Cameron,

Galvanising is generally considered to be a limited life process. When a break ocurs in the zinc coating, galvanic protection starts and continues till all the zinc is used up. Corrosion of the steel element then follows and will be rapid. Zinc can be applied in a number of ways to steel the most common being sheridising (electrolytic deposition)which gives a fairly thin coating and is generaly used on small nuts, bolts and hinges. Hot dip galvanising is a much better process since generally the part to be coated is given an acid bath to remove all corrosion and provide a good bonding surface prior to hot dipping. The downside with this is that the coating thickness is uneven and can be a problem on screw threads where big globs would make assembly difficult.

Turning to stainless steel there is a popular misconception that this does not rust or corrode. The satisfactory performance of stainless steel is very much dependent on whether it can be quench annealed and also its exposure in particular to chlorides and sulphides in a wet environment e.g. near the coast or near a coal fired power station. If this happens then stress corrosion cracking can commence on stressed components and these will then be susceptible to fatique failure over time. Failure will be instananeous and if highly stressed could be catastrophic.

Oak is high in tannic acid and even though the timber dries it is really only the water that evaporates leaving all salts and non volatile compounds behind. Some of these are capable of being re hydrated when surface moisture is present. This kind of situation happens more often than you might realise even indoors especially in the roof where for example a prolonged cold period will result in moisture condensation when there is a swing to warmer milder waether.

The old fashioned way to combine metal and oak was to use wrought iron which like cast iron tends to rust and form a protective oxide skin which slows down further corrosion. Kept reasonable dry this will last for centuries as per hand wrought lathe nails.

The best approach is, where possible, not to place a heavy reliance on brackets and bolts and instead try and design your joinery to use all wooden components. If you cannot do this then it is most likely that the design is either too esoteric, cheap or built on a grand scale like a cathedral or bridge which really is now the realms of the professional engineer.

I hope that this helps.

p.s. I am now the owner of your bisaigue !

Ken Hume