The Next Generation of Conversion Coatings

Transcription

1 The Next Generation of Conversion Coatings Terrence R. Giles and Donald Vonk Henkel Corporation, Madison Heights, Michigan USA Sergio-Luis Favero Henkel Ltda, São Paulo, Brazil Keywords: Conversion Coating, Zirconium Oxide, Corrosion, Adhesion, Cyclic Testing Abstract: Over the last few years because of skyrocketing energy costs, reductions in labor, more stringent environmental regulations and the desire for green manufacturing, the pretreatment process has changed. In 2007 the next generation of conversion coatings based on zirconium technology was introduced to the automotive industry. Initially this technology offered considerable cost savings compared to the traditional zinc phosphate pretreatments without a significant loss of performance. Since its introduction, the technology has gone through conversions at several automotive facilities, parts suppliers and general manufacturing operations. There have been many lessons learned during the introduction period and there are many still ahead. This paper will examine the advantages of a new generation conversion coating, discuss the process cost savings, examine the current level of performance and offer some suggestions as to the next steps towards improvement Henkel Corporation. All rights reserved 172

2 Introduction The method of applying a conversion coating on a steel surface to promote paint adhesion and offer corrosion protection has remained stable over the past 60 years. There have been some significant changes during this time; introduction of the Jernstadt salt, introduction of additional metal ions outside the surface being treated and a few minor modifications, but nothing really game changing until now. In the late 1990 s early 2000, the idea to replace traditional conversion coatings developed in the laboratory. New processes began to be studied in laboratories around the globe with collaboration amongst colleagues, bringing a new process to market. About the same time, new, more stringent environmental regulations implemented controlling the discharge of heavy metal ions; the manufacturing community was hit with increase costs for energy and raw materials. All of these factors, along with the need to reduce manufacturing costs, became the driving force behind a need for changing the traditional application of a conversion coating. These driving forces that pressured a need for change are summarized in Table 1. Table 1: Driving Forces to change from the Conventional Zinc Phosphate Conversion Coating Zinc Phosphate Process Driving Force New Conversion Coating Technologies Zn, Ni, Mn and P Regulated Metal Ions Non-Regulated Metal Ions, Zr, Ti Polymers, etc Processing Aluminum F-019 US Regulations F-019 Exemption Approximately 50% Sludge Generation and 90% Efficient Efficient Disposal Heated Process Energy Usage and Cost Ambient Temperature Operation Minimum of 7 Stages High Water Usage Reduced Number of Stages Environmental restrictions, along with the impact of raw material and energy costs, forced the emergence of the next generation of pretreatment chemistries. As seen in Figure 1, the movement toward commercialization began in 2005 and has advanced to the point that several automotive assembly lines are in production globally. First Full Body OEM Trials Introduced to General Industry Optimize Current Product Lines Qualification Programs Generation II Generation III and Beyond Begin Commercialization of Next Generation Ctgs. OEM Line Conversions Begin Generation II Development Begins Introduce Generation II to industry Figure 1: Historic Timeline for the Next Generation Conversion Coatings There has been a certain amount of growing pains with the introduction of the next generation of coatings into the manufacturing process. Some revolve around today s e-coat technology and others around the pretreatment process. The short history of the technology has shown this is not a drop-in application for the pretreatment and there are adjustments required for the electrocoat operation. Detailed line audits and equipment modifications to the pretreatment operation are necessary to make this technology successful. The line audits include chemistry and equipment changes, making the process more robust. An excellent multi-metal cleaner, keeping the surface wet through pretreatment and the use of rust 173

