Shaft corrosion from packing

[CarlN]

So it's time to replace the packing and I'm thinking of switching from flax to Gore GFO. My concern is that MaineSail suggests checking a shaft packed with graphite once a year for corrosion. Frankly, this is more trouble than sticking with flax -- which has always worked pretty well.

I have a lot of respect (err, maybe "fear" is a better word) for galvanic corrosion. It makes sense that this would be a bad mix with a bronze shaft. And any shaft could be damaged if it gets hot from no water. But does anyone know of a case where an Aquamet stainless shaft on a boat was corroded by Gore GFO packing?

 

[dlochner]

The issue may not be galvanic corrosion, rather it may be crevice corrosion. Stainless steel needs oxygen to remain stainless. Stainless steel has a thin layer of oxidized metal that protects the steel. Flax packing allows a small amount of water to pass through keeping the SS shaft oxygenated so that the protective corrosion stays in place, GFO does not. When the SS in the shaft depletes the oxygen in the water surrounding it the metals in the shaft begin to react with each other and the salt water to corrode. This is the layman's explanation. Those more familiar with the chemistry of metals in saltwater can no doubt add more information.

In the meantime, google "crevice corrosion."

 

[Stu Jackson]

http://forums.sailboatowners.com/index.php?threads/re-packing-stuffing-box-84-303.182431/

 

[Ken Cross]

The OP indicated a bronze shaft. The issues of SS don't apply here. I don't know if other issues do, but I just added a dripless shaft seal (PSS) on our bronze shaft. Now, no more drips and it was not much more expensive than a replacement packing system. I'll need to wait and see how it performs over time.

Ken

 

[CarlN]

I have a Aquamet shaft and that's why I'm inclined to shift to Gore GFX -- but MaineSail's excellent guide appears to say that EVEN with Aquamet shafts you should check the shaft once a year for corrosion from this type of packing. Or did I misread this? Or is this just being overly conservative? (which seems not really a bad thing when dealing with corrosion).

I assume to "check" for corrosion you need to look under the packing by pulling it or sliding the shaft. Or am I confused about this too?

 

[Charles Erwin]

I am no materials guy but some shafting are true bronze alloys, some are more like a soup of metals, the proportions of elements vary.

Many materials use the term bronze in their name but they are not really bronze which has a very high copper content. Bronze parts go under historical names like Monel, Tobin bronze, Mangnese bronze, Naval bronze, Aluminum bronze, what some of call 85 5 5 5 bronze etc.

As to bronze shafting - garden variety shaft materials typically have a relatively high zinc content. That means they are more brass than bronze. If so this is not good in the presence of graphite and especially graphite impregnated (traditional compression) packing like Gore GFO, Western GTU, Duramax UltraX.

So the short answer is avoid bronzes and carbon (graphite) in contact and submerged in water because you have no idea what the shafting is made from. If you know what the shafting is made from - contact the shaft manufacturer and ask.

Here is a picture of traditional graphite packing damage that Maine Sail published. This damage was reported to have occurred over a pretty short time span. http://forums.sailboatowners.com/index.php?threads/stuffing-box-packing.159570/&highlight=bronze shaft damage

Charles