Electrocoating Q&A: Orange Peel Effects on Steel Parts
Q. My company electrocoats steel- and zinc-plated parts in a square transfer line with a full zinc phosphate pretreatment system. We are experiencing intermittent orange peel effects on steel parts that are zinc plated before phosphate and black epoxy ecoat paint. Do you have any troubleshooting tips to eliminate the orange peel look?
A. Orange peel is a surface phenomenon that occurs on certain painted surfaces or technologies. The term “orange peel” is used because the painted surfaces resemble the look and texture of the skin of an orange. This type of surface defect is typically characterized by high film coating thickness.
Because electrocoat is a thin film technology, electrocoated surfaces do not typically show orange peel effects. I have never seen it in my professional experience, so I believe that the orange peel you described could be another effect on the electrocoated surfaces that looks like roughness and/or pinholes, resembling an orange peel.
Since the defect only occurs on the zinc-plated parts and not on the steel parts, the most likely root cause is voltage rupture of the electrocoat film. This happens when zinc-plated or galvanized metal parts are electrocoated at elevated coating voltages.
The voltage rupture typically occurs in or around micro cracks, pinholes, large porosity, or surface imperfections or impurities on the galvanizing layer. These sites sometimes enable the formation of galvanic cells between the iron in the steel substrate and the zinc in the galvanizing layer. Electrochemical galvanic cells are the consequence of the different reduction-oxidation potential (redox number) between the iron and the zinc elements. Sometimes, the galvanic cells can be created between the zinc in the galvanizing layer and iron or other impurities of metallic elements located on the surface of the galvanized surface.
When applying the electrocoat at high voltages, the galvanic cell sites generate localized high heat and a significant increase in the temperature of the electrocoat film under high ecoat application voltages. This localized high temperature burns through the ecoat film and forms clusters of small pinholes, craters or extreme roughness. These can be mistaken for orange peel.
Steel parts typically accept higher application voltages than zinc-plated parts, and voltage rupture is uncommon.
To eliminate the roughness and pinholes on the ecoated zinc-plated parts, the application coating voltage must be reduced. Separating the steel parts and the zinc-plated parts in coating batches can also enable you to do this by operating at two different voltage settings.
Outgassing is another cause of orange peel. If the zinc-plated film presents elevated porosity, the air trapped inside can be released during the ecoat’s curing cycle, leaving small pinholes as it escapes from the film.
Depending on the frequency and severity of the pinholes, the ecoat surface may look rough. Pre-baking the zinc-plated parts prior to ecoat can eliminate this issue.
Originally published in the March 2016 issue.
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