Ask Professor Zinc
May
Q: &#;Is any special preparation required for painting over galvanized steel?&#;
Answer:
Painting over hot-dip galvanized steel, known as a duplex system, has been used for decades as a means to enhance corrosion protection while allowing paint to be used for color coding or aesthetic reasons. Duplex systems are becoming increasingly common and popular because of this combination of benefits.
Successfully painting over hot-dip galvanized steel is relatively simple, but like painting on any other surface, proper surface preparation is key to creating an effective bond between the paint and the galvanized surface.
Follow ASTM D -99 Standard Practice for Preparation of Zinc (Hot-Dip Galvanized) Coated Iron and Steel Product and Hardware Surfaces for Painting.
The basic steps before painting are as follows:
1. Talk to the galvanizer the make sure they know your plans and can avoid any treatments that would impede paint adhesion.
2. Determine the condition of the surface: newly galvanized, partially weathered or fully weathered.
3. Clean the surface. Remove bump, runs and drips and remove any organic materials. Then rinse and dry.
4. Profile the surface. This final step may include sweep blasting, a wash primer, acrylic pretreatment and/or surface grinding.
Proper surface preparation is critical, but the final result is worth it when the two coatings work together in synergy.
The exterior layer of paint or powder coating will slow down the rate at which the zinc is consumed, greatly extending the life of the galvanized steel. In return, once the exterior layer has been weathered down or damaged, the zinc beneath is still available to provide cathodic and barrier protection. As a result, the substrate steel is afforded corrosion protection for 1.5 to 2.3 times the sum of the expected life of each system alone.
For example, if a galvanized coating alone on black steel would provide 50 years of maintenance-free protection and a paint coating would not require any maintenance for 10 years, the combination duplex system would provide maintenance-free protection for 90 to 138 years in the same environment. A periodic maintenance schedule can extend this synergistic lifetime even further.
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February
Q: &#;Can muriatic acid be used to clean hard water mineral deposits off galvanized steel parts in an irrigation system?&#;
Answer:
Muriatic acid is another name for hydrochloric acid. Diluted hydrochloric acid is often used as a descaling agent to remove limescale from metal surfaces in contact with hot water, such as in boilers, water heaters, and kettles. However, it disolves zinc. Zinc reacts quickly with the acid to form bubbles of hydrogen and zinc chloride, a salt. Because of this reaction, muriatic acid is actually used in a 20% solution to remove zinc coatings from steel that has to be welded after galvanizing. So the answer is &#;no&#; for using muriatic acid for cleaning galvanized parts.
CLR Calcium Lime Rust Remover would be a better choice for your task. It will remove hard water calcium, lime and iron deposits without dissolving or damaging the protective zinc coating on your irrigation system components.
If for any reason you need to touch up or repair a galvanized surface, one simple method is to use a zinc-rich paint such as Rustoleum Cold Galvanizing Compound. It can be applied to a clean, dry steel surface by either a brush or spray. Zinc-rich paints must contain either between 65% to 69% metallic zinc by weight or greater than 92% metallic zinc by weight in dry film. The coating thickness for the paint must be 50% more than the surrounding coating thickness, but not greater than 4.0 mils. Measurements should be taken with either a magnetic, electromagnetic, or eddy current gauge to ensure compliance.
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October
Q: &#;Is it possible to take steel which was recently galvanized, and re-galvanize without stripping the old coating off? The problem is bare spots from welding changes.&#;
Answer:
The short answer is &#;No&#;. Based on your description of the problem, welding through galvanizing has removed the coating at the weld area and in the surrounding heat affected zone of the weld. Zinc melts at 786˚C and during welding produces a voluminous white zinc oxide at about ˚F. Welding temperatures are in the range of 3-˚F or higher, depending upon the method.
ASTM allows for repair, but limits the allowable area. The total area subject to renovation on each article shall be no more than 1&#;2 of 1% of the accessible surface area to be coated on that article, or 36 in.2 per short ton [256 cm2 per metric ton] of piece weight, whichever is less.
The specification further states, &#;Materials that have been rejected for reasons other than embrittlement are not prohibited from being stripped and re-galvanized and again submitted for inspection and test at which time they shall conform to the requirements of this specification.&#;
The hot dip galvanized coating is formed through a diffusion reaction between iron and zinc and develops a metallurgical bond between the two metals. In order for the iron-zinc alloy to develop the steel substrate must be completely clean. Residual weld fluxes, oils, paint or any other contaminant will result in a bare area that will not galvanize. Attempting to re-galvanize without removing the existing zinc will result in a coating with bare areas and zinc that is not adherent to the pre-existing coating.
