Gary W. Dallin, P. Eng. Pre-painted metal coated steel panels for buildings have been successfully used for many years. One indication of its popularity is the widespread use of pre-painted steel roofs in Canada and around the world.
Metal roofs last two to three times longer than non-metal ones. 1 Metal buildings make up almost half of all low-rise non-residential buildings in North America, and a significant proportion of these buildings have pre-painted, metal-coated steel panels for roofs and walls.
Proper specification of the coating system (i.e. pre-treatment, primer and top coat) can ensure a service life of painted steel roofs and metal coated walls in excess of 20 years in many applications. To achieve such a long service life, manufacturers and builders of color coated steel sheets need to consider the following related issues:
Environmental Issues One of the first factors to consider when choosing a pre-painted metal coated steel product is the environment in which it will be used. 2 The environment includes the general climate and local influences of the area.
The latitude of the location determines the amount and intensity of UV radiation to which the product is exposed, the number of hours of sunshine per year and the angle of exposure of the pre-painted panels. Clearly, low-angle (i.e., flat) roofs of buildings located in low-latitude desert regions require UV-resistant primer and finish systems to avoid premature fading, chalking, and cracking. On the other hand, UV radiation damages the vertical cladding of the walls of buildings located at high latitudes with a cloudy climate much less.
Wet time is the time during which roof and wall cladding becomes damp due to rain, high humidity, fog and condensation. Paint systems are not protected from moisture. If left wet long enough, the moisture will eventually reach the substrate underneath any coating and begin to corrode. The amount of chemical pollutants such as sulfur dioxide and chlorides present in the atmosphere determines the rate of corrosion.
Local or microclimatic influences that should be considered include wind direction, deposition of pollutants by industries and the marine environment.
When choosing a coating system, the prevailing wind direction should be taken into account. Care should be taken if the building is located downwind of a source of chemical contamination. Gaseous and solid exhaust gases can have a serious effect on paint systems. Within 5 kilometers (3.1 miles) of heavy industrial areas, corrosivity can range from moderate to severe, depending on wind direction and local weather conditions. Beyond this distance, the impact associated with the polluting impact of the plant is usually reduced.
If painted buildings are close to the coast, the impact of salt water can be severe. Up to 300 m (984 ft) from the coastline can be critical, while significant effects can be felt up to 5 km inland and even further, depending on offshore winds. The Atlantic coast of Canada is one area where such climate forcing could occur.
If the corrosivity of the proposed construction site is not evident, it may be useful to conduct a local survey. Data from environmental monitoring stations is useful as it provides information on precipitation, humidity and temperature. Inspect protected exposed, uncleaned surfaces for particulate matter from industry, roads, and sea salt. The performance of nearby structures should be checked – if building materials such as galvanized fencing and galvanized or pre-painted cladding, roofs, gutters and flashings are in good condition after 10-15 years, the environment may be non-corrosive. If the structure becomes problematic after only a few years, it is wise to exercise caution.
Paint suppliers have the knowledge and experience to recommend paint systems for specific applications.
Recommendations for Metal Coated Panels The thickness of the metallic coating under paint has a significant impact on the service life of pre-painted panels in situ, especially in the case of galvanized panels. The thicker the metal coating, the lower the rate of undercut corrosion on cut edges, scratches or any other areas where the integrity of the paintwork is compromised.
Shear corrosion of metal coatings where cuts or damage to paint are present, and where zinc or zinc-based alloys are exposed. As the coating is consumed by the corrosive reactions, the paint loses its adhesion and flakes or flakes off the surface. The thicker the metal coating, the slower the undercutting speed and the slower the cross-cutting speed.
In the case of galvanizing, the importance of zinc coating thickness, especially for roofs, is one of the reasons why many galvanized sheet product manufacturers recommend ASTM A653 standard specifications for hot-dip galvanized (galvanized) or zinc-iron alloy steel sheet. dipping process (galvanized annealed), coating weight (i.e. mass) designation G90 (i.e. 0.90 oz/sqft) Z275 (i.e. 275 g/m2) suitable for most pre-painted galvanized applications sheets. For pre-coatings of 55% AlZn, the thickness problem becomes more difficult for a number of reasons. ASTM A792/A792M, Standard Specification for Steel Plate, 55% Hot Dip Aluminium-Zinc Alloy Coating Weight (i.e. Mass) Designation AZ50 (AZM150) is generally the recommended coating as it has been shown to be suitable for long term work.
