Technical Info

Specifications for Superior Corrosion
Protection & Expert Insights

Technical Info

Hot-dip galvanising is an exceptionally cost-effective and hassle-free method of protecting against corrosion. Like any production process, hot-dip galvanising must abide by specific established standards before the inspection to verify conformance with these specifications. When specifying galvanised steel in the UK and Ireland, you should be familiar with the standard for hot dip galvanising: EN ISO 1461 Standard. It specifies general properties and test methods for hot dip galvanised coatings.

Tank Size

Single Dip

If three dimensions are all close to the maximum below please contact for advice. All materials require some leeway at the galvanising bath for movement.

  • Maximum Length: 7.0m
  • Maximum Depth: 2.7m
  • Maximum Width: 1.1m

Double Dip

The Eglinton plant will double dip items, however, often we process these orders at our sister plant Silverwood Galvanising. It is cost effective for our customers.

Silverwood Bath Dimensions

  • Maximum Length: 12.0m
  • Maximum Depth: 2.7m
  • Maximum Width: 1.8m
  • Double Dipping up to 18.3m length

& Inspection

Standards-compliant manufacturing practices necessitate inspection of hot-dip galvanised steel to verify compliance with specified requirements. Inspection requires an in-depth knowledge of galvanising specifications and precise measurement techniques that enable an accurate evaluation. Ensuring compliance with relevant standards throughout this inspection process is of utmost importance.

Thanks to the zinc coating, hot-dip galvanised (HDG) products are widely known for their remarkable durability and long-term maintenance-free performance. Maintenance requirements depend on the specified thickness, which is dependent on the environment for the steel product. This will be specified by the customer.