Galvanized steel develops zinc oxide on the surface of the material within 24 hours of the galvanizing process. The coating gradually weathers to form zinc hydroxide and finally zinc carbonate. The re-galvanizing process will require that any zinc on the steel surface be removed completely before being re-coated in order to meet ASTM specifications.
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August
Q: &#;What is the best way to mark steel parts so they can be identified after galvanizing?&#;
Answer:
The identification of parts throughout the galvanizing process is an important consideration. Parts are marked for tracking during processing as well as to identify the project, customer, and/or assembly for shipping. Parts that are properly marked also simplify handling and construction at the job site. There are two categories of marking.
- Temporary identification &#; detachable embossed metal tags or printed tags especially designed for galvanizing such as the Kettle Tags® system used by Galvan (top photo), which stays on the material through all the steps in the process, including the acid dip and the molten zinc bath.
- Permanent identification &#; stamping, weld beads, and deep stencil marking (bottom photo)
Identification applied before galvanizing should made with a material or design that will be readable after galvanizing, but not disrupt the zinc coating&#;s integrity. All the methods mentioned above meet these requirements. However, the Kettle Tags system allows for more legible information and lowers the cost and time involvement for temporary ID in most applications.
Customers taking advantage of the benefits of Kettle Tags send Galvan a list of items that need to be tagged along with the information that should appear on the tag, including text, logos and/or bar codes. Galvan prints the tags and sends them to the customer to be attached to the appropriate materials before being shipped to Galvan. Once attached, the tags are not removed. They stay in place throughout the galvanizing process and through delivery to the customer or job site.
Because of the amount of information that can be conveyed and the fact that they stay where placed by the customer, Kettle Tags may be the answer to your question.
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June
Q: &#;Is electroplating per ASTM B633 equal to hot dip galvanizing to ASTM-A385?
Answer:
No, not at all. B633 is the Standard Specification for Electrodeposited Coatings of Zinc on Iron and Steel. In general terms the zinc coating is thin, up to a maximum thickness of 1 mil (25 μm), and mechanically bonded to the surface with a hardness of about a third to a half that of most steels. The specification, ASTM B633, lists four classes of zinc-plating: Fe/Zn 5, Fe/Zn 8, Fe/Zn 12 and Fe/Zn 25 where the number indicates the coating thickness in microns (μm). Most coatings are less than a half of a mil in thickness and are intended for indoor and/or non critical applications.
ASTM A385 is not actually a coating specification but the Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip), and is commonly referred to as the &#;design&#; spec for galvanizing.
ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel
Products and ASTM A153 Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware, are more likely the specifications you want to compare to.
Hot dip galvanized structural material will range from 45 μm to 100 μm as a minimum (1.8 to 3.9 mil) and hot dipped fasteners will range from 45 μm to 86 μm (1.7 to 3.4 mil), depending upon the type of material being coated.
Since zinc coating performance is linear and is based on coating thickness and conditions of exposure, a two mil coating will last twice as long as a one mil coating in the same environment. You can see from the relative coating thicknesses of electrogalvanizing and hot dip galvanizing that they are NOT equal. Hot dip galvanizing will have a service life that may be as much as 20 times the life of a B633 coating in the same application.
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April
Q: &#;What is the additional weight per square foot of surface area of galvanizing on a steel member?&#;
Answer:
Good question. Normally, galvanizing to ASTM standards will add 4% to 8% to the nominal weight of a steel member, depending upon the thickness of the steel being galvanized. Galvanizing is an alloy that is formed through a diffusion reaction between iron and zinc. The steel is allowed to remain in the galvanizing bath until the steel core reaches the temperature of the surrounding zinc bath, when the galvanizing reaction is complete. Thicker (heavier) steels take longer to heat, so the reaction continues for a longer period. Thinner (lighter) steels absorb heat more quickly and complete their reaction sooner and develop a lighter coating mass. It seems counter-intuitive, but heavier steels normally pick up a lower percentage of weight (4%-5%) due to the weight/surface area ratio, than thinner material. Light sections pick up a proportionally heavier percentage due to the same effect, and usually show a 7% to 8% increase in mass.
The ASTM Standards provide for MINIMUM coating weights according to the tables below:
Please remember that these are MINIMUM coating thickness standards. Galvanizing will normally meet or exceed these minimums in every case. A steel beam that has a nominal thickness of >1/4&#; will result in a galvanized coating of 100 micrometers, that will add a minimum of 2.3 oz/ft2 to the weight of the article.
