Proper cleaning and pretreatment of metals are critical components of the high performance paint finish used for metal roofing panels. To obtain proper long term performance of coatings used for metal roof panels, metal must be cleaned of any contaminants associated with the steel or aluminum processing/ manufacturing. Pretreatments then convert the metal surface into a non-metallic coating to obtain proper adhesion by a chemical/physical bond, increasing the durability of both the base metal and paint finish by providing an additional barrier layer and protecting areas where damage to the paint may occur during the life of the roof.
Many different types of pretreatment systems are currently available. The performance of each individual pretreatment may vary depending upon the metals that are being treated, the overlying paint system and the geographical area of the country where the metal roof panels are being installed. Most locations are identified as rural, industrial, marine or severe marine environments.
The most commonly used pretreatments for metal roof panels are as follows:
Complex oxides – used on hot dip galvanized steels
Complex oxides provide excellent flexibility in the forming operation of steel panels. Typically they do not provide the required additional barrier protection and steel surface conversion in environments subject to acid rain, industrial chemicals and fumes or marine and severe coastal locations. Their use is acceptable in areas where these contaminants are not present in high degree.
Zinc phosphates – used on hot dip galvanized steels
Zinc phosphates typically provide increased corrosion resistance of galvanized steels by the surface conversion during the pretreatment process. They have been used commercially for over 100 years, but have undergone evolutionary improvements, including the use of additives and refinement of the process which have increased the durability of steel roofing panels that have difficult forming requirements. This pretreatment is commonly recommended when the galvanized steel roof panel is subject to aggressive environments such as areas subject to acid rain, industrial chemicals and fumes or marine and severe coastal locations.
Complex chrome oxides – used on aluminum/zinc alloy coated steels and aluminum
Complex chrome oxides have been used extensively on both aluminum/zinc alloy coated steels and aluminum. They are available as conversion coatings as well as dry-in-place coatings. Complex chrome oxides provide excellent formability as well as field-proven corrosion resistance to the base metals.
New emerging technologies are forthcoming in the metal pretreatment industry. Some, such as chrome-free, are already commercially available. The new technologies are being developed to improve or maintain the performance of pretreatments while offering cleaner methods and therefore reducing the environmental impact of pretreatments. The existing technology combined with these newer emerging systems only add to the green aspects of metal roofing and their long life performance as compared to other roofing materials.
Due to the varying types of base metals, pretreatments and paint systems used, the building owner should consult with their metal roof supplier to insure all aspects of the coating system are considered prior to the selection of materials.
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