Does pitting always mean crevice corrosion?

[Tim Good]

The photos below show the base my 2 inch rudder shaft. The part which slots into the shoe. It’s a boat from 1996. You’ll notice the small pitting.

I’ve tried to use some welders crack dye but doesn’t show much up other that the pitting which is visible.

What are your thoughts on this?

 

[rotrax]

If I had a vessel of thate age and the rudder was as good as that, I would be delighted!

Vyv Cox is your main man here, but I think that is fine.

 

[KevinV]

Looks more like you're short of an anode (or a coat of paint on the exposed part) to me - but as above, vyv is your man

 

[Tranona]

Looks more like you're short of an anode (or a coat of paint on the exposed part) to me - but as above, vyv is your man

An anode won't make any difference. It could well be crevice corrosion from water trapped between the bottom of the rudder and the shoe when the boat is at rest. Probably nothing to worry about.

 

[vyv_cox]

Pitting in 300 series stainless steel can occur for a variety of reasons but in your case it seems that minor crevice corrosion could be responsible. This can take place with the tiniest of crevices, an example being the small laps that are formed when rigging wire is pulled through dies to reduce its diameter and strengthen it. The result can be rust stains if the laps face upwards. May be unsightly but rarely dangerous. In your case small scratches or imperfections can be enough to lead to pits.

I doubt very much that your rudder stock has been weakened to any significant extent or is at risk of future failure. In a rotating shaft under some load pits can be stress raisers that may initiate fatigue but I cannot imagine this in a rudder stock.

 

[Tim Good]

Pitting in 300 series stainless steel can occur for a variety of reasons but in your case it seems that minor crevice corrosion could be responsible. This can take place with the tiniest of crevices, an example being the small laps that are formed when rigging wire is pulled through dies to reduce its diameter and strengthen it. The result can be rust stains if the laps face upwards. May be unsightly but rarely dangerous. In your case small scratches or imperfections can be enough to lead to pits.

I doubt very much that your rudder stock has been weakened to any significant extent or is at risk of future failure. In a rotating shaft under some load pits can be stress raisers that may initiate fatigue but I cannot imagine this in a rudder stock.

Thanks Vyv

 

[Blue Seas]

Bearing in mind the location I would hazard a guess at differential aeration being involved. I don't think that I would be losing any sleep over it.