Metal pretreatment processes are an important step in manufacturing high quality, durable finished parts. Pretreatments are designed to clean, coat and protect metal surfaces in order to prevent corrosion and to prepare the surface for post treatment operations such as machining, forming, powder coating and painting. BONDERITE® metal pretreatment chemicals offer a wide range of conversion coatings that can be applied via dip or spray to ensure the right product fit.
What Are Conversion Coatings?
A conversion coating is a layer that is formed over a metallic surface due to a reaction between the metal substrate and a chemical solution. A conversion coating is not a separate layer on the metallic surface, but reacts to form a continuous crystalline layer that is integrated into the metal. Conversion coatings convert a raw metal surface that is irregular, conductive and susceptible to corrosion into a uniform surface that is electrically non-conductive and chemically inert. Phosphate coatings, chromate coatings, and oxide coatings are all examples of common conversion coatings used in pretreatment processes. A conversion coating is applied to a meta
l part to provide corrosion resistance, lubrication, decoration or to improve adhesion to other coatings or lubricants.
How Are Conversion Coatings Applied?
The metal pretreatment process varies depending on the type of conversion coating being formed. Phosphate coatings require the use of a metal phosphate such as zinc phosphate, iron phosphate or aluminum phosphate. We briefly provide an overview of oxide coating and chromate coating processes below, but will go into more detail about phosphate coatings as they relate to the Henkel pretreatment chemicals.
Oxide coatings are passive oxide layers on the metal surface that are formed through heat treatment, via chemical bath, or an electrochemical reaction. Oxide coatings are typically thin film layers and can be applied to several types of metal including steel, stainless steel, aluminum and copper substrates.
Chromate coatings are applied via a chemical conversion process by reacting a water solution of chromic acid or chromic salts with the metal part. Chromate coatings are commonly applied to aluminum, magnesium, zinc and cadmium metal.
Phosphate coatings are also applied via a chemical conversion process. There are several different types of metal phosphates used in pretreatment including iron phosphates, zinc phosphates, aluminum phosphates, manganese phosphates, and nickel phosphates. Phosphate coatings can be used on a variety of metallic substrates, and are commonly applied to carbon steel, low-alloy steel, and cast iron. Other metals include galvanized steel, electrogalvanized steel, zinc, zinc alloy coated steel aluminum, cadmium, tin and silver. While the pretreatment steps vary depending on the application, there are three critical steps in almost all phosphate pretreatment processes:
Cleaning & Surface Activation
Metal cleaning between production steps is paramount to ensure a proper coating and final finish. Quality cleaners are needed to remove rust, scale, dirt, lubricants and other unwanted materials to prepare the metal surface to be coated. Henkel manufactures a variety of alkaline, neutral and acid cleaners. Explore the BONDERITE® Cleaners Portfolio.
Phosphate Coating Application
After cleaning, the metal surface goes through a phosphating step to apply the phosphate conversion coating. Phosphate coatings are an excellent choice for providing corrosion protection and paint adhesion. Henkel offers multiple phosphates for dip and spray metal pretreatment processes. See All BONDERITE® Metal Pretreatments.
Sealing & Post Passivation
After phosphating, Henkel recommends a neutralization step. The neutralization step neutralizes any acidic phosphate that is leftover and reduces drag-in caused from acidic metal. The use of metal passivators also raises the temperature of the metal, thereby providing additional corrosion protection. Explore the BONDERITE® Metal Passivators.
BONDERITE® Metal Pre-Treatments
BONDERITE® M-ZN zinc phosphate coatings are traditional zinc phosphates that are used primarily for multi-metals to provide superior corrosion performance and paint adhesion. Zinc phosphate coatings are some of the most widely used conversion coatings in metal pretreatment processes. Henkel offers a wide selection of zinc phosphate technologies. We’ve highlighted several of the core products below.
*Operating time depends on the application process, contact a technical representative for more information
BONDERITE® M-NT zirconate coatings are an eco-friendly, phosphate-free alternative to zinc and iron phosphate pretreatments. BONDERITE® M-NT zirconate coatings are thin-film conversion coatings composed of nanoceramics that can be applied using spray or dip application methods. BONDERITE® M-NT coatings are free of nickel and phosphate and have zero VOC and CO2 emissions, providing a conversion coating solution that improves paint adhesion and prevents corrosion while reducing your environmental footprint. BONDERITE® M-NT zirconate coatings can be applied to multiple metal surfaces including steel, zinc, and aluminum. In summary, these are the key advantages to using BONDERITE® M-NT:
FAST Less Process Steps, Shorter Cycle Times & Shorter Contact Time
Fewer Control Parameters, Simple to Install & Less Maintenance
Minimal Environmental Impact, Less Sludge & Water Use