Dry-in-place conversion coatings creating excitement in finishing—part I
This is the first part of a two-part article on pretreatment advancements in dry-in-place (DIP), non- or low-phosphate conversion coatings. The intent of the article is to answer questions and provide information about what is available in the market and how to best evaluate this new technology. Several of the national leaders in chemical pretreatment volunteered to participate in a testing project at a custom coater's plant for this article with the hope of learning more about their products. The idea was to dispel some of the claims, obtain insights into how these conversion coatings actually work, and to run a side-by-side panel test of the varying formulations. Part I discusses the benefits and potential limitations of DIP conversion coatings, and describes the testing procedure and companies involved. Part II, which will be published in the magazine's November issue, reviews the salt-spray test results and offers more insight into this new technology.
Pretreatment advances in the past 20-plus years have been slowly emerging. We've seen an industry that primarily used three stage monorail spray iron phosphate develop into an industry that uses five or more stages. We've witnessed the evolution of alkaline cleaners, from high-alkalinity sources or hydroxide containing with high-temperature (140°F-180°F) surfactant systems to low-to-moderate alkalinity sources (noncaustic) with multiple surfactant or detergent systems operating from ambient temperature to 120°F. Conversion coatings and iron phosphates have evolved from high-free-acid and molybdate-accelerated formulations to super-accelerated and low-temperature formulations. We've also watched the use of chromium for seal rinses and subsequent salt-spray performance basically disappear and be replaced with reactive nonchrome seal rinses incorporating deionized (DI) or reverse osmosis (RO) high quality water sources.