Choosing chimney metals that can live with salt spray

Where salt collects on a chimney top

Sea salt reaches a roof as fine spray and dust, and it does not need a storm to arrive. The International Molybdenum Association (IMOA) explains in its stainless steel selection guidance that salt deposits absorb moisture when humidity is high and turn into a concentrated, corrosive solution on the metal. It also notes that local wind patterns decide how far sea salt is carried inland, so distance from the water on its own does not settle whether a particular house is exposed.

On a chimney, salt gathers where rain cannot rinse it away. IMOA points to sheltered and horizontal surfaces, which is a fair description of the underside of a cap hood, the flat top of a chase cover, the seams where a screen meets its frame and the heads of the screws that hold everything down. Those are the places to look first when you are judging how a coastal chimney is doing.

Why galvanized caps and covers give out

Galvanized steel is steel with a zinc coating, and the zinc is there to be used up. The American Galvanizers Association (AGA) explains that in marine air, chlorides from sea spray react with the protective film that normally forms on zinc, turning it into soluble zinc chlorides that rain washes off. Fresh zinc is then exposed, and the cycle repeats. The same source ranks tropical marine air close to industrial air among the most corrosive settings for zinc.

Sheet metal parts such as chase covers and many caps are made from continuously galvanized sheet. The AGA notes that the ASTM A653 coating designations for that sheet, such as G60 and G90, are significantly thinner than coatings applied to finished steel in a galvanizing kettle, and it ties service life directly to coating thickness. Near salt water, that thin coat is spent sooner, and once it is gone the steel beneath rusts, streaks and eventually perforates.

Building code minimums are not a coastal specification. Section R1003.9.2 of Florida's 2023 residential code governs a spark arrestor only when one is fitted to a masonry chimney, and for its screen it accepts 19-gage galvanized steel or 24-gage stainless steel as equal choices on heat and corrosion. That minimum is the same anywhere in the state, and it does not tell you which of the two will last longer in a spot where salt settles every week.

Stainless steel is a family of grades

A cap sold as stainless can be one of several alloys. IMOA's guidance treats Type 304 as suited to low corrosion risk and points to Type 316 or 444, with a suitable finish, for most corrosive applications. The difference is molybdenum, which is added to every grade in its comparison except 304 to improve resistance to salt, or chloride, corrosion.

Finish and shape matter too. IMOA explains that a smooth surface finish makes it harder for deposits to cling and easier for heavy rain to wash them off, while rough finishes collect contaminants. It also warns that woven wire mesh has tiny crevices where the wires cross, and corrosion can start in each one when salt is present. Welded mesh avoids those crevices. For a salt air chimney cap, ask for the grade of the hood, the grade and construction of the screen and the grade of the fasteners, because one cap can combine more than one metal.

Copper, and the mark it can leave

Copper caps and chase covers are offered alongside stainless ones by fabricators such as Rockford Chimney Supply, and many owners choose copper for its look. Copper brings its own considerations. The Brick Industry Association's guide to identifying stains lists copper among the materials above brickwork that can stain the brick below it, so the runoff path deserves a thought before copper goes over light masonry or stucco.

Copper also changes what can touch it. In the AGA's galvanic series, zinc sits toward the active end, well away from copper and stainless steel, which means galvanized steel is the metal that corrodes when the two meet in a wet, salty setting. A copper cap sitting on a galvanized chase top, or held down with zinc-plated screws, pairs two metals that work against each other.

Fasteners, flashing and where two metals meet

Galvanic corrosion needs an electrolyte, and the AGA lists ordinary rain, dew, high humidity and sea spray among them. It adds that a large cathode surface in contact with a small anode surface is an undesirable pairing, and its own example is to avoid galvanized fasteners joining copper or stainless steel. On a chimney, that describes a small galvanized or zinc-plated screw driven through a large stainless or copper part. The screw is the part that is attacked, and a failed screw is how a sound cap comes loose.

Where different metals cannot be kept apart, the AGA's remedies include painting both metals to keep moisture off the joint and separating them with electrically inert spacers such as neoprene, rubber or nylon. The simpler answer on a chimney is usually matched fasteners, meaning stainless screws in a stainless part and copper or compatible fasteners in copper.

