1. Home
  2. Electronics Design
  3. OT: red lead over zinc, good or bad?

OT: red lead over zinc, good or bad?

I've seen some people painting red lead paint over their galvanized iron sheets. They think it protects the iron for a longer time than the zinc alone.

Somehow I'm not sure about this. Is the zinc not going to try to reduce the lead-oxide into lead while offering itself up?

My chemical knowledge is not sufficient for me to answer this question, but I guess it might have to do with elektro-negativity? I vaguely remember from high school that the elektronegativity predicted the direction of a reaction, but I don't remember how that worked anymore.

Of course the more politically correct iron oxide would cause the zinc to be consumed rapidly?

Any bright mind in here knows how this works?

joe

Reply to
Joe Hey
Loading thread data ...

The paint keeps the water away from the metal. That keeps ionic reactions from happening.

--
Grizzly H.
Reply to
mixed nuts

Why do water molecules play a role in this, does it facilitate the movement of zinc, lead or iron ions?

If it's just a matter of covering, then any paint would do, no?

joe

Reply to
Joe Hey

On a sunny day (Sun, 20 Dec 2015 17:35:46 +0000) it happened Joe Hey wrote in :

I had red lead based paint, and got rid of it as it is forbidden AFAIK over here.

Reply to
Jan Panteltje

Yes. For ionic reactions to occur you need a medium capable of "dissociating" so that charge can be transferred so that ions can form and ionic exchange can proceed. Water is such a medium. There are others - including ionized gasses.

Lead oxide (aka PbO, litharge) linseed oil based paints form reactive molecules which bind chemically with metal surfaces. As a consequence, they don't peel off easily. The lead oxide also reacts with ionic species on the exposed surface (e.g. salt water) which slows the migration of reactive species through the film.

Other paints do work but the combination of chemical bonding, flexibility and toughness of the PbO-linseed oil mix is very hard to beat.

--
Grizzly H.
Reply to
mixed nuts

Ok, thanks for your expert contribution.

joe

Reply to
Joe Hey

Yeah, but he said red (Pb3O4), not litharge.

I don't know the composition of those old paints so I really don't know. It could simply be the inertness of the red lead: It takes significant effort to oxidize to PbO2, so that won't happen under environmental conditions. Nor will it be reduced to PbO, suboxides or Pb, because of oxygen, and due to its stability -- being in the spinel family.

Perhaps some reduction does happen in the presence of lipids (such as used in those alkyd/natural oil based blends), perhaps some complexes too (chemical boning of the pigment particle to the polymer network?). That would make it stronger.

For sure, you wouldn't use Fe2O3, because that is relatively easy to reduce, and in combination with water and oxygen, and especially acid rain, it catalyzes free radicals (OH, O2+, etc.), which will destroy the polymer in no time. It will also disproportionate with the base metal: x Fe2O3 + y Fe = Fe3O4 (x and y are some rather dumb fractions that I don't care to calculate right now). Fe3O4 is also fairly stable (also being in the spinel family!), but alas, it rusts under environmental conditions, leading to the same corrosion process iron has always been susceptible to.

Tim

--
Seven Transistor Labs 
Electrical Engineering Consultation 
Website: http://seventransistorlabs.com
Reply to
Tim Williams

If I understand the process (fading memory of an old Dept of Agriculture book), linseed oil was boiled with litharge until thick, an iron III oxide (essentially rouge) was added and the whole thinned to make paint (presumably for barns and other implements considering the source). I've also seen a similar recipe for artists to make a tacky substrate for murals over stone or plaster.

--
Grizzly H.
Reply to
mixed nuts

Thank you!

joe

Reply to
Joe Hey

ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.