Choosing the Right Type of Stainless Steel Screw
When using stainless steel sheet for jacketing and cladding purposes, it is essential to make sure that the quality of the screws used to secure it is just as high as the jacketing material itself. On the face of it, it would seem to make sense to use a screw of grade 304 metal with sheet of the same grade but this isn't actually the right solution as both materials are of equal strength.
Standard stainless steel self-drilling screws do not work correctly in steel sheet, whether it is stainless, galvanised or aluzinc, as they are simply not hard enough. The traditional solution is to use grade 410 screws but, though these have the requisite strength, they are vulnerable to corrosion. Bi-metallic screws are a common option: these are usually supplied as 304 grade with a carbon steel tip and silver coat. Again, corrosion is an issue here.
Grade 316 self-drilling screws have fallen from favour and are now rarely used as they will not work without pre-drilling the sheet, defeating the object. Moisture around and under the head of 304 screws make them subject to galvanic corrosion. By far the most successful solution is to use duplex screws which have been around for a while as self-tapping screws but are newly available in a self-drilling format.
Duplex screws, though with a higher initial price than other types, provide massive savings in the longer term. They are three times as hard as grade 304/316 stainless steel screws an have a high PRE value making them strongly resistant to the corrosive effects of seawater.
The duplex screws are normally supplied with a bonded metal and HNBR washers . The HNBR offers a wider temperature range than traditional EPDM or Neoprene washer they are also more resistant to chemicals and ozone. The result is an excellent waterproof seal to keep out moisture and so help and improve the prevention of CUI.
These are now successfully used by Owens Corning - Foamglas for their Pool Fire Suppression System Generation 2, used for LNG storage tanks in coastal locations such as Freeport LNG. Many of the offshore companies in Europe, have now moved to duplex screws due to the life cycle cost savings when compared to costly failures resulting in replacement cladding.