dik lang wrote:

I got the price quote everyone is waiting for. A silicon-bronze replacement comes in at $15-1600. A "naval" bronze around $900. Hot dipped galvanizing ranges wildly. All quotes I was given are not by size but by weight of the item being galvanized, 26 to 45 cents per pound. Around eighty dollars to galvanize the board sounds good! None of the galvanizers had heard of differing galvanizing schedules or standards. They all also said that the thought that an item could be double or triple dipped was absolute bunk! Now I have to figure out if the bronze pivot and stop pins would adversly affect the galvanizing. Should the newly plated surface be painted?

Dik, Make sure that the "naval bronze" they quoted is not actually "naval" brass. Naval brass has about 40% zinc content. From my Corrosion book: "Brass should not be used underwater because it "dezincifies". Dezincification is the gradual dissolving of the zinc content of brass leaving a spongy mass of copper; although the shape of the article is retained its strength is virtually nil." If you are going to make it from bronze, it must be a true bronze. I have scanned in the pertinent section on bronzes. It's a long read, but if you are going to spend $1000, you should have as much info as possible. I also included the section on hot-dip galvanizing. You only need to galvanize the cast iron. A bronze board can be just left as is, or painted with bottom paint if you have the boat in a salt-water slip. If you galvanize a cast-iron board, you still have to paint it, because the galvanic coation only lasts 1-3 years when immersed in salt water. Regards, John Fleming M-17: "Star Cross'd" -------- Zinc-Free Copper Alloys In this category come the bronzes. To most people the term 'bronze' immediately conjures up a superior metal to a 'brass', and indeed the bronzes are superior in terms of corrosion resistance, although not always in terms of strength. The essential point is that they do not contain a significant amount of zinc so dezincification is not a problem. Bronze Age bronze was an alloy of copper and tin but is not much used now because the addition of small quantities of other elements increases the strength without detriment in other respects, resulting in the 'gunmetals'. The search for higher strength alloys that retained the corrosion resistance of ordinary bronze has led to the development of copper alloys that do not actually contain tin, so they cannot really be called bronzes. Silicon bronze is an example (96% Cu, 3% Si) and so too is aluminum bronze (90% Cu, 10% Al). Other copper alloys in the same class are correctly not called bronzes - the coppernickels for example. The commonly used copper alloys are: Gunmetal 88% Cu + Tin Aluminum bronze 90% Cu + Aluminium Nickel-aluminium-bronze 85% Cu + Aluminium + Nickel Silicon bronze 96% Cu + Silicon Phosphor bronze 90% Cu + Tin + Phosphorus 90/10 Copper-nickel 90% Cu + Nickel 70/30 Copper-nickel 70% Cu + Nickel Note the high percentage of copper in all of them. Unlike brass, the bronzes are not diluted with large amounts of cheap metal (zinc). Consequently they are more expensive, but give better corrosion resistance. Gunmetal was used, as its name suggests, for making guns in centuries gone by. It is a good casting metal, it has a lovely sheen (which tarnishes in sea air), and it is not prone to dezincification, stress corrosion, crevice corrosion or pitting. Because of its relatively low strength it is not good as a fastener material. Nowadays silicon bronze has superseded gunmetal as an all-round corrosion resistant fastener material; but for castings for such items as bollards, fairleads, seacocks, stern tubes, rudder tubes, etc gunmetal has great merit. It is a material that can confidently be used underwater. There are several types of gunmetal but the common ones contain either 6% or 10% tin, in America commonly known as M bronze and G bronze respectively, but both have similar properties. Sometimes these are referred to as 'tin bronzes'. Aluminium bronze (not to be confused with aluminium brass) is also a fine material with good resistance to pitting, fatigue and wear, and it has a lovely golden colour. It casts easily and is a little stronger than manganese bronze, and of course is not prone to dezincification. It is another material that can be used underwater although it has been known to be subject to 'de-aluminification', a ghastly word for a type of corrosion similar to dezincification. This, however, is