Chemical polishing remains a widely adopted finishing process for titanium and its alloys, valued for its ability to produce bright, reflective surfaces without mechanical contact. However, non-uniform polishing-manifested as localized over-etching, flow marks, orange peel textures, or inconsistent gloss across a single workpiece-remains a persistent challenge in production environments. For industries ranging from aerospace fasteners to medical implants, surface finish uniformity directly impacts corrosion resistance, fatigue performance, and post-processing adhesion. This article examines the root causes of non-uniformity in titanium chemical polishing and provides actionable, process-level countermeasures.
1. Defect Classification and Visual Diagnostics
Before adjusting parameters, accurate defect identification is essential. Non-uniform polishing on titanium surfaces typically falls into several distinct categories, each pointing to different root causes.
Orange peel occurs when the rate of chemical attack varies between different metallurgical phases or grain orientations within the alloy. In two-phase alloys like Ti-6Al-4V (TC4), β phase dissolves preferentially under certain acid conditions, leaving a roughened surface topography. Pitting typically signals an excessively high HF concentration or an HF-to-HNO₃ ratio the optimal window. Flow marks and edge-center differences almost always trace back to fluid dynamics and thermal uniformity issues.
2. Solution Chemistry: The HF/HNO₃ Ratio as the Primary Control Variable
The HF-HNO₃-H₂O system remains the workhorse for titanium chemical polishing. HF acts as the active dissolving agent, attacking the titanium substrate and removing the native oxide layer. HNO₃ serves a dual role: oxidizing the dissolved Ti³⁺ to Ti⁴⁺ to prevent surface contamination, and promoting passive film formation that controls the overall etch rate.
Industry practice generally targets HF concentrations of 3–5% and HNO₃ concentrations of 15–30% by volume. Within this window, the HF-to-HNO₃ ratio is the critical tuning parameter. Experimental studies on TC4 have examined ratios of 1:4, 1:6, and 1:8 (HF: HNO₃ by volume). A ratio that is too HF-rich produces aggressive, uncontrolled etching with pitting and non-uniform material removal. A ratio that is too HNO₃-rich slows the reaction excessively and may induce passivation before leveling is complete, resulting in cloudy or uneven finishes.
The underlying mechanism relates to diffusion-controlled versus activation-controlled etching. When the HF concentration is properly balanced with HNO₃, the dissolution rate is limited by the transport of reactants to the surface rather than by the surface reaction itself. This diffusion-limited regime naturally produces more uniform material removal across macro-scale topography, as protruding features receive slightly higher diffusion flux than recessed areas-the leveling effect that defines true polishing.
3. Temperature Control and Thermal Gradient Management
Temperature exerts a pronounced effect on titanium chemical polishing kinetics. Reaction rates increase by approximately 1.5–2× for every 5°C rise in solution temperature. A temperature gradient as small as 3–4°C across the bath can produce visually detectable differences in polish uniformity between workpieces positioned at different locations, or even between the top and bottom of a single large part.
The recommended operating range for most titanium chemical polishing formulations is 20–35°C. However, this range is too wide for precision work. Tighter control within ±1.5°C is necessary for uniform results. Temperature excursions above 35°C accelerate HF volatilization, which alters the solution chemistry locally near the liquid-air interface. This phenomenon produces a characteristic defect pattern: over-polished upper sections of vertically immersed parts and under-polished lower sections, with a gradual transition zone in between.
Practical countermeasures include jacketed tanks with circulating temperature control fluid, immersion heaters with proportional-integral-derivative (PID) controllers, and continuous bath recirculation to eliminate thermal stratification. Thermocouples positioned at multiple depths and locations provide the feedback needed for process control.
