How salt air corrodes chimney metal and masonry

How salt reaches a chimney

Wind off the ocean carries fine droplets of sea spray inland, and when they dry, they leave salt behind on every surface they touch. A chimney stands above the roofline in the moving air, so its cap, its chase cover, its flashing and its masonry collect that salt. Moisture from rain, dew or damp air then dissolves the salt again, and the cycle of wetting and drying keeps a film of chloride-laden water on the surface for long stretches.

Exposure is heaviest close to the surf and falls off with distance and shelter, and it varies with wind, height and what stands between the house and the water. The town pages on this site say whether a place is on the coast, within about six miles of it, or farther inland, based on Census shoreline data. That is a useful first filter, but it is not a corrosion rating for any one house. What the metal on your chimney looks like after a few years tells you more.

Why galvanized parts fail first

Galvanized steel is ordinary steel with a coating of zinc. The zinc corrodes slowly and sacrificially, protecting the steel as long as the coating lasts. In clean air, the zinc forms a stable film of corrosion products that slows it further. The American Galvanizers Association explains what changes in marine air: chlorides react with those protective products to form soluble zinc chlorides, which wash away and expose fresh zinc to attack, so the coating is used up faster.

Once the zinc is gone, the steel rusts. On a chimney, that shows as rust streaks down the siding below a chase cover, rust at the fasteners of a cap, and pinholes or seams opening in a flat chase top where water sits. A chase cover that sags or has no slope holds water and salt in the same spot, which speeds the process. Paint on a galvanized top can slow surface rust for a while, but it does not bring back the zinc, and a top that has rusted through needs replacing.

Stainless steel is not one material

Stainless steel resists corrosion because of a thin, self-repairing film on its surface, but chlorides can break that film down in small spots and start pits. The grades used on chimney parts differ in how well they hold up. Type 304 is widely used for caps and chimney parts. Type 316 adds molybdenum, which gives it better resistance to pitting and crevice corrosion in chloride environments, which is why it is known as a marine grade.

In salt air, 304 can show rust-colored staining and small pits over time, especially on surfaces that rain does not wash. Whether a coastal job calls for 316 or another option depends on the maker's guidance for the part and the exposure, and not every part is made in every grade. The fasteners and any welds need the same resistance as the sheet, since a cap or chase cover is only as corrosion-resistant as its screws. We name the grade on the estimate, so you know what you are getting.

Aluminum and copper

Aluminum forms its own oxide film and holds up well to general corrosion, and it is made into chase covers and flashing, but chlorides can pit it, and it is sensitive to contact with other metals. Copper weathers to a stable patina and is durable in marine air, and it is used for caps and flashing where the maker offers it and the budget allows.

Neither is right for every detail. Aluminum is not used where the listing for a chimney component calls for steel, and copper is not paired with galvanized steel or aluminum without separation. The choice follows the part's listing, the roof it meets and the maker's installation instructions.

When two metals meet in salt water

Salt water is an electrolyte, and when two different metals touch in its presence, one of them corrodes faster than it would by itself. This galvanic corrosion is behind details that fail near the coast even when each metal is a good choice by itself. Copper flashing held with galvanized nails, a steel screw through an aluminum chase top, or a copper cap resting on a galvanized storm collar are examples of pairs that can eat at the less noble metal.

Matching fasteners to the metal they hold, or separating the metals with a barrier the maker approves, prevents it. When we replace a cap, a chase cover or a run of flashing, the fasteners come from the same family as the part, and the estimate records it.

Salt in brick and mortar

Masonry is porous, and salt carried in with water collects inside it. As the masonry dries, the salts crystallize in the pores near the surface, and that crystal growth can break down soft brick faces and weak mortar over repeated cycles. The white deposits known as efflorescence are the visible part of salts moving to the surface with water, and they point to where water is getting in.

Steel embedded in masonry is also at risk. Lintels over openings, angle irons and ties corrode when salty water reaches them, and rust takes up more space than the steel it came from, so it can crack the brick around it. The defense is keeping water out of the chimney: a sound crown with a drip edge, a cap, flashing that sheds water, full mortar joints, and, once repairs are done, a vapor-permeable water repellent that reduces how much water the brick face takes in while still letting it dry.

A coastal chimney maintenance routine

Near the coast, the yearly NFPA 211 inspection is where corrosion gets caught early. Between visits, a few checks from the yard help.

• Look for rust streaks on siding or brick below the chase cover or cap
• Check that the cap mesh is intact and the cap sits square on the flue
• Look at the chase top for sagging, standing water or lifted seams
• Watch for new white staining on the brick, which marks water moving through it
• Rinse stainless parts with fresh water where the maker's care instructions recommend it
• After a strong storm during the Atlantic hurricane season, which runs June 1 to November 30 per the National Hurricane Center, look again at the cap and chase top