Braving the Elements
by Paul Heffer and Bill Lee, info-uk@beckers-bic.com
Chromate ions, usually as zinc and strontium chromate pigments, have been the main active ingredient of high-performance anti-corrosive coatings for many years. Chromate compounds are used in pretreatments, etch primers, and a variety of other substrate-specific primers.
However, environmental legislation around the world is restricting the use of chromate pigments. As a result, there is agreement to work to eliminate hexavalent chromium compounds for corrosion protection while coil coaters are looking for maintained or improved anti-corrosion properties and continuing economy in use.
The urgency of the problem has raised demands for fast answers through accelerated corrosion tests, but there are real doubts about their value. The faster the test, the higher the acceleration factor, the lower the chances for good correlation with natural conditions. In the late 1990s the European Coil Coatings Association (ECCA) carried out an exhaustive study comparing natural and laboratory-based weathering. No usable correlations were discovered for any systems on galvanized steel.
Natural weathering is not a controlled process. We can record rainfall, temperature extremes and averages, hours of sunlight, panel temperatures, and can attempt to incorporate this data into our laboratory testing. But we cannot incorporate the element of disorder, of natural chaos.
We unquestionably need the support of real-time natural exposures at aggressive locations over extended periods of time. As a market leader in the European coil coatings market, Beckers has been supplying chrome-free coating systems in commercial quantities for more than 10 years. Beckers has exposed thousands of panels at Bohus Malmön on the west coast of Sweden and, more recently, at Hainan Island in China, where the higher temperatures make the conditions even more severe.
Chromate Alternatives and Testing
Several factors limit the choice of alternative anti-corrosives. Some are too coarse to be incorporated into conventional thin film primer systems. Some interfere with the curing mechanism or cause rapid increases in viscosity during storage. Another problem is that much of the work carried out by suppliers has been aimed at air-drying or low-stoving (low baking) coatings that are applied to mild steel, rather than pretreated galvanized steel. Less effort has been directed towards the coil-coating sector with its special requirements and specialist resin systems.
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| Figure 2: Galfan with chromate-free pretreatment (left), chromate-free polyester-polyurethane primer (middle), and PVDF topcoat. |
Early attempts to replace chromates centered on zinc phosphate, but these proved inadequate. Orthophosphate hydrates have been modified with metals such as aluminum, calcium, magnesium, and molybdenum. Polyphosphates have been developed from these. Molybdates and metaborates have been proposed, as well as the addition of organo-metallics and even ion-exchange pigments.
The basic idea behind all of these pigments is to provide a source of salts of limited solubility, which will partially dissolve to (a) form a very thin coating on the metal to be protected; (b) act as anodic or cathodic inhibitors; and (c) provide a renewable source of these salts as they are washed away or used up.
Alternative chrome-free pigments for coil primers are only part of the overall coating system. The metal pretreatment, which forms part of the cleaning and degreasing process, is traditionally chrome-based and also must be modified. Substrate manufacturers have also been looking at improvements to their products. In galvanized steel, the removal of lead from the zinc coating has improved corrosion resistance significantly.
Beckers has worked closely with major pretreatment suppliers for many years to provide a fully chrome-free package, and systems have been used commercially and in line trials across Europe over over the past seven years. In all cases, panels were exposed in both accelerated test cabinets and natural external environments.
Beckers' biggest project to date started in 1997. This compared the latest developments in pretreatments and primers applied to the three main ferrous substrates: HDG (hot dip galv) zinc coated steel, Galvalume 45/55 zinc/aluminum coated steel, and Galfan 95/5 zinc/aluminum coated steel.
Table I shows the large number of primers evaluated, both chromated and chrome-free. Substrates from different steel producers were pretreated with 15 different recipes. The most popular topcoats were then applied to the primed panels. The full coverage of all permutations would have totaled 7,965 systems, so an experimental design was used to reduce the number to almost 800 combinations of substrate, pretreatment, primer, and topcoat. Panels were subjected to:
- Accelerated testing in ASTM B117 salt spray (continuous fog of 5% sodium chloride at 35°C);
- ASTM G85-A5 Prohesion cabinets (cyclic fog of 0.05% sodium chloride and 0.35% ammonium sulfate with one-hour fog unheated and one-hour dry at 35°C);
- and natural exposure at Bohus Malmön in two orientations—traditional 45° south facing and vertical north facing "under eaves." Paint facing north under the eaves is attacked more severely because drying takes longer with no sun to warm the surface.
ECCA's lack of success in finding accelerated tests to mimic natural exposure confirms Beckers’ long-held view that salt spray testing alone does not provide sufficient evidence to launch a new product. Unfortunately, ASTM B117 salt spray is the most widely specified accelerated test for the evaluation of corrosion resistance and is the driving force behind the performance criteria of many protective coatings despite strong evidence of unreliability.
The initial findings from Beckers' project started in 1997 confirm there is little correlation between laboratory-based testing and natural conditions. Certain formulations, which perform well in salt spray, actually perform worse in the field. Other panels that performed poorly in salt spray are still performing well at Bohus Malmön after six years' exposure, illustrating why some primers that perform poorly in salt spray have become commercially accepted on the basis of their exterior performance.
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| Figure 3: HGD with standard chromate pretreatment (left), chromate acrylic (middle), and plastisol topcoat (right). |
The following (Figures 1, 2, and 3) is a selection of panels that are performing well on exterior exposure with their accelerated test counterparts. Each
series shows, from left to right, hot salt spray, Prohesion, north-facing, and south-facing panels.
This extensive work shows that:
- No single pigment will replace strontium chromate. Combinations of two or three pigments are needed to give a synergistic effect.
- Higher loadings do not automatically lead to improved corrosion resistance. Levels need to be optimized. This is commercially important due to the higher cost of the new materials.
- A combination that works in one resin system over a particular pretreatment may not work in a different resin/pretreatment combination.
- This can lead to a self-perpetuating project in search of "the best chrome-free primer"as "new and improved" raw materials are produced by suppliers. Hundreds of new permutations are currently being evaluated in this most recent phase of exposures.
Resin developments form a further piece of the jigsaw. High-build non-PVC systems are becoming more popular, generally using polyester or polyurethane primers that can be applied at higher film thickness, typically around 20 to 25 microns. This situation generates further formulation problems with differences in rates of release of the separate ions from the different anti-corrosives when combined in thicker resin films.
Summary
Beckers has many successful examples of chrome-free coating systems in the field. Chrome-free primers over chrome-containing pretreatments generally produce few difficulties.
Polyurethane primers, both conventional and high build, are generally most successful when it comes to incorporating the new chrome-free pigments. Some systems perform at the same level as chromates. Chrome-free epoxy primers can be produced, but their inherently lower flexibility introduces points of weakness for corrosion initiation at micro cracks in severely formed areas.
If a chrome-free pretreatment is to be used, Beckers' ongoing exposure series must be checked for compatibility and interactions. Chrome-free pre-treatments and primers can be produced successfully and with a high level of anticorrosive activity. The difficulty lies in getting the two to work together, as these systems seem, in general, to be more critical for substrate, paint and pretreatment combinations. Chromates are more forgiving, but chrome-free primers can be produced that are as effective as their chromate-containing analogs, providing sufficient attention is paid to these interactions and material selection.
Accelerated tests still appear to favor chromates, but, despite all efforts to date, the answer to anticorrosive performance in service can only be gained from real, natural exposures.
