DIS-TRAN Steel Blog

8 Most Common On-Site Concerns With Galvanized Steel

Posted by Brooke Barone on May 16, 2013 10:38:00 AM

When steel is delivered on-site the first thing that is noticed is the coating appearance. Upon further inspection, if discolored or lumpy areas are noticed, the most common concern is if it’s detrimental to the life span of the coating. But in many circumstances, the look tends to be more serious than the actual effects. 

Galvanized Coating

Here’s a list of 8 of the most common on-site concerns with galvanized steel, and why it occurs. 

1. Bare Spots- Smaller flaws have little effect on the service life of the coating, and can be somewhat self-healing. Some spots may require repair using such methods indicated by ASTM A 780, which includes painting with paints containing zinc, repairing with zinc-based alloys (hot sticking) or by using sprayed zinc (metalizing). But, uncoated, unrepairable spots can be grounds for rejection. Some causes of bare spots can be because of inadequate surface preparation, welding slag, rolling defects, sand embedded in castings or oxidized steel.

2. General Roughness-   This is usually due to excessive growth or unevenness of the alloy layers, which can be attributed to the steel’s chemical composition or original surface condition. Heavy coatings are usually rougher than lighter coatings because irregularity of alloy layers tends to increase with thickness. In most cases, a rough coating does not negatively affect the lifespan, as long as adhesion is good. But, there are always exceptions to the rules. For particular pieces where one surface mates with another, rough coatings can be detrimental.

3. Dross Protrusions- Dross is the zinc-iron alloy that settles to the bottom of the kettle. It produces surface protrusions when the dross layer becomes agitated from the dross inclusions. Dross protrusions tend to have little effect on the surface life since the corrosion rate is similar to zinc. However, extensive dross inclusions can be grounds for rejection because they tend to make the surface more susceptible to mechanical damage.

4. Lumpiness and Runs- A lumpy coating results when the withdrawal is too fast or when the bath temperature is too low, not allowing molten zinc to drain back into the bath. Delayed drainage from bolt holes, folds, seams or other pockets where zinc collects is a consequence of the design. When products come in direct contact with others while being withdrawn from the kettle can also cause a lumpy coating appearance. Although it’s not detrimental to the life span, some cases require a smooth finish.

5. Flux Inclusions- Flux inclusions occur when a layer of zinc-ammonium chloride floats on the top of the molten zinc. When the steel is submerged in the bath, the flux pushes to the side when the steel is pulled back out. Flux inclusions can be caused by several different scenarios, such as a stale kettle flux where it tends to adhere to the steel instead of clearly separating from the surface as the steel is dipped. If the underlying coating is sound, then flux deposits are not reasons for rejection.

6. Ash Inclusions- Similar to flux, ash may be picked up during the dipping of the steel. Zinc ash is the oxide film on the surface of the bath. Ash inclusions can occur when steel requires slow withdrawal from the bath, and has no effect on the service life. If improper skimming of the exit surface of the bath can lead to gross oxide lumps, and can reduce the effective thickness of the coating, which is not acceptable.

7. Matte Gray or Mottled Coating- Usually appears as a localized dull patch or wed-like area on a normal surface, and develops when there is a lack of free zinc layer on the coating surface during the cooling process. A matte gray coating is found mostly on steel with relatively high silicon or phosphorous content, since they are heavier sections that cool slower. Galvanizers generally don’t have prior knowledge of the steel’s chemical composition, and has no control over its occurrence.

8. Rust Stains- Surface rust stains are not cause for rejection if they are caused by seepage from joints and seams after galvanizing or steel being stored under or in contact with rusty steel. Rust stains like this are superficial and should not be confused with failure of the underlying coat.

Whenever a question arises on the advisability of galvanizing a certain weld material, fabrication or steel type, it is best to consult the galvanizer. Most of the issues can be addressed beforehand if all parties stay in contact throughout the process before the steel arrives at the galvanizing plant. Remember to keep these 8 concerns in mind next time you conduct a visual inspection to help avoid delaying projects. 

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Tags: galvanizing, galvanized steel, zinc, galvanized coating, zinc-iron alloy layers, steel coating, zinc coated steel, astm standards

A Two-Part Recipe for Understanding Galvanized Coating Appearances

Posted by Brooke Barone on Jan 30, 2013 4:05:00 PM

In last week’s post I outlined the 5 different grades of zinc used for hot-dip galvanizing; LME, SHG, HG, IG, PWG. So, following that, what changes the appearance of the galvanized steel?  

Most steel materials can be hot-dip galvanized such as cast iron, malleable iron, cast steels or hot and cold rolled steels. Structural steel shapes can also be galvanized after fabrication to provide a protective coating.

When the steel enters the kettle at 840 to 850 degrees Fahrenheit, zinc-iron alloy layers start to form. This portion normally represents about 50 to 70 percent of the total coating thickness, with the zinc being the top layer. Directly coming out of the kettle, all steel is extremely bright, despite the coating appearance.

Galvanized Coating

And if you remember, no matter which grade of zinc that’s used, all galvanized steel will start to dull after about 6 to 8 months. But if the steel is spangled, it will still have that appearance over the service life, just not as bright.

The characteristics of the galvanized coating is affected by the chemical composition of the steel material that is being used. The coating appearance can come out shiny, matte gray, spangled or shiny and dull. These are simply cosmetic issues despite the color variation.

The composition of the steel varies depending on the strength and service requirements. Certain elements in the steel can affect the galvanizing technique, as well as the structure and appearance. For example, steel with higher silicon is more reactive and tends to give that spangled look.

When purchasing steel from the mill, there are certain parameters or guidelines you should follow in order to select steel with good galvanizing characteristics. You can also request a Mill Test Report (MTR), to see if the parameters are in the requested range. When it comes to structural steel however, it is not as controlled since you’re buying from a warehouse.

The chemical content of the steel determines the zinc-iron alloy coating. If the coating appearance is bright or splotchy gray, this is due to an excessive amount of certain chemicals that you need to be careful of. However, the splotchy or matte gray appearance is not grounds for rejection, and actually, the gray areas usually have more millage than the bright silver portions because the zinc continues to grow.

Zinc-Iron Alloy

Below is a description of the characteristics of an all or nearly all zinc-iron alloy coating.

Visually The zinc-iron alloy coating will have a matte grey appearance due to not having a free zinc layer, since the free zinc layer typically is what gives the bright finish.

Adherence When compared to the “typical” galvanized coating, an all or nearly all zinc-iron alloy coating may have lower adherence, and tends to be thicker. There might be a reduction of adherence as the thickness increases.

Corrosion Resistance Galvanized coatings are specified more for their corrosion resistance rather than appearance. Fabricators and consumers should be aware that whether a gray or matte appearance occurs, it does not reduce the long term atmospheric corrosion protection of the steel. The corrosion resistance, mil for mil, of these coatings is equal.

Want to know the guidelines that usually result in the selection of steel having good galvanizing characteristics?

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Tags: hot-dip galvanizing, galvanized steel, galvanized coating, zinc-iron alloy layers, steel millage, Mill Test Report, galvanizing characteristics, galvanized coating appearances

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