Pushing the Boundaries of Conversion Coatings with an Unconventional Approach

Abstract

In the 20th century, the use of steel as the primary construction material for applications such as vehicles, bridges, and buildings led to the development of surface protection by phosphate based conversion coatings. The discovery of anti-corrosion property of phosphatized surfaces can be dated back to 1906 when iron and steel were phosphatized using a phosphoric acid solution. Over time, numerous technical enhancements were achieved in the performance of phosphate coatings such as reduction in process time and temperature and the use of oxidizing agents such as nitrates for accelerated kinetics. However, zinc phosphate is not a universally suitable system; with the current market growth of aluminum alloys and their suitability for lightweight applications, automotive industries are utilizing aluminum alloys (e.g. AA6022) to reduce the vehicle weight to improve fuel efficiency. Interestingly, running aluminum alloys in zinc phosphate baths leads to excessive sludge formation and faster depletion of pretreatment bath. The frequent bath replenishments, increased sludge disposal costs, and new environmental constraints on the usage of phosphorous, nitrates, and nickel compounds in the pretreatment solution, drive the investigation of new technologies based on titanium and zirconium compounds. Zirconium and titanium based coatings offer several advantages over conventional zinc phosphate coatings, e.g. a very thin film thickness (~100 nm), a simpler process, and an environmentally friendly pretreatment solution.During the formation of conversion coatings, the very top surface of metal alloys undergoes an oxidation reaction (e.g. M → Mn+ + ne‒). This reaction is coupled with the reduction of hydrogen ion in the pretreatment bath (e.g. H+ + e‒ → ½ H2) which leads to a slightly higher pH in the vicinity of metal substrate. If adjusted to suitable conditions, the passivating species are hydrolyzed on the substrate and determines the corrosion and adhesion properties of the alloy surface. The research at PPG Industries focuses on tuning the chemistry of pretreatment solutions, chemical modifications of subsequent layers, and engineering processing conditions which can favor the thermodynamic stability and kinetic growth of coating systems. Besides that, the utility of several analytical tools such as SEM, XPS, XRF and various modes of corrosion testing – cabinet tests, electrochemical tests, and immersion tests – are instrumental in the ongoing research. This presentation will focus on the historical development of conversion coatings, shortcomings in the phosphatized surfaces, processing and mechanism of thin film coatings, and ongoing research at PPG Industries.