Metal Finishing Guide Book

2013

Issue link: https://metalfinishing.epubxp.com/i/218436

Contents of this Issue

Navigation

Page 119 of 843

cleaning, pretreatment & surface preparation NON-PHOSPHATE TRANSITION METAL COATINGS BY BRUCE DUNHAM AND DR. DAVID CHALK, DUBOIS CHEMICALS, SHARONVILLE, OHIO INTRODUCTION Traditional iron phosphate and zinc phosphate conversion coatings have been used for more than a century as pretreatments for painting over a variety of metals. These "legacy" phosphate pretreatments have served well; however, environmental regulations restricting phosphate discharge, increased phosphate and zinc costs, and higher corrosion-resistance requirements have provided impetus for the development of non-phosphate alternatives. During the evaluations of the various technologies, it was discovered that these new non-phosphate pretreatment conversion coatings conferred significant cost savings and operational benefits along with their promised decreased environmental impact. Considered new and experimental in the New Millennium (Y2K), these nonphosphate conversion coatings have gained significant traction in the pretreatment market and are rapidly becoming the technology of choice for paint and powder coating pretreatment. The purposes of this article are to provide background information for those new to non-phosphate pretreatments, and to answer some frequently asked questions about the non-phosphate conversion coatings. WHAT ARE TRANSITION METAL COATINGS? If iron phosphate and zinc phosphate can be referenced as "Traditional Metal Phosphates", the new non-phosphorus pretreatments can rightly be called "Transition Metal Coatings" (and will be referenced as "TMC" coatings in the remainder of this paper). The term "transition metal" refers to a metal's position in the Periodic Table of the Elements, and is a term chemists use to describe the location of a group on the Table. Zirconium (Zr) is at the center of a group of elements in the Periodic Table that are considered relatively environmentally friendly. (See Figure 1) Oxides of zirconium, titanium, and/or vanadium are the most commonly used transition metal coatings, with zirconium as the most frequently encountered transition metal. Note the location of these metals relative to chromium. The closer two given elements are to each other on the Periodic Table, the more similar their properties. The first recorded application of zirconium oxide on steel was in 1996, when the first non-chrome seal rinse based on zirconium was introduced. Applied over a traditional metal phosphate conversion coating, the sealer conferred corrosion resistance that was close to that offered by the chromium seal rinse that had traditionally been used. The chemistry was then modified in 1998 to serve as a chromium replacement for conversion coating on aluminum. The first applications for steel arrived in 2002. WHAT IS THE APPEARANCE OF A TRANSITION METAL COATING PRETREATMENT? Zirconium oxide is a very versatile material, taking on such varied forms as 116

Articles in this issue

view archives of Metal Finishing Guide Book - 2013