avoided if 4% nickel is included in the mix. Added nickel also makes a stronger material, hence Nickel-aluminium-bronze (NAB for short). Similarly manganese will add strength, hence nickel-aluminium-manganese bronze. Propellers and propeller shafts and bolts can be bought in these materials and a very trouble-free life can be expected, but that old enemy manganese bronze is very much more commonly available - more's the pity. The aluminium bronzes are very resistant to pitting corrosion, crevice corrosion and water velocity effects and also stress corrosion. Another good all-rounder is silicon bronze. Screws and barb-ring nails are readily available in this alloy; for example, Gripfast nails are made of a proprietary silicon bronze called Everdur. Silicon bronze can be used with confidence underwater. Phosphor bronze castings are used for machined parts which need a bearing surface. Phosphor bronze makes a convenient corrosion resistant spring material and is found in such things as snap-shackles. It is as corrosion resistant as tin bronze since it has much the same basic composition. The copper-nickels are a family with variations in the nickel content. They are basically very strong and entirely corrosion resistant, but of course are expensive and not normally used at the moment in the boating world. The main commerical use of this alloy is in tubing for carrying seawater in condensers of power stations and desalinators. Tubing is thus readily available commercially and so is plate, although they are not generally stocked by chandlers nor used by yacht equipment manufacturers. The two varieties mostly produced are 70/30 and 90/10 (Cu/Ni). In general 70/30 is more corrosion resistant and stronger than 90/10 but more expensive. 90/10 is nevertheless as corrosion resistant as any boat owner could wish. A variety of even better copper-nickels are under development. ---------------- Zinc, Cadmium and Aluminium Coatings As zinc, cadmium and aluminium are less noble than steel, a coating of zinc or cadmium cathodically (i.e. sacrificially) protects the underlying steel. Scratches are self-healing; when wetted, a small cell is set up and the surrounding zinc is deposited onto the bare steel until it is covered. On large areas of galvanized surface the scratch can be up to about 3/16 in. (5 mm) wide and still receive cathodic protection. Various processes are used to 'stick' the coating onto the steel or iron. Galvanizing is a common process whereby the item is dipped into molten zinc, which can also be electroplated on, or alternatively sprayed in place. Sherardizing is another method of applying zinc. Cadmium is usually deposited by electroplating, while aluminium (rarely used) is sprayed on. The effectiveness, i.e. the life, of a metal coating depends largely on its thickness, and this is where hot-dip galvanizing scores because the zinc coat thickness achieved is far greater than by the other processes - except spraying, where again a good thickness can be built up so long as adhesion can be maintained and porosity minimized. Hot-dip galvanizing produces a coating thickness of at least 2 mils (50 µm) and probably 3 or 4 mils, whereas sherardizing and plating only give 1 1/2 mil (38 µm) at the most. The life of galvanizing will therefore be much longer than that of electroplating or sherardizing. The basic problem with spraying is that the coating is generally very porous compared to plating or hot-dipping. Consequently a thick coating is essential, provided that adhesion can be maintained. Zinc rather than aluminium spraying is more common, but again not as common as galvanizing. It requires good preparation by shot-blasting and skilled operators to minimize porosity and ensure an even coating. As with paint, it is difficult to achieve an even coating especially on sharp edges and joins. An unpainted zinc sprayed coating, like other bare zinc coatings, does not last long underwater. An even thicker coating than galvanizing can be achieved with spraying 4 to 8 mils (100-200 µm). Many of these processes are covered by ISO or British Standards: BS 729 Hot-dip galvanizing after fabrication IS01459-61 BS 1706 Electroplating (zinc and cadmium) IS02081-2 BS 4921 Sherardizing BS 2569 Sprayed aluminium and zinc IS02063 BS 3382 Zinc-plated nuts and bolts Since hot-dip galvanizing is generally the most satisfactory method it is worth discussing in more detail. BS 729 specifies a minimum coating weight of 305-610 g/m^2 for steel items