One aspect to keep in mind is that roll coating operations generally cannot use metal-coated sheet that has been passivated with chromium-based chemicals. These chemicals can contaminate cleaners and pre-treatment solutions for painted lines, so non-passivated boards are most commonly used. 3
Due to its hard and brittle nature, Galvanized Treatment (GA) is not used in the production of pre-painted steel sheets. The bond between paint and this zinc-iron alloy coating is stronger than the bond between coating and steel. During molding or impacting, GA will crack and delaminate under the paint, causing both layers to peel off.
Paint System Considerations Obviously, one of the most important aspects in ensuring good performance is the paint used for the job. For example, in areas that receive a lot of sunlight and intense UV exposure, it is important to choose a fade-resistant finish, while in areas with high humidity, pre-treatment and finishing are designed to prevent moisture ingress. (Issues related to application-specific coating systems are many and complex and are beyond the scope of this article.)
The corrosion resistance of painted galvanized steel is greatly influenced by the chemical and physical stability of the interface between the zinc surface and the organic coating. Until recently, zinc plating used mixed oxide chemical treatments to provide interfacial bonding. These materials are increasingly being replaced by thicker and more corrosion resistant zinc phosphate coatings that are more resistant to corrosion under the film. Zinc phosphate is particularly effective in marine environments and in prolonged wet conditions.
ASTM A755/A755M, a document that provides a general overview of the coatings available for metal-coated steel sheet products, is called “Steel Sheet, Hot Dip Coated Metal” and pre-coated by coil coating for construction products subjected to the influence of the external environment.
Process considerations for coating pre-coated rolls One important variable that affects the life of a pre-coated product in situ is the fabrication of the pre-coated sheet. The coating process for pre-coated rolls can significantly affect performance. For example, good paint adhesion is important to prevent peeling or blistering of the paint in the field. Good adhesion requires well controlled roll coating handling techniques. The process of painting rolls affects the service life in the field. Issues covered:
Roll coating manufacturers producing pre-painted sheets for buildings have well-established quality systems that ensure these issues are properly controlled. 4
Profiling and panel design features The importance of panel design, especially the bending radius along the forming rib, is another important issue. As previously mentioned, zinc corrosion occurs where the paint film has been damaged. If the panel is designed with a small bend radius, there will always be cracks in the paintwork. These cracks are often small and often referred to as “microcracks”. However, the metal coating is exposed and there is a possibility of an increase in the corrosion rate along the bending radius of the rolled panel.
The possibility of microcracks in bends does not mean that deep sections are impossible – designers must provide for the largest possible bend radius to accommodate these sections.
In addition to the importance of panel and roll forming machine design, the operation of the roll forming machine also affects productivity in the field. For example, the location of the roller set affects the actual bend radius. If alignment is not done correctly, bends can create sharp kinks at profile bends instead of smooth smooth bend radii. These “tight” bends can lead to more severe microcracks. It is also important that the mating rollers do not scratch the paintwork, as this will reduce the ability of the paint to adapt to the bending operation. Cushioning is another related problem that needs to be identified during profiling. The usual way to allow springback is to “kink” the panel. This is necessary, but excessive bending during the profiling operation results in more microcracks. Similarly, building panel manufacturers’ quality control procedures are designed to address these issues.
A condition known as “oil cans” or “pockets” sometimes occurs when rolling pre-painted steel panels. Panel profiles with wide walls or flat sections (eg building profiles) are particularly susceptible. This situation creates an unacceptable wavy appearance when installing panels on roofs and walls. Oil cans can be caused by a variety of causes, including poor flatness of the incoming sheet, roller press operation and mounting methods, and can also be the result of buckling of the sheet during forming as compressive stresses are generated in the longitudinal direction of the sheet. panel . 5 This elastic buckling occurs because the steel has low or zero yield strength elongation (YPE), the stick-slip deformation that occurs when the steel is stretched.