3 inhibitors are just some of the requirements to enhance the operation. Each line has its own personality and requirements to meet the expectations, and time needs to be spent learning the system. Initially it was thought that only some minor adjustments to the electrocoat operation were required to ensure adequate coverage and appearance. However this could not have been further from the truth. The new conversion coatings are much thinner than their zinc phosphate predecessors; see Figure 3, so the surface goes from a non-conductive, insulated type coating to one that has conductive properties. Being a conductive surface now creates a thicker e-coat film on the exterior surfaces. In order to reduce this film, e- coat suppliers change some of the operating characteristics, such as voltage and chemistry. Changing operating parameters created other problems such as maintaining the throw power to the recessed areas, hiding power for metal worked areas and whether it is best to enter the e-coat bath, wet vs. dry, and power on, vs. power off. All of the issues mentioned above have been addressed in the manufacturing operations, using some type of containment until the problem was resolved. Even with the equipment or chemistry changes in the pretreatment and e-coat operations, work continues to make the process more robust. E-coat and pretreatment suppliers have recognized these manufacturing concerns and they are taking steps to introduce new technologies. The E-coat suppliers are introducing technologies to assist in the throw power and appearance concerns, while the pretreatment suppliers are addressing performance. Even still, the current new conversion coating technology has been game changing. Two manufacturers, General Motors and PSA, have saved significant operational costs in the areas of energy, water consumption, sludge disposal and waste water treatment without sacrificing acceptable corrosion resistance or paint adhesion. Table 2 shows the total number of vehicles produced using the next generation coatings since its introduction in At the end of 2011, the plants running the next generation conversion coating technology have produced a total of 4,357,434 million vehicles or a rate of 156,540 vehicles per month based on the IHS-Automotive Forecast data for Table 2: Total New Generation Conversion Coating Vehicles Company Locations Total Vehicles General Motors South America 487,984 North America 2,427,312 Asia 222,391 Ford North America 394,016 PSA Europe 689,745 South America 135,986 Total Vehicles: 4,357,434 Source: IHS, Inc. With the introduction of the first generation of the new conversion coatings there are documented process saving for each plant in operation in the areas of energy, maintenance and reducing the environmental impact. Overall estimated savings at the two companies mentioned earlier are between a 30% cost reduction to a hard cumulative $3.5 million dollars. As a reminder, these savings are annual, not a one time hit. With all the success, supplier companies, pretreatment and e-coat strive to meet all of the requirements mandated by customer specifications. In some cases the performance is met and in other cases they meet most specifications. Currently, if the new generation pretreatments are compared directly to traditional zinc phosphate coatings, they often perform within current specifications, but perhaps at a lower level within certain corrosion specification limits. In today s environment this is an acceptable performance for some, while others are looking for equal to zinc phosphate regardless of the specification. 174

4 So the challenge has been drawn, meet the performance of today s zinc phosphate. Research continues with the developments of the second, third and even fourth generation of the new generation coatings. Generation II and beyond: The first generation new conversion coatings have met some customer s expectations in performance; however the target is to reach equal performance to current zinc phosphate regardless of the specification limits. The recognized weakness in performance from the pretreatment suppliers has been on cold rolled steel. The performance weakness is not significant, but just outside the specifications. The second generation of the new conversion coatings addresses that performance issue and is now ready for introduction to the industry. Pretreatment suppliers are introducing the Generation II based on the results of laboratory testing, and coupled with the advancements in the electrocoat technologies the results are positive. In earlier papers it was mentioned the tools used to observe and understand performance have changed with the new conversion coatings. We can no longer rely on the Scanning Electron Microscope (SEM) and X- Ray Diffraction (EDAX) to tell us what we need to know about the coating and the performance. The instruments use to understand these coatings are a Field Emission Scanning Electron Microscope (FE- SEM), Glow Discharge Optical Emission Spectrophotometer (GDOES) and the X-Ray Photoelectron Spectroscopy (XPS). Coating Characterization Generation I and Generation II: With typical chemical make-up of the bath, coating weights range from mg/m 2 and are dependent on the substrate. The deposition rate for the new coating, regardless of generation, is very rapid, producing a complete coating within approximately 20 seconds. At this rate of deposition, the overall manufacturing line could be reduced, thus freeing up additional manufacturing floor space. On the other hand, a zinc phosphate coating requires a minimum of 60 seconds deposition, is extremely thick in comparison with coating weights ranging from g/m 2, and has a defined crystalline structure from 2 15 microns. By comparison, the zinc phosphate scanning electron micrograph (SEM) shown in Figure 3 is 50 microns across. The visual appearance of the zirconium oxide type coatings varies in color and is substrate dependent. The conversion coating color ranges from a golden iridescence on cold rolled steel, a bluish/grey on electro- or hot-dipped galvanized and a near colorless appearance on aluminum. Regardless of the new conversion coating color, it is exceedingly hydrophilic, meaning that it holds a water layer very strongly, and this tightly held water sheath makes the treated surfaces very easily differentiated from untreated surfaces. Likewise, the coating color seems to have no effect on its performance. As shown in Figure 2, the new conversion coatings and its second generation are very thin compared to a traditional zinc phosphate. [nm] Coating Weight 5000 Zinc Phosphating 1-4 g/m² Iron Phosphating ~ 600 mg/m² SEM 1,000 X Zinc Phosphate 100 Chromating 50 and II Conversion Coatings mg/m² 5 Molecular Level FE-SEM 30,000 X New Conversion Coatings Figure 2: Dimensional Scale and Coating Morphology of the New Generation Conversion Coatings on Cold Rolled Steel 175

