There are two general types of electrically induced corrosion - Galvanic & Electrolytic.
Galvanic (Cathodic)Corrosion is self-generating, and occurs when two dissimilar metals are electrically connected, and immersed in an electrolyte (water).
In this instance, an electrochemical ncell is formed, and a voltage is developed between the two metals (or alloys), with water
acting as the connecting electrolyte.
The more active metal (anodic) will have a higher electrical
potential than the less active (cathodic) metal, and will deteriorate. Most Steel
is generally higher on the Galvanic Series than most Aluminum Alloys.
Electrolytic or Stray Current
corrosion refers to the erosion resulting from an “unintended” current flow, and may be DC induced, AC induced, or Telluric (a natural phenomenon caused by geomagnetic disturbances).
This unintended current flow typically occurs when an AC or DC electrical
source is connected through immersed metals to ground. Damaged wiring
& terminations, and improper grounding & bonding techniques are among the commonest causes of electrolytic errosion.
It appears that you are on the right track, in asking “how to remove the current corrosion”.
I agree with your implied assessment that you may be suffering from electrically induced electrolytic corrosion (given the “dry” conditions).
Nonetheless, I suggest that you first eliminate galvanic or cathodic corrosion, as you do have dissimilar metals (steel mounts & aluminum stringer/hull).
1. Generally speaking, Steel fittings should be electrically & mechanically isolated from Aluminum hulls. Are your mounts in electrical contact with the aluminum stringers?
A general “rule of thumb” for galvanically compatible metals (USE /W CAUTION)
1. Magnesium & it’s alloys.
2. Cadmium, Zinc, Aluminum and their alloys.
3. Iron & Steel, Lead, Tin, and their alloys (NOT Stainless Steel).
4. Copper, Chromium, Nickle, Titanium and their alloys (including Stainless & Graphite).
2. Are there salt
deposits bridging the steel to aluminum interface? This would indicate previously wet conditions - and would in itself form an electrical “couple”.
3. Is wood used as an isolator (twixt St. & Al.), and (if so) is the wood “white”, “punky”, or “foamy”?
4. Is the steel mount (& engine block) effectively “bonded” to the hull
5. Is the steel mount painted? Metalic base paints can cause galvanic corrosion.
And more ...
OK - so none of the above apply, and the corrosion is electrically induced.
6. The Aluminum hull
is the main grounding conductor and ground plate (electrode). Any metal part NOT connected to the hull (isolated as above) must have a bonding conductor attached to the hull (see 1 & 4 above).
7. Do you have a “Reverse Polarity” indicator or alarm
? I recommend that reverse polarity indicators be either: VERY high resistance circuits, or TEMPORARILY connected via a momentary switch.
8. Are you connected to shore power
? If so, install a galvanic isolator
, and check the marina’s wiring
for AC polarity, and ground continuity - and/or disconnect from shore power
except as absolutely necessary.
9. Examine your wiring (AC & DC) for frayed insulation
, chaffe, thermal injury (scorching etc), and/or poor terminations. All wiring should be run high & dry, supported, and protected against mechanical or thermal damage. All terminations should be tight, isolated (insulated) from each other & ground, and sealed against moisture.
CAUTION: DISCONNECT AC SHORE POWER, and INVERTERS (if installed) prior to performing the following tests!
10. You can check for DC POTENTIAL with a Voltmeter as follows:
-a- Turn OFF all AC & DC Loads, at their local switches.
-b- Turn ON all DC Breakers, except where the breaker directly controls a device .
DO NOT PROCEED unless you fully understand the differences between a & b above!
-c- Set the meter to 25VDC range.
-d- Connect the Positive Voltmeter test lead to the Positive Battery
Terminal, and the
Negative lead to the hull.
-f- Connect the Negative Lead to the Negative Battery
Terminal, and the Positive Lead to the Hull.
If no voltage either test (d & f) OK - If Voltage present proceed.
-g- Set the meter to “OHMS”
-g- Disconnect & Isolate the Positive & Negative Battery cables
from the Battery
-h- Connect Ohmmeter between Positive Cable & Hull, then between Negative Cable & Hull.
If infinite resistance (open circuit) OK - If continuity present (lower resistance) proceed.
-i- Test each circuit individually.
11. You can check for stray DC CURRENT with an Ammeter (20A - 10mA) as follows:
-a- Turn off all AC &DC loads.
-b- Remove the Positive Battery Cable from the Battery post.
-c- Connect Positive Lead of Ammeter to the Positive Battery Post
-d- Connect Negative Lead to the (removed) Pos. Battery Cable.
-e- Read current (starting @ highest Amps scale - preferably 20A, then switch to lower scales as practical)
-f- No current should flow, even at the lowest setting! (OK - perhaps as much as 1 milliamp = 0.001 A).
-g- If more than 1 mA, each load circuit must be individually tested.
I hesitate describing the similar testing procedures for AC systems, as they could be very dangerous! I recommend that anyone not EXPERT in electrical testing seek professional assistance.
Note re: Copper Wire & Aluminum Hulls
EXTREME caution should be taken to insure that NO copper strands, strippings or other remnants are allowed to come in contact with Aluminum!
You’ve raised a very interesting and complex subject, about which many books
and learned papers have been written. I hope the forgoing helps in some measure, but caution that a forum , such as this, cannot purport to provide comprehensive advice.
E. & O. E.