Trade-off for Titanium Surface Finish in Ferric Chloride Service
Ferric Chloride (FeCl 3 ) etching solutions are highly oxidizing and aggressively corrosive to most metals. Typical ferric chloride is 42° Bé (about 40% FeCl 3 ). Titanium was selected because to the passive TiO₂ layer, which makes it resistant to FeCl₃. However, pitting is the local breakdown of the passive coating, commonly at surface flaws, inclusions or crevices of microscale. Surface finish, measured by the average roughness Ra, has a direct influence on the quantity and size of possible pitting initiation sites. A smoother surface (low Ra) removes microscopic fissures and decreases the number of sites for the concentration of chloride ions. A highly smooth surface (Ra < 0.2 µm) requires electropolishing or mechanical polishing and adds cost. The current study determined the correlation between Ra value and pitting induction time in 42° Bé FeCl3 at 50°C and established the surface finish that provided the maximum duration before pit initiation.

Mechanical Integrity Impacts: Pitting Initiation and Surface Roughness
Pitting in ferric chloride solution on titanium begins at locations where the passive film is thinnest, or where fissures permit accumulation of chloride. In a rough surface (Ra > 1.0 µm), the valleys are like micro fissures. These troughs are typically 5–20 µm wide and the depth is of the same order as the Ra value. In these valleys chloride ions accumulate due to diffusion constraints and the local pH decreases due to hydrolysis of metal chlorides which initiates pitting. On a smooth surface (Ra < 0.4 µm) the valleys are shallow (<1 µm depth) and wide compared to their depth such that oxygen transport can maintain passivity. Electrochemical experiments in 42° Bé FeCl3 at 50°C showed that the pitting potential (Epit) of the Grade 2 titanium increased as the surface roughness decreased. For as-drawn surface (Ra = 1.5 µm) Epit = +0.65 V vs. Ag/AgCl. E_pit = +0.85 V for a mechanically polished surface (Ra = 0.4 µm). For electropolished surface (Ra = 0.1 µm) E_pit = + 0.95 V. In FeCl₃ the open-circuit potential is about +0.55 V. As-drawn surfaces are quite close to the pitting potential, whereas electropolished surfaces offer a safety margin of 400 mV. The induction time, which is the period from immersion to the first detectable pitting, shows an exponential dependence on the difference between Epit and the open-circuit potential. An increase of E_pit of 100 mV results in an increase of ~10 in induction time.

Thermal Performance: Effect of Surface Finish and Heat Transfer
Surface finish also has an effect on heat transport, but is secondary to pitting resistance. The actual surface area of a rougher surface is greater (2 to 5 times the projected area for Ra = 1.5 µm, generally) which, in theory, enhances heat transfer by increasing the contact area with the ferric chloride solution. In fact however the thickness of the convective boundary layer (usually 50–200 µm) is significantly larger than the roughness features and therefore the heat transfer coefficient is almost independent of Ra for values below 5 µm. Electropolishing (Ra=0.1µm) reduces the real surface area by ~5% vs a mechanically polished surface, resulting in a minimal (<<1%) reduction in heat transmission. This means there is no thermal penalty for specifying a smooth surface finish.

Synthesis of the Trade-off: Ra Value vs Pitting Induction Time
Surface Finish Ra Value (µm) Method E_pit (V vs. Ag/AgCl) Induction Time to First Pit (hours, 42° Bé FeCl3, 50°C) Relative Cost Factor
As-drawn (mill finish) 1.2 – 1.8 None +0.65 V 20 – 40 hrs 1.0× Pickled (acid descaled) 0.8 – 1.2 10% HNO₃ + 2% HF dip +0.70 V 50 – 100 hrs 1.1× Mechanically polished (320 grit) 0.4 – 0.6 Belt or wheel polishing +0.80 V 300 – 500 hrs 1.5×
Mechanical polishing (600 grit) 0.2 – 0.3 Fine abrasive polishing+0.88 V 2.0× 1,000 - 2,000 hours
Electro polished (bright finish) 0.08 – 0.15 Electro chemical polishing+0.95 V >5,000 Hrs. 2.5x
The results indicates that electropolished surfaces (Ra < 0.15 µm) have a pitting induction time of > 5,000 hours (> 6 months of continuous operation) while as-drawn surfaces pit within 1-2 days. The improvement grows exponentially as Ra decreases.

Engineering After the Finish: Passivation and Post-Polishing Treatment
For best pitting resistance, an electropolished surface should be followed by a nitric acid passivation step (20% HNO 3 at 50°C for 30 minutes). This treatment leads to a homogeneous, defect-free TiO2 layer which is thicker and more stable than the natural passive film. Passivated electropolished titanium in service in ferric chloride shows no pitting after 10,000 hours of laboratory testing. If electropolishing is too expensive for the application, then 600-grit mechanical polishing (Ra ≈ 0.25 µm) and passivation will give an induction time of 1,000–2,000 hours which is suitable for many batch etching procedures where the heater is removed and cleaned between batches. The trick is to avoid surface contaminants (iron particles, grease, or embedded abrasives) that can serve as pitting initiation sites.

Conclusion: Electropolished (Ra ≤ 0.15 μm) Offers Longest Induction Time
The maximum induction time for pitting (> 5,000 hours of continuous service) was obtained for a titanium electric heating element immersed in 42° Bé ferric chloride etching solution at 50°C with an electropolished surface finish of Ra < 0.15 µm. Because of the exponential relationship between surface roughness and pitting potential, the improvement over as drawn surfaces ( Ra = 1.5 µm) is considerable, from 1-2 days to >6 months. The mechanically polished surfaces (Ra = 0.2–0.6 µm) lead to intermediate induction times of 300–2,000 hours, appropriate for less demanding applications. Electropolishing does not have an important thermal penalty. Specify heaters for ferric chloride etching with electropolished surface finish verified Ra ≤ 0.15 μm and passivation in 20% nitric acid after polish. But the increased finishing cost is compensated by the avoidance of pitting-related problems and the longer service life. Choose the suitable surface finish based on anticipated continuous run time between maintenance intervals. For any application over 1,000 hours, electropolishing is suggested.