The knowledge about galvanized steel coils that you want ...

1. What is the main use of hot-dip galvanized sheet?

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Answer: Hot-dip galvanized sheet is mainly used in industries such as construction, household appliances, automobiles, machinery, electronics, and light industry.

2. What are the three types of galvanizing methods currently used in the world?

Answer: There are three types of galvanizing methods: electroplating zinc, hot-dip galvanizing and coating zinc.

3. What are the two types of hot-dip galvanizing according to different annealing methods?

Answer: They can be divided into two types: in-line annealing and off-line annealing, also known as protective gas method and flux method respectively.

4. What are the commonly used steel grades for hot-dip galvanized sheets?

Answer: Product categories include general commodity coils (CQ), structural use galvanized sheets (HSLA), deep drawing hot-dip galvanized sheets (DDQ), bake-hardened hot-dip galvanized sheets (BH), dual-phase steel (DP), TRIP steel (transformation-induced plasticity steel) etc.

5. What are the three forms of cooling towers usually used for cooling?

Answer: There are two kinds of cooling methods - air-cooled and water-cooled.

6. What are some common defects in hot-dipped zinc coatings?

Answer: The main defects include peeling off, scratches, passivation spots, zinc particles, thick edges, air knife marks/scratches/scrapes/exposed steel surface/inclusions/mechanical damage/poor base metal properties/wavy edges/cambering/out-of-size/embossing/zinc layer thickness out-of-specs/roll printing etc.

7. How many forms do zinc plating furnaces have? 

Answer：There are three forms including vertical furnace，horizontal furnace，and vertical-horizontal combined furnace。

8.What is length tolerance for a piece of galvanized steel sheet?

Answer: The length tolerance should not have a negative value, and the maximum allowable deviation is +6mm.

Galvanized Steel Coil

9. What are the main reasons for zinc layer shedding?

Answer: The main reasons for zinc layer shedding include surface oxidation, silicide compounds, dirty cold rolling emulsion, oxidizing atmosphere in NOF section and high dew point of protective gas, unreasonable air-fuel ratio or low hydrogen flow rate during production process; oxygen infiltration into furnace; too low entry temperature of strip; low furnace pressure in RWP section or air intake at door；low temperature in NOF section，incomplete evaporation of oil/grease，low aluminum content in zinc pot，too fast line speed，insufficient reduction reaction time and short residence time of strip in molten zinc bath etc.

10. What are the causes of white rust and black spots?

Answer: Black spots are formed by further oxidation of white rust. The main causes of white rust formation include poor passivation film quality or uneven thickness; insufficient drying after passivation treatment; residual moisture on the surface before coating application or during coiling operation；moisture absorption due to poor ventilation conditions during storage/transportation；long-term storage under unfavorable environmental conditions such as exposure to acid/alkaline/salt media etc.

11. How long can finished products be stored? Why?

Answer: Finished products can be stored for up to three months to avoid excessive oxidation caused by prolonged storage.

12.What is the length tolerance for hot-dip galvanized steel sheets？

Answer：The length tolerance should not have a negative value, and the maximum allowable deviation is +6mm。

13.What is big flower pattern？What is small flower pattern？

Answer：Big flower pattern refers to normal spangle which can be produced through normal production process with crystal nucleus diameter no less than 0.2 mm while small flower pattern refers to spangle with crystal nucleus diameter less than 0.2 mm which is difficult to identify by naked eyes.

14.What is the protection principle of hot-dip galvanized coating?

Answer: Zinc can form a corrosion-resistant film on its surface in corrosive environments, which not only protects the zinc layer itself but also protects the steel base.

15.What is the passivation principle of hot-dip galvanized sheet?

Answer: Passivation treatment with chromate solution can form a passivation film on the surface of galvanized sheet. The chemical reaction formula is as follows: Zn + H2CrO4 - ZnCrO2 + H2

The trivalent chromium in passivating solution has low solubility and chemical activity, acting as a skeleton while hexavalent chromium easily dissolves in water and plays a role in re-passivating when scratches occur on passivated film, thus having healing effect. Therefore, within certain limits, passivation film can prevent direct erosion of steam or humid air into galvanized sheet and protect zinc layer.