Compliance & Inspection

Galvanising – The good and the bad

Identifying Surface Conditions
Unknown Deposits on the Surface
Flux Inclusions
The inclusion of flux can occur when the galvanising hot-dip process fails to release the flux. The galvanised coating won't form under the flux spot if this happens. The area can be repaired if it is small. Otherwise, the part will have to be rejected. Flux spots may increase when the flux is applied by wet galvanising, as the flux will float on the zinc bath's surface. The interior of hollow parts, such as pipes or tubes, can't be repaired if flux deposits are present. The part must, therefore, be rejected. If the coating underneath is not damaged and the flux has been removed correctly, any flux spots or deposits picked up when the part is removed from the galvanising pot do not warrant rejection.
Dross Inclusions
Dross inclusions (or pimples) are bubbles which form on the material’s surface due to surface defects such as dross residues. They are a distinct intermetallic zinc-iron alloy that has become trapped within the zinc coating. This occurs when the material picks up zinc-iron particles at the bottom of the pot. Dross can be avoided by changing the lifting orientation or redesigning a product to allow proper drainage. When dross particles are small and entirely covered by zinc, they do not influence corrosion protection and can be accepted. When dross particals are large, they should be removed, and the area must be repaired.
Zinc Skimmings
Zinc skimming deposits occur when zinc residues cannot be removed during the steel withdrawal process from the galvanising kettle. These zinc residues get trapped on the zinc-coated surface. However, as long as removing these deposits does not damage the underlying zinc coating and the final product meets the required standards, these deposits are considered acceptable and will not lead to rejection.
Zinc Splatter
Zinc splatter refers to small splashes and flakes of zinc that stick loosely to the surface of a galvanised coating. It occurs when moisture on the galvanising kettle's surface causes liquid zinc to ‘pop’ and create droplets that land on the product. Despite the splashes, the corrosion resistance of the zinc coating remains unaffected, and there is no reason for the product to be rejected. While removing the splatter from the zinc coating is unnecessary, it can be done if a consistently smooth appearance is desired.
Welding Spatter
Welding spatter refers to lumps that can be seen in the galvanised coating near welded areas. It occurs when the welding spatter, left on the part's surface before hot-dip galvanising, gets incorporated into the coating. To prevent welding spatter, removing any welding residues before the galvanising process is essential. While welding spatter may appear covered by the zinc coating, it does not adhere well and can be easily removed. If the zinc coating is damaged, this type of defect may result in an uncoated or bare spot, which requires proper cleaning and repair.
Wet Storage Stain
Wet storage stain refers to a powdery, white deposit that appears on freshly galvanised surfaces. This happens when these surfaces come into contact with fresh water, such as rain, dew, or condensation, causing a reaction with the zinc, resulting in the formation of zinc oxide and zinc hydroxide. This type of staining is commonly found on tightly stacked and bundled galvanised items like sheets, plates, angles, bars, and pipes. The stain's appearance can vary from light to medium or heavy white powder on the galvanised steel product. One approach to prevent wet storage stains is using a chromate quench solution to passivate the product after galvanising. Additionally, avoiding stacking products in poorly ventilated and damp conditions is essential. In most cases, light or medium wet storage stains will naturally fade over time and are considered acceptable. They do not usually indicate significant degradation of the zinc coating or reduce the product's expected lifespan. However, if heavy wet storage stains are present, it's crucial to remove them either mechanically or with appropriate chemical treatments before using the galvanised part. Otherwise, the part should be rejected and re-galvanised. After leaving the hot-dip galvanising facility free of white rust, it becomes the customer's responsibility to decide whether or not to follow the storage recommendations provided by the galvaniser.
Unsealed Wet Rust Bleeding
Weld rust bleeding from unsealed crevices will stain the zinc-coated surface at the welded connections on steel. This occurs when cleaning solutions get trapped in incomplete welds, leading to corrosion of the steel surfaces concealed by the weld. To prevent weeping welds on small overlapping surfaces, fully seal the edges of the overlapping area. However, sealing the weld is not recommended for larger overlapping areas due to the risk of air volume expansion, which could cause explosions in the galvanising kettle. To avoid weeping welds in these larger areas, the best approach is to create a 3/32" (2.4mm) or larger gap between the two pieces during welding and allow the zinc to fill the gap, creating a stronger joint once the process is complete. It is important to note that weeping welds are not the responsibility of the galvaniser, and they do not warrant rejection. Galvanisers should be aware of this eventuality but are not liable for its occurrence.
Excess Aluminium in Molten Zinc
