Causes And Solutions For Uneven Chemical Polishing Of Titanium Alloys

Chemical polishing of titanium alloys uses the selective dissolution of polishing solution to preferentially etch microscopic protrusions on the surface, resulting in a smooth and bright finish. With simple equipment and compatibility with complex parts, this process is widely used in precision manufacturing. However, uneven polishing often occurs in production, such as inconsistent gloss, streaks, pitting, over-polishing, or under-polishing. It may impair appearance, corrosion resistance, dimensional accuracy, and yield rate.

 

 

(1) Incomplete Pre-Treatment and Uneven Initial Surface Condition

Large differences in surface state after casting, forging, machining, and welding are the main triggers:

• Residual oil, cutting fluid, fingerprints, etc., form shielding, causing local color differences and inconsistent gloss.
• Uneven thickness of oxide film after heat treatment and welding leads to different polishing rates and mottling.
• Scratches, deformed layers, and casting defects result in varying dissolution rates at concave-convex areas, worsening unevenness.
• Alloy composition segregation and inclusions cause regional activity differences, leading to over-etching or slow polishing.

 

(2) Unstable Polishing Solution System with Fluctuating Composition and Activity

• Improper acid ratio tends to cause over-etching, hydrogen absorption darkening, and local uncontrolled reactions.
• Aging polishing solution, accumulation of titanium ions, or impurity contamination reduce activity and cause local passivation.
• Uneven temperature or local heat accumulation leads to local over-polishing/under-polishing; deep holes and grooves are prone to over-etching.

 

(3) Poor Process Operation and Fluid Conditions Leading to Uneven Mass Transfer

• Poor fluidity of polishing solution, adhesion of bubbles and reaction products as shielding, resulting in poor polishing in deep holes and slits.
• Workpiece stacking, clamping shielding, and contact with fixtures create blind spots or black spots.
• Inaccurate polishing time control causes over-polishing in high-activity areas and under-polishing in thick oxide areas.

 

(4) Improper Post-Treatment

Incomplete or delayed cleaning after polishing leaves residual acid to continue etching and form stains; improper drying also aggravates surface unevenness.

 

 

(1) Strengthen Pre-Treatment

• Core: Unify surface condition to ensure uniform polishing.
• Multi-stage degreasing: Alkaline degreasing for heavy oil; ultrasonic cleaning with acetone/anhydrous ethanol for residual oil. Qualified standard: workpiece is continuously hydrophilic without water beads when immersed.
• Uniform deoxidation: HF-HNO₃ mixed acid pickling for 30 seconds to 2 minutes for mild oxidation; sandblasting followed by pickling for severe oxidation; rinse with deionized water for ≥3 minutes after pickling.
• Smoothing pre-treatment: Machined parts are ground with 400–800 grit sandpaper or vibratory polished to remove deep scratches and deformed layers.

 

(2) Optimize Polishing Solution System

• Standardized formula: Recommended ratio HF (40%): HNO₃ (65%): H₂O = 1:4:5, add 0.5%–1% urea and 0.2% citric acid; prepare fresh before use, replace solution when titanium ion concentration >30 g/L.
• Precise temperature control: Heat with constant-temperature bath stirring, temperature 20–35 °C (fluctuation ≤±2 °C), indirect heating for heat dissipation, control workpiece loading capacity.

 

(3) Standardize Process Operation

• Forced stirring: Mechanical/ultrasonic stirring; add circulating pump for complex workpieces to eliminate polishing blind spots.
• Optimized clamping: Single-point contact with pure titanium/fluoroplastic fixtures, workpiece spacing ≥5 mm, move/rotate workpieces during polishing.
• Precise time control: Pure titanium polishing 30 s–1 min; Gr5 polishing 1–3 min; adopt multiple short polishes, 30 s–1 min each with water washing and inspection.

 

(4) Standardized Post-Treatment

Multi-stage water washing (each stage ≥2 minutes) → weak alkali neutralization and passivation (1–2 minutes) → blow-dry and seal for rust prevention after drying.

 

(5) Quality Control and Abnormality Handling

Batch production after qualified first-piece inspection; slight unevenness can be re-degreased and re-polished; over-etched parts need grinding before re-polishing; keep workshop ventilated to prevent bath contamination.

 

 

Uneven chemical polishing of titanium alloys is caused by multiple factors including surface condition, polishing solution, process operation, and post-treatment. The key to solving the problem is unified pre-treatment, stable polishing solution activity, enhanced mass transfer, and standardized full-process control. In production, parameters should be customized according to material and structure, with refined operation to stably obtain a uniform and bright surface, improving yield rate and competitiveness.

 

 

Ruihang Group mainly produces Titanium and Titanium Alloy products with the complete industry chain,including smelting,forging,straightening,rolling,surface treating,testing process. We are a technology and innovation enterprise that integrates R&D, production and sales into one integrated system.If you have purchasing needs on hand, feel free to contact us: Sam.Rui@bjrh-titanium.com.