During rolling, the sheet attempts to thin out in the thickness direction and shrink in the longitudinal direction in the web region. In low YPE steels, the undeformed area adjacent to the bend is protected from longitudinal shrinkage and is in compression. When the compressive stress exceeds the limiting elastic buckling stress, pocket waves occur in the wall region.
High YPE steels improve deformability because more stress is used for local thinning focused on bending, resulting in less stress transfer in the longitudinal direction. Thus, the phenomenon of discontinuous (local) fluidity is used. Therefore, pre-painted steel with YPE greater than 4% can be satisfactorily rolled into architectural profiles. Lower YPE materials can be rolled without oil tanks, depending on mill settings, steel thickness and panel profile.
The heaviness of the oil tank decreases as more struts are used to form the profile, steel thickness increases, bend radii increase and wall width decreases. If YPE is higher than 6%, gouges (ie significant localized deformation) may occur during rolling. Proper skin training during manufacture will control this. Steelmakers should be aware of this when supplying pre-painted panels for building panels so that the manufacturing process can be used to produce YPE within acceptable limits.
Storage and Handling Considerations Probably the most important issue with site storage is keeping the panels dry until they are installed in the building. If moisture is allowed to infiltrate between adjacent panels due to rain or condensation, and the panel surfaces are subsequently not allowed to dry quickly, some undesirable things can happen. Paint adhesion may deteriorate resulting in small air pockets between the paint and zinc coating before the panel is put into service. Needless to say, this behavior can accelerate the loss of paint adhesion in service.
Sometimes the presence of moisture between the panels on the construction site can lead to the formation of white rust on the panels (i.e. corrosion of the zinc coating). This is not only aesthetically undesirable, but can render the panel unusable.
Reams of paper in the workplace should be wrapped in paper if they cannot be stored inside. The paper must be applied in such a way that water does not accumulate in the bale. At a minimum, the package should be covered with a tarp. The bottom is left open so that water can drain freely; in addition, it ensures free airflow to the drying bundle in case of condensation. 6
Architectural Design Considerations Corrosion is strongly affected by wet weather. Therefore, one of the most important design rules is to ensure that all rainwater and snowmelt can drain away from the building. Water must not be allowed to accumulate and come into contact with buildings.
Slightly pitched roofs are the most susceptible to corrosion as they are exposed to high levels of UV radiation, acid rain, particulate matter and windblown chemicals – every effort must be made to avoid water accumulation in ceilings, ventilation, air conditioning equipment and walkways.
Waterlogging of the spillway edge depends on the slope of the roof: the higher the slope, the better the corrosive properties of the drip edge. In addition, dissimilar metals such as steel, aluminium, copper, and lead must be electrically isolated to prevent galvanic corrosion, and drain paths must be designed to prevent water from flowing from one material to another. Consider using a lighter color on your roof to reduce UV damage.
In addition, the life of the panel can be shortened in those areas of the building where there is a lot of snow on the roof and snow remains on the roof for a long period of time. If the building is designed so that the space under the roof slabs is warm, then the snow next to the slabs can melt all winter. This continued slow melting results in permanent water contact (i.e. prolonged wetting) of the painted panel.
As mentioned earlier, water will eventually seep through the paint film and corrosion will be severe, resulting in an unusually short roof life. If the inner roof is insulated and the underside of the shingles remains cold, snow in contact with the outer surface does not permanently melt, and paint blistering and zinc corrosion associated with long periods of moisture are avoided. Also remember that the thicker the paint system, the longer it will take before moisture penetrates the substrate.
Walls Vertical side walls are less weathered and less damaged than the rest of the building, except for protected surfaces. In addition, cladding located in protected areas such as wall reliefs and ledges is less exposed to sunlight and rain. In these places, corrosion is enhanced by the fact that pollutants are not washed away by rain and condensation, and also do not dry out due to the lack of direct sunlight. Particular attention should be paid to protected exposures in industrial or marine environments or close to major highways.
