The Resistance of Fastener Materials to Copper-containing Treated Wood

A major use of copper and its compounds is the preservation of wood. It prevents decay-causing bacteria from multiplying in the wood.  Some examples are CCA (chromate copper arsenate), copper (II) chromate, and microparticulate copper. They are cheap and generally nontoxic in the levels used, but they present a corrosion hazard to normal fasteners. The more reactive a metal is, the less reactive its ionized form will be; and the less reactive a metal is, the more reactive its ionized form will be. Copper is a rather unreactive metal: this makes the ionized form used in wood preservation corrosive. The corrosion ability of copper compounds surpasses that of the hydrogen ion, which is what causes the corrosiveness of acids. This presents a unique problem to the choice of fasteners. A number of fastener compositions and their relative advantages will be discussed here.

Aluminum is a bad choice. It is a very reactive metal, although protected by a tough oxide coating which prevents reaction with atmospheric oxygen. The oxide coating is vulnerable to attack, meaning that an aluminum screw may completely disintegrate in treated wood. When a piece of aluminum is dipped in copper II chloride, it disintegrates.

Brass (or brass-plated steel) is a good choice. It has the advantage of being readily available and cheap. When brass is dipped into a solution of copper II chloride, an example of a copper compound, it acquires a durable plating of copper which protects it from further corrosion. The disadvantage is that brass corrodes in moist air. Even though it does not disintegrate like iron, the corrosion can be undesirable in many cases.

Bronze is a good choice. It doesn’t dissolve in copper compounds, and it is resistant to corrosion by atmospheric oxygen. It is rare, though.

Hot dipped galvanized steel is a very good choice. It doesn’t corrode in air, because of the thick protective coating. The coating starts dissolving in the copper II chloride, but it forms a durable coating of copper, preventing further corrosion. It does corrode in air after many years, though, and it is not resistant to acids.

Mild steel is a bad choice. It corrodes in air, disintegrating over time. It dissolves in copper II chloride, leaving copper behind. The copper doesn’t plate the screw, so it continues to corrode.

Plastic is a bad choice. Even though it is resistant to corrosion from inorganic substances such as copper compounds, it has very little structural strength.

Special-alloy coated steel (“green” screws) is a good choice. Even though it dissolves in concentrated copper II chloride, a dilute solution may not affect it. It is also resistant to atmospheric corrosion.

Stainless steel is a good choice. It does not corrode in air, and retains its shine indefinitely. But it is vulnerable to attack by concentrated copper II chloride, which is able to dissolve the protective chromium oxide coating. Dilute copper II chloride, such as found in treated wood, does not affect it. Another disadvantage is that it is very expensive.

Zinc-plated steel (or electro galvanized steel) is a bad choice. The zinc coating is thin, so it corrodes in air after a couple of years. The coating completely dissolves in copper II chloride before a durable coating of copper develops, exposing the vulnerable iron to the corrosive action of the copper compound.