Douglas S. Cinoman, Gordon L. Tullos, and Roberto Paganica of Dow Powder Coatings tell Finishing about the company’s new Xtra-Corr powder coating

Corrosion of metallic substrates exacts an enormous cost on society – not only from a  monetary standpoint but from the perspective of safety as well.  A United States Federal Highway Administration study in 2002 estimated the annual direct cost of damage from corrosion to the U.S. as $276 billion – a “staggering” amount.  

Alarmingly, the annual indirect cost was estimated to be a shocking $552 billion, or 6% of GDP.   The tragic 2007 collapse of the I-35W bridge in the U.S. state of Minnesota that killed 13 people and injured 145 may have been caused, at least in part, by corrosion and poor maintenance.  In fact, investigators found significant corrosion on the gusset plates and on other parts of the bridge that may have been contributing factors.

The use of powder coatings for corrosion control is both widely recognized and well documented.   Powder coatings are extensively used for corrosion control in such diverse applications as pipe, garden furniture, automotive wheels and architectural trim and building facades.  However, there remains a need for improved corrosion resistance in these end uses as well as in many others.  To meet this need, Dow Powder Coatings has developed a new generation of corrosion resistant standard durable polyester-based powder coatings developed especially for aluminum substrates via the use of proprietary, patent pending resin chemistry developed in our laboratories. These new coatings are available in TGIC-containing or TGIC-free chemistries – both with no compromise in exterior durability versus commercial systems.

Corrosion Control
Figure 1 presents salt spray data according to ASTM B117 for two powder coatings formulations, one prepared using a conventional, widely available polyester and the second with our engineered polyester (denoted as Xtra-Corr).

In this study, the substrate was an aluminum Q panel and the test duration was fixed at 7000 hours.  Corrosion resistance at various times was judged by measuring the creepage of the coating from the X-scribe that was placed on the panels at the beginning of the test.

This graphic clearly shows the superior performance of the Xtra-Corr system over the commercial control for a properly pretreated test panel. 

However, in the real world, many different pretreatment systems exist, varying by chemistry and complexity.  For example, an important trend in the industry is to move away from Cr(VI)-containing pretreatments for environmental and toxicological reasons.   As pretreatments become less effective and/or the number of steps in the pretreatment process is reduced to lower cost, this places additional stress on the inherent ability of the coating to provide adequate corrosion control. 

In order to assess the intrinsic ability of the novel chemistry embedded in the Xtra-Corr system, a series of salt spray studies was conducted that varied the pretreatment process from none to a commercial eleven-step process used by one of our customers.  In all cases, corrosion resistance was judged by performance in the standard ASTM B117 (ISO 9227) test protocol.  Actual parts were coated and tested to get a more realistic view of corrosion resistance for Xtra-Corr formulations as well as the standard control using commercially available powder coatings.

Figure 2 shows the results of testing for coated architectural aluminum extrusions that received no chemical conversion pretreatment whatsoever. 

After 1000 hours of salt spray testing, blisters around the X-scribe are readily visible for the standard formulation while they are noticeably absent from the Xtra-Corr  formulation, clearly demonstrating the inherent superiority of this system.
In non-chrome pretreatments, the intrinsic adhesion and corrosion advantages for our new system are also readily apparent.  Figure 3 presents salt spray data for aluminum extrusions that were pretreated with a commercial system containing no chromium.  After 300 hours, the Xtra-Corr system again shows much improved adhesion (and corrosion resistance) versus the standard formulation.

Likewise, Figure 4 shows the performance of Xtra-Corr over a very basic three-stage, non-chrome pretreatment process.  Here, the difference between the old and new technology after just 100 hours of testing is striking; the conventional polyester powder coating has almost completely delaminated from the aluminum profile.

Finally, the performance of this new technology was assessed over die cast aluminum substrates.  Figure 5 shows the performance of Xtra-Corr after 500 hours salt spray on a die cast aluminum substrate with a three stage non-chrome pretreatment while Figure 6 presents a comparison of articles with an eleven stage Cr(VI)-based pretreatment after 800 hours.  Here again, the Xtra-Corr system performed significantly better than the powder coating based on the standard technology using both Cr-containing and Cr-free pretreatments.

Accelerated and Natural Weathering
In addition to the corrosion studies discussed above, we have also investigated the performance of powder coatings containing this new technology under QUV-B (313 nm irradiance) accelerated weathering.  Figure 7 below shows 60 degree gloss retention curves for three blue RAL-5010 formulations; viz., two commercial formulations and one using Xtra-Corr chemistry.  The cycle employed was 5 hours at 50°C and 4 hours condensation at 40°C.  This plot shows very similar results for all systems tested.  In addition to this RAL 5010 blue formulation, similar results were obtained for RAL 3005 red, RAL 8014 brown, and RAL 9010 white.

It should also be noted that Xtra-Corr polyester powder coatings have already received a Qualicoat Category 3 Class 1 approval.

Summary
Significant improvements in corrosion resistance of polyester based powder coatings have been obtained by the use of novel resin chemistry and will be commercialized this year as Xtra-Corr.  In addition to the standard durable system described above, Dow Powder Coatings has also developed a superdurable version that will be introduced to the market in late 2009.
We believe that this technology will find broad acceptance in a number of end use markets and demonstrates the commitment of Dow Powder Coatings to this industry.

Acknowledgments
The authors wish to acknowledge the many contributions made by Ms. Marina Bastarolo, Mr. Andrew Daly, Mr. Orfeo Ganassin, and Ms. Diane Pennington in the course of carrying out this work.