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Advances in Automotive Conversion Coatings During Pretreatment of the Body Structure: a Review

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Advances in Automotive Conversion Coatings During Pretreatment of the Body Structure: a Review coatings Review Advances in Automotive Conversion Coatings during Pretreatment of the Body Structure: A Review Mark Doerre 1,*, Larry Hibbitts 2, Gabriela Patrick 2 and Nelson K. Akafuah 1 1 IR4TD, College of Engineering, University of Kentucky, Lexington, KY 40506, USA; [email protected] 2 Paint Production Engineering, Toyota Motor North America, Inc. (TMNA), Georgetown, KY 40324, USA; [email protected] (L.H.); [email protected] (G.P.) * Correspondence: [email protected] Received: 7 September 2018; Accepted: 10 November 2018; Published: 15 November 2018 Abstract: Automotive conversion coatings consist of layers of materials that are chemically applied to the body structures of vehicles before painting to improve corrosion protection and paint adhesion. These coatings are a consequence of surface-based chemical reactions and are sandwiched between paint layers and the base metal; the chemical reactions involved distinctly classify conversion coatings from other coating technologies. Although the tri-cationic conversion coating bath chemistry that was developed around the end of the 20th century remains persistent, environmental, health, and cost issues favor a new generation of greener methods and materials such as zirconium. Environmental forces driving lightweight material selection during automobile body design are possibly more influential for transitioning to zirconium than the concerns regarding the body coating process. The chemistry involved in some conversion coatings processing has been known for over 100 years. However, recent advances in chemical processing, changes in the components used for vehicle body structures, environmental considerations and costs have prompted the automobile industry to embrace new conversion coatings technologies. These are discussed herein along with a historical perspective that has led to the use of current conversion coatings technologies. In addition, future directions for automobile body conversion coatings are discussed that may affect conversion coatings in the age of multi-material body structures. Keywords: design for environment; conversion coating; zirconium; lightweighting 1. Introduction In a 1911 US Patent [1], Thomas Coslett described the use of six ounces of zinc, a pint of water, and a pint of phosphoric acid as ingredients for making a conversion coating concentrate. Its efficacy was demonstrated when iron objects boiled in a water solution made from this concentrate displayed a significant reduction in corrosion as a consequence of a protective zinc phosphate coating that had been formed on the surface of the iron. The definition of a conversion coating is a coating formed during surface-based chemical reactions that include the base metal and other ions present in solution. Being a direct consequence of surface reactions differentiates a conversion coating from other coatings such as paint: paint is really a covering. Since 1911, vastly improved conversion coating compositions have included various elements in the periodic table and now include numerous organic and inorganic compounds. Although the evolution of conversion coatings started with iron requiring more corrosion protection, other metals such as zinc and aluminum are also now beneficiaries of conversion coating technologies. For example, automotive bodies have incorporated Zn as a sacrificial anode in the form of galvanneal (GA), or electrogalvanneal (EG)-treated steel and Al, a lightweight substitute for steel. Paints, or other organic coatings, have improved adhesion characteristics because of a roughened Coatings 2018, 8, 405; doi:10.3390/coatings8110405 www.mdpi.com/journal/coatings Coatings 2018, 8, x FOR PEER REVIEW 2 of 17 Coatings 2018, 8, 405 2 of 16 Paints, or other organic coatings, have improved adhesion characteristics because of a roughened surface that is created by the conversion coatings; in other words, a conversion coating also functions surfaceto improve that the is created mechanical by the anchor conversion profile coatings; for paint in. other words, a conversion coating also functions to improveThe canned the mechanical food and beverage anchor profile industr fories paint. have exerted significant influence on the development of newThe metal canned ion food chemistries and beverage for conversion industries coatings have exerted that have significant improved influence coating on theappearance development after ofcontainer new metal pasteurization ion chemistries and forthe conversion adhesion coatingsof decorative that have organic improved coating coatings. By the appearance 21st century, after containerenvironmental pasteurization concerns weighed and the adhesionin on chromates, of decorative heavy organicmetal sludge, coatings. phosphates By the, 21st and century, energy environmentalconsumption. The concerns long- weighedterm combination in on chromates, of these heavy forces metal have sludge, nudged phosphates, conversion and coating energy consumption.technologies to The Zr long-term-based materials combination, a trend of these noted forces by have David nudged Chalk conversion of Dubois coating Chemical technologies as the to“shrinking Zr-based periodic materials, table” a trend [2]. noted by David Chalk of Dubois Chemical as the “shrinking periodic table” [2]. 