Coated carbon steel fasteners perform reliably in most metal roofing projects. But the corrosive environments found in coastal zones and agricultural and industrial buildings can damage coatings and accelerate fastener failure.
Salt spray, ammonia, and industrial fumes attack fastener coatings. Once the barrier is breached, corrosion can spread beneath the surface, leading to red rust, washer seal failure, and reduced holding power.
That’s why stainless steel roofing fasteners are recommended for corrosive environments. Stainless steel is more resistant to corrosive chemicals. It can also “self-heal” so that damage to the fastener surface doesn’t result in accelerated corrosion.
In this article, we’ll explain how each type of fastener protection works, where coatings reach their limits, and what you should use instead.
How Coatings Protect Roofing Fasteners
Coated carbon steel fasteners rely on a layer of corrosion-resistant material around the steel to keep moisture and oxygen from reaching it. The thicker and more durable the barrier, the longer the fastener lasts.
For example, Atlas carbon steel fasteners are mechanically zinc plated across the shank, threads, and head as the base layer of protection. On top of the plating, customers can specify long-life coatings and color-matched paint. Each layer adds time to the fastener’s working life.
But the barrier only does its job for as long as it stays intact, and that is harder to guarantee than it sounds. Driver bits can grind the head during installation. UV can break coatings down over years of exposure, and environments containing corrosive chemicals can attack and degrade the coating. Once the protective layer is breached, bare steel is exposed, and corrosion spreads sideways underneath the surrounding coating, damaging threads and weakening the connection.
How Stainless Steel Resists Corrosion
Stainless steel resists corrosion through metallurgy rather than a mechanical barrier. Chromium is added to the steel, and it reacts with oxygen to form a thin chromium oxide passive layer. Unlike zinc coatings and paint, the chromium oxide layer reforms when a roofing screw is scratched or damaged. Some stainless steel fasteners may include a surface treatment to reduce friction or improve installation performance, but their corrosion resistance comes from the stainless steel itself.
The two stainless grades Atlas uses in roofing fasteners serve different roles:
- 304 stainless is a chromium-nickel alloy. In addition to steel, it contains 18% chromium with around 8% nickel. That composition gives 304 strong corrosion resistance against ammonia, organic acids, and other industrial chemicals. It outperforms coated carbon steel in environments where coatings degrade quickly, including agricultural buildings and coastal applications with occasional saltwater exposure.
- 410 stainless is a straight-chromium alloy at around 12% chromium with no added nickel. The lower chromium content gives it less corrosion resistance than 304, but it can be heat-treated to a hardness suitable for self-drilling roofing fasteners. However, reduced corrosion resistance makes 410 stainless steel unsuitable for use in coastal environments with saltwater exposure.
Choosing between the two types of stainless steel fasteners is a question of priorities. When corrosion resistance is a priority, 304 is the right call. However, when chemical exposure is moderate, 410 delivers a step up in corrosion resistance over coated carbon steel, with the added benefit of improved hardness.
Galvanic Corrosion: Matching the Fastener to the Panel
Stainless steel is not the end of the corrosion conversation. You should also consider the risk of galvanic corrosion, which occurs when two dissimilar metals are in contact, and a film of moisture bridges them.
The moisture acts as an electrolyte, and the metal pair behaves like a small battery, with the less noble metal corroding faster than it would on its own. Stainless sits near the noble end of the galvanic series; therefore, a stainless fastener paired with a less noble panel can drive corrosion of the panel around the screw, especially in coastal or wet environments.
In general, there are three rules you should follow to limit the risk of galvanic corrosion.
- Match fastener metal to panel metal where possible.
- Avoid bare stainless against bare galvanized steel.
- Use a non-conductive washer.
Choosing the Right Fastener for the Exposure
When panels are rated for decades, the fasteners need to last just as long. Atlas’ Long-Life Fastener line is built for environments where coatings cannot keep up. Explore Metal Ultimate®, 304 stainless fasteners, and 410 stainless fasteners to spec the right protection for your next project.