Bare spots and black marks on the surface are defects that may be caused by excessive aluminium in the galvanising bath. According to international product specifications, galvanisers must maintain a bath of 98% pure zinc, with the remaining 2% comprising additives chosen by the galvaniser. One common additive is aluminium, which helps enhance the coating's appearance. However, an excess of aluminium can lead to bare spots and black marks on the steel surface. To avoid excessive aluminium, it is vital to ensure proper control of its level in the galvanising bath through regular sampling and analysis, making adjustments in a consistent and controlled manner. The affected part may be repaired for small areas with bare spots as specified in the guidelines. However, if this condition affects the entire part, it must be rejected and re-galvanised. Galvanisers should pay attention to the aluminium level to prevent this issue and maintain the quality of the galvanised coating.
Brown Staining
Following hot-dip galvanising, a material may develop a surface issue called brown staining which is caused by the oxidation of iron in the zinc-iron alloy layers. This leads to a change in appearance from the typical grey of galvanised steel to a brown colour. Brown staining occurs when the pure zinc eta-layer or most of the eta-layer matrix surrounding the zeta layer has been consumed by corrosion in the affected areas. Laboratory tests and field surveys reveal that zinc coatings on reactive, high-silicon steels are more prone to brown staining than those on low-silicon steels. This is due to differences in the structural layout of the zinc coating after galvanisation. The eta-layer is thicker for low-silicon steels, and the intermetallic structure is tightly compact, making it difficult for free iron particles to penetrate and oxidise within the zeta layer. However, in high-silicon steels, the eta-layer is thinner, and the intermetallic structure is less compact, allowing free iron particles to migrate to the top of the zinc coating and form brown stains upon oxidation. Distinguishing between red rust on the base steel and brown staining can be challenging visually. However, a coating thickness gauge can help differentiate between the two. Red rust will only form on the base steel if no zinc is present at the specific point of the defect. If only brown stains are present, the base steel is not rusting, and the galvanised steel's corrosion performance remains unaffected. As per the international specification, brown staining is acceptable since zinc is still present under the brown stains.
Bare Spots
Uncoated areas on the steel surface, known as bare spots, are common surface defects caused by various factors like inadequate surface preparation, welding slag, sand in castings, excessive aluminium in the galvanising kettle, or lifting aids hindering coating formation in certain spots. The standard allows for repairing very small bare areas, less than 1 inch in the narrowest dimension, and totalling no more than 0.5% of the accessible surface area. However, larger bare areas exceeding one-inch square must lead to re-galvanising. To prevent bare spots, the galvaniser must ensure thorough surface cleaning and the absence of contaminants during pretreatment. If rejection occurs due to the size of the bare spot or total surface area, the parts can be stripped, re-galvanised, and then re-inspected to comply with the necessary standards and specifications.
Surface Contaminants (paint, oil, wax, lacquer)
Contaminants like paint, oil, wax, and lacquer on surfaces are challenging to remove during the standard caustic solution cleaning/degreasing step in the surface preparation of the steel for hot-dip galvanising. These contaminants are often not visible before or after immersion in this initial cleaning process. Consequently, certain areas remain unclean and are not properly cleaned during the pickling step either. Consequently, the iron in these areas does not react with the molten zinc in the galvanising kettle, resulting in ungalvanised spots or bare areas that are easily noticeable. If these bare spots' total surface area exceeds the specified allowance, the part must undergo re-galvanising. This ensures the surface is appropriately cleaned and coated to meet the necessary standards.
Weld Slag
Weld slag on the black steel surface cannot be eliminated during the galvanising cleaning process. Therefore, all weld slag must be removed mechanically before sending the steel to the galvaniser's facility. If the slag is overlooked and the steel undergoes the galvanising process, it forms bare spots. According to standard specifications, these bare spots must be repaired. The specifier and fabricator must ensure the removal of weld slag unless they have made prior arrangements with the galvaniser. Taking this precaution ensures a successful galvanising process and prevents the occurrence of bare spots that need additional repairs.
Sand Inclusions in Castings
Another form of surface defect arises when sand gets embedded in castings, resulting in rough or bare areas on the galvanised steel surface. These sand inclusions cannot be removed through the usual acid-pickling process. Therefore, it is essential to perform abrasive cleaning at the foundry before sending the products to the galvaniser. When this type of defect occurs, it creates bare spots that necessitate cleaning and repair. If the issue is significant, the part may have to be rejected, stripped of the existing coating, and re-galvanised. Taking proactive measures, such as abrasive cleaning at the foundry, helps prevent these defects and ensures a smoother galvanising process with better surface quality.
Weld Blowout
