The electroplating industry is one of the most aggressive environments for any mechanical component immersed in a treatment tank. Chromic acid solutions at 60°C, pH-controlled nickel plating baths, pickling solutions with hydrochloric and hydrofluoric acid, hot alkaline degreasing baths: each tank presents a different combination of chemical aggressiveness, temperature and mixing requirements.
An inadequate agitator in an electroplating tank does not simply perform poorly — it compromises the quality of the electrodeposit. Insufficient mixing causes non-uniform ion distribution, hot and cold zones in the tank, accumulation of contaminating metal ions near the cathode, and surface defects on the workpiece (porosity, burning, coating delamination).
This guide analyses the specific agitation requirements for each stage of the electroplating process and explains how to select the correct agitator for every type of bath.
Why agitation is critical in electroplating processes
In an electroplating bath, metal deposition occurs through electrochemical reduction of metal ions on the cathode surface (the workpiece to be coated). Deposit quality depends directly on three factors that agitation controls.
Uniform ion distribution
During deposition, metal ions are consumed near the cathode, creating a depleted zone (diffusion boundary layer). If the solution is not adequately agitated, the ionic concentration near the workpiece drops below the critical level and the deposit becomes porous, rough or non-uniform. Agitation constantly renews the boundary layer, maintaining uniform ionic concentration across the entire workpiece surface.
Temperature uniformity
Many electroplating baths operate at controlled temperatures: chrome plating at 50–60°C, nickel plating at 45–55°C, alkaline zinc at 25–35°C. Heating elements or coils create thermal gradients that, without agitation, can reach 5–10°C between different zones of the tank. This non-uniformity causes differences in deposit thickness, aesthetic defects and variations in coating mechanical properties. The agitator maintains temperature uniformity within ±1–2°C.
Gas bubble removal
During electrolysis, hydrogen gas evolves at the cathode. If hydrogen bubbles adhere to the workpiece surface, they prevent metal deposition at those points, creating porosity (pitting). Agitation promotes bubble detachment and removal, improving the surface quality of the deposit.
The effect on cathode efficiency
Agitation has a direct impact on process efficiency. In a chrome plating bath, cathode efficiency with agitation reaches 70–90%, while without agitation it drops to 50–60%. This means that for the same treatment time, the agitated workpiece receives a significantly thicker and more uniform deposit.
Electroplating baths: specific requirements for each treatment type
Each stage of the electroplating cycle has different agitation needs. Let us examine the main ones.
Alkaline degreasing
Degreasing is the first stage of the cycle and uses strongly alkaline solutions (NaOH 30–50 g/l, carbonates, phosphates, surfactants) at temperatures of 50–80°C. Agitation must be vigorous to promote detachment of organic contaminants from the workpiece surface, keep surfactants in emulsion and ensure uniform cleaning across the entire surface.
For this stage, a high-speed EV series agitator with a three-blade marine propeller impeller in PP is the standard choice. The high speed generates the turbulence needed to “strip” oily residues from the metal surface.
Acid pickling
Pickling removes oxides and scale from the surface through immersion in strong acids: 10–20% hydrochloric acid (the most common), 10–25% sulfuric acid, or hydrochloric/hydrofluoric acid blends for stainless steels.
Agitation must be moderate: sufficient to maintain uniform acid concentration and remove reaction products (metal salts) from the surface, but not so violent as to cause excessive base metal removal. The EVR series with pitched blade impeller at low speed (50–150 rpm) is ideal: it generates recirculation without excessive turbulence.
For baths with hydrofluoric acid, the material must be PVDF. For dilute hydrochloric and sulfuric acid, PP is adequate.
Alkaline zinc bath
Alkaline zinc plating uses sodium zincate solutions in a strongly alkaline environment (NaOH 80–140 g/l) at ambient or slightly heated temperature (25–35°C). Agitation must be gentle and uniform to avoid localised turbulence that would cause non-uniform deposits.
A low-speed EVR or KVRL series agitator at 30–80 rpm with PP pitched blade impeller is ideal. PP has excellent resistance to caustic soda, even concentrated — one of the few applications where PP outperforms PVDF.
Nickel bath (Watts)
The Watts bath (nickel sulfate, nickel chloride, boric acid) operates at 45–55°C and pH 3.5–4.5. Agitation must be uniform and low-turbulence to prevent air incorporation and ensure a bright, pit-free deposit.
The EVR series at 40–100 rpm with PP impeller is the typical configuration. For electroless nickel baths that operate at 85–90°C with more aggressive pH, PVDF offers greater reliability.
Chrome plating bath
Chrome plating is the most critical electroplating process from both the agitation and materials standpoint. The bath contains chromic acid (CrO₃ 200–400 g/l) and sulfuric acid as catalyst, at temperatures of 50–60°C.
Chromic acid is a strong oxidiser that attacks both PP and PVC. PVDF is the only adequate thermoplastic material for agitators immersed in chrome plating baths.
Agitation must be moderate and uniform: sufficient to maintain homogeneous temperature and constant ion distribution, but without excessive turbulence that would promote evaporation of chromium compounds (an environmental and health concern). The EVR or KVRL series with PVDF impeller at 30–60 rpm is the optimal configuration.
