DIS-TRAN Steel Blog

8 Most Common On-Site Concerns With Galvanized Steel

Posted by Brooke Barone on May 16, 2013 9:38:00 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 When products come in direct contact with others while being withdrawn from the kettle can also cause a lumpy coating appearance.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

What You May Not Know About Hot-Dip Galvanizing

Posted by Brooke Barone on Jan 25, 2013 2:26:00 PM

When you think of a galvanizing kettle, you might only think it’s a bunch of hot melted silver stuff. But in fact, there are several different types of zinc used for hot-dip galvanizing, as well as other materials that make this protective coating.

Here are 5 types of zinc for continuous hot-dip galvanizing:

    1. London Metal Exchange (LME) Grade
    2. Special High Grade
    3. High Grade
    4. Intermediate Grade
    5. Prime Western Grade

    Zinc Abbrevations

    Terminology of the 5 different grades

    LME is a grade of zinc containing a minimum of 99.995% zinc

    SHG is a high purity grade of zinc containing a minimum of 99.990% zinc

    HG is a grade of zinc containing a minimum of 99.95% zinc

    IG is a grade of zinc containing a minimum of 99.5% zinc

    PWG is a grade of zinc containing 0.5 to 1.4% lead, and a minimum of 98.5% zinc

    Galvanizers were mainly using PWG up until the 90s when the industry started to gravitate towards high grade or special high grade due to more talk and discoveries about lead. Today, galvanizers try to limit the amount of lead that is used in the process, but the percentage is tilted and doesn’t necessarily mean that all galvanizers practice this.

    The chart below shows the composition breakdown of each grade of zinc.

    Zinc grade pic

    There are distinctive looks and markings on the finished galvanized product depending on the grade used. For example, PWG lends itself to a more spangled look, whereas SHG has a more continuous bright coating.

    No matter which type of zinc is used, all galvanized steel will start dulling after about six to eight months. While zinc certainly has a “say so” in the look of the galvanized product, the chemistry of the steel is the most important determining factor.

    Service life of galvanized steel?

    There are two factors that influence the service life for hot-dip galvanizing:

    1. Amount of millage
    2. Geographical location

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    Tags: steel structures, hot-dip galvanizing, galvanized steel, zinc, continuous hot-dip galvanizing, lead, special high grade, prime western grade

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