Welding blowout refers to a bare spot that appears around a weld or overlapping surface hole. This happens when pre-treatment liquids seep into the sealed and overlapped areas, leading to boiling during immersion in the liquid zinc. As a result, localised surface contamination occurs, preventing the proper formation of the galvanised coating. To prevent welding blowouts, it is essential to carefully inspect weld areas to ensure there is no fluid penetration. Additionally, preheating the products before immersion in the galvanising kettle can help dry out overlap areas, reducing the risk of blowouts. When welding blowouts occur, they create bare areas that must be repaired before the part is deemed acceptable. Taking measures to mitigate this issue is the responsibility of the specifier and the fabricator.
Rough Surface
Drainage Spikes
Drainage spikes or drips refer to spikes or teardrops of excess zinc that form along the bottom edges of the product. They occur when the product's surfaces are processed horizontally during galvanising, hindering proper zinc drainage as the product is removed from the kettle. During an inspection, drainage spikes are typically removed through buffing or grinding. The excess zinc does not impact corrosion protection. However, they can pose a safety hazard to people handling the parts. Therefore, these defects must be eliminated before the part can be accepted. Ensuring proper drainage and removing spikes or drips is crucial to maintaining product quality and safety.
Clogged Threads
Clogged threads occur when a threaded section does not drain properly after removing the product from the galvanising kettle. To address this issue, post-galvanising cleaning methods — like using a centrifuge or heating the threads to around 800 F (420 C) and brushing them with a wire brush — can be employed to remove the excess zinc. It is essential to clean the clogged threads before the part can be accepted. Taking appropriate measures to ensure proper drainage and removing excess zinc from the threads is crucial to meeting quality standards for the product.
Clogged Holes
Clogged holes refer to holes partially or fully obstructed by zinc metal. This happens because liquid zinc has high surface tension and does not quickly drain from holes smaller than 1/2" (12mm) in diameter. To reduce the occurrence of clogged holes, it is advisable to make all holes as large as possible. Several methods can remove the trapped zinc, such as active fettling while the part is in the galvanising kettle, vibrating the crane hoist chains to agitate the parts, or blowing compressed air onto the galvanised products. Having clogged holes is not a reason for rejection unless it hinders the part from fulfilling its intended purpose. Ensuring proper hole sizing and utilising the mentioned techniques to address clogged holes helps maintain the quality and functionality of the galvanised products.
Sandpaper Effect
Steel Chemistry Silicon
Rough surface condition or appearance refers to a textured and uniformly rough coating that covers the entire product. This roughness is caused by using hot-rolled steel with high silicon content. To avoid this rough surface condition, it is recommended to purchase steel with a silicon content of less than 0.03% by weight. Interestingly, the rough surface condition can benefit corrosion performance because it leads to a thicker zinc coating. However, a rough coating may be grounds for rejection if it occurs on handrails. Generally, the corrosion performance of galvanised steel with rough coatings remains unaffected by the surface texture. It is essential to consider that even if the steel's silicon level is less than 0.03%, other elements like phosphorous may still influence the coating thickness and result in a rough surface. The combined effect of phosphorus and silicon can accelerate the growth of zinc-iron alloy layers during the galvanising process, acting as catalysts for the metallurgical reaction between zinc and iron.
Steel Chemistry Phosphorus
A rough surface condition or appearance refers to a textured and uniformly rough coating that covers the entire product. This roughness is caused by using hot-rolled steel with a high phosphorus content. To avoid this rough surface condition, purchasing steel with a phosphorus content of less than 0.04% by weight is advisable. The rough surface condition can benefit corrosion performance because it produces a thicker zinc coating. However, one situation where a rough coating may lead to rejection is if it occurs on handrails. Generally, the corrosion performance of galvanised steel with rough coatings remains unaffected by the surface texture. It is essential to consider that purchasing steel with lower phosphorus content can help prevent the rough surface condition and ensure a smoother coating. The increased zinc coating thickness from the rough surface can improve corrosion resistance in most applications.
Steel Surface Condition
Rough surface condition or appearance refers to a consistent and textured appearance that covers the entire product. Suppose it is not caused by the steel's chemistry, such as high silicon or high phosphorus content. The rough surface condition may arise from the mechanical cleaning process, like blasting, before reaching the galvaniser. Abrading the black steel during mechanical cleaning creates a profile that encourages rapid growth of the intermetallic layers, particularly a column-like crystalline growth of the zeta layer. This outcome leads to a rough coating, which is not a reason for rejection. In summary, a rough surface condition can be caused by mechanical cleaning methods like blasting, resulting in a textured appearance of the product. While it may not be due to the steel's chemistry, it is still not a cause for rejection in the galvanising process.
