Anode Cell Best Practices

Contents[Show]

Downloadable Copy

Anode surface area is estimated using one or more criteria (Rule) listed below:

4:1 Ratio: Anode area is equal to 2 minutes worth of painted through-put / 4. For automotive systems, use 2.5 minutes.
Anode Current Density: When total amp amount is divided by the amount of anode area, the figures should not exceed 35 amps/SF (3.5 amps/SM) for high through-put systems that need redundancy; or as much as 5 amps/ SF (50 amps/SM) for industrial E-Coat paint systems.
Center-to-Center Cell Spacing: The center-to-center spacing be-tween Anode Cells should be no more than 5 x & no less than 1 x the circumference (or arc length) of the anode.

Some common terms are defined below:

Anode: Positive side of the DC rectifier
Anodic Paint: Ware is the anode
Cathode: Negative side of DC rectifier
Cathodic Paint: Ware is the cathode
Electrode: The metal part connected to the bus bar. For cathodic paint, it is the anode. For anodic paint, it is the cathode.
Electrolyte: Fluid inside the Anode Cell. For cathodic paint, it’s called anolyte. For anodic paint, it’s catholyte.
Ware: Object or part being painted

There are several different types of Anode Cell designs or configurations, including:+

Open Top: The most common and typically the least expensive since it has fewer parts.
Closed Top: Sometimes called Low Profile or Pressurized, has a Bulkhead Fitting. Does not require as much space above the rim of the tank. Use PTAN membrane to minimize anolyte loss.
Horizontal (i.e. Roof or Floor): Note the electrolyte discharge must be at the 12 o’clock position and sloped upwards at 2% to allow oxygen to leave the Cell. Use 16 lpm / sm (4 gpm / 10 SF).
Geometries: Crescent, Box & Tubular: Box Cells have the least amount of membrane; tubular have the greatest amount.

24/7 motion of the anolyte promotes better cooling to ex- tend anode life & expel the oxygen that can cause corrosion:

8 lpm / sm (2 gpm/10 SF) + 20% to size electrolyte pump
Size supply manifold at 3-5 fps (1-1.5 mps) velocity.
Size return manifolds for no more than 3/4 full & 1-2% negative slope.
Do not mix up the supply & return tubing connections for Closed Top Cells as the Cell can fill with oxygen & current will go to zero.
PVC tubing with 2.3 mm (3/32”) min. wall thickness
Use 90° hose barbs to avoid kinked PVC tubing.
Limit supply pressure for Closed Top Cells to 7 psi (0.5 bar) to avoid over pressurization & membrane failure.

Stagnant, warm anolyte fluid will promote the growth of fungus. Some preventive actions to take:

Anolyte Pump: Do not turn off, keep operating 24/7.
Raise Anolyte Conductivity: The higher the conductivity, the less attractive the food source. Talk to your paint supplier to discuss an increase to 1,800 or 2,000 micro S/cm to help reduce fungus.
UV lamp: If your DI water or RO water is stored in a tank, is there a UV lamp on the recirculation loop?
Approved Biocides: Use approved biocides in the anolyte tank. Avoid those with halides such as chlorine and bromine as they may accelerate corrosion of the 316L anode.
Limit Hydrogen Peroxide: To 3% strength and no more than 2 or 3 consecutive cleanings with paint company approval.

Several activities can be performed during the year that may pay benefits, reduce surprises, & lessen down-time:

Do not mix new and old Anode Cells together. Keep new Cells away from older Cells, so they all share the work as equally as possible.
Keep electrical connections tight with use of spring washers.
Train personnel on Confined Space & Lockout/Tagout procedures.
Repair membrane tears less than 25 mm (1 in.) with membrane repair kit. Avoid membrane contact with strong oxidizers.
Use a carbon filter as part of the DI water system & discuss with paint vendor use of nitric acid to reduce 316L corrosion.
Record mass loss of SS Anodes, rotate Tubular Anodes 120 deg 3 x a year, or visually inspect precious metal anodes.

Anode Cells are made from ion-exchange membrane, metal anode, and a plastic housing:

PTAN Membrane: Pressed sheet style with the best mechanical properties and the least amount of water permeability.
PTAR Membrane: Reduced cost membrane produced in 30 m rolls. Not recommended for Low Profile applications since the water permeability is 2 x that of PTAN membrane.
316L Stainless Steel: The primary choice for anode material. Do not use 304, 302, or 17-8 stainless alloys as they have short lifetime.
Precious Metal: Either ruthenium or iridium oxide coated over a thin titanium tube. Cannot be used as a cathode.
Housing: Fiberglass, PVC, or polypropylene is normally used.

Every system will change as it is operated. When trouble occurs, baseline data provides a valuable basis for comparison:

Voltage: Use the TruIDL Voltage Logger to investigate the voltage profile of the ware as it moves through the E-Coat bath; especially after major rework.
Electrical Current, Amps: The ability of the Anode Cell to deliver work and a measure of how resistive the Cell is. Readings should be recorded every 3 months for the largest ware.
Conductivity & Temperature: Monitor these two parameters of the anolyte circulation system and record daily.
Mark on the Label when Shells are installed with a Sharpie.