What is White rust on HDGI?
White rust is wet storage stain is a white or gray powdery deposit that can develop on newly galvanized articles. It occus when galvanized steel is exposed to rain, dew, or high humidity conditions. White rust is visible formation of zinc oxide and zinc hydroxide on the surface of the galvanized steel. It is important to note the hot-dip galvanizing process does not contribute to the formation of wet storage stain. The stain occurs after the coating reacts with the environment, not during the coating process.
What is weathering of Zinc Coating in HDGI and how does it slow corrossion?
Natural Weathering of the Zinc Coating Zinc, like all metals, begins to change when exposed to the atmosphere. As the fresh zinc surface interacts with the wet and dry cycles in the environment, a protective layer of zinc corrosion products, collectively known as the zinc patina, builds on the surface.
Once the galvanized zinc coating is exposed to free flowing air, a thin layer of zinc oxides forms on the surface starting the natural weathering process. Then, when the piece comes in contact with moisture – rainfall, dew, humidity – the zinc oxide particles capture the water to form porous, gelatinous zinc hydroxide. As the surface of zinc hydroxide and zinc oxide interacts with carbon dioxide in the atmosphere, a thin, compact, and tightly adherent layer of mostly zinc carbonate is formed. The rate of the patina formation varies according to the environmental conditions, but typically takes 6-12 months to fully develop. The fully developed patina is a passive, stable, water-insoluble film on the zinc surface that does not wash off in rain or snow.
As the zinc patina develops, the galvanized coating appearance will turn a matte gray color and the protective film of the patina will slow the corrosion rate to about 1/30th the rate of steel in the same environment. Zinc patina formation is critical to the long lasting corrosion resistance of hot-dip galvanized steel in atmospheric exposures.
How does white rust (wet stain) develop)?
When newly galvanized articles are deprived of adequate airflow, moisture can be trapped on the coating surface altering the natural development of the zinc patina. The trapped moisture presents a different set of conditions creating a reaction that will rapidly create a white powdery zinc corrosion product on the surface. When a drop of water is flattened between two surfaces, the zinc surface near the middle of the water drop gets a different supply of oxygen than the zinc surface on the edge of the water drop. The varied exposure leads to a difference in the electrolytic potential of the zinc. The central area can become anodic and the edge area can become cathodic, creating a potential corrosion cell (Figure 2). If this corrosion cell forms, the anodic area begins to corrode, while the cathodic area is unchanged. Corrosion products similar to the soluble zinc hydroxide of the zinc patina form on the surface (Figure 3). Because zinc hydroxide remains fairly stable under these conditions, zinc ions continue to leave the coating to bond with water, forming a thick and visible white powdery substance. As long as the area is restricted of oxygen, it is also limits the flow of carbon dioxide hindering the conversion to zinc carbonate. The extent of the zinc coating consumption by the wet storage stain depends on the duration of exposure to trapped moisture without free flowing air. If the trapped moisture contains chlorides – from sea water, sulfur compounds, flux residues – the attack will be accelerated because these contaminants will increase the water’s electrical conductivity, increasing the attack on the zinc metal of the galvanized coating. During transit, chloride rich moisture from road salts can be trapped on the steel, even under a tarp, and cause wet storage stain. However, even water alone can cause wet storage stain formation in a relatively short period of time. Exposure to heavy rains, dews, or very high humidity conditions after the galvanized coating is formed can start the visible formation of zinc oxides and zinc hydroxides.
Does white rust effect the life of the zinc coating?
Wet storage stain corrosion is voluminous – about three to five times greater than zinc metal – so any attack may appear more serious than it actually is. Wet storage stain is very unsightly, but often results in very little zinc metal loss; and therefore, will not have a significant effect on the life of the coating.
How does one minimize the chances of white rust (water stains)?