WRuns are concentrated thick patches of zinc on the surface of a product. They happen when zinc solidifies on the product's surface during removal from the zinc bath. This is more likely to occur on thinner sections with large surface areas that cool rapidly. If possible, adjustments can be made to the dipping angles to prevent runs from changing the drainage pattern to a more acceptable form. In cases where runs cannot be avoided and may affect the product's intended use, they can be smoothed out by buffing. It's important to note that runs are not a reason for rejection in the galvanising process.
Inconsistent Coating Appearance
Steel Chemistry
The variation in the appearance of galvanised steel is primarily influenced by the steel's chemistry, particularly the levels of silicon and phosphorus. Both elements promote thicker coating growth, leading to different appearances. The amount of silicon added during the steel-making process can create distinct appearances in galvanised products. The recommended silicon composition is either less than 0.04% or between 0.15% and 0.25%. Steels outside these ranges are considered reactive and tend to form thicker zinc coatings. Highly reactive steels with elevated levels of silicon and phosphorus often have a matte grey or mottled appearance, unlike the typical bright coating. The rapid growth of the zinc-iron intermetallic layer is responsible for this appearance, which is beyond the galvaniser's control due to limited knowledge of the steel's composition. Nevertheless, the increased coating thickness can offer advantages, as it directly influences the time to first maintenance. The Sandelin curve illustrates the relationship between zinc coating thickness and the percentage of silicon in steel. The ‘I’ area, known as the Sandelin range, exhibits thick and dull grey coatings due to the silicon content, roughly between 0.05% and 0.15%. The ‘II’ area represents steel with over 0.25% silicon, where coating thickness increases with silicon content and levels of around 0.4%. Phosphorus also affects the reaction between liquid zinc and steel, leading to specific coating characteristics. Steel with high phosphorus levels may result in smooth, dull coating areas and thicker coating ridges due to increased intermetallic growth, resulting in a rough surface with ridges. Although galvanised products may have varying appearances due to steel chemistry, they all maintain equal corrosion resistance and are considered acceptable.
Different Cooling Rates
A visually dull or shiny coating on a product can result from variations in the cooling rate of the product. Rapid cooling at the outer edges allows a free zinc or eta layer to form on top of the intermetallic layers. However, the zinc in the centre of the product, which would have formed the eta layer, cools more slowly and may be consumed in a reaction with the iron after removing the product from the galvanising kettle. As a result, the outer layer of the coating in the centre appears intermetallic, giving it a dull grey appearance. Over time, as the product weathers, these differences in appearance will even out, and the entire product will eventually develop a dull grey colour throughout.
Stress Induced During Steel Production
Apart from the temperature and chemistry of the steel, the way the steel is processed can also influence the appearance of galvanised products, whether bright or dull. For instance, in the example photo, the top rail shows a winding pattern of dull grey areas resulting from the tube-making process. Stresses in the steel during processing affect the formation of intermetals and can create this striped appearance. However, it's crucial to note that despite the visual difference, the corrosion protection remains unaffected, and these parts are deemed acceptable.
Oxide Lines
Oxide lines refer to light-coloured oxide film lines visible on the surface of galvanised steel. These lines occur when the product is not withdrawn from the galvanising kettle at a constant rate, which could be due to the product's shape or drainage conditions. Over time, these oxide lines will naturally fade as the entire zinc surface oxidises. It's essential to understand that while they may impact the initial appearance, they have no effect on the corrosion performance of the steel. Therefore, the presence of oxide lines is not a reason to reject hot-dip galvanised parts.
Touch, Chain, and Wire Marks
Touch marks are a form of surface defect characterised by damaged or uncoated areas on the product's surface. These marks occur when galvanised products come into contact with each other or the material handling equipment during the galvanising process. If touch marks meet the size criteria for repairable areas, they are not grounds for rejection. However, it is crucial to repair these marks before accepting the part, as per standard specifications, which do not permit any bare spots on the finished galvanised product. Ensuring the repair of touch marks guarantees that the galvanised part meets the required standards and specifications.
Finished Product in Contact
Another surface defect may arise when steel parts touch each other or become stuck together during the galvanising process. This can happen when multiple small products are hung on the same fixture, leading to the possibility of products getting connected or overlapping during galvanisation. The galvaniser is responsible for handling all steel parts correctly to prevent such defects caused by products in contact. Ensuring proper handling and separation of the steel parts helps avoid these issues and ensures a smooth galvanising process with high-quality results.
Fish Boning
