Q. A few years ago, I used Type 304 stainless steel trim screws to face-fasten composite decking to pressure treated framing for a new back porch. My clients reported that the screws had rusted after one season, during which deicing salts were used on the deck (we work in the upper Midwest). We replaced the fasteners with Type 316 stainless-steel screws from the same manufacturer, and the same thing happened the next season. Is this typical for stainless steel, or is something amiss with the manufacturer’s version of “stainless steel”?
A. Foster Lyons, an engineer and building-science consultant, responds: If the fasteners are truly rusty, they aren’t stainless steel. If they are just stained (dark), or streaking a dark stain, that’s to be expected in the presence of salt. My guess is that is what you are seeing—stain, not rust.
Here’s the thing with stainless steel: It’s stain less, not never-stain steel. The 304 and 316 alloys of steel are known to stain in the presence of chloride salts, whether from exposure to salty air at the beach or from salty deicers. This is a common irritation for homeowners who build on the oceanfront, who believe they are buying a material (for railings, for example, as well as for fasteners) that will never stain and then are surprised when it becomes blotchy and stained after the first storm.
While 317LMN or 904L alloys won’t stain as much, I doubt that you’ll have much luck finding a fastener manufacturer that makes screws using these higher-priced alloys. A better option is to educate your clients about the difference between stain and rust and suggest the use of a chloride-free ice melt. You could also recommend replacing the face-mounted screws—which create a small recess that allows salty snow melt to puddle around the fastener heads—with plugged screws. The fastener heads might still stain, but they won’t corrode, and they won’t be visible underneath the plugs.
