Are current ASTM galvanizing standards suitable for use with the new chemicals being used in pressure treated wood? Some people have told me I need to use double or triple dipped fasteners. Is this true?
Since the new chemicals (alkaline copper quat [ACQ], copper azole [CBA-A and CA-B], and sodium borates [SBX]) now being used in pressure treated wood have been shown to be up to two or three times more corrosive than the previously used chromate copper arsenate (CCA), the recommended galvanizing thickness for materials being used with the new pressure treated wood was increased from about 0.75 to 1.56 mils thick. However, ASTM specifications A153 and F2329 say that any individual specimen above 3/8″ diameter must have an average galvanizing thickness of 1.7 mils. This means any material galvanized to either A153 or F2329 will be more than adequate for use with pressure treated timbers.
The idea that double or even triple dipped galvanized fasteners are required for corrosive environments is a frequent misconception. Fasteners with external threads cannot be double or triple dipped galvanized. This is because it would lead to excessive build up of zinc on the threads and not allow the nuts to screw on to the bolt. Galvanized nuts and washers are already tapped oversize to account for the thickness of the zinc coating, and would have to be custom made to accommodate a double or triple dipped fastener.