Fish boning is an irregular fish bone-like pattern found all over the surface of a steel part. It occurs due to variations in the surface chemistry of a large-diameter steel piece and differences in the reaction rate between the steel and zinc during the galvanising process. These reactions lead to distinct zones with varying thicknesses of the galvanised coating across the surface, creating the fish-boning pattern. It's essential to note that fish boning does not impact the corrosion protection the zinc coating provides. Therefore, it is not a reason to reject the hot-dip galvanised part. Despite the irregular appearance, the zinc coating ensures effective corrosion resistance, making the part suitable for its intended use.
Striations are identified by raised parallel ridges in the galvanised coating, mainly appearing in the longitudinal direction. This occurrence is caused when certain sections of the steel surface are more reactive than the surrounding areas. Such sections are typically associated with the segregation of steel impurities, particularly phosphorus, formed during the rolling process in steel manufacturing. The presence of striations is related to the type of steel used for galvanising, and while it affects the appearance, it does not impact the corrosion protection performance. In most cases, striations are considered acceptable on various parts. However, if striations happen to appear on handrails, the parts must be rejected and re-galvanised. There are instances where re-galvanising does not improve the situation. In such cases, it may be necessary to reconstruct the handrail using higher-quality steel to eliminate the striations.
The mottled appearance occurs when the zinc on the coating surface combines with the iron in the steel. When using reactive steel, which contains high levels of silicon and phosphorus, this reaction between zinc and iron continues briefly after the steel is taken out of the zinc bath, as long as the temperature remains sufficiently high.
Blasting Damage
Blasting damage refers to blistered or flaking areas that appear on the surface of galvanised products. This damage occurs due to incorrect abrasive blasting procedures carried out before painting the galvanised steel. It leads to shattering and delamination of the alloy layers in the zinc coating. To prevent blasting damage, special care must be taken while preparing the product for painting. Moreover, it is crucial to significantly reduce blast pressure, following the guidelines outlined in the governing standard. It's important to note that since blasting damage results from a post-galvanising process, the responsibility for the damage lies with those handling the product after galvanisation, not with the galvaniser.
Flaking occurs when thick galvanised coatings (normally 12mm or more) develop during the galvanising process. This creates high stresses at the interface between the steel and the galvanised coating, leading the zinc to flake and detach from the surface of the steel. To prevent flaking, it is essential to minimise the immersion time in the galvanising kettle and ensure rapid cooling of the galvanised steel parts. If possible, using a different steel grade may also help prevent flaking. The micrograph provides a close-up view of flaking. In cases where flaking is limited to a small area, repairs can be made, and the part can be accepted. However, if the flaking area exceeds the specifications' allowable limit, the part must be rejected and re-galvanised. Taking these precautions will help ensure high-quality galvanised products without the issue of flaking.
Peeling (Delamination)
Peeling or delamination results in a rough coating on steel where the zinc has separated from the surface. Several factors can lead to zinc peeling. For instance, with large or thick galvanised parts, extended cooling time in the air can form zinc-iron layers after removal from the galvanising kettle. This can create voids between the top two layers of the galvanised coating. If numerous voids form, the top layer of zinc may detach from the rest of the coating and peel off the part. The part remains acceptable if the remaining coating meets the minimum specification requirements. However, if the coating fails to meet these requirements, the part must be rejected and undergo re-galvanisation. It's important to note that the galvaniser is not accountable for the defect if delamination occurs due to post-galvanising fabrication processes, such as blasting before painting. Proper care during fabrication can help prevent such issues.
Distortion refers to the buckling of a thin, flat steel plate or similar material. This occurs due to the varying thermal expansion and contraction rates between two different steel thicknesses. To prevent distortion, consider using a thicker plate, ribs, or corrugations to reinforce flat sections or construct the entire assembly using the same steel thickness. Distortion is generally acceptable as long as it does not render the part unsuitable for its intended purpose. These measures will help minimise distortion-related issues, allowing the part to function effectively as intended.
Coating Thickness
ISO 1461
The International Standards Organization (ISO) has combined the ASTM standards A123 and A153 into a single standard called ISO1461. Regarding coating thickness, ISO1461 generally requires a lower amount of zinc coating than ASTM for similar steel thicknesses. For example, when specifying a part with steel thickness greater than 6mm, ISO requires an 85µm mean /75µm local coating, whereas ASTM requires 100µm (for structural shapes), 85µm (for wire), and 75µm (for plate, strip, pipe/tube) coating thickness for steel greater than 6.4mm. It's important to note that a perfect comparison is not possible as the steel thickness categories are not exactly the same. In ISO1461, there are only two material categories for items not centrifuged: steel and castings. The minimum coating thickness requirements can be found in the tables below.

