the surface which hinders the transport of reactants or products of the reaction into the bulk of the supporting electrolyte; consequently, peaks are typically etched at higher rates than valleys.

The key benefits of chemical and electrochemical methods versus traditional mechanical polishing are: superior corrosion resistance due to the reduction in stress placed on the sample; no direction lines; reduced contamination; and the ability to treat a variety of complex shapes.

Titanium electropolishing is used at CERN primarily to polish electrodes, which require an ultra-smooth surface to avoid sparks during operation. The new method uses a chemical bath formed of sulphuric acid, hydrofluoric acid and acetic acid. The bath can also include a cationic wetting agent. Many electropolishing bath compositions are available in the literature. However, CERN’s patented method enables the electrochemical process to be more carefully controlled. By an appropriate choice of the electrical parameters, the metal surface roughness can be either reduced or increased. Furthermore, the method is less sensitive to temperature changes and works effectively up to 30º C, resulting in lower power consumption. Consequently, the method may be cost-effective even when used on large, complex structures.

A bath composition and working parameters for non-alloyed titanium are described in patent WO 01 / 00906. However, proprietary bath compositions and working parameters for other titanium alloys are also used at CERN.

The metal can be polished down to the nanometer level, depending on the starting surface roughness of the part being treated, thus creating a shiny, mirror-like appearance. The process can be run with low power consumption. It provides easy maintenance of hygienically clean surfaces due to reduced particle adhesion. Metallic purity and chemical passivity are very high, and the process can be used on a wide range of sample sizes and geometries.

All electropolishing methods can be hazardous if not handled correctly. As the method involves the use of strong acids, appropriate facilities must be in place and staff must possess the required level of skill and experience. Furthermore, the method contains Hydrogen Fluoride (HF) gas, which is released during the process, meaning that the air in the treatment area must be suitably extracted and processed before being released.

There is practically no size limit on the sample to be treated, making the method appropriate for a wide range of applications, including vacuum technology, medical industry (e.g. implants, tools), jewellery, spectacle frames, watches, aerospace (e.g. turbine blades), or electronics and storage discs.

Source: BD Chrono