Cleaning Zinc Anodes

D

davidmh

Well-Known Member
Joined
30 Jan 2015
Messages
476
Visit site
The pear Anode on my yacht gets covered in a whitish/brown deposit, it is hard and will not scrub off, it does respond a little to chipping. The anode does erode in places and the prop is in good condition. Anyone now what the crust is, do you think it may dissolve in a weak acid such as Brick Cleaner. I have a used one which is off the boat, and I could just soak the anode and its backing plate in the acid. The backing plate seems to be a galvanised steel strip.

Any thought from the chemists on the forum

Regards
David MH
 
Usually a mix of zinc oxide and calcium deposits, with additional fouling growth. You can just abrade it with either a file or coarse (80 grit) sandpaper. The calcium and hard fouling will dissolve in vinegar over night, or faster in some stronger hydrochloric acid (brick cleaner). Not sure what that does to the zinc though. I just replace mine, as they tend to also get porous and crumble.
 
Yngmar said:
Usually a mix of zinc oxide and calcium deposits, with additional fouling growth. You can just abrade it with either a file or coarse (80 grit) sandpaper. The calcium and hard fouling will dissolve in vinegar over night, or faster in some stronger hydrochloric acid (brick cleaner). Not sure what that does to the zinc though. I just replace mine, as they tend to also get porous and crumble.
Click to expand...

The zinc will react with the acids at least as fast as the unwanted deposits. Don't do it! The mechanical solution will give you more control without wasting the zinc.
 
I clean mine up with a wire brush on an angle grinder.
But be sure to wear goggles or visor and a face mask, not good to breathe the dust in or get particles in your face
 
I thought you should not use a ferrous metal object or brush on a zinc anode as it leaves traces behind which prevent the zinc working. Old wives tale or not?
 
Never heard this one and can't see any reason why it should cause any problem. As long as a sufficient area of the anode is directly exposed to sea water (electrolyte) and the anode is connected to the protected metal parts with minimal electrical resistance, it is going to work. Some traces of other metals on the zinc can't make any noticeable difference.
 
jiris said:
Never heard this one and can't see any reason why it should cause any problem. As long as a sufficient area of the anode is directly exposed to sea water (electrolyte) and the anode is connected to the protected metal parts with minimal electrical resistance, it is going to work. Some traces of other metals on the zinc can't make any noticeable difference.
Click to expand...

Anodes are made of a high purity zinc alloy, usually to a US military specification. The iron content , in particular, is very low, as it has been found that iron in any significant amount as an impurity adversely affects the performance of the anodes.

That is the basis for not using a steel wire brush to clean the anodes. Valid or not I am not in a position to judge

Zinc anodes : MIL-A-18001K

Cadmium . . . . 0.025% - 0.07%
Copper . . . . . . .0.005% Max.
Iron . . . . . . . . . 0.005% Max.
Lead . . . . . . . . .0.006% Max.
Aluminium . . . . 0.1% - 0.5%
Zinc . . . . . . Remainder (special high grade 99.995%)​
 
VicS said:
Anodes are made of a high purity zinc alloy, usually to a US military specification. The iron content , in particular, is very low, as it has been found that iron in any significant amount as an impurity adversely affects the performance of the anodes.

That is the basis for not using a steel wire brush to clean the anodes. Valid or not I am not in a position to judge

Zinc anodes : MIL-A-18001K

Cadmium . . . . 0.025% - 0.07%
Copper . . . . . . .0.005% Max.
Iron . . . . . . . . . 0.005% Max.
Lead . . . . . . . . .0.006% Max.
Aluminium . . . . 0.1% - 0.5%
Zinc . . . . . . Remainder (special high grade 99.995%)​
Click to expand...

Funny how the zinc anode is held on with a steel strip, and that doesn't seem to affect it's performance.
I make my own anodes for my Bruntons Autoprop, and although they are made by melting down old discarded anodes in an iron ladle, they work perfectly well. When melting the zinc, simply skim off any floating impurities before pouring.
 
I'd say there is a difference between the iron being present in the alloy or being a "trace" of it on the surface - from where it quickly disappears anyway, as the anode is being consumed...
 
NormanS said:
Funny how the zinc anode is held on with a steel strip, and that doesn't seem to affect it's performance.
I make my own anodes for my Bruntons Autoprop, and although they are made by melting down old discarded anodes in an iron ladle, they work perfectly well. When melting the zinc, simply skim off any floating impurities before pouring.
Click to expand...

The steel strip is not "alloyed" with the zinc as an impurity.

I dont think I have the expertise or the facilities to test any home cast anodes so would not be able to make any judgement on their performance although it would be an interesting project given the right facilities.
 
An early paper (1956, abstract here: http://www.corrosionjournal.org/doi/abs/10.5006/0010-9312-12.7.53 ) reported that ‘Zinc containing a higher iron concentration will develop a high resistance corrosion product film which will render the zinc less effective as an anode material’. [My italics.]

Zinc ions adsorb quite strongly on ferric oxide/hydroxide – even when the surface charge on the solid is positive, such is the strength of chemical bonding between the zinc and hydroxyl groups in the solid. I suspect that the same chemical bonding could maintain a thin coating of insoluble ferric oxide/hydroxide on a zinc surface.

Like VicS I cannot judge the validity of the warning not to use a steel wire brush to clean an anode. But given a plausible mechanism by which iron might affect the zinc, I’d use another abrasive and/or vinegar as Yngmar suggests.
 
I was given to understand that the coating will disappear over a couple of weeks in the water, but that the anode will not be functioning during this time and that exposing bare metal was absolutely necessary.
 
johnalison said:
I was given to understand that the coating will disappear over a couple of weeks in the water, but that the anode will not be functioning during this time and that exposing bare metal was absolutely necessary.
Click to expand...

Was that a coating that appeared only after the boat had been lifted out?

Halo said:
I tried brick acid on an anode and ruined it. Don't try it ! I find a wire brush on a drill works fine
Click to expand...

Brick acid is a lot stronger than vinegar, but I accept the point that any acid/complexing agent that attacks a calcium carbonate deposit (if that is what it is) will also attack the zinc, so perhaps better to use an abrasive – but not a steel one if you are concerned about the iron issue.

I did once briefly use a steel wire brush on (calcium carbonate, I assumed) on a pear anode before recalling the advice not to do so, without to my knowledge affecting the zinc. But as other abrasives are readily available it seems sensible to use one of them and remove the concern.
 
You must log in or register to reply here.

Share this page

Top