Examples of Corrosion

To protect the hull, metal fasteners or fittings from corrosion, some form of protection needs to be fitted. This is usually in the way of zinc anodes. Although in fresh water, magnesium anodes are needed.
However it is not simply a matter of placing anodes randomly on a hull. Poor bonding of the anodes and so called over zincing whereby too many anodes are fastened to the hull, are the more common problems found with anodes.

A mix of anodes from different manufacturers can also cause problems. On one occasion I surveyed a boat hull with similar sized anodes fitted to either sides of the hull. Anode ‘A’ was working reasonably well but anode ‘B’ was not. The bonding was correct but the composition of the anode was different. Anode ‘A’ had a higher zinc composition. Try and match your anodes and source from the same supplier. If an anode is not working, find out why. Do not paint anodes, this effectively seals it off and prevents it from working.

Shown below are some common examples of corrosion found in boats of different construction.

	The stainless steel bolts in this chain plate are fully corroded. Chain plate fasteners are a common fault in yachts as the chain plate deck fittings are hard to seal and often leak water. Stainless steel is not immune to corrosion and if the bolts are in low oxygen, wet situation, crevice corrosion will occur. Even bolts that show early signs of corrosion and are discoloured should be removed for inspection.
	Do not do this! Painting over a zinc anode (centre of rudder) will ensure that the zinc anode will not work. The rudder in this photo and probably the rudder shoe are not protected from electrolytic action. I once surveyed a boat with corrosion problems that had all the zinc anodes nicely painted. The owner assured me the anodes lasted longer that way!
	The spotted effect on this propeller is the result of selective leaching. One of the less noble elements in the casting is being leached out of the bronze, leaving pockets of copper behind. This propeller will eventually corrode away if the anodes are not corrected. The propeller needs replacing.
	The small screw driver has been inserted in a crack to the stainless steel chain plate. It is probable that the crack has occurred from stress on a small area of corrosion. The chain plate is constructed from 304 grade stainless steel, which is prone to corrosion.
	This alloy bracket is fastened with stainless steel bolts. In the centre of the photo there is a circular disc surrounded by a yellow halo. The disc is a stainless steel pin that is securing the chain plate seen in the photo above. The yellow halo is corrosion to the alloy caused by galvanic corrosion. This bracket was attached to the chain plate shown in the photo above.
	The rudder stock on this yacht was not bonded to a zinc anode. The stock has severely corroded as seen in the photo and a new rudder is needed.
	This yacht hull is constructed from Ferro-cement. The rust in the photo is from protruding wire ties that have not had sufficient cement cover. The ties have corroded away and sea water has entered the internal hull structure. The armature (steel frame) of the hull is seriously corroded.
	The sea cock in this photo is on a production yacht that is only a few years old. In order to reduce manufacturing costs, builders often use inferior fittings. Note also the corrosion on the clips securing the water inlet hose. The clips are a low grade stainless steel and have zinc coated worm drives.