5 The coating weight is a critical measurement of current zinc phosphate technology and is important in the new conversion coatings as well. The coating weights and/or coating thickness are measurements for the new conversion coatings to help understand performance and composition. One supplier relies on coating thickness as a measurement of the coating but it does not tell the end user, what is the coating composition, which is more critical. The coating weight is measured two ways, one outside the laboratory using a hand held X-Ray Diffraction instrument and collecting counts for the respective elements, in this case mostly zirconium. These counts are then converted into a milligram per square meter (mg/m 2 ) reading. This unit can be used in a plant environment and provides instant feedback as to how the system is operating. The second method involves stripping the coating off in the laboratory and analyzing the stripping solution using ICP for the major components. Knowing the coating composition is a key attribute and coupled with a depth profile of the coating, one begins to understand the mechanism. This understanding helps in troubleshooting any on-line problems. An XPS depth profile tells the user the coating composition at a given depth. Figure 3 shows a depth profile for the current technology and one for Generation II on cold rolled steel (CRS). 100 New Conversion Coating CRS 100 New Conversion Coating I CRS Coating Layer Coating Layer 70 Co nc en 60 tr ati 50 on, 40 at CRS Substrate C1s O1s F1s Fe2p Cu2p3 Zr3d Coating Thickness ~ 44nm Sputter Depth, nm vs. SiO 2 Zirconium Coating Weight = 70mg/m Figure 3: XPS depth scans of Generation I and II New Conversion Coatings 70 Co nc en 60 tr ati 50 on, 40 at Coating Thickness ~ 98nm Sputter Depth, nm vs. SiO2 CRS Substrate Zirconium Coating Weight = 87mg/m2 C1s O1s F1s Fe2p Cu2p3 Zr3d Knowing the coating weight and the depth of the components is essential in determining performance. The depth of the coating is measured from the surface until the base substrate reaches a 50% atomic weight in nanometers. From the XPS scans seen in Figure 3, Generation II coating in significantly thicker, 2.5 times than that of Generation I, while there is a slight difference in coating weight, 70 mg/m 2 compared to 87. The fact that the coating weight does not change significantly, but the thickness shows a two fold difference, is important to performance. As seen in Figure 4, the appearance between the two coatings using a FE-SEM further shows significant differences in coating appearance. New Conversion Coating New Conversion Coating I Figure 4: FE- SEM s at 30,000 magnification 176

6 The conversion coating seen in Generation I exhibits a coarser structure on the surface, while Generation II is more refined. The Generation II structure lends to an increase in surface area and a larger foothold for the electrocoat. With a thicker coating layer, a tighter coating structure and slightly higher coating weight, performance in cyclic corrosion testing is improved. Performance The primary measurement of any new pretreatment is overall performance. Corrosion and adhesion performance with any conversion coating depends on three factors, substrate, pretreatment and the paint system. All three factors must work as one to meet today s corrosion and adhesion specifications. Depending on the specification and its ranges, the new generation coatings are capable of meeting specification requirements that were written for zinc phosphate. Generation I, in many cases met the specification limits with regard to adhesion and corrosion, and hence considered equal. However, Generation I system itself, based on laboratory data, was not robust enough on cold rolled steel, so Generation II was developed. In a qualification program, panels of cold rolled steel (CRS), electrogalvanized (EG), hot-dipped galvanized (HDG), galvanneal (HIA) and two aluminum alloys (AL), 6111 and 6016 were prepared using Generation I, II and a control, zinc phosphate. Two sets of panels were prepared in a laboratory environment, one set was treated with electrocoat and the second the full layering system. Three paint systems from the three major OEM paint suppliers were used in the evaluation. Afterwards the treated and painted panels were tested under General Motor s GMW cyclic corrosion test. Figures 5-7, exhibit the results of the comparison. In this test there are two ratings for each pretreatment. The first rating is the maximum creepage from both sides of the scribe. The second rating is the mean average across the scribe. Results and Discussion: 14.0 Generation I, II and Zn 3 (PO 4 ) 2 GMW14872 (28 cycles, 3.7g mass loss) Paint Supplier 1 - Topcoat System Corrosion/ Scribe Creep (mm) Maximum Average CRS Spec. Max < I I I I I CRS HDG EG HIA Al6111 Al6016 I Figure 5: Cyclic Corrosion Results, Paint Supplier 1 Full Paint As one can see on the cold rolled steel panel, Generation II and a full layering system exhibits performance improvement over Generation I. The general trend seen across all substrates is an improvement moving from Generation I to II. With the exception of the aluminum alloys, the overall performance can be shown as Gen I < Gen II < Control. On the aluminum alloys the pretreatment system did not matter; all performance was equal. 177

