Titanium and titanium alloys are attractive materials in a variety of industries such as aerospace due to a high strength to weight ratio and a surface which is naturally resistance to many forms of corrosion. However, tribological properties such as poor abrasion resistance and high friction during metal-to-metal contact have proven difficult to handle without additional treatments. Application of coatings onto the material’s surface is one means of protecting it from those poor tribological properties.

Electroplated nickel coatings retain much of the corrosion resistance desired of titanium alloys while also exhibiting better abrasion and wear resistance. Electroplating onto titanium and its alloys is prohibitively difficult due to the propensity of the material to form a tenacious oxide coating on any fresh surface instantaneously after its exposure. The oxide surface acts as a barrier between the solution and the titanium surface reducing the adhesion strength of the resulting deposit. The oxide layer is most commonly removed and the surface modified by hydrofluoric acid (HF) based treatments. HF is considered highly dangerous to workers and to the environment and work is being done to heavily limit its use.

For this work, a proprietary chloride-based solution was used during the preparatory steps. Brush or selective plating was chosen as the application technique due to its ability to perform the entire operation with the substrate exposed to solution and under potential control. In most applications of electroplating, the surface/part to be plated must be transferred between multiple tanks containing each preparatory solution used to etch and activate the surface which would involve exposure to air at several points. In addition, the brushing action inherent in brush or selective plating creates a surface environment which can be optimized to promote strong adhesion at the nickel/titanium interface.

This presentation will discuss how SIFCO’s brush plating process technology and automation capabilities were used to achieve strong adhesion to the titanium surface without the use of HF. Focus was specifically placed on achieving strong adhesion of a nickel pre-plate onto the titanium surface which allows for a multitude of other coatings to be applied thereafter. The method was performed using a single anode through which the solutions for each step flowed. The samples were under applied potential and immersed in solution throughout the entire process. Applied potential and solution delivery were automated to maximize process consistency.

Samples were measured for tensile adhesion strength via the specification ASTM C633. Several variables related to the preparatory procedure were optimized during the study including the flow rate of the solution, the voltage/current conditions, and the evaluation of preparatory steps taken before the electrochemical procedure.