AS 3566 Class 3 & Class 4 Mechanical Galvanized Screws

AS 3566 is the Australian standard that outlines the performance requirements for corrosion-resistant fasteners especially used in roofing, wall cladding, and framing applications. It ensures that screws and fasteners can withstand long-term exposure to environmental factors such as moisture, salt spray, and acidic industrial pollutants.

AS 3566 categorizes fasteners into different corrosion resistance classes, based on the severity of the intended environment. Among these, Class 3 and Class 4 coatings are the most widely used for exterior applications, as they offer enhanced protection for screws used in outdoor structures exposed to varying levels of environmental stress.

Class 3 – Medium Corrosion Resistance

• Suitable for moderate environments, including areas not directly exposed to harsh weather or industrial pollution.

• Commonly used in inland areas, sheltered roofing, and light steel framing applications.

• Offers excellent performance in non-coastal, non-industrial environments.

Technical Requirements for Class 3 Galvanizing

In Australia, most Class 3 screws use a mechanically plated zinc-tin coating to meet the corrosion resistance requirements outlined in AS 3566.2:2002.

According to the standard:

• The minimum coating thickness must be 25 μm.

• The coating composition must contain 20–30% tin, with the balance being zinc.

• After coating, the fasteners must undergo a passivation process in accordance with AS 1791, to enhance corrosion resistance and surface stability.

This coating system provides a reliable level of protection suitable for moderately corrosive environments, such as inland or sheltered urban areas.

Click here for more specifications and testing criteria under AS 3566.2

Class 4 – High Corrosion Resistance

• Designed for severe environments, including coastal areas, regions with high humidity, and locations exposed to industrial pollution.

• Ideal for use in marine-influenced zones, unprotected roofing, facades, and structural applications where maximum corrosion protection is critical.

• Delivers long-term durability and reliable performance in harsh outdoor and aggressive environmental conditions.

Technical Requirements for Class 4 Galvanizing

In Australia, most Class 4 screws also use a mechanically plated zinc-tin coating to meet the corrosion resistance requirements outlined in AS 3566.2:2002.

According to the standard:

• The minimum coating thickness must be 40 μm.(while Class 3 is 25μm)

• The coating composition must contain 25–30% tin, with the balance being zinc.

• After coating, the fasteners must undergo a passivation process in accordance with AS 1791, to enhance corrosion resistance and surface stability.

This coating system provides the highest level of corrosion protection as defined by the AS 3566 standard, making it suitable for highly corrosive environments such as coastal regions, areas with high humidity, or locations exposed to industrial emissions. It ensures long-term durability and structural integrity even under aggressive environmental conditions.

Click here for more specifications and testing criteria under AS 3566.2