Dissimilar Metals Corrosion on Sheet Blocks? [Solved]

[Christian Williams]

They're "Surfline" twin genoa sheet blocks. I've been able to keep them turning by periodic lube, but apparently missed the window once too often. The sheaves are stuck. I've tried soaking for days in PB Blaster, soaking in hot water detergent, heat gun, tapping with hammer, pounding with hammer, clubbing hard enough for a baby seal.

But they're history, frozen irredeemably. Never heard of Surfline fittings, and although nicely machined these were apparently designed with tolerances too close for the yachting environment. Oh well.

I've lost several fittings to stainless/aluminum corrosion welding: an outhaul slug in a boom channel, a pedestal guard corroded to the YS binnacle connector plate, and lesser components too. Tef-Gel solves the issue for new installations. Otherwise it is necessary to pay attention and manipulate the fitting periodically, as we have all learned as the only way to keep our headliner zippers alive.

"A stitch in time saves nine."

 

[Nick J]

Bummer. 40 years was a good run though. What's the plan for replacement?

Although the failure wasn't due to dissimilar metals, the cars on my 25+froze and the sheets started waring a groove in the sheave. I "fixed" it by lashing a low friction ring to the body of the car. Unfortunately, I sold the boat before I was able to put any real time on them and haven't heard from the new owner. Probably a little light for the cars on a 38'.

 

[Christian Williams]

The rings are a good idea. I figure if the sheet isn't moving back and forth a lot, a smooth bearing surface is pretty close to OK. And my recently replaced half-inch genoa sheets had been through a lot and the covers showed almost no wear.

I'll probably put the frozen blocks back on the boat for now. I only have to switch to the outer genoa track when reaching, and since the frozen Surfline sheaves are smooth and slippery, let's see what happens. "See what happens" can be a good plan, as long as you're not heading off transAtlantic...

 

[Kenneth K]

View attachment 45858
But they're history, frozen irredeemably. Never hear

Seems a shame to lose them.

Can you:
- hammer a putty-knife blade between the surfaces?
- try prying the "jaws" apart and working a small wire or blade in between the surfaces?
- use a small, stranded wire like dental floss between the two surfaces?
- try heating the whole thing up to 300 deg in your oven and then "shocking" one of the jaws with dry ice? (wear thick gloves & eye protection)

I've purchased similar on eBay, which used to have good prices on this stuff, but not so much any more.

 

[gareth harris]

Soaking in acid has worked for me. Galvanic corrosion is caused by the dissimilar electron potential between the metals, and the acid removes some of both metals either side of the bond to free up the joint.

Gareth
Freyja E35 #241 1972

 

[Christian Williams]

They're soaking in vinegar at the moment and showing some hopeful signs of residue.

From what I read hydrochloric (muriatic) acid ruins anodized finishes quickly, so I'm holding off on that.

 

[u079721]

They're soaking in vinegar at the moment and showing some hopeful signs of residue.

From what I read hydrochloric (muriatic) acid ruins anodized finishes quickly, so I'm holding off on that.

Hydrochloric acid will work for sure, but it also dissolves aluminum. I would try some Naval Jelly (phosphoric acid) and see if I could get it down the cracks without coating the whole part with it.

 

[gareth harris]

Hydrochloric acid will certainly ruin a finish but it may succeed where vinegar (ethanoic acid) fails since it has a much lower pH.

The table on the below link shows how strongly different metals react with each other when in contact via an electrolyte such as sea water. The further apart the potentials on the table, the greater the reaction, and as Steve has pointed out, aluminium is extremely vulnerable. Zinc is above iron on the table which is why pieces of zinc are attached to steel parts to protect them below the waterline.

Hydrogen is also on the table to show how strongly the metals will react with the hydrogen ions in sea water, and also in acid. Copper is below hydrogen on the table so is stable from attack by hydrogen ions, as are its alloys bronze (copper and tin) and to a lesser extent brass (copper and zinc). All metals above hydrogen are vulnerable and will dissolve in acid, although only slowly in the case of lead and tin.

The reason for the difference in potentials between metals is university level physics involving the quantum energy levels of free electrons in a metal lattice.

Since HCl is made from constituents of sea water you would think it environmentally friendly to dispose of in small quantities, although regulations may say otherwise.

Gareth
Freyja E35 #241 1972

Galvanic Corrosion vs. Electrode Potential

Introduction to electro chemical series and corrosion of metals.

www.engineeringtoolbox.com

 

[Christian Williams]

To my surprise, problem solved. And it apparently wasn't dissimilar metals corrosion after all.

Although PB Blaster and hammer violence did nothing, three days of soaking in vinegar freed the sheaves. It seems by the residue to have been a buildup of salts and dirt in inaccessible areas.

In this design, the sheave turns on a stainless insert mated to it.



The insert bears on the hole in the cheek. The insert was apparently pressed in during manufacture, so disassembly isn't possible. A machine screw penetrates the insert on each side.

The tolerances are small and something built up in there to freeze the sheaves. A dental pick helped dislodge the crusty glop. If this were actual corrosion, I don't think vinegar would've worked.



So: I will spin the sheaves more often. Hose water every few weeks didn't do the job.