[TowerTalk] Aluminium Paint versus Cold Galv

[Donald Chester]

Reviewing the archives,  I see many posts over the years on the subject Cold Galvanising paint.  Some say it works wonders while others have reported it being no better than ordinary paint.  Here is my experience.

I once used a spray can of Cold Galv on another outdoor steel structure, and recall rust spots creeping through within just a few months. I have used aluminium paint on unprotected steel, and on galvanised steel showing signs of rust, and it always seems to last a long time before more rust appears.  The steel casting on the salvaged broadcast station base insulator I use with my tower was not galvanised at the factory. After erecting the tower I painted the ungalvanised casting with aluminium paint, and in the 33 years that I have had the tower up, I have re-painted it only once. Aluminium paint seems to be almost as good as hot-dip galvanising for warding off rust, and I believe it is a more effective product than Cold Galv for protecting steel.

Here is a brochure from ALCOA touting the advantages of aluminium paint.  See
https://ia601502.us.archive.org/33/items/AluminumPaintATreatiseOnThePhysicalPropertiesOfAluminumPaintAndIts/AluminumCoOfAmericaCca39891.pdf

According to the brochure, "Many power companies are finding aluminium paint more satisfactory than galvavizing for protecting the steel towers used in  supporting power lines... In painting towers, bridges and structures in general, the painter appreciates the difference in weight...  Since the corrosion of iron and steel is caused by moisture and oxygen, it is obvious that a highly impermeable paint film is  desirable for their protection"

 ALCOA goes on to say, "The metallic aluminium flakes, called aluminium bronze powder, which are its pigment portion, have many unique and useful properties which no other paint pigment has. The simple mixing of this aluminium bronze powder with a suitable oil or varnish vehicle at once provides an aluminium paint which  renders unusually satisfactory service in many and varied applications... Aluminium bronze powder for paints is made by stamping aluminium into very small and thin flakes... The shape of the particles of aluminium bronze powder is of particular interest. The ordinary pigment materials like zinc oxide, red lead, white lead, etc., are composed of particles distinctly granular in form even though they be exceedingly small. Aluminium bronze powder is, however, essentially flake-like in character... little particles of aluminium swirl about in the liquid and many of them come to the surface of the liquid and remain there... Very quickly an almost continuous film of metallic aluminium is formed at the surface of the varnish by the little flakes... which arrange themselves, in layer upon layer, much like fish scales... descriptively called LEAFING..."

Of course, one must keep in mind that ALCOA is promoting their product, but still this is a credible explanation. Cold Galv uses tiny particles of zinc suspended in the vehicle/bonding agent of the paint to coat the steel, on the theory that the zinc particles will protect the steel in the same manner as the solid layer of zinc in real galvanising. But with hot-dip galvanising, the zinc coating is directly bonded to the steel like solder, making electrical contact with the steel substrate. By galvanic action (hence the term "galvanising"), the zinc sacrifices itself in the presence of corrosive substances like sulphuric acid in polluted air, thus protecting the steel from rust. With zinc paint, the particles are suspended in the varnish-like vehicle of the paint, which continues to serve as a bonding agent after the paint dries. Since varnish is an electrical insulator, not a conductor, each zinc particle is effectively isolated from the steel it is supposed to protect, so how could any galvanic action occur?


Don k4kyv