Stainless bolts for bronze fittings?

[rajjes]

Hi all,

I need to refit bronze rudder fittings below the waterline. I'm aware that ideally I should use the same material below the waterline and use bronze bolts, however I'm having difficulty in finding anything in Bronze here in Malta. (I have found Brass but I'm not too keen on using them).

Could I use SS fittings with sealant around each bolt, or is it a definite no no?

 

[AndrewB]

Different metals in proximity underwater causes electrolysis between them, which will weaken one. How serious this is depends on how far apart the metals are on the galvanic scale. IF you are using SS grade 316 or A4 bolts, and IF the bronze is proper phosphor-bronze or silicon-bronze (i.e. not a high zinc so-called bronze), then the metals are close together on the scale and you should get away with it. The sealant won't make any difference, but you should keep an eye on the assembly from time to time when you haul, for signs of any pitting.

Incidentally, on the whole it is best to avoid using stainless bolts underwater, even 316 (A4) grade, as they can suffer from crevice corrosion over time. But definitely avoid 304 or A2 grade SS for this reason.

 

[lutraii]

My boat was built in 1971 with a bronze 'shoe' to support the bottom rudder bearing, and this was held in place with a number of stainless steel screws. In 2001 the bronze shoe was starting to break up, apparently due to galvanic corrosion, but it had lasted nearly 30 years.

I would suggest you could use A4 or 316 stainless screws (definately not A2 grade) and for peace of mind screw a zinc anode to the fitting.

Definately do not use brass screws, because the zinc in the brass will disappear due to galvanic corrosion. leaving you with a crumbly copper residue.

Sealant probably won't make any difference, as it is unlikely to electrically isolate the fitting from the screws.

(In my case, I had a 316 stainless steel shoe fabricated, fitted with A4 stainless screws and a small disc zinc anode on each side. The zincs last 2 years and there is no deterioration in any of the stainless parts).

Good luck,
Alan

 

[TigaWave]

Stainless bolts for bronze fittings are fine, use 316L A4 bolts.
We fit all our smaller rope cutters with a bronze spool and stainless bolts.

The rope cutters are stainless cast and are attached to the bronze P bracket with stainless bolts, they are all protected by the same anode that protects your prop and other underwater connected metal bits.

The prop is prbably bronze and the shaft/nut etc are stainless.

 

[DJE]

I recently replaced a bronze seacock which had been fixed with stainless bolts 17 years ago and there were no signs of galvanic corrosion.

 

[AndrewB]

[ QUOTE ]
The prop is prbably bronze and the shaft/nut etc are stainless.

[/ QUOTE ]True. One time I couldn't find the correct replacement shaft anode, so relaunched without fitting one. Next time I hauled, 18 months later, the bronze (AB2) prop had to be replaced. So some care is needed.