E-Coating Zinc Stampings
Q. We are running an automatic electrocoat system coating zinc stampings. What are the best operating practices to successfully electrocoat these parts? J.W.
A. Galvanized coatings applied over steel substrates provide excellent corrosion protection in most environments, and are used as a protective coating in many different application conditions and exposures by many different OEMs. In addition to chemical corrosion properties, galvanized coatings have the electrochemical ability to provide supplemental cathodic corrosion protection by acting as sacrificial metals or anodes.
When galvanized substrates are further finished with an additional electrocoat coating, the final “triplex” corrosion system provides the unmatched corrosion performance and durability used by the automobile industry.
Quality or processing challenges associated with running galvanized substrates in e-coat lines depend on the percentage of galvanized materials processed through the e-coat system compared with other substrates such as steel or aluminum. Electrocoat lines that run 100-percent galvanized materials would be set up and operated differently than lines that only run 10-20 percent of galvanized materials. This percentage is calculated based on surface area, not on number of parts or weight.
Some processing or quality issues associated with running galvanized materials through e-coat systems may be poor or no adhesion, pinholes, out-gassing, voltage rupture, nubbing, and poor salt spray.
The optimal application process and quality for a typical automotive e-coat system over galvanized substrates will depend primarily on four critical process elements or steps: substrate, cleaning, conversion coating and e-coat.
There are several methods of applying zinc coatings over steel substrates. The most commercially available galvanized coatings are batch hot-dip galvanizing, continuous sheet galvanizing, electrogalvanizing, zinc plating, mechanically plating, zinc spraying and zinc painting. For this discussion I will focus on zinc plating, which is the galvanized coating most encountered by custom electrocoaters. Most of this advice applies to other types of galvanized coatings as well.
These characteristics play an important role in the application and corrosion performance of the galvanized e-coat system: zinc coating thickness and porosity, iron content, type of passivation, and ultimately the degree of oxidation of the galvanized coating. The more thickness the zinc plating has, the more capacity it will have to develop voids within the coating (porosity). Those voids within the substrate can hold or trap process chemicals and air, and play a significant role in the creation and formation of many surface-quality defects. Only improvements to the coating or the application process made by the plater can eliminate the porosity or void problem altogether. But the electrocoater can improve the quality by pre-baking the parts to boil out liquids or squeeze out the air before e-coating to compensate for excessive porosity.
The presence of iron in the galvanized coating such as in zinc-iron or galvannealed coatings, or by contamination, can lead to further failures in corrosion tests. This can be improved by optimizing the chemistry and application of the galvanized coating.
Most zinc platers apply a further protective coating to the zinc coating called a passivator. A fresh zinc coating is quite reactive to the atmospheric air, and some platers will passivate or protect the fresh zinc by applying a clear or yellow dichromate coating. This final dichromate layer must be removed with mild acids prior to e-coat, or the final system will have poor or limited adhesion.
Zinc surfaces without passivators are still quite reactive when exposed to the atmosphere and will oxidate rapidly, which results in the formation of corrosion effects that start with zinc oxide and transform into zinc hydroxide and zinc carbonate as times passes. This is what it is typically referred as patina. Degrees of oxidation are defined as newly galvanized, partially weathered and fully weathered.
Understanding the degree of oxidation or age of the zinc surface prior to painting is critical for selecting the right cleaners and conversion coatings for the processing of galvanized materials in e-coat lines.
Once we know the thickness and porosity of the substrate, the iron content of the zinc plated part and the age of the coating, and confirm the absence of passivators prior to e-coating, then we can determine an optimum process for e-coating. This will vary of course, dependent on the specific parts to be coated and the specs outlined.
In general, the cleaner must be alkaline in nature but below a pH of 12.5 to prevent etching a significant amount of zinc from the substrate. Those cleaners are called “tri-metal cleaners” because iron, zinc and aluminum can be processed through them. They also can be referred to as “neutral cleaners” in reference to their mild pH. Mild temperatures less than 150°F (65°C) and cycle times slower than 3 min provide the best processing results for galvanized substrates. Silicated cleaners can also provide excellent results but they introduce other process issues which I don’t have space to mention here.
Galvanized coating must be phosphated prior to e-coat to improve adhesion with the e-coat and also provide additional protection against corrosion creep. This phosphating can be iron phosphate, zinc phosphate or tri-cationic phosphate composed of zinc, nickel and manganese.
The best operational and corrosion performance for electrocoaters running galvanized substrates is accomplished using tri-cationic phosphates with coating weights of 175-300 mg/sq ft (1.5-2.75 gr/sq m), crystal sizes of 2-15 microns and complete coverage of the surface without voids. It’s best to deposit the minimal amount of phosphate capable of meeting acceptable corrosion levels.
The phosphate layer should be applied at less than 140°F (60°C) and cycle times between 1.5 and 4 min. If the e-coat line is running an elevated percentage of galvanized parts, then a low-zinc concentration, tri-cationic phosphate replenishment must be used to compensate for the additional zinc incorporated into solution by etching of the substrate. The bath’s zinc concentration must be closely monitored by the phosphate supplier to properly adjust the phosphate replenishment into the bath to match local zinc conditions. Too much zinc in the bath can lead to adhesion and corrosion issues.
The best e-coat for galvanized surfaces is a cathodic-epoxy e-coat applied at 18-25 microns and cured at 325-375°F (160-190°C). The application time is between 2.5 and 5 min in an emulsion bath with 10–20 percent solids.
Typically the application of e-coat over galvanized substrates requires low application voltages compared with the voltages used if the line was only running iron or steel substrates. Zinc has a lower melting point than iron or steel, and thus is more susceptible to voltage rupture. The best quality is typically accomplished by applying the e-coat at the lowest operating voltage as possible.
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