Crusty tough white deposits on anchor

[C08]

I am wanting to paint an anchor and am finding it difficult to remove the white deposits with a drill and wire brush attachment. I could use a finger sander but I do not want to sand into the galvanising where it is still good. Chemically what are the deposits and how best to remove without further abrading the surface.. Thanks.

 

[sarabande]

Zinc Oxide .

Brush some vinegar on to the deposits and that should loosen and clean off the "white rust".

 

[C08]

Thanks for that SB.

 

[Neeves]

A less aggressive way of treatment is simply to use the anchor in some nice clean sand as the vinegar will also attack the galvanising. Galvanising is quite robust, and abrasion resistant but not particularly resistant to acid (though an angle grinder will remove anything!). If you use acid, do it quickly - then wash with fresh water. If needs be - do it again - don't leave it to soak.

I'm not sure that white rust is zinc oxide and not zinc hydroxide and the normal and recommended method of removal does not include any acid (though vinegar is very mild) but water (and light abrasion). White rust forms in the presence of moisture (I'm not sure if salt accelerates the process) but washing the anchor with fresh water and keeping it dry (unlikely on a bow roller ) but it is also a problem with chain - will reduce white rust formation. If left, I understand, you can consume all of the external zinc coating (not the zinc alloys) - though I have not actually heard of this happening nor seen evidence. The pure zinc external coating does not last long, being soft, but it does offer some temporary protection (and you have paid for it). Being soft it is said to protect the chain on rocky seabeds and as it passes through the gypsy - but as it is transitory its not that critical.

The problem is common and is usually caused by storing in a nice damp locker, which anchor locker is not damp!? and is seen also on chain. Its the surface, shiny, zinc left at the end of galvanising (often spun or shaken off, as much as possible) reacting in a damp environment. Normally as this exterior zinc is soft it soon wears off in use and as far as I know (I stand to be corrected) the alloys created during decent galvanising are stable and wear by abrasion (but are susceptible to acid).

If you still have superficial zinc you really should not need to paint the anchor (unless its purely decorative) - the paint will not last long and the gal should last for a few years. You might lose gal on the 'working' edges of the anchor which may then rust (red) - but that should be worn off every time you use the anchor.

If the issue is, red, rust (as well as white rust) - you do need to attack it with something, abrasion, chemical (acid), to get a decent clean surface for primer - or it will continue to (red) rust

Jonathan