Horizontal sections of wall cladding must have a sufficient slope to prevent the accumulation of water and dirt – this is especially important for basement ebbs, since insufficient slope can cause corrosion of it and the cladding above it.
Like roofs, dissimilar metals such as steel, aluminium, copper and lead must be electrically insulated to prevent galvanic corrosion. Also, in areas with heavy snow accumulation, corrosion can be a side siding problem – if possible, the area near the building should be cleared of snow or good insulation should be installed to prevent permanent snowmelt on the building. panel surface.
Insulation should not get wet, and if it does, never allow it to come into contact with pre-painted panels – if the insulation does get wet, it will not dry quickly (if at all), leaving the panels exposed to prolonged exposure to moisture – - This condition will lead to accelerated failure . For example, when the insulation at the bottom of the side wall panel gets wet due to water ingress to the bottom, a design with the panels overlapping the bottom appears to be preferable rather than having the bottom of the panel installed directly on top of the bottom. Minimize the possibility of this problem occurring.
Pre-painted panels coated with a 55% aluminium-zinc alloy coating should not come into direct contact with wet concrete – the high alkalinity of the concrete can corrode the aluminum, causing the coating to peel off. 7 If the application involves the use of fasteners that penetrate the panel, they must be selected so that their service life matches that of the painted panel. Today there are some screws/fasteners with an organic coating on the head for corrosion resistance and these are available in a variety of colors to match the roof/wall cladding.
INSTALLATION CONSIDERATIONS The two most important issues associated with field installation, especially when it comes to a roof, can be the way the panels move across the roof and the influence of workers’ shoes and tools. If burrs form on the edges of the panels during cutting, the paint film may scratch the zinc coating as the panels slide against each other. As mentioned earlier, wherever the integrity of the paint is compromised, the metal coating will begin to corrode faster, which negatively affects the life of the pre-painted panel. Similarly, workers’ shoes can cause similar scratches. It is important that shoes or boots do not allow small stones or steel drills to enter the sole.
Small holes and/or notches (“chips”) are often formed during assembly, fastening and finishing – remember, these contain steel. After work is completed, or even before, the steel can corrode and leave a nasty rust stain, especially if the paint color is lighter. In many cases, this discoloration is considered to be the actual premature degradation of the pre-painted panels, and apart from aesthetic considerations, building owners need to be sure that the building will not fail prematurely. All shavings from the roof must be removed immediately.
If the installation includes a low pitched roof, water may accumulate. Although the slope design may be sufficient to allow free drainage, there may be local problems causing standing water. Small dents left by workers, such as from walking or placing tools, can leave areas that cannot drain freely. If free drainage is not allowed, standing water can cause the paint to blister, which can then cause the paint to peel off in large areas, which can then lead to more severe corrosion of the metal underneath the paint. Settling of the building after erection can lead to improper drainage of the roof.
Maintenance considerations Simple maintenance of painted panels on buildings includes occasional rinsing with water. For installations where the panels are exposed to rain (e.g. roofs), this is usually not necessary. However, in protected exposed areas such as soffits and wall areas under eaves, cleaning every six months is helpful in removing corrosive salts and debris from panel surfaces.
It is recommended that any cleaning be done by first “trial cleaning” of a small area of the surface in a place that is not too open in order to obtain certain satisfactory results.
Also, when using on a roof, it is important to remove loose debris such as leaves, dirt, or construction runoff (i.e. dust or other debris around roof vents). Although these residues do not contain harsh chemicals, they will prevent the rapid drying that is critical for a long-lasting roof.
Also, do not use metal shovels to remove snow from roofs. This can lead to severe scratches on the paint.
Pre-painted metal-coated steel panels for buildings are designed for years of trouble-free service. However, over time, the appearance of all layers of paint will change, possibly to the point where repainting is required. 8
Conclusion Pre-painted galvanized steel sheets have been successfully used for building cladding (roofs and walls) in various climates for decades. Long and trouble-free operation can be achieved through the right choice of paint system, careful design of the structure and regular maintenance.
Post time: Jun-05-2023