When Heating Ferric Chloride Etchant (42° Bé) at 50°C, Why Does Titanium Grade 7 Exhibit Lower General Corrosion Rates (0.01 mm/year) Than Grade 2 (0.2 mm/year)?

Ferric chloride (FeCl₃) etchant at 42° Bé (around 38-40% FeCl₃ by weight) is the typical medium for etching copper-clad circuit boards and for the chemical milling of metals. The solution is very oxidizing, intensely acidic (pH 0.5-1.0) and includes about 200-250 g/L chloride ions. Titanium is used extensively for immersion heaters in FeCl 3 etching lines due to its excellent resistance to chloride pitting under oxidizing conditions. But there are important distinctions between commercially pure Grade 2 and palladium-stabilized Grade 7 titanium. In long term service at 50°C the general corrosion rate of Grade 7 is 0.01-0.03 mm per year while Grade 2 corrodes at 0.15-0.25 mm per year, a factor of 10 difference. This paper describes the electrochemical mechanism of this performance gap and defines when the extra cost of Grade 7 is justified by its reduced corrosion rate.

Ferric chloride as oxidizing agent
Ferric chloride is a strong oxidant due to the Fe 3+ /Fe 2+ redox pair with a typical reduction potential of +0.77 V vs. SHE. The effective potential is +0.6 to +0.8 V vs SCE in concentrated solution at 50°C. This should be a very oxidising environment and would be expected to fully passivate titanium . Both Grade 2 and Grade 7 are passive . The passive layer on titanium is not static, but is continuously dissolving and reforming. The rate of dissolution is controlled by the chemistry of the titanium-solution interface, in particular the concentration of complexing anions (chloride) and the presence of noble metal catalysts.

FeCl3 Grade 2 Corrosion Mechanism
The passive TiO₂ coating on Grade 2 titanium in FeCl₃ solution is slowly chemically dissolved TiO₂ + 4Cl⁻ + 4H⁺ → TiCl₄ + 2H₂O. The TiCl₄ is hydrolyzed to soluble species. Simultaneously with disintegration, the film is re-formed by interaction of titanium with water or oxygen. The steady state corrosion rate at 50°C is 0.15–0.25 mm per year, the result of the difference between dissolution and repassivation rates. The rate is temperature dependent: at 40°C Grade 2 corrodes at 0.05-0.10 mm/year, at 60°C the rate jumps to 0.4-0.6 mm/year.

Corrosion is widespread (universal) and not pitting. A Grade 2 tube wall will thin slowly and uniformly during the months of operation. A 1.65 mm thick tube in use at 50 °C will lose about 0.2 mm per year, leading to a service life of 5–8 years until the minimum permissible wall thickness is reached (usually 1.0 mm for mechanical integrity).

Grade 7 Cathodic Treatment
Titanium in grade 7 contains between 0.12 to 0.25% palladium, which is dispersed as fine islands throughout the titanium matrix. Titanium has a good cathodic site and is less noble than Palladium. The main cathodic reaction on Pd in FeCl3 solution is . Fe3+ + e- -> Fe2+ This reaction is initiated at a lower overpotential on palladium than on titanium, suggesting that the majority of the cathodic current occurs at the palladium sites. The titanium matrix polarises to a more noble potential, often +0.3 to +0.5 V more noble than Grade 2 under the same conditions.

As this more noble potential the TiO₂ film dissolving rate drops exponentially. Electrochemical tests indicate that the passive current density of Grade 7 in FeCl3 at 50°C is 0.5-1.0 µA/cm2 , which is equivalent to a corrosion rate of 0.005-0.010 mm per year . The palladium does not need to cover the whole surface, because 0.5-1.0% palladium islands on the surface are enough to ensure cathodic protection.

Comparison of Long-Term Performance
Field validation is provided by field data from PCB etching facilities using 42°Bé FeCl 3 at 50°C. Grade 2 titanium heaters (1.65 mm wall) normally have a wall thinning life of 5-7 years and need to be replaced. Grade 7 heaters (1.65 mm wall) have been in service for 15+ years with measured wall thickness losses of less than 0.2 mm total. Grade 7 costs more upfront (about 30-40% more than Grade 2), but it pays for itself with longer service life and less downtime.

Material General corrosion rate @ 50°C Time to 50% wall loss (1.65 mm to 0.8 mm)Application Relative Cost Factor Cost-Effective
Grade 2 titanium 0.15-0.25 mm/yr 3-5 years 1.0x Low-volume, intermittent etching
Grade 7 titanium 0.01-0.03 mm/year 15-25 years 1.3-1.4x High-volume, continuous process
Hastelloy C-276 0.02-0.05 mm/year 10-15 years 2.5-3.0x Only specialized applications
FeCl₃ Heat Source Selection Application Matrix
Etching Operation Temperature Bath Turnover RateTitanium Grade RecommendedHeater Life Expected (1.65 mm wall)
Low-volume PCB prototyping 50°C Intermittent, bath change monthly Grade 2 5-7 years
high volume PCB fabrication 50°CContinuous, bath regenerated Grade 7 12-15 years
Chemical etching (thick metal) 55-60°CHigh use, continuous Grade 7 8-10 years
Regenerated FeCl₃ (low Fe²⁺) 50°C Continuous Grade 7 15+ years
Used FeCl₃ (high Fe²⁺, less oxidizing) 50°C Batch disposal Grade 2 4-6 years
The cathodic modification by palladium in titanium grade 7 leads to total corrosion rates of about one twentieth of that of grade 2 in ferric chloride etchant at 42 degrees B and 50 degrees C. The palladium sites modify the titanium surface potential to a more noble value, leading to a decrease in passive film breakdown. For continuous high-production etching operations where heater dependability and long service life are of primary importance, the higher starting cost of Grade 7 is warranted. Grade 2 remains the cost effective choice for intermittent or low volume use where a 5-7 year heater life is acceptable. When specifying ask for the actual corrosion rate recorded in the specific FeCl3 solution. Trace contaminants (copper, nickel) can impact performances.