How do I remove galvanize

Factual Questions
21

As a commercial fisherman and an aluminum boat builder, I can speak with some authority on this question. Electrochemical corrosion is caused by two dissimilar metals in sea water making a primitive battery. When the electrical current flows between them, one of the metals is eroded. The erosion is proportional to the voltage between them, which is determine by the difference in the “galvanic potential” of the two metals.

Here is the relevant part of a table of the galvanic potential of metals in sea water:

ELECTROCHEMICAL CORROSION POTENTIALS
The following table lists the corrosion potential (in volts) for various materials measured against a saturated calomel electrode in sea water at 25°C. The potential difference between any two materials should not exceed 0.50 volts for equipment installed inside, subject to salt free condensation, and **0.25 volts for equipment installed outside at any location. **

The material with the more negative potential will erode away.

Example, combination of stainless steel (CRS316) and galvanized Iron:
From table, stainless steel (CRS316) = -0.35 V, galvanized iron = -1.05 V
Potential difference = -0.35 - (-1.05) = 0.7 V
Therefore the galvanized iron will exhibit accelerated corrosion

Material Potential (volts)

**Zinc & its alloys **
Zinc die casting alloy -1.10
Zinc plating on steel -1.10
Zinc plating on steel, chromate passivated -1.10
Zinc coated (galvanized) iron -1.05
Tin-Zinc (80/20) alloy plating on steel -1.05
Cadmium plating on steel -0.80

**Aluminium & its alloys **
Wrought aluminium-alloy-clad aluminium alloy -0.90
Cast aluminium -0.75
Wrought aluminium -0.75
Aluminium-manganese alloy -0.75
Aluminium-magnesium alloy -0.75
Aluminium-silicon-magnesium alloy -0.75
Duralium (unclad) -0.60
**Irons & steels **
Non corrosion resisting -0.70
Stainless steel (CRS304) -0.45
High chromium stainless steel (CRS316) -0.35
Austenitic -0.20

Some things to note here:

  1. Zinc is the most negative, so it protects the steel/aluminum combination, and will erode in preference to the aluminum or the steel. That’s why it’s on there, you should not remove it. The voltage between aluminum and galvanized steel is 0.35V, but the zinc erodes rather than the steel or the aluminum.

  2. The difference between aluminum and steel is only 0.05V, which is not enough to cause a problem. Corrosion will be very slow, and generally not significant.

  3. Unlike what you’d expect (or at least I’d expect), steel and aluminum get along much better than stainless steel and aluminum.

  4. In addition to the problem of galvanic corrosion (from two dissimilar metals), we also have the problem of plain old rust. Galvanization also protects against rust, because the zinc doesn’t rust and the steel is coated with the zinc.

Because of these factors, removing galvanization from steel in a marine environment is A Bad Idea™, regardless of whether it is used in contact with aluminum or not. I’ve used galvanized steel clamps on aluminum at sea a number of times, and haven’t had a problem.

To help slow any possible corrosion, rinse the connection with fresh water when you come back into port, and remove the clamp when it’s not needed.

w.

PS - Note that both copper and its alloys (bronze, brass) and silver (galvanic potential -0.20) are all extremely reactive with aluminum, so be very careful with those. The difference between them and aluminum is 0.5V, which makes for a strong reaction, with the aluminum being eaten away. I know people with aluminum yachts who make guests stow all their coins in a drawer when they come aboard, because if you drop a copper penny in the bilge, after while it will eat a hole right through the aluminum … and not a long while, either.

22

PS - One more note. A curiosity is that carbon (galvanic potential plus 0.1 V, used for that reason as one of the poles in an ordinary “C” cell battery) reacts even more strongly with aluminum than copper. If you draw construction lines on an aluminum boat with a pencil, they will erode the surface and leave a permanent etching of the lines on the surface. Unless this is what you desire, use a magic marker instead …

23

Isn’t it zinc that is a plant killer, and a strip can be put up to help prevent alge on an exterior surface below the strip?

24

So doesn’t this mean that its best to use the ungalvanized steel?

anyway it’ll be painted.

25

“Zincs” are pieces of zinc attached to a boat below the waterline. Because of the galvanic effect explained above, it will corrode first, before any other submerged metals. It’s called a “sacrificial anode” for that reason, and needs to be replaced periodically. It has no effect on the algae.

“Bottom paint” is poisonous paint used to keep algae from growing on the bottom of the boat. It often contains a metallic compound, with either copper or tin, but this is because the compound is poisonous, not because of its galvanic effects.

Copper based bottom paints cannot be used on aluminum boats, for the galvanic reasons I listed above.

w.

26

No, the galvanized is better. Ungalvanized steel rusts so fast in the marine environment as to be nearly useless.

Paint is good. Painted galvanized steel can last a while at sea, and most steel vessels are made of painted un-galvanized steel. Be sure to use a marine paint, though, as regular paint won’ cut it. There are 2-part marine epoxies that have added zinc to slow corrosion, for example here , that are specifically designed to do the job. Pricey, but you only need a little bit.

Because of the temperature extremes, the salt, the moisture, and the motion, the marine environment is one of the harshest on the planet. You are wise to be thinking carefully about your choices of materials and treatments.

w.

27

The nautical environment around here ,Iowa, doesn’t include sea water, so its not quite as harsh as yours.
thanks for the info.

28

Well thats what a couple of beers will do for you.
thats a marine environment.

29

Galvanic corrosion in fresh water is not much of a problem. I worked last summer as a sport salmon guide on the Kenai River in Alaska. I used C-clamps to hold my autopilot on all summer, and had no problem.

Stay safe on the water,

w.

30

You had a pretty cool job.
thanks.
You stay safe too
just

31

I had an epiphany today and slapped myself on the head. Just to clarify the chemistry going on here. Yes aluminum produces a nearly impenetrable barrier of aluminum oxide. In this case however it is not atoms that need to cross the barrier but electrons. We all know that a peice of aluminum will conduct electricity, so the barrier is irrelevent and a galvanic cell is easily set up.

In my defence I am an organic chemist.

ehhh OK. Its still not going to dissolve the aluminum oxide layer, but thats a nitpick. I would have guessed the oceans were acidic.