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

Proper Draining and Venting Provisions for Steel Structures

Posted by Brooke Barone on Aug 7, 2014 2:33:00 PM

Something that might be viewed as a small, insignificant venting hole on a 10,000 pound steel structure, if not well thought out, could really have an adverse effect on production.

These mistakes, big or small, can delay or even put a halt to jobs. It’s key that all along the process, from engineering to detailing and quality control, there are people in place who know what to look for. Once the structure gets delivered to the galvanizer it can become difficult and more costly to make modifications because plates might already be cut, or everything might be welded up.

When creating fabrication drawings for galvanized structures, it’s important, as well as valuable, to know proper draining and venting provisions for these steel structures. If adequate venting and draining holes are not provided, the structures can run into many problems.

5 Negative Effects:

1. Air pockets can form, causing structures to rust out from the inside

2. Excess galvanizing buildup

3. Lead to longer fabrication times

4. Welded plate can blow out, causing safety concerns

5. Poor coating

Not having adequate venting and draining holes can really have an intangible effect: it’s hard to put a dollar amount on what happens when a structure either doesn’t have proper venting, or one of the five stated above occurs. It’s usually not too hard to correct if it’s caught up front, but the further it gets in the process, and closer to the delivery date, is when the scrambling might start. All the man hours it takes to call the engineer on record to approve revisions, or contact customers, plant personnel, the galvanizer, etc. can really put a stop to production, causing low production numbers and possibly delayed shipping. (But working with a trusted steel fabricator, can help to avoid these issues.)

excess galvanizing buildup

Some standard shape structures, such as square and rectangular tube columns and beams, are hollow, so provisions need to be made in order to allow galvanizing to easily flow and coat the inside portion of the structure. Sometimes fabricators will provide a small bar with a removable cover plate, attached with two (2) small stainless steel self-drilling screws. However, if the customer doesn’t feel this is sufficient enough, then the next suggestion could be to use a thicker bar with drill and tap holes, and two (2) A307-TAP bolts.  Some might suggest the use of expanded metal, but excessive build up can take place, which is unsightly and also impairs vision into the tube, hindering the Quality Control Department from being able to adequately determine if interior galvanizing coating is sufficient.

Other standard shape structures like channels, wide flanges and angles, are solid, with just the outside receiving coating. Some issues that can arise with this are air pockets and excessive galvanizing buildup. For these shapes, you need to watch where stiffeners, connection plates and brackets are welded that could form large pockets of air as the section is dipped into the kettle. Tapered tubular structures are also hollow like square and rectangle tubes.

As a designer, you are always trying to find the balance of putting enough holes for galvanizing while not putting too many to affect the structural integrity of the steel member. For example, if dealing with corners in a square and rectangular tube, slots or holes can be provided near these corners to prevent air pockets from forming, which can decrease the amount of galvanizing coating in the area.

The more you understand how the member is lifted and dipped in and out of the galvanizing kettle, the better you can locate the venting and draining provisions.

For more information about galvanizing and how it works, click here to read past articles.

  Dive Deeper Into the Transmission World

Tags: standard shape steel structures, galvanized steel, steel fabricator, galvanized structural steel, rectangular steel tube, steel square tube

Why Galvanized Structural Steel Craves Proper Venting and Draining

Posted by Brooke Barone on Oct 23, 2013 3:39:00 PM

Something that might be viewed as a small, insignificant venting hole on a 10,000 pound structure, if not well thought out, could really have an adverse effect on production.

We all make mistakes every now and then; no matter if you’re on the engineer side, or project coordinator, quality control or detailing side, mistakes happen. But these mistakes, big or small, can delay or put a halt to jobs. I know it’s easy to sometimes overlook something when we’ve done it so many times to where we could practically do it in our sleep.  But nonetheless, you have to make sure to keep an eye out and to catch mistakes before it gets too far down the production line.

When creating fabrication drawings for galvanized structures, it’s important, as well as valuable, to know proper draining and venting provisions for these steel structures. If adequate venting and draining holes are not provided, these structures can run into many problems.

5 Negative Effects:

1. Air pockets can form, causing structures to rust out from the inside

2. Excess galvanizing buildup

3. Lead to longer fabrication times

4. Welded plate can blow out, causing safety concerns

5. Poor coating

It’s key that along the process, there are people in place who know what to look for or have an eye for knowing what will work when it goes to the galvanizer. But if it passes through the line of engineering, detailing, quality control and then is delivered to the galvanizer, it might be harder and more costly to make modifications. Plates might already be cut or everything might already be welded up, making it more difficult to modify.

Not having adequate venting and draining holes can really have an intangible effect. It’s hard to put a dollar amount on what happens when a structure either doesn’t have proper venting, or one of the five stated above occurs. It’s usually not too hard to correct if it’s caught up front, but the further it gets in the process and closer to delivery dates, is when you might start scrambling. All the man hours it takes to call the engineer on record to approve revisions, or contact customers, plant personnel, the galvanizer, etc. can really put a stop to production, causing low production numbers and possibly delayed shipping. But working with a trusted steel fabricator, can help to avoid these issues.

excess galvanizing buildup

Some standard shape structures, such as square and rectangular tube columns and beams, are hollow, so provisions need to be made in order to allow galvanizing to easily flow and coat the inside portion of the structure. Other standard shape structures, like channels, wide flanges and angles, are solid so just the outside receives coating, which means air pockets and excessive galvanizing buildup can form. For these shapes, you need to watch where stiffeners, connection plates and brackets are welded that could form large pockets of air as the section is dipped into the kettle. Tapered tubular structures are the same as square and rectangle tubes, in the sense of being hollow.

For standard shape and tapered tubular structures, using removable cover plates on the ends of beams is a good option instead of welding solid plates to the ends. This allows for faster flow through the member and more adequate galvanizing.

Now, also to keep in mind, ensuring proper venting doesn’t mean place a bunch of holes all over the structure, but rather strategically supply the venting and drainage provisions. For example, if dealing with corners in a square and rectangular tube, slots or holes can be provided near these corners to prevent air pockets from forming, which can decrease the amount of galvanizing coating in the area.

The more you understand how the member is lifted and dipped in and out of the galvanizing kettle, the better you can locate the venting and draining provisions. As a designer, you are always trying to find the balance of putting enough holes for galvanizing while not putting too many to affect the structural integrity of the steel member.

 

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Tags: standard shape steel structures, galvanized steel, steel fabricator, galvanized structural steel, rectangular steel tube, steel square tube

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