16. What are the methods for testing the corrosion resistance of hot-dip galvanized sheets?

Answer: There are three methods for testing the corrosion resistance of hot-dip galvanized sheets: salt spray test, wet test, and immersion test.

17. Why is rust prevention treatment necessary for hot-dip galvanized products?

Answer: When hot-dip galvanized sheets are exposed to humid air, especially when the air contains acidic substances such as SiO2, CO2, NO2 and NO, a loose white rust will quickly form on the surface of the zinc coating. The main components of this white rust are ZnO and Zn(OH)2. This kind of white rust not only affects aesthetics but also brings great difficulties to future use.

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18. What are the reasons for white rust formation?

Answer: The reasons for white rust formation include putting hot-dip galvanizing together with other corrosive media such as acid-base or salt; damage to passivation film or anti-film; poor passivation or oiling effect; poor ventilation in storage warehouse causing humidity; water splashing on galvanized sheet during transportation; transportation at low temperature and storage at high temperature resulting in condensation.

19. How do you calculate coating thickness?

Answer: The formula for coating thickness is Dz = Gz (Sz * dz), where Dz represents coating thickness in mm, Gz represents weight per square meter of coating in g/m² , Sz represents area covered by coating in mm² , dz represents density of zinc. This formula can be simplified as Dz = 0.000139Gz.

20.What causes zinc particles to appear?

Answer: Problems with raw materials mainly involve cleanliness and iron content levels within them；the temperature being too high which causes slag floating up from bottom；Aluminum content being too high affecting solubility level of iron.

21.What does it mean when a hot-dip galvanized sheet is smooth?

Answer: With the continuous expansion of the use of hot-dip galvanized sheets, in modern strip steel continuous hot-dip galvanizing units, the following objectives can be achieved by smoothing the strip:

① Improve the flatness and straightness of thin plates. At the same time, zinc particles on surface can also be flattened to make it smooth. This is particularly advantageous for deep drawing and other high-precision applications in future.

② The smoothing roller used has been pre-treated with shot blasting, so that after being smoothed, there is a certain roughness on the surface of plated plate. It can improve coating adhesion and store a certain amount of grease which will benefit lubrication during deep drawing processing;

③ For plated plates that will undergo painting treatment later on, even if their surfaces are controlled to have small zinc flowers but these small zinc flowers still show through paint layer. Therefore, for some high-demanding galvanized sheets, their surfaces need to go through further smoothing treatment so as to obtain a more uniform silver-white appearance;

④ Through smoothing process，yield point can be lowered，making yield platform disappear or less obvious，which could prevent slip lines from appearing when stretching or deep drawing processing takes place in future thus improving its deep drawability.

Galvanized Steel Coil

If you want to import high quality steel coils from, please contact us at

- Email: jameson@fadasteel.com

Hope you can forward this message to your purchasing manager or related directors if you are not the intended one.

Thank you and best regards,

Mr Jameson / Sales Manager / FADA STEEL CO., LTD

If you’re a contractor who installs standing seam metal roof and wall systems, you significantly rely on coil manufacturers and suppliers to provide high-quality material for your projects.

Whether you’re rollforming your own panels on-site or purchasing panels from a regional manufacturer, you might run into a supplier that is advertising a low price for what seems like a quality metal sheet and coil product.

However, these low-cost coil suppliers are often cutting corners, including lacking proper quality assurance procedures, supplying a lesser quality product, or offering little to no post-sales support, all of which can negatively affect you and your contracting business.

At Sheffield Metals, we don’t claim to be a low-cost coil supplier. We offer our metal products at a competitive price level, which allows us to:

• Buy domestic steel the majority of the time.
• Coat the metal in a high-performance paint system.
• Slit and process the coil in-house.
• Provide valuable warranties, panel profile engineering, technical and design assistance, and a variety of other services.

