What are common galvanized coating defects?
- Bare Spots
- Dull Gray or Mottled Coatings
- Dross Pimples/Inclusions
- White Storage Staining
- Ash Staining
- Deformation
- Spangled Coatings
- Lumpiness and Runs
- Delamination
- Black Spots
Galvanizing is one of the most cost-effective protective coatings for metal products like iron and steel. When done right, it creates a strong barrier against rust and corrosion. But, like many other metal working processes, it can be easy to mess up, especially if you lack knowledge and experience in dealing with galvanized coating defects.
Several factors play a role in the galvanizing process and how each metal piece turns out. In this article, we provide some insight and explanation for common defects, how they affect your metal product, and how you can avoid them. Read on!
Why Do Defects Happen During the Galvanization Process?
Factors like the composition of the metal, bath temperature, alloying elements in the bath, and dipping time are just a few things that can affect your galvanization coating process. Without proper attention and inspection, they can cause design and functional defects to the final product.
Bare Spots
These are uncoated areas on the surface of your metal piece, caused by not preparing the surface properly before beginning the coating process. It can also be caused by pickling, welding slag, spatter, grease, cavities, and other imperfections on the metal’s profile.
While smaller bare spots don’t have a huge impact on your metal product’s usability (and can be somewhat self-healing), larger spots will require repair. Some methods use paints containing zinc to coat the bare spot, or by using sprayed zinc (metalizing). However, some bare spots are unrepairable, which means your metal part would need to be discarded.
Dull Gray or Mottled Coatings
This defect looks like matte patches that cover most or all of the surface of the galvanized coating. This can be caused by the use of reactive steel grades (such as those with high silicon and phosphorus content), which will produce thicker galvanized coatings duller than regular coatings.
It can also happen when there is a lack of zinc in the coating during the cooling process. This is a common result of steel metallurgy and is usually beyond the control of the galvanizer, especially if they have no prior knowledge of the steel’s chemical composition before the galvanizing process.
Dross Pimples/Inclusions
Dross is the zinc-iron alloy that settles at the bottom of the metal bath. Dross can become agitated during the dipping process and end up trapped in the coating. This produces little dimples or protrusions on the metal’s surface.
While this defect has little effect on the metal product’s corrosion resistance, it can still render your metal product unusable. This is because the dimples make the surface rougher, and therefore more susceptible to mechanical damage.
White Storage Staining
After galvanizing, your metal product could be placed in storage rooms. Under humidity or improper ventilation or stacking, white storage staining can appear on the surface. This is because the coating reacts to the extra moisture in the air, and will develop “white ash,” (zinc hydroxide), which becomes bulky deposits on its surface.
Ash Staining
In metalworking processes, your metal product likely goes through other steps before arriving at the galvanization stage. If so, your metal surface likely has ash, which should be cleaned before dipping.
If this ash is not completely removed, then it will be trapped and stuck to the outer side of the coating as the metal is removed from the bath. This creates light brown stains, a dull finish, or oxide lumps on the surface. This defect can reduce the thickness and evenness of the coating, affecting its protective quality.
Deformation
In hot dipping galvanization, the metal is put in a liquid zinc bath with a temperature of 450° C. This temperature meets the stress-relieving temperature points for some metals, like iron. At these temperatures, it’s possible that the metal eliminates the previous stresses it experienced, such as those from the rolling and welding process.
This “reverses” it, so to speak, which causes material changes during the galvanization process. When this happens, your metal product could come out of the bath deformed, which is difficult to repair.
Spangled Coatings
In some cases, your metal could come out of the galvanization process with a silver-gray coating, with a noticeable patterned structure. This is called “spangling,” and is caused by the zinc crystals that form on its surface. This is common with galvanizing alloys created in specific smelting processes and in hot-dip galvanizing. This is another cosmetic defect, and the anti-corrosion performance of the coating is not affected.
Lumpiness and Runs
A lumpy, runny, and uneven surface is another possible defect for your galvanized coating. This happens when the metal is pulled too quickly from the coating process, or when the bath temperature is too low. This causes slow draining of the coating material from components like bolt holes, folds, seams or other pockets where the coating can collect. This defect is also caused when the metal accidentally touches another metal piece during the coating process.
Smaller lumps and runs won’t affect the metal’s life span. However, if your metal products require a smooth finish, then you would need extra work to file down the imperfections.
Delamination
If your product needs a very thick coating, then delamination may occur. Coatings that are more than 250 µm in thickness tend to be more fragile than the standard galvanized coating. This can cause delamination, which is when the coating completely separates from the metal surface. Delamination could happen upon impact, so manufacturers and users need more caution when handling these pieces during transport and use.
Delamination can also be caused by the metal material itself. Metal that is thin, cold-rolled, or has very smooth surface finishes can lead to this defect, which is also the case with reactive steel. This is because the coating is unable to stick to the surface as it should, and therefore easily separates under stress.
Black Spots
Your galvanized coating could also have scattered black spotting, caused by residual galvanizing flux settling on the surface. This could happen if your rinsing water is contaminated by flux, or by poor rinsing after galvanizing. If the flux deposits don’t affect the soundness or thickness of the coating, then the product is still usable, though its appearance is impaired.
How Can These Defects Be Prevented?
Inspection of the metal, assemblies, castings, and other components is needed to reduce the likelihood of defects.
An experienced metal manufacturer, such as Regan Industrial Sales, Inc., has experience as galvanizers and fabricators to ensure that all the metal pieces are suitable for galvanizing before starting the process. They will closely examine if the size, shape, and grade of metal are fit for processing.
Key Takeaway
Galvanizing is critical in metal preparation. With galvanized coatings, your product gains both functional and cosmetic benefits, which improve the value and use of your product. But, if galvanized coating defects occur, this could create plenty of wasted material and money for your project. This is why doing your galvanization process right the first time around is important.
If you’re interested in fabricated galvanized steel products or want to source the best metal products for your upcoming project, then contact Regan Industrial Sales, Inc. today. We have a wide array of galvanized steel, as well as many other metal products and fabrication services for a variety of applications.
