Products Finishing magazine is the No. 1 industrial finishing publication in the world. We keep our readers informed about the latest news and trends in plating, painting, powder coating, anodizing, electrocoating, parts cleaning, and pretreatment.
Issue link: https://pf.epubxp.com/i/792756
PFonline.com/experts 44 MARCH 2017 — PFonline.com ELECTROCOATNG C L I N I C JOSE A. TIRADO Consultant Jose has a master's in industrial chemical engineering and has been involved in the electrocoat industry since 1984. 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 accom- panied 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 applica- tion 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 oxida- tion 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 Almost any process or equipment variable can influence surface quality. 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? How to Identify and Control Poor Ecoat Film Appearance 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 incrus- tations 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 incompat- ibility 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, intro- ducing 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 sensi- tive 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