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Although galvanized coatings are highly abrasion resistant, coating damage may occur once the galvanized steel leaves the galvanizer’s facility due to extremely rough handling, installation techniques, or in-service conditions. Occasionally, small areas of the galvanized article may be bare due to unseen contaiminants the cleaning process is unable to remove. There are methods available to ensure post-galvanizing coating integrity and ASTM A780, Standard Practice for Repair of Damaged and Uncoated Areas of Hot-dip Galvanized Coatings authorizes three accepted touch-up and repair methods:

  • Applying zinc-rich paint
  • Coating with zinc solder
  • Spraying with molten zinc (metalizing)

The touch-up and repair method chosen should consider the specific use of the galvanized steel and the performance characteristics of each of the three methods. Corrosion protection should always be the primary consideration, but certain uses and conditions may warrant selection on the basis of other performance characteristics. Applying zinc-rich paint is often erroneously called “cold galvanizing,” and may be appropriate for touchup and repair but just as is the case for applying any paint, surface preparation procedures outlined in D6386 should be closely adhered to in order to ensure the best overall performance. It is important to note none of the three repair methods provides the same corrosion protection as hot-dip galvanizing. 


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Comments

Matt Filcek

What types of applications would you recommend to use a zinc-based alloy (A 780 A1) in lieu of paint with zinc dust (A 780 A2)? I've always viewed zinc solder as superior to zinc paint, especially for high stress locations exposed to de-icing chemicals. thanks, Matt

Alana Hochstein (AGA)

Hi Matt, It is possible to achieve a successful and very durable coating repair using any of the three repair methods when applied per the specification requirements. However, there are pros/cons when using each type of repair material. The AGA has reports of studies detailing the variations of their performance, ease of application, appearance, etc.

Jerry Kott

Hi I have an antique watering trough that has some rust challenges. I would like to restore it for decorative purposes. It is 12 feet long 15 wide and 15 high. I have been researching on line for direction. My resources are limited but I do have elbow grease. I have a power washer and a palm sander. Thanks for any direction Jerry

Alana Hochstein (AGA)

Hi Jerry, You can repair hot-dip galvanized coating with any of the materials listed in the article. For information and instructions on how to apply each repair material, see our instructional video series: https://galvanizeit.org/specification-and-inspection/repairing-hdg-steel As well as our listing of suppliers offering HDG repair materials: https://www.galvanizeit.org/uploads/publications/Touch_Up_and_Repair_of_Galvanized_Steel.pdf

So-Young Kim

In case of hot dip galvanized piping which is zinc coated on internal piping as well as external piping, both internal and external zinc coating will be damaged(burned-off) during welding procedure. Damaged zinc coating on external piping could be repaired as you explained according to ASTM A780. However, damaged zinc coating on internal piping could be hardly repaired because of difficulties of operations especially for small diameter piping. It is concern that remaining as un-repaired zinc coating would result to corrosion of piping. I would like to know the special welding procedure that not cause any damage of zinc coating, so it would not require any additional repair of zinc coating. I would be appreciated if you could reply on this. Thanks.

Alana Hochstein (AGA)

Hi So-Young, Yes, this issue is a known design concern when working with welded hot-dip galvanized steel pipe. Unfortunately, there is no method to repair coating on the internal side of a pipe after field-welding. The bare area will receive some cathodic protection from the nearby zinc, but the effect is limited by zinc and is explained further in the following article: https://galvanizeit.org/education-and-resources/resources/technical-faq-dr-galv/cathodic-protection-on-bare-spots-of-galvanized-steel When such pipe assemblies are required to be welded, this should be performed prior to hot-dip galvanizing (if possible). There is no welding technique that will prevent removal of the zinc in the nearby area of the weld except for resistance welding which can only be used on very thin sheet products <1/4” (6 mm) and with a zinc coating lighter than 1 oz/ft (305 g/m). No, it is strongly recommended to remove the zinc prior to welding to prevent zinc from contaminating the weld puddle, and also to prevent vaporization of the zinc which can cause zinc metal fume fever and health issues for the welder. The muggyweld technique appears to be soldering two pieces together and does not provide the strength of welding the steel components together. I cannot advise if this is structurally suitable for your specific project. Please let me know if you need any further assistance.

George/

We are using hot dipped galvanized screws to install Galvalume corrugated metal roofing (24 and 26 gauge) to wooden purlins and rafters. However, What I am worried about that drilling the screw into the Galvalume metal will scratch of the hot dipped galvanization from the surface of the threads and the shaft of the screw. and then it would be installed in treated wood. As far as I researched that scratches from hot dipped galvanized is fine since the Zink will corrode thus providing a sealant cover on the scratches. However, this would happen in the existence of air around the Zink. But our screw will be inserted inside the treated wood, thus the Zink won't react and corrode to produce this protective layer. Question 1 So the question is, does drilling through the Galvalume metal, removes the hot dipped galvanized layer from the screw and will jeopardize the screw as it would be considered Uncoated? Question 2 is there any anode-cathode would happen between Galvalume corrugated metal and Hot dipped galvanized screws? Question3: is there any anode-cathode would happen between Galvalume corrugated metal and stainless steel screws?

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