Minimum Coating Thickness on Steel Samples not Centrifuged.

Steel Thickness (mm) Local (Single) μg/m Mean (Avg) μg/m
>6 70 85
>3 to ≤6 55 70
≥1.5 to ≤3 45 55
<1.5 35 45

Minimum Coating Thickness on Castings Samples, not Centrifuged.

Steel Thickness (mm) Local (Single) μg/m Mean (Avg) μg/m
>6 70 80
<6 60 70

Unlike the ASTM A153 specification for the hot-dip galvanising of small parts, which has four material classes, ISO1461 specifies just two material categories, threaded and other, for centrifuged articles. See the tables below for minimum coating requirements for small parts under ISO.

Minimum Coating Thickness on Centrifuged Articles with Threads.

Steel Thickness (mm) Local (Single) μg/m Mean (Avg) μg/m
>6 40 50
≤6 20 25

Minimum Coating Thickness on Other Articles Centrifuged Including Castings.

Steel Thickness (mm) Local (Single) μg/m Mean (Avg) μg/m
≤3 45 55
<3 35 45
Sampling Procedures
A standard sampling protocol to ensure the quality of galvanised products has long been developed, as inspecting the coating thickness for every single piece in a project would be impractical. According to the standard, for products with a surface area equal to or less than 160 in² (1032 cm²), the entire surface of each test product is considered a specimen. A product containing more than one material category or steel thickness range will have multiple specimens. Products with surface areas greater than 160 in² (1032 cm²) are classified as multi-specimen products. There are four important terms used in the specifications, and each one is defined below. • Lot – a unit of production or shipment from which a sample is taken for testing • Sample – a collection of individual units of product from a single lot • Specimen – the surface of an individual test product or a portion of a test product that is a member of a lot or a member of a sample representing that lot • Test Product – an individual unit of product that is a member of the sample
Multi-Specimen Articles with Surface Area > 160 in²)
A multi-specimen product is a product with a surface area larger than 160 square inches (1032 square centimetres), having multiple steel thicknesses or containing more than one coating category. To test the coating thickness of such products with larger surface areas, they are divided into three continuous local sections with equivalent surface areas, and each section is considered a separate specimen (refer to the first figure above). If any of these local sections contain more than one material category or steel thickness range, that specific section will have more than one specimen. The second figure demonstrates how a test article is separated into three specimens. When following the hot-dip galvanising specifications, the table provided in the specifications determines the minimum number of specimens for sampling from a given lot size.