In the article, we’re going to discuss the top six problems that your contracting business could encounter if you choose the low-cost metal coil supplier.

Problem #1: The metal used to make the coils/sheets could be low-quality.

The most apparent problem with buying from a low-cost coil supplier is that you run the risk of getting lower-quality metal to work with. This can cause issues in the long and short run, including premature failure, forming problems, little to no offered warranties, and more.

While this affects your customers directly, problems that stem from cheap, low-quality materials also reflect poorly on your business. It could stir up bad reviews, money and time lost in repairs, or worse.

Some common reasons that metal coil is offered at a lower price include:

• The metal is imported and/or is difficult to track the original manufacturing source.
• The metal isn’t mill certified, and the metal make-up (i.e., steel, aluminum, etc.) can’t be verified. This metal could have been acquired on the secondary or reject market. 
• The metal is not intended for use in architectural standing seam metal panel systems.
  • For example, lower quality and thinner metal sheet/coil are often used for agricultural projects, exposed fastener roofs, gutters, and other applications.
  • Some companies buy one type of metal for dual purposes and the metal might not be ideal for one of those end use purposes.
• The metal doesn’t have an adequate coating weight for a standing seam metal roof.
  • A painted Galvalume sheet/coil coating weight is typically AZ50, and an unpainted Galvalume sheet/coil coating weight is typically AZ55. There are coil suppliers who use AZ40 or lower, which is a lower quality when compared to AZ50.
• The supplier is using paint, rather than metal, to meet the minimum advertised thickness or gauge.
• The metal is not easily formed into a profile and may even create issues with a standing seam rollforming machine.
• The ideal steel used is going to be a structural grade steel (Grade 50) with a minimum yield of 50 KSI, which is in accordance with UL testing. Here’s a quick breakdown of varying steel grades:
  • Grade 33 – Construction grade, or Grade 33 steel, is most commonly used for rain-ware goods, such as gutters. It has a lower yield and is not as strong as structural grade steel (above 50 KSI).
  • Grade 50 – Anything within the 50 to 65 KSI range, which should result in elongation in the 20%+ range. This allows for the rigidity and tight bends needed for engineered standing seam metal roofing systems. UL commonly recognizes structural grade steel for engineered applications in metal roofing systems.
  • Grade 80 – A stronger, higher carbon steel most commonly used in agricultural panels and face-fastened systems. However, it’s not a good alternative for standing seam because it can’t make the tight bends that Grade 50 steel can make. 

Problem #2: The paint system coating the metal might be low-quality.

Beyond problems with the metal substrate, a low-cost coil might also have a cheap or low-quality paint system coating the metal. The paint quality relies on the resin type, which affects the longevity of the color and overall system performance.

Best Resin: PVDF (Polyvinylidene Fluoride)

In the architectural standing seam metal roofing industry, most metal coils are expected to have a PVDF (polyvinylidene fluoride) resin coating system, commonly referred to as Kynar 500® or Hylar 5000®. PVDF resins are the best protective coatings available to stand up to extreme temperatures, sun exposure, oxygen, humidity, and even pollution particles in the atmosphere.

The minimum requirement for a PVDF paint system is 70% PVDF and 30% acrylic, but some suppliers sell products with less than 70% PVDF in the paint system. The lack of PVDF will cause the paint to fade faster and affect the overall color integrity, allowing a manufacturer to sell the metal at a low price.

Better Resin: SMP (Silicone-Modified Polyester)

SMP coatings are a step below PVDF in terms of quality. This resin consists of a combination mixture of polyester and silicone materials. SMPs are known to be a great option to defend against extreme weather conditions.

SMP is still a good paint system for many metal roofing applications, but in terms of warranties and longevity, PVDF is the clear winner. It’s critical to watch out for businesses that might be marketing or selling SMP coated coil as PVDF, which has happened with less credible suppliers.

