(A version of this article was printed in Practical Boat Owner in 2010)
The subject of bronzes and brasses, and which to use for various marine applications, is one that comes up on a weekly basis on yachting forums. Here’s a short explanation of what they are and where they should be used.
The excellent properties of copper-tin alloys have been known since – well, since the Bronze Age. Alloys of copper and tin, along with small amounts of other elements, have been used for thousands of years where strength, corrosion resistance, good appearance and ease of manufacture are important. Cast tin bronzes have been widely used in marine applications, perhaps the most widely known of these being gun-metal, although large numbers of fittings that are immersed in seawater continue to be made from this versatile material. Where strength is important, such as in bolting, it is common to use a slightly modified version, called Phosphor bronze.
The one disadvantage of this versatile alloy is its cost. Tin has always been a high-priced metal, with the result that for more than 100 years there have been many attempts to find a cheaper alternative. Although originally gunmetal contained 10% of tin, modern ‘gunmetal’ standards BS 1400-LG2 and LG4 specify a tin content between 3 and 7%, with similar amounts of lead and zinc.
It is increasingly rare to find tin bronze used in modern yacht fittings, although 25 years or more ago this was the material of choice for skin fittings, seacocks and suchlike. Beware that some bronze valves have been found to have chromium plated balls and stem fittings that are made from brass, although the body is bronze.
A number of other bronzes are in common use for underwater purposes, including silicon bronze, aluminium bronze and nickel-aluminium bronze. Silicon bronze is perhaps the most common alloy for fastenings in modern wooden boat building thanks to its strength and corrosion resistance. ‘Bronze’ rigging screws may be made from either silicon or aluminium bronze. Aluminium and nickel-aluminium bronzes are widely used for ship’s propellers but the alloy seems to have fairly limited use for this purpose in yachting applications.
Brass is also copper-based but there the similarity ends. Its main alloying element is zinc, at anything between 0 – 50%. Brass is widely used in marine duties, where its resistance to atmospheric corrosion is reasonable, provided that tarnishing is either accepted or taken care of by regular polishing. However, for underwater use the unmodified alloy is not acceptable. The most commonly used alloys are 70/30 (70% copper, 30% zinc) which is ductile and 60/40 (60% copper, 40% zinc) which is rather more brittle. Typical duties for 70/30 brass are forgings, sheet, tubes and wires, whereas 60/40 is normally cast, for plumbing fittings and similar products.
Since the 19th Century, many attempts have been made to improve the resistance of brass to seawater corrosion. Small additions of other metals have brought about significant improvements, although perhaps not quite as much as their names would suggest, for example neither Admiralty Bronze nor Manganese Bronze are bronzes at all, although Naval Brass is rather more modest in its claims. Naval Brass is sometimes sold under the standards MS56 (CuZn44Pb2, CZ130) or MS 58 (CuZn39Pb3, CZ121).
Very many yacht fittings are made from Manganese Bronze; propellers, P-brackets, skin fittings, being some examples.
All of these modified brasses have been susceptible to a particular type of corrosion known as dezincification. When immersed in seawater there is a tendency for the zinc-rich phase in brass, and its modified versions, to be leached away, leaving the copper-rich phase behind. Copper has little strength, especially as loss of the zinc leaves it spongy, and the component will fail when this happens. In a well-known case a boat named the Random Harvest sunk when a skin fitting failed due to dezincification. However, the answer to the dezincification problem has been known for 50 years and DZR (dezincification resistant) brasses have been available for almost that length of time.
DZR is a leaded brass with a small arsenic content. Its copper content is very carefully controlled, at about 63%. Over 63% brass is normally single alpha phase, so more malleable and used for forgings, etc. Below 63% is two-phase, mostly used for castings. In the case of DZR the copper level is such that components can be made by hot stamping but converted from duplex to alpha phase by subsequent heat treatment.
Although people think the alloy is only necessary in seawater service it is also used in soft waters where dezincification can be a problem. (I can vouch for this as I have seen the problem at home). Valves and fittings in DZR are available at plumbing suppliers, no doubt cheaper than in chandleries. They should be marked as DZR,
Most good quality fittings are now made from DZR, Blakes seacocks being one example, giving them Lloyds approval for use in a marine environment. If buying underwater fittings and components from a chandlery you should insist that they are DZR, preferably with some form of certification. In 2013 skin fittings and hose tails marked with the CR logo became available.
Dezincification in used components is fairly easy to see. After abrading away any surface paint and oxidised layers, inspect the bright surface for any sign of pinkness. If this is present you are looking at the copper that remains. The problem affects the surface first; so further abrasion may well remove the entire affected layer. Although mainly seen on underwater components it also occurs on engine water pumps, where the cover is often made from 60/40 brass.
How to tell copper alloys apart
The short answer is that there is no simple way. A spectrographic analysis is probably the cheapest way if you really need to know, the likely cost being £100 – 200. A good guide may be to consult Metalreference where composition can be estimated, based upon the colour of the alloy.
|This bronze casting has been in situ in a marine environment for 60 years. The colour of the verdigris corrosion product is typical for a bronze and is slightly darker than that on a brass, as seen on the valve actuator below.|
Composition of copper alloys
The table shows typical compositions of various copper alloys. Note that these can vary considerably and the table should not be considered definitive. For instance, Admiralty bronze is a 70/30 alloy with additions that may be either 1% tin or 2% aluminium plus a small amount of arsenic. There is even one grade of manganese bronze that does not contain manganese!
|BS 1400 – LG2||85||5||5||5|
|BS 1400 – LG4||88||7||2||3|
|Phosphor bronze (wrought)||93.7||6||0.2|
|60/40 Brass (Muntz metal, Tonval ) (CW 617N)||60||37 – 40||up to 2.5|
|Naval brass (CW619N)||about 60||1||39 – 44||1 – 3|
|MS 58 (CW 614N)||About 60||39||3|
|DZR brass (CW602N, )||62||0.7||35.2||2||0.1|
|The colour of every copper alloy is characteristic, enabling identification simply by inspection. Perhaps the best guide to copper alloy composition without paying for analysis is to use the Metalreference website, which shows the colour of a large number of alloys for comparison with a sample.|
Early stages of dezincification in a skin fitting
| Removal of the antifouling paint reveals the surface of the skin fitting flange. The unmistakeable pinkness associated with dezincification can be seen.
Photo: Jill Wood
|Dezincification of a stern tube|
| Many stern tubes are made from inappropriate materials. This one shows advanced dezincification that could conceivably cause loss of the boat within a relatively short time.
Photos: Guido Villarosa
Dezincification of a small valve actuator in seawater service
|Pronounced pink colour due to severe loss of zinc|
|End view of the same fitting. The green colour is common to most copper alloys in seawater|