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February
Q: &#;Is there an appropriate/recommended methodology to obtain an acceptable non-slip surface for a galvanized steel plate deck, either during the galvanizing process or after?&#;
Answer:
Yes, there are actually both pre-galvanizing and post-galvanizing options.
One of the most effective and reliable methods of creating a non-slip galvanized surface is to apply a thermal metallizing spray to the steel before galvanizing. In this process, the non-slip surface is produced by applying molten or semi-molten particles of metal onto the steel plate. This creates a permanent, rough random matrix on the steel for increased traction. The bond strength of the metal particles to the steel plate is 5,000 psi or more.
The steel should be blasted to SSPC Surface Preparation Specification 10 and be clean and free of oils and grease before it is metallized. After application, the metallized steel is hot-dip galvanized in accordance with ASTM A 123, providing decades of rust free, slip-resistant service.
Another option is the application of a special non-slip paint or epoxy after galvanizing. This may be a less expensive approach initially, but could require maintenance or reapplication later, increasing life-cycle cost.
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December
Q: &#;Can structural steel ASTM A36 and steel pipe ASTM A53 be hot dipped galvanized to a minimum thickness of 6 mils?&#;
Answer:
The company is the world’s best Hot Dip Galvanized W-Beam Guardrail supplier. We are your one-stop shop for all needs. Our staff are highly-specialized and will help you find the product you need.
Yes, technically it is possible, depending on the chemistry of the steel and the thickness of the material itself, but a 6-mil minimum coating thickness is well beyond the requirements of ASTM A123/123M for Structural Steel Products, which galvanizers use as a standard.
The ASTM A123/A123M specification covers individual steel pieces as well as assemblies of various classes of material. The six material categories covered include structural shapes, strip and bar, plate, pipe and tubing, wire, and reinforcing bar. Per this standard, the required minimum coating thickness for structural steel greater than 5/8-inch in thickness is 3.9 mils. For pipe greater than 5/8-inch in wall thickness, the minimum is 3.0 mils.
It is the responsibility of the galvanizer to ensure compliance with these specifications, as long as the product is in accordance with the supporting design and fabrication specifications.
It should also be noted that excessive coating thickness can cause problems. Although a thick coating is associated with a longer time to first maintenance, coatings developed beyond 8-10 mils tend to be very brittle and are susceptible to flaking both at the galvanizing plant or after delivery to the job site upon exposure to impact forces during handling, transportation, or installation by the customer.
The ASTM A123/A123M specification has stood the test of time. Sticking with a proven standard is always a good idea.
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November
Q: &#;Why does hot dipping make welds seem to pop up?&#;
Answer:
The answer is most likely the difference between the chemistry of the steel and the weld metal. Galvanized coating thickness primarily depends on the silicon content of the iron or steel part. The major difference between the weld metal and the structural steel is the amount of silicon in the weld rod. Excessive silicon in the weld filler material can accelerate the growth of the hot-dip galvanized coating. Because some weld rod metal contains nearly 1% silicon, the difference between the coating thickness on the weld metal and the surrounding structural steel can be significant. Excessive silicon in the weld material to be galvanized causes an accelerated formation of the zinc-iron layers that make up the hot-dip galvanized coating, greatly increasing coating weight.
When the fabricated structure is immersed in the zinc bath long enough to achieve a coating that meets the minimum thickness of the galvanizing standards (such as ASTM A 123/ A 123M, Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products), the coating on the high-silicon weld metal can be more than two-times thicker than the surrounding coating. This thick coating on the weld detracts from the appearance of the fabricated structure and increases the possibility of the zinc coating becoming damaged in the weld area with further handling of the assembly or part.
For typical welding processes, such as shielded metal arc welding (SMAW), submerged arc welding (SAW) and flux-cored arc welding (FCAW), there are weld rod materials that will not cause excessively thick coatings.
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September
Q: &#;Can you dip something measuring 9&#; 2&#; wide x 15&#; long x 3&#; thick? It is made of I-beams and channels. Here is a photo.&#;
Answer:
Absolutely. When a fabrication is too large for a single immersion, it is often possible to galvanize it by progressive dipping, immersing one end of the work at a time in the galvanizing bath. Galvan Industries&#; hot dip galvanizing kettle is 42&#; long x 4&#;6&#; wide x 8&#;6&#; deep. With &#;progressive&#; or &#;double dipping&#;, our large bath size will allow us to hot dip galvanize almost any structure for greatly reduced maintenance costs and extended life.
Click here to see our double-dipping chart.
Because hot-dip galvanizing is a total immersion process, it is always a good idea to verify kettle constraints with your galvanizer in advance when dealing with a large items like this one.
