Breathable covers support efforts to minimize corrosion by removing moisture from beneath the protective cover. Breathable covers should prevent water from getting in while allowing water vapor to escape. These two characteristics, waterproofing and moisture permeability, are mutually contradictory. So how does that work?
First, let's clarify one thing: Breathability should not be confused with wicking. We've all heard of wick-away apparel for athletes. The fabric used in these clothes works by moving water (sweat) away from the surface being protected (the body). Breathable covers do not work this way.
The fabric in a breathable cover works differently. The water, in vapor form, is allowed to release either through inter-yarn spaces or through the individual fibers. Tightly woven fabrics made from natural hydrophilic (water absorbing) fibers are more efficient in transmitting water vapor than fabrics made from hydrophobic (water adverse) fibers of a similar construction. For over 60 years it has been known that water vapor permeability decreases as the fabric thickness increases . Once the water is absorbed into the fiber, the importance of breathability begins.
The mechanism switches from a capillary action (wicking action) to gas diffusion where gas molecules flow into areas of lower concentation and flow out through tiny openings. The charge along the molecule varies, and weak bonds between water vapor molecules and the membrane may form. This bond may be broken, allowing another water vapor molecule to takes its place. This process continues, and water vapor molecules migrate from the side with higher relative humidity (inside the cover) to the side with lower relative humidity (outside the cover). Fabric used in breathable covers should be woven tight enough to prevent water penetration, but loose enough to allow a high transmission rate of water vapor. These fabrics should be protected with durable, breathable coatings, to enhance the life span of the fabric.
Covers that employ breathable fabric allow moisture, a key cause of corrosion, to migrate away from the item being protected.