Why Coat Titanium?
Titanium alloys in and of themselves have outstanding corrosion resistance so the question is often asked why would anyone have to coat titanium? As a diverse metal finishing house offering nearly 100 different coatings and finishes on all sorts of metal and non-metallic components, Anoplate can emphatically state that corrosion resistance is only one reason for coating parts. We coat parts to improve their appearance including making them shiny or changing their color to give them greater appeal. Parts that go into optical assemblies often get a dull black finish to make them less reflective. Black coatings are also used to impart particular thermal characteristics. Similarly, specific coatings such as silver and gold are used to increase surface conductivity while others are used to render a surface non-conductive or more electrically resistant. Anoplate also coats many parts with tin-lead or nickel or silver to assist in joining parts using solder or brazing techniques. Often the coating we’re asked to apply is more related to the desired surface characteristics of the assembled component rather than the individual piece, thus surface engineering is probably a more appropriate description of what Anoplate provides.
With specifics to titanium, its corrosion resistance is compromised by exposure to halides such as chlorides or fluorides. Residual chlorides lead to stress corrosion cracking while fluorides readily attack the natural oxide that protects titanium from atmospheric corrosion. In addition, due to titanium’s extremely passive nature, when titanium components are in contact with more electrochemically active materials such as aluminum, zinc or copper, where the materials meet there is such a galvanic charge generated due to the dissimilar metal junction, galvanic corrosion is wildly accelerated beyond what either metal by itself would experience.
Another shortcoming and that most often associated with titanium is galling. When titanium slides against titanium it tends to gall or stick more so than any other metal combination. Similarly, titanium to titanium parts in threaded combination “cold weld” making disassembly impossible without destroying the parts. In order to prevent this, typically dry film lubricants are applied over suitably prepared titanium (i.e. either anodized or conversion coated).
A good way to answer “why coat titanium?” would be to recap applications Anoplate has seen over the course of it’s 44-year history. One of the earliest applications was caustic anodizing of titanium aircraft engine turbine blades. While this job migrated south with the turbine blade manufacturer, we’re continue to do this same coating on large pumps used aboard Navy ships. This coating is also used on surgical trays used in the medical field that must withstand repeated sterilization in extremes of temperature and chemical exposure (e.g. bleach, hydrogen peroxide, and peracetic acid). Caustic anodizing of titanium is crudely detailed in the specification AMS 2488. Anodic treatment of titanium in nearly neutral electrolytes such as sodium bicarbonate, phosphoric acid or TSP result in a rainbow of colors depending on the voltage applied. This type of coloration is used for identification in the medical field and for decorative purposes by jewelers, sculptors and bicycle manufacturers. Anodic treatment in sulfuric acid results in a pronounced deep blue to purple oxide coating on titanium. This process is detailed in the specification ISO 8080 and is used in various military vehicle, aerospace and weaponry applications. Lastly, the fluoride – phosphate conversion coating on titanium detailed in AMS 2486 has been used on optical components to make them optically dull and less reflective. Both the conversion coating and caustic anodize processes have been used as pre-treatment for subsequent dry film lubrication on titanium for increased adhesion and durability.
Obviously there are many benefits apart from corrosion resistance to coating titanium. Based on the many applications Anoplate has encountered, the fact that the specifications exist indicate that designers and engineers derive some benefit from their use. Furthermore, titanium specific finishing firms such as Tiodize in California and Titanium Finishing in Pennsylvania attribute their very existence to the benefits derived from coating titanium.
If you’d like to discuss your specific titanium application with Anoplate, please feel free to call or E-mail myself or call Renee Ventresca in our Sales department. Thank you for your continued interest in Anoplate.
Milt Stevenson, Jr.
Milt Stevenson, Jr.
Chief Technology Officer