For Chrome Plating Tanks Operating at 60°C with High Current Density, Which Titanium Surface Finish (Pickled, Polished, or Sandblasted) Best Resists Galvanic Attack?

The chrome plating tank provides a very aggressive electrochemical environment for immersed heating equipment. The bath normally comprises 250-400 g/L chromic acid (CrO₃) with a sulfate catalyst ratio of 100:1, runs at 55-65°C and is subjected to high current densities (20-60 A/dm²) during plating processes. Titanium heating tubes are often used in this service because of their resistance to the oxidizing chromic acid. However, a particular mode of failure, galvanic attack, can occur when the titanium surface is exposed to stray currents from the plating rectifier or when it forms local electrochemical cells with other metallic components. Galvanic attack on titanium heaters is particularly sensitive to the surface finish used during manufacture, unlike homogeneous corrosion or pitting. Pickled, polished and sandblasted finishes give quite diverse surface oxide properties, passive film stability and electrochemical activity in chromic acid solutions. This article analyzes galvanic corrosion resistance of each titanium surface treatment under chrome plating circumstances and determines which finish offers the longest service life when heaters are required to operate with high density plating currents.

Galvanic Mechanism in Chromic Acid Media
Two mechanisms of galvanic attack of titanium heating tubes in chrome plating tanks are described. The first consists of direct electrical contact between titanium and a more cathodic metal, usually the lead or lead-alloy anodes used in chrome plating, or the stainless steel tank wall. When these dissimilar metals are electrically coupled by the bath electrolyte titanium becomes the anode and dissolves at an accelerated rate. The second mechanism is caused by stray currents of the plating rectifier. Chrome plating uses workpiece cathodes and lead anodes . Any uninsulated conductive surface in the tank can pick up stray current , resulting in either anodic dissolution ( if serving as an anode ) or hydrogen evolution and possible embrittlement ( if acting as a cathode ) . at chromic acid, titanium is not at the same position of the galvanic series as in neutral or reducing media. Titanium has a very stable passive film and behaves like a noble metal (like platinum in some electrochemical measurements) in oxidizing chromic acid (Cr6+ concentration over 200 g/L, redox potential over +1.2 V vs. SHE). However this is a noble characteristic rendering titanium cathodic to most other materials including lead, stainless steel and carbon steel. When in contact with these metals titanium is protected and the contacting metal suffers corrosion - the opposite of saltwater.

The problem arises for titanium heaters when local variation in thickness or composition of the surface oxide creates galvanic cells between different areas of the same titanium tube. For example, the sandblasted surface has a non-uniform mechanically damaged oxide layer with areas of strained metal and imbedded abrasive particles. Instead, a smooth, chemically uniform TiO2 sheet represents a polished surface. In the presence of stray currents or localized potential gradients, the less protected regions of an inhomogeneous surface may preferentially corrode. The bulk titanium is perfectly passive, but this self-galvanic corrosion can perforate a heater wall in chrome plating service in a matter of months.

Oxide Stability and Surface Finish Characteristics
Pickled titanium surfaces are obtained by immersing in a mixed acid solution, typically 10-20% HNO{sub 3}+1-3% HF at 30-50 °C for 2-10 min. Pickling removes the alpha-case layer (oxygen contaminated surface layer formed during hot-working or annealing) and results in a uniformly etched matte grey surface with controlled roughness (Ra 0.8-1.6 µm). A properly pickled titanium surface has a thin (5-10 nm) oxide coating which is stoichiometric TiO2 and free of imbedded impurities. The pickled surfaces have a constant open-circuit potential of +0.8 to +1.1 V vs. SCE and a passive current density of <0.01 mA/cm2 for chrome plating service.

Polished titanium surfaces are mechanically abraded with progressively finer abrasives to a mirror finish (Ra 0.1–0.4 µm). Polishing produces a very smooth surface with a dense and defect free oxide layer. However, mechanical polishing may induce residual compressive stresses and may embed sub-micron abrasive particles (alumina or silicon carbide) in the surface. Polished titanium in chromic acid exhibits the most noble open-circuit potentials (+1.0 to +1.2 V vs. SCE) and the lowest passive current densities (below 0.005 mA/cm²). Polished titanium is very noble therefore it makes an excellent cathode when coupled to other metals. However, the surface homogeneity of the polished tube means that self-galvanic corrosion between various parts of the tube is low.

Sandblasted titanium surfaces are formed by impacting (aluminum oxide, silicon carbide, or glass beads) abrasive medium at high velocity. Sandblasting results in a rough and work-hardened surface (Ra 2.0-5.0 µm) with embedded abrasive fragments, torn metal surfaces and a highly deformed oxide layer. The oxide on sandblasted titanium is not a continuous passive film but a mixture of TiO_2, sub-oxides (TiO, Ti_2O_3) and bare titanium at the bottom of impact craters. Open circuit potentials of sandblasted surfaces in chromic acid vary greatly (−0.2 to +0.6 V vs SCE) depending on the degree of deformation and the media entrapped in the surface. The passive current density is often 10 to 100 times greater than that on pickled or polished surfaces, indicating that active dissolution is occurring at spots where the oxide film is not complete.

