True or false ?

[Leighb]

The engineer who is doing work on my MD7A pointed out that the bolts holding the sacrificial anode were very corroded, and suggested I got stainless replacements.

I enquired at Fox's and was told they only sold mild steel, as stainless would have too high a resistance and would not work "I might just as well paint over the anode"

I find it hard to believe that there is much difference in electrical conductivity between the 2 types of steel.

Which is correct please?

 

[europe172]

My shaft anode has stainless steel bolts and it works well, slowly over a season it disappears, so that works for me, why not try it yourself?

 

[BAtoo]

FWIW http://www.ndt-ed.org/GeneralResources/MaterialProperties/ET/ET_matlprop_Iron-Based.htm

Don't know what it means (!!) but I would think its good enough to use SS; & I would prefer it on my boat....

 

[BAtoo]

Also just though- the connection from engine to my shaft annode is via a SS shaft - so if its alright for that......

 

[philip_stevens]

MGDuff are the people who make anodes, and also supply both steel and stainless steel bolts. They should know what they are doing.

I suspect the swindlery were only trying to make a sale on what they had in stock - better a sale than no-sale.

I've got mild steel studs on our boat, as it came with mild steel studs. When they are to be replaced next year, (and they will be replaced), SS studs will be used.

 

[Leighb]

Thanks all, more or less what I suspected, but it is nice to have it confirmed.

 

[VicS]

It is not necessary to fit anodes with stainless steel bolts or studs because the anode will protect a mild steel bolt. If they don't protect the bolts they they wont give anything else much protection.

What's more the cathodic protection removes the oxide that gives stainless steel its corrosion resistance so in the end it is just protected by the anodes in the same way that mild steel would be.

It is true that stainless steel has a higher electrical resistivity than plain carbon steel. Whether or not that translates into a significant difference in the resistance of bolts or studs made from the two materials you can calculate from the data that you will find HERE

 

[graham]

The anode will protect the bolts from electrolosis but not from plain rust.

 

[Billjratt]

Rust is oxidisation, not cathodic depletion

 

[William_H]

I would guess at the resistance of a M8 bolt 75mm long from end to end at in the order of .01 ohm compared to a mild steel bolt of the order of .0001 ohm (don't quote me)
In an anode circuit the voltage will be around .3 volt while current will be in microamps. This gives a characteristic impedance of the circuit in megohms. So even a quite large resistance of the bolt will make no difference.
olewill

 

[ccscott49]

Not true, in 30 years I have never had a mild steel bolt on my anodes rust.

 

[lw395]

Stainless can have high contact resistance, its resistance from corrosion is mostly due to chromium oxide forming on the surface iirc, which is why some stainless can corrode very badly in low oxygen environments. But I think in reality it will be ok, if you use some sort of shakeproof washer to get a good contact. I've only got a shaft anode, I tend to clean the shaft well and bed the anode firmly on to it, to be sure there is metal to metal contact, and always use the zinc plated fasteners supplied. They come undone easily after two years.