A coating is often applied to metal substrates, especially metal substrates that contain iron, such as steel, prior to the application of a protective or decorative coating. The coating can help to minimize the amount of corrosion to the metal substrate, if and when, the metal substrate is exposed to moisture and oxygen. Many of the presently known and used pretreatment coating compositions are based on metal phosphates, and some rely on a chrome-containing rinse. The metal phosphates and chrome rinse solutions produce waste streams that are detrimental to the environment. As a result, there is the ever-increasing cost associated with their disposal.
Coating compositions which can be applied without chrome rinse solutions are known. For example, the post-treating of phosphated metals with zirconium-containing rinse solutions has been described and is known in the art. However, such chrome-free, zirconium-containing rinse solutions are generally only suitable for use over a limited number of metal substrates, and the generation of metal phosphate waste streams is not alleviated.
Non-chrome coating compositions containing a fluoroacid such as fluorotitanic acid, silica, and a water-soluble polymer such as an acrylic acid polymer and/or a polymer with hydroxyl functionality have been described. By heating the silica and fluoroacid, the silica is dissolved, or at least partially dissolved until the solution is clear. As a result of their dissolution, the silica particles used in these coating compositions are not considered acid-stable particles. The pH of these compositions is very acidic, and ranges from 0 to 4, preferably from 0 to 1. The coatings compositions can enhance the corrosion resistance of steel and galvanized steel substrates.
Other coating compositions have been described for forming a coating on metal substrates, except aluminum. Such a coating composition can include an oxidative compound such as nitric acid or hydrogen peroxide, silicate or silicon dioxide particles, and a metal cation, oxymetal anion, or fluorometallate anion of Ti, Zr, Ce, Sr, V, W, and Mo.
Another non-chrome coating composition that contains a metallic surface-treating agent, water-dispersible silica, and one or more of a zirconium or titanium compound, thiocarbonyl compound, and a water-soluble acrylic resin has also been described. The metallic surface treating agents are silane coupling agents that are typically used in the coating industry to improve adhesion between the pre-coating and the decorative coating.
Another non-chrome coating composition that contains a cross-linked polymer system, which includes a copolymer with acrylic and hydroxyl functionality or the reaction product of an acrylic polymer and a polymer with hydroxyl functionality has also been described. A fluoroacid such as fluorozirconic acid or fluorotitanic acid can be added to these compositions. Such compositions with the addition of dispersed silica and an ammonium carbonate containing a group IVB metal have been described as well.
Unfortunately, while the use of chrome rinse solutions has decreased, and while various coating compositions have been suggested and used in place of phosphate compositions and chromate-based coatings, phosphate- and chromate-free pretreatment coatings for substrates, particularly multi-metal substrates, which exhibit the corrosion protection, adherence and other coating performance properties of known phosphate and chromate coatings have not been adequately provided in the art. In addition to failing to meet the corrosion, adherence and coating performance standards of prior phosphate- and chromate-containing coatings, known phosphate- and chromate-free coatings generally require large amounts of water and/or energy in use, and generally produce large amounts of sludge in application baths, the sludge having to be removed which results in less efficient operation and higher costs.