Electrocoating Anodes

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HOW E-COAT WORKS

Electrocoating is a painting method which uses an electrical current to deposite paint. The process works on the principal of "Opposites Attract"...

The electrocoat system applies a DC charge to a metal part immersed in a bath of oppositely charged paint particles. The paint particles are drawn to the metal part and paint is deposited on the part, forming an even, continuous film over the entire surface, until the coating reaches the desired thickness. At that thickness, the film insulates the part, where attraction stops and the process is complete. Depending on the polarity of the charge, electrocoating is classified as either anodic or cathodic.

 

In anodic electrocoating, the part to be coated is the anode with a positive electrical charge which attracts negatively charged paint particles in the paint bath. During the anodic process, small amounts of metal ions migrate into the paint film which limit the performance properties of anodic systems. The main use for anodic products is interior or moderately exterior environments. Anodic coatings are economical systems that offer excellent color and gloss control.

In cathodic electrocoating, the product has a negative charge, attracting the positively charged paint particles. Cathodic electrocoat applies a negative electrical charge to the metal part which attracts positively charged paint particles. Reversing the polarities used in the anodic process significantly reduces the amount of iron entering the cured paint film and improves the cathodic properties. Cathodic coatings are high-performance coatings with excellent corrosion resistance that can be formulated for exterior durability.

WHAT IS THROWING POWER?

Perhaps the most advantageous aspects of the electrocoating process is "throwing power"- the ability to achieve complete coating everywhere on the product.

Each charged coating particle in an E-COAT solution seeks out the point of greatest opposite attraction. At the beginning of the process, this could be anywhere on the bare metal suface. However, as coated particles are deposited, they lose their charge and insulate the tiny areas they cover, forcing additional coating to find a location - even in out of the way corners of on exposed edges.

In this way, an electrocoat finish builds at an extremely even rate - layer by microscopic layer until the insulating nature of the coating is sufficient to halt further attraction. Thickness can be closely controlled through regulation of the voltage applied to the coating solution.

Partially enclosed areas and inside corners are well coverd because electrocoating is not influenced by the Faraday Cage Effect, as is spray coating. Repulsive electric fields produced at metal corners can greatly hamper spray penetration into inside spaces. Because E-COAT throwing power results in such a strong, uniform finish everywhere on the product, reduced paint useage in a thinner coating still offered unparalleled corrosion resistance.

Each charged coating particle in an E-COAT solution seeks out the point of greatest opposite attraction. At the beginning of the process, this could be anywhere on the bare metal suface. However, as coated particles are deposited, they lose their charge and insulate the tiny areas they cover, forcing additional coating to find a location - even in out of the way corners of on exposed edges.

In this way, an electrocoat finish builds at an extremely even rate - layer by microscopic layer until the insulating nature of the coating is sufficient to halt further attraction. Thickness can be closely controlled through regulation of the voltage applied to the coating solution.

Partially enclosed areas and inside corners are well coverd because electrocoating is not influenced by the Faraday Cage Effect, as is spray coating. Repulsive electric fields produced at metal corners can greatly hamper spray penetration into inside spaces. Because E-COAT throwing power results in such a strong, uniform finish everywhere on the product, reduced paint useage in a thinner coating still offered unparalleled corrosion resistance.

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INSOLUBLE ELECTROCOATING ANODES

A comparison with stainless steel anodes

Stainless steel is the common material chosen for electrocoat anodes. It’s relatively inexpensive and readily available. But stainless steel has a major drawback: It dissolves during e-coating, allowing iron into the anolyte system or paint tank.

Figure 1 illustrates the dissolution process, in which iron in the stainless steel reacts with the acid and water in the anolyte. Dissolution occurs because the electrical potential needed to form an iron oxide is less than that needed to initiate water electrolysis.

The effects of iron oxides

The release of iron oxides into the anolyte system and/or paint tank can cause problems. In the anolyte system, the electrolytic formation of iron oxides on the anode surface increases the resistivity of the anode, affecting localized current density. And at just 50 parts per million (ppm), iron in the paint tank can change the paint color, especially the whites and beiges. At 100 ppm, iron in the black and gray primers used on auto bodies reduces the smoothness of the coat. This reduces the smoothness of the spray coat that follows. And because ultrafiltration doesn’t remove iron, iron is coated out with the parts. This high iron content in the paint film increases corrosion potential.

Insoluble anodes

The ideal anode doesn’t dissolve at all, and very few iron oxides enter the tank or anolyte system. With an insoluble anode, the reaction during e-coating corresponds to Figure 2.

In fact, insoluble anodes are available.  Instead of using bare stainless steel, insoluble anodes have an inert material on their surface.  Usually, this inert material is from the platinum-group of metals or the oxides of these metals.  For e-coating anodes, ruthenium oxide is the most common choice.

But a solid sheet of ruthenium oxide would be too expensive to justify its use, so only a thin coating of ruthenium oxide is applied to the surface of a titanium substrate.  Titanium is used because of its resistance to corrosion, electrochemical properties, availability, and cost.

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Low iron oxides and long life

An anode cell with a ruthenium-oxide-coated electrode virtually eliminates iron contamination.  Low iron contamination means no color changes and no danger of decreasing the corrosion resistance of the part.  In addition, there’s less sludge in the anode cell and no iron deposits on the membrane.  See Figure 3, which compares the iron content of two anolyte systems, one with stainless steel anodes, and one with ruthenium oxide anodes.

Although ruthenium-oxide-coated anodes cost more at first compared with stainless steel anodes, they cost less over the anode’s life cycle. Expect a ruthenium oxide anode to last about three times as long as a stainless steel anode.  Typical service life is 3 years.  Some anodes have lasted 7 years.

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FACTS ABOUT ANODES:

  1. Cathodic acrylics are especially aggressive on stainless steel anodes.  Consider insoluble anodes.
  2. When insoluble anodes were introduced to e-coating over 10 years ago, some people said they could handle double or triple the current densities (amps per square foot) compared with stainless steel anodes.  While this is true at low voltages (100 volts or lower), the high voltage of e-coating reduces this ability.  A 25-percent increase in current density is more reasonable.
  3. Like stainless steel anodes, excessive current densities and exposure to chlorides shorten the life of insoluble anodes.  Under such conditions, an alternative coating, such as iridium oxide, could be used.  In addition, always handle anodes properly.  Be sure not to abrade the anode’s ruthenium oxide coating. This could expose the titanium substrate and shorten the anode’s service life.
  4. Large electrocoat installations sometimes use two anolyte systems, one for insoluble anodes and one for stainless steel anodes.

De Nora Tech, LLC

7590 Discovery Lane
Concord Ohio 44077
SurfaceFinishing@denora.com