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July
Q: &#;What is the best arrangement of vent holes in a sealed rectangular tube that is 10 inches by six inches? Length varies.&#;
Answer:
ASTM A385/A385M specifies that a minimum vent opening of 30% of the cross-sectional area of the tubular structure must be included on each end of the capped tube. This is so that no air or solutions are trapped inside the assembly during galvanizing. In this case, the cross-sectional area is 60 sq. in., which means the vent openings must equal 18 sq. in. (30%).
The diagrams here show three different ways to achieve the 30% vent opening. All three meet the requirements of the spec. The best arrangement is the one that works best for your situation or application.
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May
Q: &#;What is hot dip galvanizing CLASS B-2?&#;
Answer:
Class B-2 is a materials classification from ASTM A153 for hardware products such as castings, fasteners and miscellaneous threaded objects that are centrifuged, spun, or otherwise handled to remove excess zinc. Class B is refers specifically to rolled, pressed, and forged articles. B-2 identifies such articles under 3/16 inch (4.76mm) in thickness and over 15 inches (381mm) in length.
The requirements for ASTM A153/A153M are very similar to those for ASTM A123/A123M, except for the addition of threaded products and embrittlement requirements.
ASTM A153/A153M Requirements
&#; Finish &#; continuous, smooth, uniform
&#; Embrittlement &#; high tensile strength fasteners (>150ksi) and castings can be subject to embrittlement
&#; Appearance &#; free from uncoated areas, blisters, flux deposits and gross dross inclusions as well as having no heavy zinc deposits that interfere with intended use
&#; Adherence &#; the entire coating should have a strong adherence throughout the service life of hot-dip galvanized steel
&#; Threaded Products &#; areas with threads are not subject to the coating thickness requirement
&#; Coating Thickness/Weight &#; depends on the material category and steel thickness, as listed in Table 3 of the specification. For Class B-2, the requirements are as follows:
Weight (Mass) of Zinc Coating,
Coating Thickness, mils
oz/ft
2
(g/m
2
) of Surface, minimum
(microns), Minimum
Class of Material
Average of Specimens Tested
Any Individual Specimen
Average of Specimens Tested
Any Individual Specimen
B-2
1.5 (458)
1.25 (381)
2.6 (66)
2.1 (53)
For more information, visit the Galvanizing Standards section of the AGA web site, www.galvanizeit.org.
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March
Q: &#;We have a supplier of an endfitting overseas that is having trouble galvanizing the inside of a cast fitting due to a deep pocket in the fitting. I have attached a few pictures. Could you explain the difficulty of getting the galvanization to work in this area?&#;
Answer:
A.There are two possible explanations for the bare spots inside the castings. The first is that they were not properly cleaned before dipping. Castings must be abrasive cleaned by shot blasting prior to the galvanizing process. If there is any sand or other residue left from the casting process inside the part, it will not galvanize correctly. At Galvan, we blast clean every casting prior to galvanizing.
The second possible explanation is the formation of an air pocket inside the part. If this part is not hung at exactly the right angle when it is being dipped, an air pocket will form at the back of casting and it will not get full coverage of zinc. The air will not allow the zinc to reach the back of the part.
A fix could be an added hole through the back of the part to allow the air to escape. However, if the purpose of this part is to stop the flow of a liquid or gas, adding a hole would create another problem.
If that is the case, the answer to the problem is greater care in cleaning to eliminate sand inclusions and greater care in dipping to ensure that air inside the casting is allowed to escape.
Regardless of what caused the problem, these parts as shown should be cleaned and repaired, or rejected, stripped, and regalvanized.
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January
Q: &#;Does the cost of galvanizing really pay off for transportation infrastructure projects like bridges and airports?&#;
Answer:
A. Airports, bridges, highways and mass transit systems are major public investments. It is critical to protect these large investments with a sustainable, durable, maintenance-free corrosion protection system that will withstand the effects of constant rough usage and environmental exposure. Hot dip galvanizing the steel used in infrastructure projects from rebar and structural steel to guard rail and sign posts will lead to significant savings in maintenance and repair costs. The extended the life of projects in question will also save or delay replacement costs.
The annual direct cost of corrosion for highway bridges alone is estimated to be $6.3 to $10.15 billion, a cost that has to be paid by state and federal taxes. Building every new bridge &#; structural steel or reinforced concrete &#; with galvanized steel would keep those costs from rising further and reverse them in the future as older bridges are replaced. Optimizing the return on an infrastructure investment means building things to last as long as possible, safely and maintenance-free. Given that, galvanizing definitely pays off.
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