Choosing the Right Fasteners for Rooftop PV Mounting: Addressing Corrosion and Longevity.
Yellow zinc coated screws may come at a better price than stainless-steel screws, but they do not offer much in terms of . . .
What is Galvanic and Atmospheric corrosion?
Galvanic corrosion
Galvanic corrosion, sometimes referred to as bimetallic corrosion, is a process whereby dissimilar metals (one acting as an anode and the other as a cathode) causes corrosion when placed in the presence of an electrolyte.
This phenomenon becomes evident when the screws, used to fasten mounting brackets to steel roof cladding, are exposed to moisture or saltwater spray.
Atmospheric corrosion
What is a Class 1 - Class 5 environment?
Class 1 will be considered a mild environment such as office buildings or residential interiors.
Class 2 will be considered Low-moderate environments such as warehouses or manufacturing facilities with controlled humidity.
Class 3 environments will typically be considered moderate and can be found in urban areas or light industrial facilities.
Class 4 environments are considered severe and are commonly found in coastal regions and chemical processing plants.
Class 5 is an extremely harsh environment considered by engineers as worst-case scenario, these will typically be offshore platforms or industrial sites with aggressive chemical use.
How are the screw coatings tested for classification?
Many different testing methodologies may exist, but the common test to preform to verify the category rating of a fastener / screw will be a Salt Spray test (ASTM B117, ISO 9227), whereby the fastener will be exposed to an intermittent or continuous salt spray to simulate corrosive conditions. The duration of exposure and the appearance of corrosion are observed and used to assess the coating’s performance.
Stainless-steel vs. yellow zinc screws: Advantages and disadvantages.
Yellow zinc coated screws
Yellow zinc coated screws may come at a better price than stainless-steel screws, but they do not offer much in terms of atmospheric corrosion resistance. With yellow zinc coated screws typically having a coating of 5 – 25µm, they only offer a mild resistance. Better suited to class 1 or class 2 environments, these screws may not survive extended periods of outdoor use.
Stainless-steel screws
Stainless-steel screws, being made from a noble metal, boast an impressive resistance to atmospheric corrosion, but opposed to popular belief, these screws may cause galvanic corrosion when inserted into zinc coated (galvanised) steel sheeting. The galvanised sheeting, being less noble, may corrode when placed into direct contact with stainless-steel in the presence of an electrolyte.
Coil and sheet suppliers: What do they say?
Acelormittalsa, a dominant supplier of galvanised or pre-pained coil, does not allow the use of Stainless-steel fasteners / screws as can be seen on the table below found on Datasheet C1.1 (arcelormittalsa.com) and derived from SANS 10237.
In addition to the above, Global Roofing Solutions (GRS), a widely known roof cladding supplier, also does not allow for the use of stainless-steel screws as can be seen on GRS installation manual 01/21. (www.globalroofs.co.za)
Furthermore, Safintra, a well-known and respected supplier of roof cladding also disallows the use of Stainless-steel fasteners on their sheeting as can be seen on the Safintra product guide (www.safintra.co.za)
“Only Class 3 (concealed fix) and Class 4 fasteners are suitable for use with Aluminium-Zinc coated steel (unpainted or pre-painted).”
Conclusion
Given the above-mentioned information, Lumax Energy does not support the use of Yellow Zinc coated or stainless-steel screws on galvanised steel roof sheeting.
Lumax Energy supplies class 4 coated self-piercing screws: FS-S-SP-25X5.5-C4 as a standard product offering.
