DIS-TRAN Steel Blog

What Separates Hot-Dip Galvanizing From the Pack

Posted by Brooke Barone on Aug 26, 2013 11:36:00 AM

"Galvanizing is galvanizing. Hot-Dip Galvanizing is the same as any other type of galvanizing."

stop sign

After fabrication, hot dip galvanizing is specified by ASTM A123 and generally refers to fabricated assemblies typically used in petrochemical facilities, power transmission applications and offshore equipment assemblies, although, other materials such as unfabricated (stock) members are also included in the specification.  ASTM A123 clearly identifies these materials in section 1.

This specification covers unfabricated and fabricated products such as:

  • Assembled steel products
  • Structural steel fabrications
  • Large tubes already bent or welded before galvanizing
  • Wire work fabricated from uncoated steel wire
  • Steel forgings and iron castings incorporated into pieces fabricated before galvanizing or which are too large to be centrifuged

 *It does not apply to wire, pipe, tube or steel sheet which is galvanized on specialized or continuous lines, or to steel less than 22 gage (0.0299 in.) [0.76 mm] thick.

galvanizing kettle

Different types of zinc coatings:

  • Hot Dip Galvanizing – Hot dip galvanizing is achieved by immersing cleaned steel products in molten zinc at 830 0F – 850 0F.  When the base metal comes into contact with molten zinc at this temperature, the zinc and iron combine in a “diffusion reaction” forming new metallurgically bonded zinc/iron alloy layers.  The new layers (galvanizing) serve as a super-bonded abrasion resistant barrier, as well as providing cathodic protection. A metamorphosis takes place when the steel and molten zinc react together, forming a series of zinc-iron alloy layers which are: Eta (100% zinc), Zeta (94% Zinc, 6% Iron), Delta (90% Zinc, 10% Iron), and Gama (75% Zinc, 25% Iron).
  • Zinc Metallizing – Metallizing or zinc spraying is accomplished by feeding the zinc in either wire or powder form to a spray gun where it is melted and sprayed onto the steel surface.  Metallizing allows coating of fabricated items, which cannot be galvanized due to their size.  Metallizing is an ASTM approved method of repair of damaged or uncoated areas on galvanized steel.  Before metalizing, abrasive cleaning of the steel to white metal is required.  Metallizing provides cathodic protection but does not develop the zinc/iron alloying (metallurgical bonding) found with hot dip galvanized products.
  • Zinc Rich Paint – Zinc rich paint consists of zinc dust suspended in organic or inorganic binders.  Zinc rich coatings are barrier coatings, which can also provide some limited cathodic protection.  The binder must be conductive or the zinc particles must be in contact with the steel substrate to provide cathodic protection.  The coating does not develop zinc/iron alloys.  Suitable zinc rich paints are approved by ASTM as repair coatings for damaged galvanized coatings.
  • Continuous Galvanizing – Continuous galvanizing is a hot dip process, although usually limited to steel mill operations.  The process consists of coating sheet steel, strip or wire on machines over 500 feet long, running material at speeds of over 300 feet per minute.  The mil thickness is minimal compared to that of hot dip galvanized after fabrication, with minimal zinc/iron alloy layers; however, barrier and cathodic protection is provided.
  • Zinc Electroplating – Electroplating generally refers to a very thin layer of zinc coating applied to steel sheet and strip by electro-deposition in a steel mill facility.  There are no zinc/iron alloy layers, however, barrier and cathodic protection is provided.

The American Galvanizers’ Association (AGA) maintains a website with access to a wealth of information about the industry.  Newsletters, press reports, specification and fabrication guidance, technical papers, and more is available. 

**The most important thing a designer and/or fabricator can do to maximize the probability of achieving a quality galvanized product, is to consult the galvanizer during the design process.

 

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Tags: galvanizing, hot-dip galvanizing, hot dip galvanizing process, hot dip galvanized steel, zinc alloy metal, electroplated zinc, zinc electroplating, structural steel fabricators

The Sustainable Truth About Hot-Dip Galvanizing

Posted by Brooke Barone on Jul 25, 2013 2:54:00 PM

Over the past couple of decades, environmental awareness has been on the rise as more people are paying attention to how they can help preserve natural resources and lighten their carbon footprint. In the energy sector, this has been a controversial topic for many years. Refineries have been shut down, as well as nuclear plants and other industrial facilities due to greenhouse emissions and other environmental impacts.

More recently, concerns have been raised about whether or not hot-dip galvanizing is sustainable. Just to refresh your memory, hot-dip galvanizing is the process of coating fabricated steel by immersing it in to a bath of molten zinc that metallurgically bonds the zinc to the steel.  This practice has been around for over 150 years, and provides maintenance-free corrosion protection for decades. Also, galvanized steel used in public construction is an efficient use of tax dollars, due to the no maintenance for decades.