Good Resins: Fluoroethylene Vinyl Ether (FEVE), Plastisol, & Polyester

All three of these resins, which you can read more in-depth about in this article, are available as coating options for metal sheet and coil. These products are all significantly cheaper than SMP and PVDF coated material because they don’t offer the same protection level.

Again, some suppliers may falsely advertise their coil as PVDF or SMP when it’s coated in a plastisol, polyester, or FEVE resin. 

How to Make Sure The Paint System is Valid

The good news is that there are some ways to tell if a paint system is trustworthy, including:

• Checking to make sure the metal supplier uses a reputable paint supplier.
  • For example, the well-known and world-renowned paint manufacturer Sherwin-Williams supplies the paint that coats Sheffield Metals’ sheet and coil products. If a metal sheet and coil supplier get their products coated by an unknown paint supplier, you should question their products’ reliability. 
• Asking the coil supplier what paint company supplies their coating(s).
  • Most reputable metal suppliers are transparent about what company they use as their paint supplier. If the supplier doesn’t know or is reluctant to tell you who their paint supplier is, it could be because the supplier has a bad reputation or doesn’t have quality paint/resin systems.
• Ensuring the paint supplier offers a finish or paint warranty. 
  • When it comes to paint systems on flat metal products, warranties are one of the most significant selling points. A paint warranty is a guarantee from a manufacturer/paint supplier that the paint system applied to the metal substrate will not degrade to a certain degree within a specific time frame. This is why it’s critical to make sure a quality paint warranty is offered, especially because you can use it as a valuable selling tool.
• Checking that the paint supplier has a good track record of honoring and responding to claims.
  • Beyond offering a warranty, you’ll want to verify the paint supplier is responsive to claims, especially where the paint system was defective. You can find this out by asking around the industry or asking for warranty claims policies and procedures.
  • Even if a paint supplier offers a warranty on their product, it’s important to know that they will actually stand behind that warranty if a problem arises. Problems can happen in any manufacturing process, but how the company handles it is what sets them apart.

Problem #3: The material doesn’t adhere to testing standards or is too inconsistent for engineering requirements.

The third problem with cheap metal sheets and coils relates to engineering. Most reputable manufacturers test their metal coil and offer panel profile engineering, which means they adhere to many of the stringent testing standards and requirements used for residential and commercial projects. 

Engineering requirements dictate the type of material used in testing. With lesser grade material that might have more inconsistencies due to a lack of quality control processes, it is hard to know if the material you are getting is what was actually tested, if there was any testing done at all.

Engineering is designed to not only keep your customer and their property safe, but it also keeps your contracting business safe. Having engineering can substantiate the performance of the roof you’re installing, so why wouldn’t you always want to use an engineered product?

For reference, make sure to ask if the supplier’s metal sheet or coil is usable for profiles that have passed and received the engineering reports for:

• Wind uplift testing
• Air infiltration testing
• Water penetration testing
• Water submersion testing
• Impact resistance testing
• Fire rating
• Florida Building Code (FBC) Approval (if applicable)
• Texas Department of Insurance (TDI) Approval (if applicable)

Problem #4: Warranties are NOT offered with the metal and/or the warranties are poor or misleading.

Another indicator of if a coil supplier is questionable or if their metal coil is subpar is whether they offer warranties, including paint/finish, substrate, and weathertight warranties. 

It’s simple: Metal roofing should have a warranty of some kind. Metal is a high-quality product that comes at a higher price point than many other roof types, which is why reputable manufacturers will stand behind their products with warranties. Plus, not only should there be warranties offered, but the warranties should be straightforward as to what is and is not covered, and for how long.

As a contractor, warranties also set your business apart from your competitors, as warranties strengthen your credibility and trustworthiness to prospective customers. 

