SV Pilgrim - 1979 Morgan 382 - Homeport: Beaufort, NC

Monday, September 15, 2014

Corrosion Discovered on Backing Plates

This post is a continuation of previous post– Morgan 382 Chainplates.

The only significant corrosion we discovered was on the aluminum backing plate for the port cap shroud chainplate. This plate is accessed via the outboard cabinet in the head.  The visible side of the backing plate and the six, ½ x 2 ½ ” stainless steel fasteners appeared fine, but removing the plate we discovered…
White powdery residue from aluminum vs. salt water corrosion caked on plywood bulkhead.
This type of corrosion typically occurs when salt water is trapped against aluminum.  Despite recent heavy rains, the area was not wet when we removed the backing plate. So, I’m guessing this corrosion is from Pilgrim’s early days in south Florida.  I’m also speculating that the water intrusion came from leaking port windows that have since been replaced.

 Fortunately the plywood bulk head is not damaged and the pitting on the plate is not serious enough to warrant replacement. 

Using  a scraper followed by 80 grit sand paper, I removed the white powdery residue from the bulkhead and backing plate.  The backing plate then when home with me for an overnight soak in vinegar.  The next morning, I used a wire brush chucked into a cordless drill to clean the plate.
The aluminum backing plate after a visit from the wire brush.
I followed this up with a soap and water wash down.  Finally, nearly 24 hours after it’s extrication, the backing plate was ready for a few coats of zinc chromate.  The zinc chromate chemically etches the aluminum.  The etching process forms a chemical bond that makes the surface much more resistant to corrosion.
Allowing three coats of zinc chromate to air dry.
After allowing the zinc chromate to dry for 48 hours, I applied a few coats of tan spray paint to the backing plate. 
The backing plate painted and re-installed.  Note the white creamy substance on the bolts is
Utra-Systems Tef-Gel
While in the process of painting the salon, I removed the aluminum angle brackets that join the salon bulkheads to the outboard cabinetry.  The salon cabinetry serves and an anchor point for the aft lower chainplates.  Thus these short sections of angle aluminum are critical.  Unfortunately I found similar salt water corrosion along the back side of all the brackets.
Corrosion on the backside of the port forward angle brace in salon.  Note - Inadequate 1/4" dia x 3/4" long fasteners to left of bracket.
Much to my dismay, I also discovered that the screws in all the angle brackets were only ¾” long.  Hmm… the aluminum angle is 1/8” thick + the ¼” teak veneer on the bulkhead = 3/8”.  This means that at best the screws only protruded 3/8” into structural material!  Guess I need to add replacing the salon aluminum angle brackets to my project list.

But first re-bedding the chainplates…

For more images and notes check out our Chainplates Photo Album.

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