There are three fundamental steps in hot-dip galvanizing: surface preparation, galvanizing and inspection. During step one, surface preparation, the steel goes through three cleaning steps; degreasing, pickling and fluxing. It is pertinent that the steel surface is clean because zinc will not react. During the second step, galvanizing, the steel is dipped into a molten bath at 840 F, that is made up of 99 percent zinc. In the final step, the steel can either be visually inspected to find areas that did not react or a magnetic thickness gauge can be used to verify if coating thickness meets requirements.

galvanizing kettle

Sustainability

Sustainable Development (SD), according to the American Galvanizers Association (AGA), is the social, economic and environmental commitment to growth and development that meets the needs of the present without compromising the ability of future generations to meet their needs. The two most well-known and accepted ways for measuring sustainability are the combination of life-cycle inventory (LCI) and life-cycle assessment (LCA), and the US Green Building Council’s Leadership in Energy and Environmental Design (LEED).  

galvanized steel

The two primary elements in hot-dip galvanized steel are zinc and steel, which both have high recycling rates. The recycled content is determined by weight and then the recycled fraction is multiplied by the cost of the assembly to determine recycle content value. Hot-dip galvanized steel becomes one product when the zinc metallurgically reacts with the iron, becoming more than 70 percent combined recycled content, which definitely meets the requirements of Credits 4.1 and 4.2 of the Materials & Resources Credit 4: Recycled Content category.

recycled steel(a)International Zinc Association (IZA), Zinc Recycling, 2004. (b)Steel Recycling Institte, Steel Takes LEED with Recycled Content, March 2009

To further explain, let’s break down what zinc and steel actually are. Zinc is the 27th most abundant element in the Earth’s crust and exists naturally in water, air and soil. Annually, about 30 percent of the world’s zinc supply comes from recycled sources, and about 80 percent that can be recycled is reclaimed. Zinc is also essential for humans, and aids in kidney function, breathing, digestion, diabetes control and much more. Zinc can also be found in a number of products such as cosmetics, tires and in treatments like sunscreen, diaper rash, burns and more. But also to make note, an abundant amount of zinc to humans can be very harmful.

Steel is the most recycled material in the world, with 70 percent made from recycled material. It has also been vital in construction since the industrial revolution took place back in the early 1800s. But, if steel is left unprotected, it will fall victim to corrosion.

Waste Created?

LCI also examines how much solid waste is created throughout melting the material, casting into pieces and then fabricating the steel. The figure below, provided by AGA, shows just how little of solid waste is created. 

steel characteristics

Hot-dip galvanizing isn’t something new. It’s been around for centuries, and has been proven to protect steel from corrosion, while having minimal environmental, economic or social impacts. 

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Tags: galvanize, galvanizing, hot-dip galvanizing, galvanized steel, American Galvanizers Association, recycled steel

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

What You May Not Know About Hot-Dip Galvanizing

Posted by Brooke Barone on Jan 25, 2013 3:26:00 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

    Benefits of Galvanizing Steel Structures

    Posted by Brooke Barone on Sep 28, 2012 2:34:00 PM

                                                                                                                             IMG 0221 resized 600

    Every year the U.S. spends $350 billion replacing corroded steel.

     BIG PROBLEM: Corrosion increases taxes $2.2 trillion annually, which is 3% of the GDP (gross domestic production).  And what’s even more frightening is the $1.6 trillion needed investment in order to bring up the nation’s infrastructure, which translates to 27% of the bridges in the U.S. To dig deeper into that number, about 160,000 bridges across the country are in need of repairs. Over the decades, many have done the “patch and pray” method, instead of properly maintaining steel structures.

    Clearly you can see that corrosion is a major problem. So what is the solution?

    Galvanizing is the process of applying a protective zinc coating to steel in order to prevent rusting and corrosion. The zinc acts as a protective coating by forming a physical barrier and by acting as a sacrificial anode even when the barrier is damaged.

    The most common method used today is hot-dip galvanizing, in which steel parts are submerged into a bath of molten zinc. Galvanizing creates a coating that is actually stronger than steel and has an extremely long maintenance-free service life.

    Benefits of galvanizing:

    • Barrier protection resists corrosion by isolating the steel from electrolytes in the environment
    • Alloy and zinc iron alloy layers are stronger than steel, providing superior durability
    • Extremely economical-less expensive when compared to the maintenance costs that would have been needed
    • Corners and edges protected because coatings grow perpendicular to the surface
    • Coating applied inside and out (ex: tapered tubular poles)
    • Environmentally friendly compound-zinc is a natural element in the earth’s crust
    • Sustainable-zinc and steel are 100% recyclable
    • Availability-24/7/365 without temperature or humidity requirements
    • Variety of shapes and sizes
    • Process is inclusive of all cleaning, materials and labor

     Poles in Kettle 1

     

    If you look around, we encounter galvanized steel on an everyday basis. The Texas Motor Speedway, the Botanical Gardens or the Baltimore Ravens Stadium, and even down to irrigation systems and street signs; all of these steel structures are galvanized.

    One big misconception about galvanizing is that it is a “paint job.” It’s assumed that if one portion of the steel comes out bright and shiny and the other is dull and grey, then the duller area must be defected or not up to code. But the truth is that it just depends on the steel composition, and there is a good chance the duller portion contains more zinc. The discoloration has nothing to do with the sustainability of the steel, it is strictly aesthetics.

    There are 4 different types of coating appearances:

    1.)    Shiny

    2.)    Matte grey

    3.)    Spangled

    4.)    Shiny and dull

    Galvanizing has been a proven method and has been around since the 17th century, and was patented by Stanislas Sorel of Paris, France in 1837. The process of galvanizing is very thorough and unique, and has an unmatched reputation. To learn more about the 4-step process of galvanizing, click the link below.

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    Tags: steel structures, galvanize, galvanizing, corrosion resistant, protective zinc coating, hot-dip galvanizing, maintenance-free

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