While paint and substrate warranties are available with the metal sheet and coil upon purchase, the opportunity to offer a weathertight warranty for commercial roofing projects is a game-changer. It not only certifies that the products used on the roof are high-quality, but it verifies that your business correctly installs the high-performance roofing system. So, if a coil supplier can offer a weathertight warranty option, it’s more likely that they’re a credible business.

Additionally, all warranty documents should be easily accessible and/or available on a supplier’s website. If you’re unable to find this information or the supplier is vague about what they can offer, you should question their ability to provide quality products.

Problem #5: Potential issues with forming or fabricating panels.

Some low-price coils and sheets are known to be incompatible with in-plant and portable rollforming machinery. Alternatively, if the cheaper coil will run through a rollforming machine, it might produce deformed or unusable panels.

Some examples of these occurrences include:

• The metal shows signs of oil canning – Before you rollform any panels, it’s best to check for signs of oil canning on the coil or sheet. For reference, oil canning is the perceived waviness of a metal panel and is an inherent characteristic of light-gauge, cold-rolled flat metal products. If the low-cost coil already has oil canning, running it through a rollforming machine to make panels will not make the oil canning go away; it might even make it worse.
• There is a camber in the coil – According to New Tech Machinery, camber is when sheet metal has a stretched edge and curves slightly from side to side. If you’re buying low-cost material, it’s critical to check for any cambering, as it shouldn’t be run through a rollformer. It will create uneven panels and can affect the machine by causing a need for adjustment.
• The metal and or paint cracks or fractures when rollformed or folded – On occasion, issues like paint cracking or fracturing can happen when fabricating metal. 
• The metal is too thin to be rollformed – Rollforming machines have specific thicknesses that are used to form panels. If you get a low-cost coil that is too thin, it likely won’t form correctly or could even create a problem within the machine, like metal getting stuck or jammed up in the forming rollers.
• The metal is not a proper width for a rollforming machine – Some low-cost metal suppliers might not slit the metal to the correct width for a rollforming machine to make a specific profile. Or, the width might not be consistent throughout the entire length of the coil, which could also cause forming issues.

Problem #6: The supplier doesn’t offer support for quality issues, resources, and more.

Finally, one of the most prevalent problems associated with buying low-cost metal is the lack of resources and assistance you can expect from the supplier. There is always a reason that the material is cheaper, and a lot of it relates to the support offered when you become a customer.

Some of the standard support issues we’ve come across with low-cost suppliers include:

• Uneducated or unhelpful sales representatives.
• Lack of help when there’s a quality or processing issue with the metal sheet or coil products. 
• No technical department to assist with installation questions, engineering requirements, warranties, project bidding, and other technical topics.
• Little to no complimentary value-added services, such as a take-off service, marketing support, architectural assistance, a color visualization tool, and more.
• No training or educational resources available.

Final Thoughts on Problems with Low-Cost Metal Suppliers

Choosing the low-cost metal sheet and coil supplier could lead to more money lost in the long run. Plus, it’s typically not worth risking your reputation on choosing materials that are the lowest price.

So, if you’re in the market for buying metal sheet and coil products, the best thing to do is research and learn about the different manufacturers and options available to you. Then, use your best judgement to pick the supplier that suits you and your business.

To get you started, here are some questions you might consider asking:

• Where do you buy your raw materials from? Foreign or domestic suppliers?
• Is your metal mill-certified? 
• What company supplies your paint or coatings?
• What resin is used on your sheet and coil products?
• Has this metal been tested for use on engineered panel profiles? Which ones?
• What are the warranties I can offer my customers or clients with this metal?
• Do I have access to a sales representative, any complimentary tools, training opportunities, or other resources?

Our main goal at Sheffield Metals is to help you grow your manufacturing or contracting business. We do this by providing you with the high-quality products you need to install and fabricate standing seam metal panel systems, as well as the engineering, support, and tools to help take your business to the next level.

Looking for a reputable coil supplier? Contact one of our helpful roofing specialists today to discuss how we can help your business succeed!

If you are looking for more details, kindly visit what is cold rolled steel used for.