Rust or “tea stains” are common for stainless steel deck hardware installations; they are usually easily avoidable.
Rust stains running across a deck or hull are nothing new. While unsightly, they are ignored or begrudgingly accepted as part of modern boat ownership in many cases. However, in some cases, that rust can be a message, and it’s not a very good one.Â
If the staining occurs adjacent to stainless steel hardware, then the “rust” is often the byproduct of a process known as crevice corrosion, whereby normally passive, corrosion-resistant stainless steel becomes active (see SDMC Marine Systems Excellence Miracle Metal). “Active” for stainless steel is analogous to rusting for ordinary mild steel. In both cases, the metal is compromised, although, in the case of stainless steel, it’s much more insidious as the flaking and shale associated with conventional steel is not present. Thus, whether it’s for cosmetic or structural reasons, preventing this staining is well worth the effort.
PREVENT STAINS
To prevent staining, sometimes referred to as “tea staining”, of this sort, it’s essential to understand the process by which it occurs. For stainless steel to remain rust-free, it must be continuously exposed to a source of oxygen. In doing so, stainless steel can replenish and maintain the tough oxide layer that allows it to resist rusting. Stainless steel used on the deck is exposed to air, providing ample oxygen to maintain the oxide layer. Even stainless steel used below the waterline, provided the water moves freely and regularly, may be exposed to enough oxygen to remain passive. However, if the stainless steel is exposed to an oxygen-poor environment, stagnant water, for instance, it’s likely to enter an active state.
STAINLESS STEEL RUST
Where deck and above the waterline hull fittings are concerned, brown staining is usually a result of low corrosion resistance, i.e. 304 or 18-8 stainless alloy and/or exposure to stagnant water. This alloy, by the way, is what’s used for 99% of all common fasteners, bolts, screws and nuts. In the first case, short of replacing the hardware, there’s not much that can be done.Â
In the latter case, however, there is a solution. If you look closely, what’s often clear about the staining is that it emanates from the perimeter of, or under, the offending hardware.
This is because water becomes trapped beneath the hardware, where it becomes stagnant and thus oxygen-depleted, where it then leads to “activation” of the stainless steel; it begins to corrode. This phenomenon extends to fasteners as well; if the holes through which they pass in decks, cabins and hulls, are filled with stagnant water, they will become active.
The solution for this problem involves removal, thorough cleaning, de-waxing, and fully bedding the hardware. That is, instead of applying bedding compound only to the areas where fasteners pass through, the entire base of the hardware, be it a cleat, portlight trim ring, grab rail flange, chock or any other hardware, should be liberally bedded with sealant that will both create a watertight seal and exclude water that could migrate beneath the hardware base. Exclusion of the water is the key to preventing staining.
Stainless Steel Rust
In addition to bedding, polishing otherwise rough or irregular stainless steel will minimise surface staining (polishing removes micro-pits where corrosion gets its start).Â
Avoiding water traps in recesses where (often undersized) fasteners are installed will also aid in the prevention of crevice corrosion and staining. Fastener heads should fill the hole and countersink in any hardware, always use the largest screw that will fit, and it should be fully bedded. Even then, fastener recesses can form water traps which lead to staining. Using 316 stainless alloy fasteners and polishing fastener heads will reduce the incidence of staining.
Article courtesy of boattest.com
