Salt is carried by wind, so the same lock can be in very different conditions depending on where it sits. A door facing the water, taking onshore gusts with no porch or fence in front of it, collects salt on every surface. A door set well back from the shore, or tucked into a covered entry on the sheltered side of the house, may see so little that an ordinary lock lasts as it would anywhere else. Cylinders, pin springs, and strike plates are the small parts of an exterior lock where salt-air damage starts, because they are thin, sit in tight gaps, and are handled every day.

That variation shapes the decision. Near the water, every metal in the lock deserves attention. Inland, a sound solid-metal lock that is kept clean may be all the door needs. A locksmith can read the signs of exposure from the old hardware: pitted screw heads, staining around the keyway, and a latch that has gone rough all indicate the metal has been corroding.

Solid brass is a copper and zinc alloy. Left bare, it dulls, then darkens to a brown or bronze tone, and the lock keeps working while it does. A salt deposit can be wiped off, and the darkened metal underneath is sound.

A clear coat sprayed over brass to hold its yellow shine is the first thing salt air breaks down. The damage appears as dark specks that join over the months, though the brass beneath remains sound. Uncoated brass changes colour evenly, from bright to a mellow brown, which is the easier look to live with by the water. Close to the sea, ask for brass with no clear coat and plan on the colour moving.

Brass also makes a sound base under a plated finish. A handle or cylinder collar that is solid brass all the way through gives the locksmith something to work with years later: it can be cleaned, and the internal parts can be rekeyed or serviced without the case breaking up.

Zinc alloy is a cast metal. It can be moulded into detailed shapes, which is why it turns up in handles, escutcheons and some lock cases, and under a chrome, nickel or brass-look plating it can pass for a solid brass part on the shelf. The trouble starts when a scratch breaches the plating, a thin patch or an edge. On a door exposed to salt air, salt that gets through a breach in the plating attacks the zinc alloy beneath, causing the plating to blister and lift from that point.

Zinc alloy has its place. On a protected interior door or at a house well back from the water, a lock with zinc-alloy parts can provide reliable service. For an exposed exterior door near the sea, treat plated zinc alloy as lower-grade hardware for any part facing the weather, and ask about it whenever a handle is priced well below its solid brass equivalent. Ask what the body is cast from, because the finish name will not say.

Stainless steel is a family of alloys, and the grades differ in the additional elements they contain besides iron and chromium. Grade 316 contains molybdenum, which helps it resist pitting from salt; grades 201 and 304 contain none, and they are the ones that spot first on a door facing the sea. Grade 316 is the one to look for on handles, hinges, and locks that face the weather.

A stainless label on its own tells you little. A lock described as stainless with no grade may be a lower grade, and the difference does not show on the shelf because all of them are bright and grey when new. Ask for the grade by number, and ask which parts of the lock it applies to. A 316 outer plate over an internal mechanism in another metal is possible, with the pin springs and cylinder body being the parts out of sight.

Stainless suits a door where a silver-grey look is wanted, which brass cannot offer once it darkens. That preference has to be weighed against getting the grade right, since the grade is what the extra expense of stainless is buying.

A lock can be made of the right metal and still fail at the screws. When two different metals come into contact in a damp, salty environment, the less noble one corrodes faster. A plain steel screw in a brass plate is the part that suffers, while a steel screw in an aluminium or zinc alloy fitting eats into the fitting around it, so those pairs need a matched screw or a barrier between the two metals. Check that the screws, the strike plate and the cylinder retaining screw match the lock body in metal, or are of a grade chosen to suit it.

The strike plate and keeper deserve their own attention. They sit in the door frame, take the thrust of every closing and face the weather on the edge of the jamb. A corroded strike plate can roughen the latch's path, so the door starts to catch or feel stiff. Replace it in the same metal as the lock rather than with whatever is in the toolbox. In a timber frame, corroded screw heads also stain the surrounding paint and wood.

A finish is the skin over the base metal, and the plated names on a carton, such as polished chrome, satin chrome, or satin nickel, say nothing about what lies beneath. The same plating can be laid over solid brass or over zinc alloy, and the difference shows only once the skin is breached, so ask for the base metal by name, whatever finish is chosen.

Black hardware can be powder-coated, which is a baked paint layer, or plated or vapour-coated, so ask which. A powder coat over iron or steel protects only while it is unbroken, because the coat is all that stands between that metal and the salt. Clear-coated brass is the finish to avoid outdoors near salt, and uncoated or chemically darkened finishes have no layer to fail.

The listing or the carton gives a finish name and little else, so the useful details have to be asked for. These questions settle the choice:

- Body metal: Is the lock body and cylinder solid brass, a stainless grade, or a zinc alloy?

- Grade: If stainless, which grade, and does it cover the internal parts or only the plate?

- Coating: Is the brass lacquered? Is the finish plated, and over which base metal?

- Fixings: What are the screws made of, and are they supplied, or do they need to be sourced to match?

- Spares: Can the cylinder be rekeyed or replaced on its own later without replacing the whole lock?

A locksmith can answer these from the product and also check how the lock sits in the door. A corrosion-resistant lock that is slightly out of alignment still wears, since the latch grinds on the strike with every closing.

Weighing the answers is simple enough. Solid brass, bare, gives long service with a changing colour. Grade 316 stainless steel retains a silver-grey appearance and resists pitting. Zinc alloy belongs on sheltered doors. Any of the three can be undone by fixings in the wrong metal.

Salt air is patient, and a lock that is the right metal from the cylinder to the last screw does not need to be fought over every year. The metal is chosen once, at fitting, and the lock repays that choice in every season that follows.