Metal Pretreatments

What Are Conversion Coatings?

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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 metal part to provide corrosion resistance, lubrication, decoration or to improve adhesion to other coatings or lubricants.
 

How Are Conversion Coatings Applied?

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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

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

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

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:
 

BONDERITE^® Metal Pre-Treatments

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BONDERITE^® M-ZN

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

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: