We talk a lot about the benefits of metal roofing such as energy efficiency, durability, corrosion resistance, etc. Many of these qualities and benefits, especially corrosion resistance, comes from the special coatings that are applied to the steel panel. In this article we're going to take a look at these coatings to better understand why some are better than others, and why some are not even suitable for residential use.
Galvalume was first commercialized in 1972 and has since become the dominant coating for steel metal roofing products. Galvalume is a registered trademark of BIEC International, a wholly owned subsidiary of Blue Scope Steel. It's likely you've heard of "galvanized" coatings before, and as you might expect from its familiar name, Galvalume is also designed to prevent steel corrosion from occurring -- much like its galvanized cousin.
Galvanization is still used today to protect steel roofing, though it is now less common than Galvalume. "Galvanized" is the term we use to describe a protective layer of zinc that is applied over steel to protect it from corrosion. While galvanized coatings are made up purely of zinc, Galvalume coatings use a specific combination of zinc, aluminum and silicon. Because of the addition of aluminum which prevents oxidation, Galvalume has superior corrosion resistance and can last twice as long as its galvanized counterpart.
Now we enter the world of residential metal roofing. Even under your painted metal roof, there is a layer of Galvalume protection that is necessary to prevent corrosion. Metal roof paints are permeable and will retain and release moisture (a completely normal process). This makes the corrosion resistant coating that is under the paint extremely important.
This is also where buyers should pay close attention because they can easily be fooled by misleading advertising. Your paint layer can be a high quality paint, but if the corrosion resistance layer is subpar, your metal roof is at risk of failing prematurely. Many companies, even large, reputable ones, tout a 40-year paint warranty but are applying that high quality paint on top of subpar corrosion resistance coatings. This allows them to offer a lower price to consumers with the perception that it is a quality product.
What makes a corrosion resistance coating subpar?
We've described Galvalume in this article and have shown how it protects steel from corrosion. In order for an aluminum-zinc substrate to be considered "Galvalume" it must have a specific formulation of aluminum and zinc, and must be a certain weight.
Perhaps you are familiar with galvanized coating weights being measured by G40, G60, G90, etc. Similarly, Galvalume weights are measured with the prefix "AZ", which stands for Aluminium-Zinc. Acrylic coated Galvalume metal roofing (that is the shiny, silvery metal roofing sometimes called "mill finish") typically carries a corrosion resistance weight of AZ55 (the 55 represents 0.55 ounces of aluminum-zinc coating per square foot). This is an extremely durable coating and most mills even offer a 20 to 25-year corrosion resistance warranty on this product without it even being painted. A quality prepainted metal roofing product will have a corrosion resistance weight of G90 if galvanized is used, and AZ50 if Galvalume is used. A common subpar coating that has unfortunately made its way into the residential metal roofing market is AZ35. This would be a corrosion resistance coating of 0.35 ounces of aluminum-zinc coating per square foot.
A residential metal roof product with a Galvalume corrosion resistance layer weight that is less than AZ50 is a subpar product and should not be installed on your home.
In fact, AZ35 coatings are not even technically considered "Galvalume". As Steelscape points out:
"Due to the length of the zinc columns in the aluminum-zinc coating, a thinner AZ35 coating can create pathways for corrosion. As a result, AZ35 coatings cannot carry the Galvalume trade name and will not carry a corrosion warranty." 
Further, AZ35 does not meet residential building code requirements. Material providers that market AZ35 coated material as a residential product, and installers who install such products could be opening themselves up to liability.
The 2018 International Residential Code (IRC) lists AZ50 Galvalume as the minimum acceptable weight for coated steel used in exterior roofing for residential applications. 
See this excerpt from the IRC, Chapter 9 Roof Assemblies:
The minimum acceptable corrosion resistance listed for 55% aluminum-zinc-alloy-coated steel is AZ50 Galvalume. For prepainted steel we see ASTM A755 referenced (ASTM is an international standards organization that develops and publishes voluntary consensus technical standards for a wide range of materials, products, systems, and services). Take a look at this excerpt from the ASTM A755 publication that is referenced in the IRC table above:
The publication speaks for itself as it outlines AZ50 as the minimum recommended coating and goes on to highlight the importance of adhering to these minimum coating weight designations.
Why would someone use AZ35?
AZ35 brings the cost of materials down. For interior liner panels and other interior applications where moisture exposure is kept to a minimum AZ35 could find acceptable uses. AZ35 was not designed for exterior roof use and certainly not for use as a residential product. Material providers that market AZ35 as Galvalume are either being deceptive or are just ignorant of the facts. AZ35 is not Galvalume. Suppliers that market AZ35 as a residential roofing product are attempting to sell materials at a cheaper cost and at the end user's expense.
As you shop for your metal roof, evaluating high quality paint systems is very important, but paint doesn't tell the whole story. As we've now demonstrated, you can have a high quality paint systems on a subpar corrosion resistance layer and still experience premature product failure. Ask your supplier if their materials have a corrosion resistance coating of less than AZ50. If you find they are using AZ35, you should consider purchasing elsewhere.