Red oxide and zinc chromate are common chemicals used in primers for metals.
Both substances have similar uses but differ in their appearance and ability to inhibit corrosion.
The key difference between them is that zinc chromate primer provides a greater amount of corrosion protection compared to red oxide.
What is Red Oxide Primer?
Also known as Minium and red lead, red oxide is a tetraoxide of lead and is not a natural substance.
You will often find red oxide (Fe₂O₃) in paints and other products used to protect ferrous metals from rusting.
How it works starts when the red oxide comes into contact with a surface made of iron or iron oxide.
The contact with red oxide creates plumbate, which are insoluble compounds that help it adhere to the surface.
Even if there is rust forming on the surface, the red oxide will still form a bond.
This will often stop the oxidation process, assuming the amount of rust is not extensive.
However, despite the effectiveness of red oxide, it has fallen out of use in recent years because of its lead content and flammable nature.
While red oxide is still used in some formulations of glass, that is only because the lead poses no threat to public health.
Otherwise, in its standard form as a rust preventative, red oxide does pose a small threat of lead poisoning.
What is Zinc Chromate Primer?
This is a chemical substance that primarily consists of zinc and is often used to coat aluminum and iron materials when used as part of paint.
It’s not conductive and is quite effective as part of paint. And compared to zinc phosphate, it’s more robust.
The US military used zinc chromate (ZnCrO4) extensively during the 1930s on aircraft to prevent them from rusting.
In fact, for aerospace along with automotive paints, zinc chromate has been quite popular.
Zinc chromate is usually thinned with toluene and is primarily used as a rust preventative, especially when coating aluminum.
However, one big downside is that zinc chromate is considered highly toxic. Care must be taken whenever it is used to coat metals.
Because it is often used for artistic and spray paints, along with being used in varnishes as a pigment and creating linoleum – zinc chromate remains highly popular.
Difference Between Red Oxide and Zinc Chromate Primer
It may seem that both substances are highly similar because they primarily protect against rusting and corrosion.
The main visual difference is the bright red color of the red oxide as compared to the yellow-green color of the zinc chromate.
But the main difference is the increased effectiveness of the rust preventative qualities found in zinc chromate as compared to red oxide.
If you are shopping for a rust-preventative in your primer, you may run across zinc oxide primer.
This has been in wide use since World War II. It has the familiar yellow-green tint but can also come in a wide variety of pigments.
The one you choose should be based on its compatibility with the metal you want to protect.
Plus, protecting yourself from the toxic compounds prevents you from being harmed while applying the primer.
Painting Over Zinc Chromate and Red Oxide Primers – Things to Know
Zinc chromate and red oxide primer don’t require any special paints to topcoat, except that you must use paints that are compatible with rusty metal surfaces.
You can generally pretreat the surface by cleaning, sanding, and priming – and then applying the paint once the surface and primer are fully dry.
Depending on the condition of some metals, flame cleaning or a method such as grit blasting may be required before you apply the primer and paints.
Red oxide primers are not recommended for galvanized or nonferrous metals such as aluminum, copper, or brass since it is only suited for usage on ferrous metals (metals that contain iron).
You should also avoid using them on asphalt or bituminous surfaces.
It’s good to test the surface with a magnet if you’re unsure whether your metal is ferrous or nonferrous.
So, in the end – which is better red oxide or zinc primer?
Zinc chromate, in my opinion, is an anti-corrosive and anti-rust primer that provides more corrosion resistance than red oxide, making it the better choice for those concerned about corrosion.
Red oxide, on the other hand, is excellent for exposure to sunlight and exterior exposure, so it is better suited for those applications.
When picking the one, always consider your needs and the environment that the item will be used in most.
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Jack Luis is a semi-retired painter who loved painting his clients’ ideas on their walls.
He had worked as a painter for more than a decade to serve the customers in areas such as Charleston, Mount Pleasant, Beaufort, Georgetown, SC (South Carolina). Today in his free time, he likes to read and write about the newer techniques that are being implemented in his profession. You may read more about him here or get in touch with him here.
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