Crevice corrosion

[Jacana]

I have suffered what I believe is called crevice corrosion on the propshaft of my Malo 38 under the rope cutter. this resulted in having to change the prop shaft! Not for the feint hearted on a yacht with a skeg. Should I replace the cutter on a bed of silicone or fit it dry? /forums/images/graemlins/confused.gif /forums/images/graemlins/confused.gif

 

[oldharry]

Crevice corrosion occurs in Stainless as a result of the exclusion of oxygen. Bedding the cutter on anything may make it more difficult to remove next time, and will exclude even more oxygen, so could make matters worse, not better!

 

[MarkG]

I read in one of the sailing mags about a company that can rebuild a shaft by adding metal by using something like electrolysis. Can't remeber the exact details
but they did it for a prop shaft that had wear around the seal.
Not any use now as you already have a new shaft but something for the future...

 

[Guest]

The only time I get Crevice Corrosion is after a curry! /forums/images/graemlins/crazy.gif /forums/images/graemlins/wink.gif

 

[copterdoctor]

Any good engineering company should be able to metal spray and machine a finish. I would stay away from marine companies they will probably just send it out and charge you extra.

 

[rogerthebodger]

I worked for a company 25 - 30 years in oldbury West Midlands who would metal spray hard steel coatings onto plastic molding extruder screws.

This is a process that could be used to build up the corroded prop shaft. I don't know if my old company is still around but there must be people who still do this

As far as I understand crevice corrosion could be due to galvonic action or oxidation action of the Iron in the stainless. If it is due to oxidation sealing the joint will help.

If due to galvonic IMHO it will not. The need for an oxigan rich enviromrnt is due to the fact that stainless is protected by a layer of chromium oxide on the surface of the stainless created and maintained by the oxigin in the water. If this is eliminated crevice corrosion could result.

this is simular to the way anadizing protects aluminium.

 

[scuby]

Oldharry is correct but misses the point that crevice corrosion results if oxygen is excluded from wetted surfaces, you can read an excellent piece on SS corrosion here http://www.mcnallyinstitute.com/04-html/4-1.html

To answer the original question, the first point is silicone sealant should not be used below the waterline. The cutter was probably an excellent fit on a nice shiny shaft so once the water got in it stayed there and bacame deoxygenated. You could either exclude the water using an appropriate sealant such as a Sika primer or possibly make the cutter fit less well so that water can be replaced by fresh, the cutter manufacturer may have useful advice.

 

[TigaWave]

Scuby is right on this one, its wetted surfaces where oxygen is excluded that can suffer. A solution that we use to protect the threads of A4 stainless bolts in Bronze P bracket and prop is coating them in epoxy, this prevents ingress of salt water.

Removal or dimsantling is easy as epoxy softens as it is heated. We use epoxy to hold composite bearings in place and they slide out when the bearing carrier is heated.