Background: High performance fluoropolymer (Kynar) coatings have enjoyed unparalleled success as decorative and protective coatings for aluminum building products including balcony railings, windows and sliding glass doors for over five decades. Introduced into the architectural community in the mid 1970’s, this new generation of coatings offered the combination of exceptional durability and a wide spectrum of UV stable colors. Initially these were targeted to prestigious monumental projects around the world where color afforded new aesthetic design opportunities. By contrast, the new fluoropolymer coatings – typically polyvinylidene fluoride (PVDF), were resistant to alkali attack and available in a full palette of hues from earth tones to bright blues, greens, reds and yellows.
Throughout the 1980’s and 90’s PVDF resin based coatings continued to gain popularity. These were being specified for residential and commercial aluminum building product applications. These coatings were marketed towards the exclusive resorts, hotels and condominiums located along the coasts, where long term UV stability and wear resistance and durability was, and continues to be a primary concern. It is in these coastal environments (generally considered to be within 1000 yards of the high tide line) which is where the harshest conditions for organic coatings are found and through the combined effects of:
- Salt Laden Air
- High Humidity
- High Ultraviolet (UV) Exposure
- Wind Driven Abrasive Sand
These aluminum products when properly pretreated, primed and coated, with PVDF have an excellent performance long term history for functional use in these coastal environments. The evidence of their performance is in the many shoreline projects constructed during this period that to this day still remains unblemished and not in need of repair or replacement.
For the maximum durability in these hostile environments, where decorative protective coatings take the brunt of the environmental impact, a third clear top coat of PVDF provides maximum protection. By adding this third clear top coating, parts coated can be covered by a 20 or 25 year warranty for film integrity, resistance to chalking, color and gloss retention by the coating manufacturer.
The introduction of the PVDF resin based fluoropolymers embraced the emerging “wet on wet on wet” coating technology which enabled the 3 coatings ( the corrosion inhibiting primer + fluoropolymer color coat + PVDF clear coat) to be applied consecutively in a single production cycle on our automated overhead line. The molecular composition of the PVDF clear coat provides a seal that precludes UV rays, high velocity sand impact, chemical cleaning from eroding the color or performance of the coating. Contractors have reported that this surface lubricity has protected the finish during the installation stage of a project.
During the last 50 years several million feet of PVDF coated railings, canopies, awnings, and wind/sun screens were installed along the Eastern seaboard, from the Carolinas through the Florida peninsula and around the Gulf states without failure. These regions boast the most punishing shoreline environment for salt water corrosion.
The original development and testing of the liquid PVDF coating with a corrosion inhibiting primer was applied over the amorphous chromium phosphate conversion coating. “On-fence” exposure test panels with this combination have been under continuing evaluation in South Florida testing stations for over 70 years and still exhibit excellent coating adhesion. Amorphous chromium phosphate was the mandated pretreatment in the American Architectural Manufacturers Association (AAMA) high end specifications from 1979 – (AAMA 605.2) “Voluntary Specification for High Performance Organic Coatings on Architectural Extrusions and Panels” through 2005.
Chromium Phosphate Conversion Coating: The chromium phosphate conversion coating is recognized as the superior metal pretreatment for use in severe coastal and industrial environments. However, it is associated with demanding process controls in production. Chromic acid, used in the chemical conversion of the aluminum surface is subject to very strict health and industrial hygiene controls. equipment to ensure compliance as well as the implementation of strict monitoring and record keeping of employee exposure levels. The net effect of the revised standard for metal pretreatment described in AAMA 2605-05 and the new OSHA regulations initiated the search for less demanding alternatives to the time tested and proven chromium phosphate conversion coating. With the increase in the focus on chrome containing materials by the Federal Government, the costs for companies to operate with chromium pretreatment processes has increased substantially over the last 15 years. Most companies that had this pretreatment capability have discontinued the use to avoid paying the costs to control emissions and have moved to lower performing pretreatment technologies.
Coating Failure: It is important to note that the most common failures of organic coatings in coastal environment are the result of galvanic and filiform corrosion. The aluminum substrate becomes anodic and corrodes. The corrosion produced undermines the coating’s adhesion and delamination occurs. To minimize this reaction with aluminum balcony railings the concrete deck is core drilled at post locations. The anchoring posts should be set in insulating epoxy cement. If it is necessary to floor mount the railings directly to the concrete pad it is imperative the aluminum foot plate be separated from the stainless steel fasteners with nylon or other insulating sleeves or washers.
What is Filiform Corrosion?: The porosity of organic coatings varies considerably with fluoropolymers being the least permeable by a significant factor. Filiform corrosion is initiated when the organic coating ruptures or when moist salt laden air penetrates the pores of the coating down to the aluminum substrate. Without an appropriate pretreatment (especially with a coating system that does not employ a corrosion inhibiting primer) catastrophic coating delamination can occur.