2. The Automotive Coating Stack 2. The Automotive Coating Stack Figure 1 illustrates a typical stack of coatings that are applied onto an automotive body during the paintingFigure1 illustratesprocess [3] a; typicalthe layers stack of ofthese coatings coatings that add are beneficial applied onto attributes an automotive as noted body in the during left-hand the paintingside of the process Figure. [3]; However, the layers a of customer these coatings in an addauto beneficialmobile dealer attributes showroom as noted only in thesenses left-hand the upper side ofthree the layers Figure. of However, the “paint a job customer”, which in include an automobile a clear coat, dealer base showroom coat or color only ( sensespossibly the with upper optional three layersmetal offlake thes) “paint and a job”, primer which; these include coverings a clear coat,are typically base coat organic or color in (possibly nature. withLayer optional 4 below metal the flakes)coverings and is a a primer;n electro these-deposition coverings (ED) are coating, typically and organic layer in 5 is nature. the phosphate Layer 4 below conversion the coverings coating. is The an electro-depositionactivation layer below (ED) coating,the phosphate and layer layer 5 is— thei.e., phosphate between the conversion phosphate coating. layer Theand activation the steel layerbody belowpanel— thetypically phosphate consists layer—i.e., of Ti or between Zn but thedoes phosphate not have layera thickness and the per steel se bodybecause panel—typically it is made of consistsnoncontiguous of Ti or particles Zn but does applied not haveunder a thicknessspecifications per se that because call for it isan made application of noncontiguous based on a particles weight- appliedper-unit under area. specifications that call for an application based on a weight-per-unit area. FigureFigure 1.1. Vertical stackstack onon an automotive body panel with a phosphate conversion coating (not(not drawn toto scale)scale) [[3]3].. 3. The Automotive Conversion Coating Timeline and Purpose 3. The Automotive Conversion Coating Timeline and Purpose A timeline for phosphate conversion coating technologies is given in Figure2; starting at around A timeline for phosphate conversion coating technologies is given in Figure 2; starting at around 1940, it notes some significant milestones [4]. The conversion coatings serve two primary purposes on 1940, it notes some significant milestones [4]. The conversion coatings serve two primary purposes vehicle bodies. First, they increase corrosion protection when compared to an untreated metal surface; on vehicle bodies. First, they increase corrosion protection when compared to an untreated metal second, they promote adhesion between the paint and metal, creating a more robust bond than in the surface; second, they promote adhesion between the paint and metal, creating a more robust bond case of paint applied directly to metal. Together, these effects act as a system that is more tolerant of than in the case of paint applied directly to metal. Together, these effects act as a system that is more chemical and mechanical attacks on the body panel surface. tolerant of chemical and mechanical attacks on the body panel surface. Coatings 2018, 8, 405 3 of 16 Coatings 2018, 8, x FOR PEER REVIEW 3 of 17 Coatings 2018, 8, x FOR PEER REVIEW 3 of 17 FigureFigure 2. 2. SignificantSignificant milestones milestones in in the the progress progress of phosphate phosphate conversion coating technology technology.. Figure 2. Significant milestones in the progress of phosphate conversion coating technology. 4. The Foundation of Zinc Phosphate Conversion Coatings 4.4 T. Thehe Foundation Foundation of of Zinc Zinc Phosphate Phosphate ConversionConversion Coatings Figure3 is presented because discussions found later in this article require some familiarity with Figure 3 is presented because discussions found later in this article require some familiarity with the chemicalFigure 3 reactions is presented associated because with discussions the application found later of a in conversion this article coating; require thesesome reactionsfamiliarity include with the chemical reactions associated with the application of a conversion coating; these reactions
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