Flashing belongs in the same picture. The Brick Industry Association's chimney guidance notes that most codes require corrosion-resistant flashing and that a metal rain cap should be corrosion-resistant. When one part at the top is replaced, look at what it will touch.

The damper below the salt

An open flue lets damp, salty air sit on whatever metal is inside the chimney. A steel throat damper above the firebox can rust in that air, and rust flakes on the hearth are one sign of it. A top-sealing damper shuts the flue at the chimney top instead, and National Chimney describes its gasketed lid as sealing out weather when shut. That lid and its hardware then live in the salt themselves, so their material matters, and National Chimney's listings note that some top-sealing models are not for chimneys venting gas logs.

A rinse-and-look routine you can keep from the ground

IMOA's guidance says frequent cleaning by heavy rain or by hand washing keeps corrosive deposits from building up, and that sheltered parts get less of that rain. For a homeowner, coastal chimney maintenance is mostly about looking, because the parts that collect salt sit on the roof. Keep your feet on the ground and use binoculars.

• After a dry, windy stretch and again after any storm, look at the cap, the screen and the edges of any chase cover.
• Note orange bloom at seams and screw heads, streaks running down the chase or brick, screen wires that have parted and any cover corner that has lifted.
• Rinse with fresh water only metal you can reach while standing on the ground, and never take a hose up a ladder.
• Leave washing and checking the top to a maintenance visit, and ask what metal each part is made of so you know what you are watching.

What to ask for when a part is replaced

A replacement is the moment to get the metal right, and the written estimate should say what you are buying. Ask for the metal and grade of the cap body, the screen and the fasteners, and whether the screen is woven or welded. Ask how the part will be attached and what it will touch. On a factory-built fireplace, ask whether the cap is the one listed for that chimney system. Section R1005.1 of the Florida residential code has a factory-built chimney installed and terminated to its maker's instructions.

Fit still comes first. Section R1003.9.3 of the 2023 residential code sets the minimum open area under a rain cap on a masonry chimney, and a well-chosen metal does not excuse a cap that is too small for the flue. The damage that salt does to masonry, rather than metal, is covered in our spalling brick guide.

What we record about each metal part

Start with a free inspection of the metal at the top of the chimney, taken part by part. We note what each piece is made of where it can be identified, where rust has started, how the fasteners are holding and whether two metals are working against each other. From there, the estimate we write names the material proposed for any chimney cap replacement or new chimney chase cover, with its price set out in advance, so the choice is yours before any work starts.

Sources

• International Molybdenum Association, Which Stainless Steel Should Be Specified for Exterior Applications?: How salt deposits corrode metal, sheltered surfaces, smooth finishes, woven mesh crevices, grade selection and washing.
• American Galvanizers Association, hot-dip galvanized steel in the atmosphere: How sea spray chlorides consume a zinc coating, and marine atmospheres ranked among corrosive settings.
• American Galvanizers Association, continuous sheet galvanizing: ASTM A653 coating designations for galvanized sheet and the link between coating thickness and service life.
• American Galvanizers Association, dissimilar metals in contact: Electrolytes that drive galvanic corrosion, zinc's place in the galvanic series, avoiding galvanized fasteners on copper or stainless steel and ways to separate metals.
• American Galvanizers Association, galvanized steel in contact with other metals: Why a small anode against a large cathode is an undesirable pairing.
• Florida Building Code, Residential, 8th edition (2023), chapter 10: Spark arrestor screen corrosion resistance in section R1003.9.2, rain cap open area in section R1003.9.3, and factory-built chimneys installed and terminated to the maker's instructions in section R1005.1.
• Brick Industry Association Technical Note 19B, Residential Chimneys: Corrosion-resistant flashing and metal rain caps.
• Brick Industry Association Technical Note 23, Stains: Copper above brickwork as a source of stains on the brick.
• Rockford Chimney Supply chimney chase covers: Replacement chase covers in stainless steel or copper, and why original galvanized tops rust.
• National Chimney top-sealing dampers: How a top-sealing damper closes the flue and the gas log restriction on some models.
• National Park Service Preservation Brief 2, Repointing Mortar Joints in Historic Masonry Buildings: Salt crystallizing inside masonry units and pushing off the outer surface.