Galvanized vs Galvannealed vs Phosphatized:

Coated Products Explained

We sell a variety of different coated flat-rolled products. The overwhelming majority of our steel is Galvanized, followed by Galvannealed. Here is a breakdown of the differences.

Galvanized steel is cold-rolled carbon steel with a zinc coating on either side. The zinc coating weight is denoted with an G60 or G90, which means there is .60 or .90 oz.of zinc applied per sq.ft. The zinc provides great corrosion resistance against rust, and will hold up very well if left exposed to the elements. However, it does not provide a great paint surface. It CAN be painted but only after the metal has been exposed to the elements for a period of time. It can be painted sooner, but not without a pretreatment. With its shiny zinc appearance, galvanized is the most common, cost-effective, corrosion resistant metal for HVAC.

Galvannealed is cold-rolled carbon steel coated with zinc/iron alloy on both sides. The zinc\iron coating weight is denoted with an A40 or A60, which means .40 or .60 oz./sq.ft. The steel is immersed in a zinc bath (just like the Galvanizing process). As the steel emerges from the bath, it enters the galvanneal furnace about 10-15 feet above air knives. Before the zinc can solidify, reheating of the strip begins. As the strip temperature rises, the zinc-iron diffusion reaction restarts and breaks down the aluminum-zinc-iron inhibition layer that formed in the zinc pot at the steel zinc interface. After 5-7 seconds of heating, and up to about 10 seconds of soaking, enough iron diffuses into the coating to convert it to a dull matte gray appearance. The result is a zinc/iron alloy that allows for better paint adherence. The iron in the alloy coating provides something for the paint to “grab onto”. It is a product that is intended to be painted, and should not be left unpainted when exposed to the environment. Galvannealed has a light dull gray appearance and is sometime mistakenly referred to as “paintgrip” – not to be confused with another coated product.

Phosphatized is technically the true “Paintgrip”. It is galvanized steel that undergoes an additional electrochemical phosphate conversion. This treatment bonderizes the zinc and makes it paintable. It has a dull gray appearance that is darker than galvanized. This phosphatized (also known as “bonderized” or “phos-bond”) has all the corrosion resistant benefits of galvanized but it can be painted.  However, this product is only  produced by a few domestic mills and thus becoming less and less specified in our industry.

Note: When I refer to “rust”, I am referring to the “red-oxide” rust that forms when iron and oxygen react in the presence of water. I am not referring to “white-rust”, which is not really rust at all but rather a storage stain due to zinc, water and a lack of oxygen.