_ Corrosion is the bane of all metal building products and yet the standard measure of corrosion resistance is still most commonly the neutral salt spray test (ASTM B-117) that virtually everyone concedes is a very poor indicator of field performance. However, the industry has years of data gathering experience with salt spray, and so there is strong tendency to stay with what you know, even when you know it isn’t ideal. It is worth remembering that the original purpose of the salt spray test was to monitor product performance consistency, not to predict years of field performance. The best measure of field performance is, of course, actually monitoring buildings over many years under many different field conditions, but this has the practical disadvantage of requiring decades of monitoring.
___ Realizing this situation, a group of companies involved throughout the value chain of metal building and roofing manufacture worked together over the past decade to construct and inspect a series of building roofs with the goal of determining which roofing features lead to the highest degree of corrosion. Armed with this information, could these features be combined to construct a panel test rack that would accelerate natural field corrosion to a more manageable time frame, say less than five years?
In 2000 and 2005 U. S. Steel, Henkel, AkzoNobel, Precoat, and Morton Buildings cooperated to build a series of metal buildings in different locations around the US using well characterized GALVALUME™ coated steel sheet, chrome-containing and chrome-free pretreatments, chrome-containing and chrome-free paints, and coil coating processes. A series of different pretreatments, primers, and topcoats were coil-applied and buildings erected in the Northeast, Southeast, and Midwest of the US. This allowed real life corrosion characterization of different painted metal systems to be evaluated. The pretreatment-paint systems included both commercial and experimental products at the time. The systems also included both hexavalent chromium-containing systems as well as chromium-free systems. Additionally, these materials were put in outdoor exposure testing at the Battelle site in Daytona Beach, Florida at 120 feet from the ocean as well as in neutral salt spray and cyclic accelerated corrosion testing. This would allow future comparison of all the test methods.
___ It was also important to have buildings in different geographical locations, as corrosion mechanisms are different depending on the local environment of temperature, humidity, acid rain, etc. The focus of the building inspection was the roof, and specifically the lap and drip edges of metal roof. Typical corrosion levels can be seen in Fig. 1, where the drip edge (top) shows slight corrosion starting and the lap edge (bottom) shows some white rust stain on the side of the major rib and red rust and water under the lap edge.
___ One of the difficulties in defining the best conditions to test painted metal performance is that optimal test conditions depend on what is to be learned. To measure paint chalk and fade, it is best to have vertical edges and scribes using a high slope. However in the present corrosion study, the more informative system turned out to be lap edges and drip edges with a low slope. When all the buildings were inspected the generalizations listed below could be drawn.
1. The north side of the roof shows more corrosion than the south side.
2. Lower slope shows more corrosion than higher slope.
3. Sheltered edges that spend more time in the shade show more corrosion than sunny edges.
4. Lap edges show more corrosion than drip edges.
___ The common feature of all these observations is that the locations which remain wet longer show more corrosion. This can be due to slower water runoff when there is low slope or slower water drying due to shade or less wind, or capillary action holding water under a lap edge. It was noteworthy that the corrosion tendencies after 6-11 years exposure were more dependent on the location and four conditions listed above than on the actual pretreatment and paint system. Thus, the degree of corrosion was more dependent on the conditions the roof experienced than on the different chemistries being employed. These observations led to the following design features in constructing a test panel rack to mimic these conditions.
1. Panel design. Assemble a pair of panels so that both a drip edge and a lap edge are available for rating the extent of corrosion.
2. Low slope. The rack is designed with a low 10° slope.
3. Sun/wind shelter. Panels are sheltered from all direct sun and most wind by placing the rack against a north facing wall with additional wind protection from the prevailing westerly wind.
4. Rain shelter. Panels are placed on lower levels of the rack so that a covering roof layer reduces any direct rinsing action of rainfall. This will increase the exposure to naturally occurring atmospheric corrosives and minimize rinse-off by rain.
This test and rack have been referred to as the Sheltered Exposure Roof Corrosion (SERC) test. A test panel rack incorporating all these features was built and installed on the roof of the Henkel facility in southeastern Michigan (Figure 2). The first rack was built with four levels of polypropylene pegboard for attachment of the panels, leaving room for a roof over the top shelf so that the fourth level could also hold test panels. The rack was installed adjacent to a north facing wall and sheltered to the west by another wall extending above the roof section on which it was placed. The rooftop location also meant that uncontrolled road salt would not be scattered on the rack, which could otherwise happen with ground level installation during Michigan winters.
The test panel design was also optimized to use a single standard 4″ x 12″ panel which was cut into a 2 ½” x 12″ and a 1 ½” x 12″ piece. These two pieces then present a freshly cut lap edge as well as a drip edge when fastened together in an overlapping fashion. The two panel pieces are taped together on three sides to ensure they remain tightly layered, as overlapping roofing panels would, and they are then attached to the pegboard shelves of the test rack with all plastic fittings to avoid initiating any galvanic corrosion. The panel pair is held slightly off the test rack surface to avoid any random capillary effects under the bottom of the panel pair and the drip edge. Figure 3 (top) shows a schematic of the panel attachment to the rack and (bottom) a photograph of a typical assembled test panel attached to the test rack.
A variety of test panels have been on the Henkel test rack since June 2012. On the rack we are currently comparing a matrix of substrates (CRS, HDG, GALVAUME™ coated steel sheet and Aluminum) and pretreatments (Cr(VI), Cr(III), and non-Cr) with a standard coating of primer and paint. Precoat has built a similar SERC test rack for comparing products from their coil coating lines. Figure 4 shows a series of test panels of hot dipped galvanized (HDG) coated with a variety of pretreatments under a chrome-containing primer and a siliconized polyester topcoat. The panels photographed in Figure 4 had only been on the test rack for 18 months, and after this short time did not show any measurable difference between the Cr(VI), Cr(III), and non-Cr pretreatments.
___ The next steps in this cooperative effort between all five companies are to perform a follow-up building inspection and correlate the results with Florida exposure panels, standard lab salt spray and cyclic corrosion tests, and the new SERC test rack panels. Now that the buildings are 9 and 14 years old, this second building inspection three years after the first inspection will let us determine if the corrosion trends seen earlier continue or whether new corrosion tendencies are noticed. After the 6 and 11 year inspection, the specific location on the roof was more important in determining the extent of corrosion than was the actual selection of pretreatment or paint system. This meant that the combination of factors that defined the SERC test (remaining wet longer) were more important than the actual pretreatment-paint system employed on the metal. However, it is possible that after three years longer in the field, the pretreatment-paint systems will start to differentiate themselves more. Secondly, a data comparison of building exposure and the various accelerated tests versus the SERC test will let us see how effectively they correlate with observed building exposure or other tests. The SERC test panels will only have two years of outdoor exposure, so it may be too early to see demonstrable differences. However, any indication at this early date would be very promising as it would show the value of an outdoor test rack to better predict real life corrosion in a more reasonable 2-3 year time frame.