There are a number of simple guidelines to consider following when storing or transporting newly formed galvanized steel
- * Whenever possible, avoid nested stacking
- * Provide adequate ventilation between stacked pieces
- * Incline parts to allow for maximum drainage
- * Use spacers and tarp material during shipping if there is the likelihood of condensation or moisture
- * Stagger or cross stack galvanized pieces
- * Elevate and separate articles stacked outdoors with strip spacers (poplar, ash, spruce)
- * Avoid stacking on wet soil or decaying vegetation
- * Thoroughly dry small items that are quenched before packing in storage containers
- * Include a dehumidifying agent in sealed containers
- * Whenever possible, store material under cover in dry, well-ventilated conditions, away from doorways open to the environment
- * Ensure the final product is free of flux residues
- * Treat with a passivating agent
- * Remove road salts from galvanized articles
Removing Wet Storage Stain
To remove wet storage stain, brush with a stiff-bristled, nylon brush and a cleaning solution. There are five products identified that do not damage the coating appearance: CLR®, lime juice, Naval Jelly® Rust Dissolver, Picklex® 10G, and white vinegar. After removing the storage stain with the nylon-bristle brush and cleaning solution, the surface should be rinsed with tap water and dried. Finally, it is best to measure the zinc coating thickness after cleaning to ensure an adequate coating remains on the base steel.
What is flaking and peeling in HDGI?
Flaking occurs when galvanized layers detach from the galvanized part. If a coating thickness measurement is taken in the area where the coating has flaked, the measurement will be near zero or even zero in some cases, indicating only the gamma layer is left on the steel. Flaking is caused by excessively thick galvanized coatings that are more brittle than coatings of a normal thickness.
The Galvanized coating can flake off the steel when the part is struck, such as during transport or assembly in the field. Pieces of coating that have flaked off the steel usually have very long and distinct edges. Because flaking of the coating causes large, nearly bare areas, it is usually cause for rejection and the steel must be repaired or regalvanized.
Some ways to reduce the galvanized coating thickness during regalvanizing include leaving it in the bath for a shorter time period, decreasing the bath temperature, and blast cleaning the steel. Ensuring your customers provide steel with recommended chemistry for galvanizing (see ASTM A385) goes a long way to decreasing coating thickness and thereby also reduces the chances for a coating that flakes off the steel. When you know your customer has provided you with reactive steel, blasting the steel prior to galvanizing can reduce the galvanized coating thickness that develops and thereby limits the possibility for flaking. It might also be prudent to have clauses in your purchase orders limiting your responsibility when galvanizing reactive steel that does not meet the recommended chemistry of ASTM A385.
Some ways to reduce the galvanized coating thickness during regalvanizing include leaving it in the kettle for a shorter time period, decreasing the kettle temperature, and blast cleaning the steel. Ensuring your customers provide steel with recommended chemistry for galvanizing (see ASTM A385) goes a long way to decreasing coating thickness and thereby also reduces the chances for a coating that flakes off the steel. When you know your customer has provided you with reactive steel, blasting the steel prior to galvanizing can reduce the galvanized coating thickness that develops and thereby limits the possibility for flaking. It might also be prudent to have clauses in your purchase orders limiting your responsibility when galvanizing reactive steel that does not meet the recommended chemistry of ASTM A385.
Peeling of the galvanized coating is different from flaking and occurs when the outer free zinc layer separates from the intermetallic layers. When a coating thickness measurement is taken in an area that has peeled, there is some coating left. One cause of peeling is when a newly galvanized piece cools extremely slowly or when the steel is subjected to high temperatures (excess of 400F) for prolonged periods. When galvanized steel cools very slowly and the part remains in excess of 600F, the galvanizing reaction can continue. The outer layer serves as the source of zinc for further reactions and can be consumed by the galvanizing reaction.
This consumption of the outer zinc layer can create voids between it and intermetallics, which then causes peeling of the outer zinc layer. When peeling is caused in this manner it is called the Kirkendall Effect.