Many metals that rely on a passive film for their corrosion resistance suffer from the dual problems of pitting and crevice corrosion, stainless steels being just one example. If a crevice, such as is formed in bolted joints, swaged fittings or nuts and bolts, is immersed in water, the oxygen content of the ‘stagnant’ water deep inside the crevice is lower than that of the fully aerated water on the outside. This difference causes the metal within the crevice to become active, corroding preferentially due to the fact that the active and passive metals have different galvanic potentials. Pits are themselves small-scale crevices, as is the surface roughness in drawn rigging wire, accounting for the rust stains that can develop on them.
A perfect example of crevice corrosion in several locations. Rust stains can be seen in the roots of the threads, between the nut and the bracket, between the bracket and the tube, and even to a small extent between the tube and the plastic plug. However, where the bedding sealant is in place between the bracket and the GRP the water has been excluded and staining has not occurred. There seems to be some staining near the lower edge where the sealant is not present, although this may be run-off from above.
An example of crevice corrosion in a bolt and washer. They were set in wood, with a sealant.
Crevice corrosion of a stainless steel keel bolt.
The majority of builders fit stainless steel bolts (Moody being an exception) and failures are rare. In the event that sea water penetrates to the bolts due to inadequate or faulty sealant, a serious situation can result. Replenishment of oxygen is almost impossible and the water becomes 'stagnant'. The metal becomes active in some areas, reacting galvanically with other areas where a higher oxygen content allows the passive oxide film to remain intact. The result can be dramatic metal loss, as shown here.
Leakage of water to the bilge, or rust staining at the keel to hull joint, should be investigated by removing a keel bolt.
Photo: a poster on Sailnet forum
|Crevice corrosion of a prop shaft at the cutless bearing|
| ||Blackening (sometimes known as contact staining) in the unmistakeable rectangular shape of the lands of a water-lubricated bearing. The black colour is caused by the corrosion product in a network of very small pits. |
|On another part of the shaft the pits are considerably deeper and continuous, possibly due to the presence of a rope cutter.It would appear that some galvanic corrosion may also be involved here.|
|Crevice corrosion due to inferior materials selection|
The shaft was installed on a 9 year old Jeanneau. The damage was found when the boat was hauled out by its new owner after an extended time in the water. All of the damage shown is crevice corrosion. During the investigation it was found that the shaft was attracted to a magnet, indicating that it is not 300 series. It seems likely that a 400 series material has been used, having considerably less resistance to this form of corrosion.
This photograph shows a deep crevice formed between prop and shaft at the shaft taper.
| Fluted crevices formed between the cutless bearing and the shaft.|
Corrosion damage on a part of the shaft not in contact with any other components. It seems likely that crevice corrosion formed beneath either an antifouling splash or an area of fouling.
Photos: Mike Wylde
|Crevice corrosion of a shaft in way of the propeller|
Severe crevice corrosion of a propeller shaft beneath its propeller. The location of the corrosion at the widest part of the tapered section suggests that the propeller may have slightly overlapped the taper, creating a crevice there. Having formed the circumferential groove the corrosion continued inwards towards the keyway. The propeller was easily removed, suggesting that it may have not been fully tightened on installation.
|Crevice corrosion between a rope stripper and shaft|
It appears that this rope stripper was assembled in a misaligned manner to create a crevice. Note that there is corrosion between the faces at one bolt hole, indicating that this was bolted tightly as installed.
|Stainless steel propellers with plastic bushes|
VP F5 propellers. Replaced by VP after two years in service. The replacements corroded in the same way but after filling the damage with 'Molecular Metal' no further corrosion occurred in seven seasons. This may indicate that surface defects were at least partly to blame as initiation sites.
The crevice corrosion took place primarily at the interface of the plastic bush and the outer hub.
Photos by Graham Roberts
|Volvo C3 propeller corrosion|
Persistent corrosion issues with Volvo C3 propellers fitted to 290 DPE drives. The photo shows the third set to be affected in about two and a half years. This set is 7 months old.
The propellers are suffering crevice corrosion in the centre near to the bush, caused by the plastic washer Volvo insist should be used when fitting the props. The corrosion is isolated to the area under this washer, as can be seen in the photo. An interim solution might be to fill the space between the propellers and plastic washer with a sealant.
Photo by Yabs
The latest props to replace the earlier alloy ones to DPE drives are now a one piece manufactured design, it has no centre bush and no isolation from the shaft. There also coated in a special material to stop corrosion. Called the J series when replacing a B series prop. (This text by volvopaul)