Q. We are experiencing textured ecoat on galvanized service parts and were told the galvanized coating is electrolytically applied with 60/60 zinc weights. The textured ecoat surface is made up of little bumps in various sizes. This has never been an issue from the customer, but they have told us to improve the textured look. How can we identify the cause?
A. The goal is to identify all potential variability influenced by:
- Materials used, such as the specific type and design of the pretreatment and ecoat system;
- Equipment, including maintenance and operation;
- Local manufacturing processes used;
- Environmental conditions affecting both systems;
- Human influences; and
- Potential variability introduced by measurement systems or methodologies used to obtain process control, quality metrics or product performance results.
Although the term textured describes the visual appearance of the final electrocoated galvanized parts, terms such as bumpy, wavy, peely, pimpled or pinched can describe surfaces that show minor geometrical profiles and are not accompanied by any film rupture.
Measurements like surface profilometry average roughness values (Ra) with long and short wave scans, dry film thickness (DFT), gloss and other parameters provide data to define the problem at the start and measure individual impacts during testing.
Unfortunately, almost any process or equipment variable can influence surface quality and therefore be a potential root cause to the issue. Raising the bar on visual surface quality requires that almost every variable and parameter be studied and analyzed.
Metal substrate. The ecoat process starts with the quality of raw parts. Electrolytic or electrogalvanized (EG) metal substrates—due to the nature of the application process, the softness, reactivity and high conductivity of the zinc metal—are more sensitive and challenging surfaces than steel.
Zinc oxidation. The EG surface used cannot come from freshly applied and unoxidized EG. Some air oxidation is necessary to stabilize zinc oxide development on the surface prior to ecoat or any other coating, as the final adhesion can be compromised. Sometimes, this needed oxidation can be days or weeks, depending on the zinc materials and application used.
The galvanized substrate must also have limited direct exposure to water or humidity. Zinc coated surfaces enable the formation of hydrated zinc oxides or zinc hydroxides (white spots or rust) under contact with water or exposure to high humidity. White rust can be difficult to remove with conventional alkaline cleaners, depending on the age, severity and conditions of the water exposure.
EG surfaces. The EG surface smoothness and structure must be free of small imperfections, porosity or incrustations of embedded iron or other metal particles. Also, the EG metal parts must be fabricated and manufactured using compatible lubricants and working fluids. These can contribute to contamination sites, surface imperfections or even galvanic cells that could enable telegraphing through both the conversion coating and ecoat film.
Passivating materials. The zinc substrate must be free of any passivating treatments that could present incompatibility with typical alkaline cleaning or conversion coating technologies or materials used in pretreatment. Chrome and other passivating materials are difficult to remove in alkaline conditions and are often responsible for marginal adhesion, low appearance levels and poor corrosion resistance.
Also, proper metal correction or rework procedures and ecoat-friendly feathering techniques must be used to minimize any potential metal mapping and to eliminate exposing the cold-rolled steel under the EG layer, introducing contamination or creating excessive metal profiles.
Stereo and scanning electronic microscopes can identify and measure surface characteristics, condition and quality. Optical profilometry using surface scans is also a great tool to obtain measurable units of metal surface roughness.
Cleaning. A clean and smooth surface is necessary to obtain a perfect ecoated part. Metal substrate and cleaning are the two biggest sources of variability encountered in pretreatment and ecoat systems. The potential variability in EG metal substrates exponentially compounds the work and thus potential variability of the cleaning stages.
Weak or aggressive cleaning can affect appearance issues. EG surfaces, like any metal, can be under-cleaned and over-cleaned. A softer metal than steel, its surface is more sensitive to temperature and chemical exposures.
Over-cleaning can be the result of high etch rates created by high temperatures, pH, impingement, free alkalinity or all of the above.
Under-cleaning is the incomplete removal of metal working lubes, oils and soils from the EG surfaces and typically the result of lower parameter values. Cleaner oil loadings and age can also introduce significant variability in the process, depending on specific operating procedures.
Equipment-related variables like filter size and filtration rates, recirculation turns per hour and others are also sources of variability. Proper magnetic filtration is typically required for high surface appearance properties.
Filter selection and rates are location-specific, depending on the type of oil loads and soils of each specific tank and system (immersion or spray), tank size and configuration, temperature and time, cleaner materials used, and overall condition of the incoming metal mix. Filter sizing is always an important factor when looking at raising the bar on appearance quality.
Phosphate or chemical conversion. Conversion coating mapping and/or conversion coating sludge or residues left on the surface of parts are common appearance defects. Although the conversion coating should only telegraph the base metal profile, it can create its own contour and special profiles.
Free and total acid, pH, temperature, ionic concentration levels and potential contamination of the conversion coating bath are also key factors.
Filtration system and operation parameters, along with rinse stages (rinse rates and conductivity) prior to ecoat are also typical sources of variance. 10 to 20 micro-Siemens or better rinse bath conductivities are always desired.
Stereo and scanning electronic microscopes are great tools to use when identifying and measuring conversion coating surface quality and appearance.
Ecoat application. Ecoat application and materials should be the next process to look at. Typically, high pigment-to-binder ratios, high pH or paint conductivity, or low solvent levels can contribute to the final appearance. High pigment conditions are often the source of textured ecoat films.
Contamination. Elements like zinc, aluminum, iron, sodium and calcium—or elements and molecules like phosphates, nitrates, chlorides or any alkaline or caustic materials—can be carried into the ecoat bath by parts, racks or conveyor chains and contaminate the paint bath.
High carry-over in general and cleaner carry-over contribute to surface defects due to the caustic reactive incompatibility with the acidic cathodic ecoat.
Equipment-related variability like filter size and filtration rates, recirculation turns per hour of the ecoat bath, permeate and RO/DI rinses are typically sources of process variability on ecoat appearance. Filter sizes between 5 to 25 microns in the ecoat tank and post rinses are normal.
Electrodeposition. Correct use of the DC deposition power cycle volts, amps and time can be a major contributor to the final appearance of the film.
Rectifier equipment must provide low DC voltage ripple effects with all anodes operating under the recommended current densities. High film deposition rates and high DC voltage current densities can affect final film quality.
Zinc substrates are almost ten times more conductive than steel substrates, so they are more sensitive to DC voltage/amperage interactions, and generally high voltages and or fast voltage ramps are responsible for bumpy, textured, rough and even ruptured films. Extremly high voltages can result in ecoat film rupture on galvanized substrates, due to the high localized heat and gases generated during electrodeposition.
High film thickness resulting from high voltages can contribute to bumpy or wavy films. Ecoat materials are primers and not designed for excessive film thicknesses.
Cure Oven. The way the wet ecoat film is dried, flowed, crosslinked, hardened and transformed in the oven is often the source of defects. Dirt, gloss and drips are typically defects generated in ecoat ovens. However, the bumpy appearance of your textured ecoat could be more related to fast oven ramps or high oven fan velocities that can lead to textured films.
Originally published in the March 2017 issue.