7 14.0 Generation I, II and Zn 3 (PO 4 ) 2 GMW14872 (28 cycles, 3.7g mass loss) Paint Supplier 2 - Topcoat System Corrosion/ Scribe Creep (mm) Maximum Average CRS Spec. Max < I I I I I I CRS HDG EG HIA Al6111 Al6016 Figure 6: Cyclic Corrosion Results, Paint Supplier 2 Full Paint Again, one can see on the cold rolled steel panel, Generation II and a full layering system exhibits performance improvement over Generation I. The general trend seen across all substrates with the full layering system is an improvement moving from Generation I to II. With the exception of the aluminum alloys, the overall performance can be shown as Gen I < Gen II < Control. On the aluminum alloys, it did not matter what the pretreatment system was, all performance was equal Generation I, II and GMW14872 (28 cycles, 3.7g mass loss) Paint Supplier 3 - Topcoat System Corrosion/ Scribe Creep (mm) Maximum Average CRS Spec. Max < I I I I I I CRS HDG EG HIA Al6111 Al6016 Figure 7: Cyclic Corrosion Results, Paint Supplier 3 Full Paint Again, one sees improvement on the cold rolled steel panel, Generation II and a full layering system exhibits performance improvement over Generation I. The general trend seen across all substrates with both paint systems is an improvement moving from Generation I to II. With the exception of the aluminum alloys, the overall performance can be shown as Gen I < Gen II < Control. On the aluminum alloys, the pretreatment system did not matter, all performance was equal. 178

8 General Conclusion of Cyclic Test Performance: Overall, the performance of Generation II was better on cold rolled steel regardless of the paint supplier. The general trend was improvement across the balance of the substrates with the exception on the aluminum alloys. Paint supplier 3 performed slightly better than the other two suppliers in this testing. Summary The current technology, Generation I, has shown promising performance under today s specifications to the point of automotive companies running the technology under full production. The pretreatment and paint suppliers agree there is room for improvement both in performance and appearance. Both supplier bases are working to refine their respective technology to address each concern. The current generation coatings are running in production, allowing companies to reap the additional benefits offered by the technology. These benefits include: 1. Reduced environmental impact since the coatings are based on mixed metal oxides; using zirconium or polymers as a base, deposit a conversion coating that is free of regulated heavy metals and phosphate. 2. Reduced Waste Water treatment by eliminating heavy metals allows the system to remain virtually sludge free and free of the wastewater constraints and costs associated with today s zinc phosphate processes. 3. The rinse water with the new generation coatings can be more efficiently reused, saving on water usage. 4. Energy cost savings because the bath operates at ambient temperature, thereby eliminating the need for heat. 5. As represented by the data, Generation I new conversion coatings met most of the specification requirements and depending on the substrate, they equal the zinc phosphate control. Generation II research has shown improvements in conversion coating performance on cold rolled steel and zinc substrates. There is still a way to go. The movement continues and as more and more lines convert to the technology, we increase our knowledge of the next generation of conversion coatings. Acknowledgements: The author s would like to acknowledge for their participation: Al Bobadilla, Barbara Kayitmaz, Bruce Goodreau Henkel Corporation Renato Barao, Diego Garcia, Marcio Berton - General Motors Corporation S. America Kevin Cunningham - General Motors Corporation N. America Rob Schiller - BASF Corporation DuPont Corporation 179

9 References 1. Phosphating and Metal Pretreatment, D.B. Freeman, published by Woodhead-Faulkner Ltd, Cambridge, England, Emerging Technologies in Pretreatment, T. Giles, B. Goodreau, W. Fristad, J. Krömer, P Droniou, Henkel Corporation, SAE 2007, Detroit MI April Update of the New Conversion Coating for the Automotive Industry, T. Giles, B. Goodreau, W. Fristad, J. Krömer, M. Frank, Henkel Corporation, SAE 2008, Detroit MI April BASF Laboratory Evaluation, R. Schiller, Southfield, MI Dec New Metal Pretreatment Technologies, B. Goodreau, Henkel Corporation, SAE 2010, Detroit MI April Laboratory Study internal to Henkel Corporation, D. Vonk

10 About the Authors: Terrence R. Giles; Terry currently works as a Business Development Manager in the Metal Pretreatment Group at the Henkel Adhesive Technologies, Madison Heights, Michigan, USA. He is responsible for introducing the Next Generation conversion coatings to the OEM and auto supplier markets. Terry has been at Henkel for 31 years holding various positions in Research & Development, Technical Service, Marketing and Business Development. Terry.Giles@henkel.com Donald Vonk; Don currently works as a Sr. Research Chemist at Henkel Adhesive Technologies, Madison Heights, Michigan, USA. He is responsible for developing the Next Generation conversion coating for the metal finishing industry. Don has been at Henkel Adhesive Technologies for 5 years and has 23 years in the Metal Finishing Industry. Donald.Vonk@henkel.com Sergio-Luis Favero; Sergio currently works as the Technical Service Manager Automotive at Henkel Ltda, São Paulo, Brazil. He is responsible for field service activities in the areas of metal pretreatment, sealers and adhesives at the OEM s. Sergio has been at Henkel for 20 years holding various positions in Technical Service. Sergio-Luis.Favero@henkel.com 181