Acid copper bath
Acid copper plating (copper sulfate in sulfuric acid) operates at ambient temperature with H₂SO₄ concentrations of 5–10%. Agitation must be vigorous near the cathodes to uniformise ion distribution, especially on workpieces with complex geometries (holes, recesses, corners).
The EV series with PP impeller at medium-high speed (300–800 rpm) provides the turbulent flow needed. For plants with large tanks, side-entry agitators from the LVO series may be preferable.
Rinsing and washing
Rinse tanks between process stages require vigorous agitation to rapidly remove chemical drag-out from the workpiece surface. High-speed EV series in PP, with the possibility of multiple installations for large tanks.
Installation configurations for electroplating tanks
Electroplating tanks have specific geometries (typically rectangular, long and narrow) that require particular installation solutions.
Top-entry installation
Top-entry installation is the most common for small and medium tanks. The agitator is mounted on the tank rim with a bracket support. The shaft descends vertically into the solution with the impeller positioned in the lower third of the tank.
For electroplating tanks, Nuova Darimpianti offers the EVL and EVRL series with extended shafts, specifically designed to reach the bottom of deep tanks without requiring intermediate guides.
Side-entry installation
For long tanks where a single top-entry agitator cannot guarantee uniformity along the entire length, side-entry installation (LVO or LRO series) generates a helical flow involving the entire volume. This configuration is particularly effective for electroplating lines with tanks 2–5 metres in length.
Multiple installation
For very large tanks or processes requiring particularly uniform agitation, multiple agitators can be installed on the same tank, distributed evenly along the length. In this case, it is important that the impellers generate complementary (not opposing) flows to avoid zones of excessive turbulence or dead zones.
Fume resistance: an often-overlooked aspect
In hot electroplating tanks, the zone immediately above the liquid level is exposed to concentrated acid fumes and vapours. The agitator shaft passes through this zone and suffers chemical attack that is different from — and often more aggressive than — the attack in the solution.
Chromic acid fumes, for example, attack PP even when the solution below would not (because the fumes are more concentrated and oxidising). For this reason, in chrome plating tanks it is advisable to use PVDF also for the exposed portion of the shaft, or to provide a protective sleeve in the liquid-vapour interface zone.
Agitator maintenance in electroplating environments
The electroplating environment is particularly aggressive for maintenance. Here are the recommended practices.
The impeller and shaft should be visually inspected every 3–6 months for signs of chemical attack (whitening, surface roughness, swelling). Bath concentration and temperature must be constantly monitored: a concentration drift can shift the fluid outside the material’s compatibility zone. Bearings and seals (in gear-reducer versions) must be protected from acid vapours with bellows or sleeves.
Thanks to solid-block CNC machining, Nuova Darimpianti components have no internal stresses that could accelerate chemical degradation: this translates to longer service life compared to moulded components, especially in cyclic environments (tanks heated and cooled daily).
Frequently asked questions
What agitation speed is needed for an electroplating tank?
It depends on the bath type. For chrome and nickel plating, low speeds (30–100 rpm) with pitched blade impellers are ideal to avoid excessive turbulence. For degreasing and rinsing, higher speeds (300–800 rpm) with propeller impellers promote mechanical cleaning. The guiding principle is: deposition baths → low speed; preparation baths → high speed.
Can I use compressed air instead of a mechanical agitator?
Air agitation is a traditional alternative, but has disadvantages: it introduces oxygen into the bath (problematic for many processes), creates acid mists above the tank, does not allow precise control of mixing intensity, and has lower energy efficiency. Mechanical agitators offer more precise control and better efficiency, and are preferred in modern plants.
Which material should I use for an agitator in a chrome plating tank?
PVDF is the only adequate thermoplastic material for chrome plating baths. Chromic acid is a strong oxidiser that attacks both PP and PVC. PVDF resists chromic acid at all concentrations and operating temperatures of chrome plating (50–60°C).
Can the agitator damage workpieces in the tank?
If the impeller is too close to the workpieces hung on the cathode frame, excessive flow can cause non-uniform deposits on exposed edges (the “dog bone” effect). The impeller should be positioned at least 15–20 cm from the workpieces, in the lower third of the tank, where it generates circulation without directly impinging on the cathode surfaces.
Does Nuova Darimpianti supply agitators for complete electroplating lines?
Yes. Nuova Darimpianti designs agitation solutions for the entire electroplating line: degreasing, pickling, deposition baths, rinses and post-deposition treatments. Each tank is individually analysed for size, fluid, temperature and process objective, with personalised selection of series, material and speed.
The right agitator for every stage of the electroplating cycle
The quality of an electroplating treatment depends on many factors, but agitation is one that is too often underestimated. A correctly sized agitator built from the right material improves deposit uniformity, reduces rejects, increases cathode efficiency and extends bath life.
Nuova Darimpianti manufactures high-speed agitators (EV, EVK, EVL, KVL series) and low-speed agitators (EVR, EVRK, EVRL, KVRL series) in PP, PVC and PVDF specifically designed for the electroplating and surface treatment industry. Every component is machined from solid blocks on CNC centres to guarantee maximum chemical resistance and durability even in the most aggressive baths.
Planning a new electroplating line or looking to improve agitation in existing tanks? Contact Nuova Darimpianti’s engineering team for personalised advice.