DIS-TRAN Steel Blog

Tips for Successful Galvanizing Touch-Ups in the Field

Posted by Wendy Gintz on Jun 3, 2015 9:26:43 AM

“How do I successfully touch-up galvanizing in the field?”

This is a common question about substation and transmission structures.  There could be a number of reasons why touch-ups may be necessary in the field.

  • Extremely rough handling
  • Installation techniques
  • In-service conditions

galvanied-1

There are certain factors to consider when repairing galvanizing in the field, such as: the size of the area to be repaired, the ease of use of the repair material and the performance of the repair method.  There are also standard specifications to follow for the repair of galvanized coatings.  ASTM A780 covers methods used to repair damaged hot-dip galvanized coating on hardware and structural shapes as well as required coating thicknesses.  There are three acceptable forms of touch-up.

  1. Zinc Based Solders - achieved by applying zinc alloy by stick or powder form.  Most common alloys are zinc-tin-lead, zinc-cadmium and zinc-tin-copper.  To prepare the surface use a wire brush, lightly ground, or mildly blast clean and remove all weld flux and spatter.  The area being repaired needs to be preheated to 600 F.  This is the most difficult method of repair.
  2. Zinc Rich Paints - either by brush or spray is applied to a clean, dry surface. The paint must contain between 65% to 69% metallic zinc by weight or greater that 92% by weight in dry film.  To prepare the surface the area must be cleaned either be blasted, power tool cleaned or even hand tools (wire brush). This is the most commonly used field repair method and can easily be done without a need for blasting or power tools. Zinc-rich painting should be avoided in high humidity and/or low temperatures.
  3. Metallizing achieved by melting zinc power or zinc wire in a flame or electric arc. The zinc used is minimum of 99.5% pure.  To prepare the surface it must be blasted cleaned to SSPC-SP10/NACE No.1 near white metal and must be free of oil, grease, weld flux residue, weld splatter and corrosion products.  The cleaning must include surrounding, undamaged coating.  Spraying should be done by a skilled worker in horizontal overlapping lines to create a uniform thickness.  Not recommended for high humidity locations.

The coating thickness of the repaired area must match the coating thickness of the surrounding area. If zinc-rich paint is used, the coating thickness must be 50% higher, but not greater than 4.0 mils.

Maximum_Size_of_Repairable_Area

Be sure to discuss any touch-ups with your steel fabricator or galvanizer. They may have suggestions on which method has worked best for certain circumstances.  Final coating thicknesses need to be agreed upon between customer and vendor and be measured by the methods in ASTM A 123/A 123M.  Remember that the surface of the repaired coating should be free of any lumps, course areas and loose particles.

For more about galvanizing check our our free and easy resource - Galvanizing Ebook below.

Galvanizing eBook

*References used for this article are from www.galvanizeit.org.  

 

 

Tags: galvanized structural steel, galvanized coating, galvanized coating appearances

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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