Combining Ecoat and Plating for Next Generation Edge Protection
Efficient corrosion protection of the sharp edges of manufactured steel fasteners has long been a goal for coaters and coating formulators. Numerous commercial processes exist to achieve this goal, but all involve large tradeoffs between cost/practicality and performance. A method is described and proven by which a high-purity and well-coalesced layer of aluminum metal is electroplated onto a steel substrate from an ionic liquid electrolyte. The aluminum plated fastener is then coated with a layer of cationic epoxy electrocoat. This combination provides exceptional efficiency, adhesion and corrosion protection – especially on sharp edges – while the process similarities between the aluminum plating and e-coat make the concept an ideal candidate for commercialization.
The automotive and industrial benchmark for cost-efficient corrosion protection of steel is a layer of zinc phosphate followed by a layer of cationic epoxy electrocoat. The electrocoat can serve either as a stand-alone coating or as a primer for successive paint layers. Both the phosphating and electrocoating are high-throughput, high-efficiency processes which allow in-line coating application with minimal labor input per square meter finished, and very little waste.
This process provides excellent corrosion resistance, particularly over smooth, flat or gently curved surfaces. The sharp edges prominently featured on steel fasteners, however, are a different story. Thermosetting by nature, all electrocoat products must go through a semi-liquid ‘melt’ stage (McMillan, 2002) before building enough heat to chemically unblock their crosslinkers, evaporate the blocking agent, and finally crosslink, or ‘cure.’ This melt phase is necessary for the coating to achieve optimum coalescence before cure. Without it, the cured film would be rough, low gloss, and porous, ruining both its aesthetic and protective properties. But too much flow before cure results in “pull-away” from sharp edges, leaving little or no protective layer after crosslinking (Sample, 2007).
One way to battle loss of sharp edge protection during cure is to reduce or eliminate the edges themselves before coating. This approach can be very effective, but also very expensive in terms of engineering and handling. The second way is through electrocoat formulation – designing the coating to achieve the desired balance of appearance and edge protection (Corrigan, 1992). This approach can also be effective, but it poses technological challenges and adds formulation cost. The large-batch, scale efficiencies used by electrocoat manufacturers also limit tailoring products to the needs of the individual industrial coater.
This article describes a third approach – electroplating a layer of pure aluminum metal between the base steel substrate and the electrocoat layer.
Deposition of Aluminum from Ionic Liquid
Aluminum Plating Background
Unlike more easily electroplated metals (tin, silver, zinc, etc.), the reduction potential of aluminum in aqueous solution (Al3+) is more negative than the reduction potential for water. This means that any attempt to electroplate aluminum from an aqueous medium will be unsuccessful, as water will simply electrolyse to pure hydrogen (at the cathode) and oxygen (at the anode) gases while the aluminum salt remains in solution.
If aluminum clad steel is desired, an alternate means of aluminum deposition must be sought. A technological process was developed over the latter half of the 20th century by Siemens and the Max Planck Institute, and then commercialized in 1995 by a new American company called Alumiplate. The process involves electrodeposition of aluminum from an organic bath comprising an aluminum salt, a stabilized electrolyte, and organic solvent.
In the Alumiplate process, the obvious technical advancement of electroplating aluminum onto steel is tempered somewhat by the need for flammable, volatile organic solvent in the plating bath. Using an ionic liquid electrolyte, however, eliminates the need for volatile organics in the deposition bath, reducing both the fire hazard and the VOC-management demand on the inert-atmosphere plating system. Zero VOC in the plating bath means less evaporation, lower emissions, and no need for replacement or monitoring over time.
Ionic liquids are a broad class of chemical compounds which are both ionic in nature and liquid in neat form at T < 100° C (SERDP, 2011). Importantly, most ionic liquids also exhibit nearly zero vapor pressure. This property is very interesting from an industrial perspective because it allows for the conservation of raw materials, since none can be lost to evaporation.
For comparison, common ionic solids often involve either a metal or small organic cation coupled with an anion, comprising either a simple halogen or transition element in any of several oxidation states. Sodium Chloride (NaCl) is one of the most familiar examples. Copper nitrate (Cu(NO3)2) and Ammonium Sulfate ((NH4)2SO4) are two more.
The atomic size and electronic structure of this kind of compound typically results in a pure material which forms a highly-ordered, crystalline solid. They usually exhibit good solubility in strongly polar solvents like water, and limited solubility in non-polar, organic solvents. In contrast, ionic liquids generally involve larger organic cations, and larger, more complex anions. The classic example is the imidazolium cation, shown in figure 1 compared to neutral imidazole.