Mast Step Corrosion

[wingssail]

I have evidence of corrosion under my mast step.

A thick white paste has been exuding from under the mast step where is rests on the fiberglass floors.

As shown in the photo the mast, foot and step are Aluminium alloy. The floors (ribs) are bare fiberglass.

There doesn't seem to be deterioration of either the step or the floors but I worry about that.

There is little or no corrosion anywhere else in the boat except paint blistering and some white powder on bolts around the partners.

The mast and step structure are bonded to a keel bolt with the cable shown. The chain plates are also connected to the same keel bolt with heavy cable,

There is usually moisture in the bilge under the step, mostly from rain water coming down the mast.

What is causing this corrosion and white paste? What can be done about it?

[bil56]

water, different aluminium alloy, SS screws, ...
a mini electric battery

./.

[Bowdrie]

White powder=Alumina, mix with water=white paste.
On thing about aluminum is that it's always more than ready to return to its natural state.
Those cables connecting chainplates/mast/keel bolts; is any other wiring connected into that loop?
If so, I would suspect some current leakage from somewhere.
As per above post, perhaps a battery of sorts has been established.
Or, years of salt water is having its way with the mast/mast step.

[wingssail]

Originally Posted by Bowdrie

White powder=Alumina, mix with water=white paste.
On thing about aluminum is that it's always more than ready to return to its natural state.
Those cables connecting chainplates/mast/keel bolts; is any other wiring connected into that loop?
If so, I would suspect some current leakage from somewhere.
As per above post, perhaps a battery of sorts has been established.
Or, years of salt water is having its way with the mast/mast step.

There is no other wiring connected to the mast grounding system.

[Bowdrie]

Originally Posted by wingssail

There is no other wiring connected to the mast grounding system.

Ok, fair enough.
Just throwing this out there;
Is there any chance that some internal mast wire is chafed, or somehow perhaps feeding current into the mast when it is energized?
If so, now we have a return path to the battery.
Chafed wire>mast>keel>thru water>prop>engine>batt negative.

[wingssail]

Originally Posted by Bowdrie

Ok, fair enough.
Just throwing this out there;
Is there any chance that some internal mast wire is chafed, or somehow perhaps feeding current into the mast when it is energized?
If so, now we have a return path to the battery.
Chafed wire>mast>keel>thru water>prop>engine>batt negative.

Oh, that's interesting. If so I should be able to measure voltage between the mast and battery neg. And it should go away if everything on the boat gets turned off. I'll try it, Now where is my multimeter?

[wingssail]

Did some meter readings, Mast to batt neg. Not sure what to make of it all

DC voltage (2000m setting) got 339 . must be 339 mv with all switches off

DC Voltage (2000m setting) same reading, with Batt positive disconnected,

AC Voltage Mast to Battery, No reading

[misssherry]

I would try disconnecting the bonding cable from the mast and run your multimeter test again.

[donradcliffe]

A common problem with mast grounding is that for lightning protection you need a strong ground to seawater. That aluminum to water potential is going to be different than your prop shaft ground potential which is based on bronze and SS. This makes white powder.

[Bowdrie]

Originally Posted by donradcliffe

A common problem with mast grounding is that for lightning protection you need a strong ground to seawater. That aluminum to water potential is going to be different than your prop shaft ground potential which is based on bronze and SS. This makes white powder.

You're right, and I looked at the pic of the cable connected to the mast but didn't say anything about it.
I've put it in a post before; what I did on my boat was take the cable from a keel bolt to a 1/2" bronze bolt that's mounted on a simple bracket.
The point of the bolt is ~1/8" from the mast step, (under the sole,) this keeps the mast isolated, but if lightning strikes the airgap will ionize and carry the strike into the keel, (hopefully).
They do make expensive devices for such purpose, infinite resistance until a strike, then they conduct.