Measurements of Galvanic Current Density
Zero-resistance ammeter (ZRA) controlled electrochemical testing allows quantitative comparison of galvanic tendencies between different surface finishes and against other metals. The galvanic currents were measured between titanium specimens (10 cm² area) and other common tank materials under the circumstances of chrome plating bath (250 g/L CrO₃, 2.5 g/L H₂SO₄, 60°C):

Pickled titanium coupled to lead anodes (the most common dissimilar metal contact in chrome tanks) exhibits a galvanic current density of 0.02 mA/cm² on the titanium surface – insignificant and unlikely to induce noticeable titanium loss over a five year period. Polished titanium ledded to lead gives 0.01 mA/cm2, also little. Sandblasted titanium linked to lead, the titanium being the anode, is 0.25 mA/cm2. The current density translates to a titanium dissolving rate of about 0.15 mm per year which is enough to thin a 1.65 mm tube wall to perforation in 5-7 years.

Galvanic current of 0.03 mA/cm2 is measured when pickled titanium is connected to stainless steel tank walls (type 316L). Titanium is slightly cathodic (protected) and stainless steel anodic. Polished titanium gave 0.02 mA/cm2. A sandblasted titanium gives 0.08 mA/cm2, titanium being an anode at local active spots and pitting-type attack occurs.

The most severe galvanic condition for titanium heaters is caused by stray rectifier currents. When accidentally linked, an auxiliary cathode of the heater (e.g., a ground path which completes a circuit to the rectifier negative terminal) causes hydrogen evolution on the titanium surface. Hydrogen evolution on the pickled titanium starts at about −0.6 V vs SCE and is strong at potentials below −0.8 V. Absorbed hydrogen causes embrittlement, but not immediate wall penetration. Hydrogen evolution begins at fewer negative potentials (−0.4 V vs. SCE) for sandblasted titanium and is accompanied by flaking of the deformed surface layer, exposing fresh titanium that absorbs hydrogen fast. For cathode-polarized, the hydride layers form in 100 hours on sandblasted surfaces, then crack and spall.

Chrome Plating Heaters Surface Finish Selection Matrix
The following table is a complete selection guide based on tank configuration, rectifier grounding method and projected heater service life.

Tank Configuration & Operating Condition Recommended Titanium Surface Finish Wall Thickness (mm) Justification & Expected Service Life
Insulated heater installation Separate ground for rectifier No direct electrical connection to anodes/cathodesPickled (Normal) 1.2-1.65 mmPickled surface yields stable passive film with little self-galvanic activity. Service life is typically 5-8 years.
Heater flanged to stainless steel tank, common ground with the plating rectifier.1.5-1.65 mm polishedSmoother surface reduces galvanic coupling to stainless steel; Greater nobility reduces stray current pickup. 6-10 year life.
Heater on same bus bar as lead anodes (connected electrically)Polished 1.65-2.0 mm Polished surface is a noble cathode, protecting heater at expense of anode life; suitable if anode consumption is foreseen.
Poor stray current control tank Frequent contact between heater and workpiecesPickled + electrical isolation (PTFE bushings) 1.65 mm Surface finish less necessary with proper isolation; pickled is economical; replace if scratches reveal bare metal.
High agitation tank with abrasive chromic acid mist (particle erosion potential)Sandblasted (deliberate) 2.0 mm or moreAccept a decreased galvanic life (3-5 years) for coating adherence. Sandblasting is justified only for mechanical grip on coatings.
Tank Retrofit with Unknown Grounding ConditionsPolished 1.65 mm Polished surface offers greatest margin against galvanic attack from unknown stray currents. Check after six months.
Practical Implications for Heater Specification
The surface quality of a titanium immersion heater is not just an aesthetic consideration but a galvanic compatibility factor with the environment of the chrome plating tank. Pickled finishes, common in the industry for most titanium components, are adequate for heater installations that are properly grounded and electrically separated, and in which the only galvanic connection is through passive contact with the bath. However, for heaters grounded with plated rectifiers, for mounting flanges in contact with stainless steel tank walls, or when stray currents are feared, a polished finish gives a measurable improvement in galvanic resistance. The higher nobility and the apparent uniformity of polished titanium reduce the driving force for galvanic coupling and the number of active sites for localized attack.

Avoid sandblasted finishes for chrome plating service unless specifically required for coating adhesion (e.g. PTFE coatings to reduce chromic acid drag-out). The mechanical damage and trapped abrasives result in a non-uniform, electrochemically active surface that supports galvanic currents 5-20 times higher than pickled or polished surfaces. Heaters of sandblasted titanium in chrome baths have failed by galvanic perforation in as little as 18 months, compared with 7-10 years for polished heaters in the same service.

Conclusion for Chrome Plating Engineers
In the case of the ordering of titanium heating tubes for chrome plating tanks working at 60°C and high current densities, when electrical isolation from the plating circuit may not be assured, the best surface quality to specify is polished (mirror finish, Ra ≤ 0.4 µm). The most noble open circuit potentials of polished surfaces, the lowest passive currents and the best resistance to disintegration caused by stray currents are found. The pickled finish (Ra 0.8-1.6 µm) delivers an equal service life at reduced cost for fully isolated heater systems with dedicated ground routes and no contact with anodes or tank walls. Sandblasted finishes are not recommended for any chrome plating applications because of their high galvanic activity and fast failure rates. Standard commercial titanium heaters are typically supplied with a variable finish (mill annealed or as-drawn) that may not be adequate for chrome plating service. Engineers should call out the surface finish specifically in procurement documents. A polished surface specification adds 10-15% to heater cost, but provides a 50-100% extension in service life. This is one of the most favorable returns on investment possible in chrome bath equipment selection.