depending on the size of the item, 610 g/m^2 for iron castings, and 305 g/m^2 for threaded work which in the process is spun to 'clean' the thread. These awkward metric figures originate from the old Imperial coating of 1 oz/sq ft ( = 305 g/m^2) which in turn is equivalent to a thickness of 1.7 mils ( = 43 µm) Extra clearance must be given on threads to allow for the coating thickness. With nuts and bolts, the bolt is threaded normally and galvanized while the nut is tapped 0.02 in. (0.4 mm) oversize after galvanizing. The bare steel thread on the nut is then protected by the zinc on the bolt thread. A roughened surface, caused by shot-blasting for instance, will give a thicker coating. If a really thick coat is required this is a more effective method than double dipping. Before galvanizing the article must be clean and free of paint or grease, but light rust does not matter; it is also acid pickled. If the steel has not been cleaned sufficiently a bare area will result. There are numerous small galvanizing plants around the country and the process is generally quite inexpensive, but for all exterior or underwater items it is vital to state that hot-dip galvanizing is required, otherwise electrogalvanizing (another common term for electroplating) might be employed resulting in a very thin coat of zinc. Ideally ask for 'hotdip galvanizing to BS 729' or an equivalent national standard. Any article to be galvanized should not have sealed voids, for instance a length of tube should not be sealed at either end, because of the possibility of an explosion when it is dipped in the hot molten zinc. A hole should be drilled at each end. Galvanizing does not affect the strength of structural steels. Electroplating steel to BS 1706 involved asking for one of three coating thicknesses. For marine work Class A should be used (the thickest), which will give a minimum of 0.6 mils (15 µm) for cadmium and 1.5 mils (38 µm) for zinc. Again, zinc plating is preferable to cadmium. If something is merely 'commercially plated' rather than to this standard very thin coatings may be given which are quite useless for marine work even inside the cabin. The British Standard for sherardizing has two classes of thicknesses, Class 1 at 1.2 mils (30 µm) and Class 2 at half that thickness. A sprayed coating to BS 2569 Part 1 should have an average thickness of 4 mils (100 µm) In the sherardizing process the item is put in a sealed container together with a quantity of zinc dust and heated to a controlled temperature. A uniform coating is obtained which is very resistant to knocks. No etch-priming is required when painting. Clearly it pays to specify a coating to a standard and not just to ask for 'galvanizing' or 'plating'. How does zinc perform in seawater? The corrosion rate of a zinc coating does not vary much according to how it was applied so long as adhesion is good. In static seawater a bare galvanized coating wastes away at about 1 mil per year (25 µm) so even a thick coating of 3' mil (86 µm) will only last three to four years. Polluted seawater and mud containing hydrogen sulphide corrodes bare zinc at an accelerated rate. Zinc also corrodes away much faster as the water starts to move. At 2 knots the rate is 3 mils per year (75 µm) Consequently, on immersed surfaces it is essential to paint the coating. As long as the paint is 'keyed' to the zinc (with an etch-primer for example) galvanizing will last a very long time, and the underlying steel will be completely protected. On deck, a heavy galvanized coating (e.g. to BS 729) will last many years - over ten and perhaps more, even unprotected. The rate of corrosion is about five to eight years per mil. Painted, the coating life is extended several times. A painted electroplated surface will also last a long time on deck providing the electroplating is to a good thickness, e.g. BS 1706 Class A. Bare thin electroplating will start to rust through in no time on deck. A zinc, aluminium or cadmium coating creates a very sound base for painting - far sounder than bare steel - but it is essential to free the surface of grease and preferably to use an etch-primer. A galvanized surface in damp conditions (rather than fully immersed or fully open to the wind and rain) can develop 'white rust'., a voluminous white powder. Painting can prevent this, but good adhesion of the paint is very important. The greasy galvanizing flux must be neutralized with an etch-primer before the normal primer and top coats are applied.