Rust on Zinc Substrate

Q. When we test electrocoated zinc panels in our salt spray system they come out with red rust in the creep, edges and other locations. Our experience is that zinc substrates rust to the white, so we are a little puzzled. Can you please explain why we see this red rust? Do we have iron contamination in the salt solution used in our salt spray cabinet?—D.F.

A. The simple reason why you see red rust in your tested electrocoat zinc panels after salt spray testing is because you are testing a type of zinc-coated steel called galvanneal, designated by the code GA steel. This type of zinc-coated steel is characterized by having greater amounts of iron alloyed within the zinc coating than regular galvanized coatings, which are mostly zinc. This level of iron in the substrate coating is responsible for the red rust color you see after salt spray testing.

When zinc coatings are deposited by immersing the workpieces into a molten zinc bath, we can obtain either galvanized (GI) or galvanneal (GA) zinc-coated steels. Both materials can be obtained by either a batch process (for low volumes or unusual-shape steel pieces) or continuously through automatic conveyorized systems.

The main difference in the production process between GI and GA zinc-coated steels is that, with GA, the zinc-coated steel is further heated by passing it through a furnace immediately after it passes through the molten zinc-coating bath but before the molten zinc completely solidifies. This process is referred to as annealing.

The annealing process consists of heating the zinc coating to approximately 840-1,100°F (450-570°C) and holding the temperature for a specified amount of time. During this holding period, more iron from the steel substrate diffuses into the already-applied semi-solid or semi-moten zinc coating, and increases the amount of iron that alloys within the zinc.

Depending on the time and temperature of the annealing process, four distinct layers/phases of zinc/iron (Zn/Fe) ratios can be found on GA-coated steels. The Zn/Fe phases not only differ in their chemical composition (more iron in the base layers and less on the top), but also in their microstructure, ranging from monoclinic structures to cubic and hexagonal structures.

Galvanneal-coated steels are less galvanically active in most corrosive environments than the galvanized -coated steels because of the greater amount of iron they contain. GI-coated steels are quite soft and easily scratched, while GA coatings are hard, brittle and not easily scratched during handling. If spot welding is involved, then GA is typically preferred over GI. Both coatings have a greyish dull or matte appearance, and are heavily used in automotive applications.