Anodic Versus Cathodic Electrodeposition

Q. What is the difference between anodic and cathodic electrodeposition? Can I use aluminum and steel parts in both?

A. To keep the chemistry of these two processes straight, first consider the metal part you are coating. In cathodic electrocoat, the part is the cathode and in anodic electrocoat, the part is the anode. Follow the four E’s of the electrocoat process to keep track of the major process steps: electrolysis, electrophoresis, electrodeposition and electroendosmosis. 

Electrodeposition coatings are initiated by the electrolysis of water, which makes the current flow possible and starts the process. The reduction of water occurs at the cathode and the oxidation of water occurs at the anode. 

Cathodic Case (Reduction of water at the negative charged part): 2H2O(l) + 2e− γ H2(g) + 2OH−(aq)

Anodic Case (Oxidation of water at the positive charged part): 2H2O(l) γ O2(g) + 4H+(aq) + 4e−

In cathodic electrocoating, the part has a negative charge to which positively charged polymer is attracted, a movement called electrophoresis. The anodic case is similar, but the polymer has a negative charge and the part has a positive charge.

The electrodeposition coating occurs directly at the part, which is initiated by the oxidation or reduction reactions of water at the part. In cathodic electrocoat, hydroxide is produced at the part, which neutralizes the salting acid of the positively charged polymer and the polymer coating is deposited onto the part. It is important to note that, in this electrodeposition process, there is no oxidation or reduction of the polymeric coating—only a change in the solubility that results in the coating being deposited onto the part. In anodic electrocoat, the hydron, produced by the oxidation of water, neutralizes the base of negatively charged polymer.

Following the deposition of the polymeric coatings, the water moves away from the part—termed electroendosmosis—and the part is insulated and deposition is completed. Insulation is the major factor to limit the film thickness of the final coating. You can use steel and aluminum in both processes. 

Remember that, in anodic electrocoat, since the oxidation occurs at the part, many metals can be oxidized in this process. This can contribute to weaker corrosion resistance, so special consideration needs to be given for the quality of corrosion protection that is needed for the application. The use of cathodic electrocoat is very common in automotive applications, for example, when strong corrosion resistance is essential. Special pretreatment may be required if you use aluminum and steel in the same cathodic bath.

 

Surface Tension of Liquid Coatings

Q. What does the surface tension of my paint need to be in order to wet the substrate?

A. The simple answer is that the surface tension of the liquid must be lower than the surface energy of the substrate for wetting to occur. This is easy to remember if you think about the low surface tension liquid covering the high surface energy substrate to result in an overall lower free energy of the system.

However, liquid coatings with an application and drying step are very complex. The paint formulations with surface tension additives and rheology control agents add more complexity. Nonetheless, it is important to keep the fundamentals in mind as you formulate your coatings.

The surface tension of a liquid in air results from an imbalance of forces. The molecules have a stronger attraction to each other (cohesion) than to the molecules in the air (adhesion), creating a net inward force and surface tension. The dimensions of surface tension are a force exerted in the surface perpendicular to a line (the same types of forces are also seen in solids). The interactions of the surface tension can be described by dispersive and polar components. The polar components are the permanent dipoles (forces), induced dipoles and hydrogen bond forces. 

A good example of surface tension is how water will bead up on a freshly waxed car finish. The water has high surface tension (72.8 millinewtons per meter) and will minimize the energy over the low surface energy waxed surface by forming a spherical shaped droplet. The sphere has the smallest surface area to volume ratio, and hence, the lowest energy.

The two main behaviors of the coating must be kept in mind when studying wetting behavior: the surface tension effect to minimize the area exposed to the air interface, and the effect of the flow of a lower surface tension liquid over a high energy substrate forwetting.  

 

Tim December is a technical expert for BASF. For information, visit basf.com.