Minimum Number of Specimens

Number of Pieces in Lot Number of Specimens
Three or less All
4 to 500 3
501 to 1200 5
1201 to 3200 8
3201 to 10.000 13
10.000 + 20
Single-Specimen Articles with Surface Area ≤160 in²)
Each randomly selected product serves as a specimen for products with a single specimen. In thickness measurement tests, five measurements are taken at different locations across the specimen's surface to represent the total coating thickness. The mean value of these five coating thickness measurements for one specimen must have a minimum average coating thickness grade that is at least one grade below the minimum average coating thickness for the specific material category. The figure above illustrates how a lot is divided into a sample and individual specimen. When dealing with products hot-dip galvanised according to the standard specifications, the data table in the specifications determines the minimum number of specimens to be sampled from a given lot size.
Number of Pieces in Lot Number of Specimens
Three or less All
4 to 500 3
501 to 1200 5
1201 to 3200 8
3201 to 10.000 13
10.000 + 20
Articles with Multiple Material Thickness
Where products consisting of various material thicknesses or categories are galvanised, the coating thickness grades for each thickness range and material category shall meet the specified coating thickness grade.
For hot-dip galvanised rebar following the standard, the following information is utilised to determine the minimum number of samples per lot, the number of measurements per sample, and the total number of measurements required for various coating thickness measurement methods. This data is essential to ensure quality control during the galvanisation process.
    Magnetic Thickness:
  • •Three samples per lot
  • •Five or more measurements per sample
  • •15 measurements, at the minimum, comprise the average
  • Microscopy Method:
  • •Five samples per lot
  • •Four measurements per sample
  • •20 measurements, at minimum, comprise the average
Measurement Procedures and Devices
The term ‘coating thickness’ refers to the amount of zinc applied to steel, while ‘coating weight’ refers to the quantity of zinc applied to steel per unit surface area. Two main methods are used to measure the coating thickness of hot-dip galvanised steel. The first method involves using magnetic thickness gauges, which come in three types and can be easily used in the galvanising plant or field. The first type is a small gauge with a spring-loaded magnet, resembling a pencil. The gauge is placed on the steel surface and slowly pulled off, with a graduated scale indicating the coating thickness. It should be used in a true vertical position for accurate measurements, as gravity can affect readings. Multiple readings are recommended for better accuracy. The second type is the ‘banana gauge’, which measures coating thickness by rotating a scale ring until the magnetic tip breaks contact with the coated surface. It can measure thickness in any position without calibration adjustments due to gravity. The third type is the electronic or digital thickness gauge, considered the most accurate and easy to operate. After placing the magnetic probe on the coated surface, it displays the coating thickness on a digital readout. Calibration with shims of different thicknesses is needed to ensure accuracy. ISO 1461 provides information for measuring coating thickness using magnetic or electromagnetic current. It offers guidelines for obtaining precise measurements and addresses potential interference from physical properties, structure, and coating. The requirements aim to make coating thickness measurements using magnetic or electromagnetic methods as accurate as possible.

Standard Requirements

1.Measurements on large products should be made at least four inches from the edge to avoid edge effects.

2.Measurement readings should be as widely dispersed as possible.

Some general guidelines, as seen below, are for reducing error and ensuring the most accurate readings are collected when using magnetic thickness gauge instruments.

Guidelines for Reducing Error

1. Recalibrate frequently, using non-magnetic film standards or shims above and below the expected thickness value.

2. Readings should not be taken near an edge, a hole, or an inside corner

3. Readings taken on curved surfaces should be avoided if possible

4. Test points should be on “regular areas” of the coating

5. Take at least five readings to obtain a good, “true” value that is representative of the whole sample.

The second method for measuring coating thickness is optical microscopy. This technique is destructive and usually reserved for inspecting the coating of individual samples that couldn't be accurately measured using magnetic methods or for research purposes. As it's not widely used, the accuracy heavily relies on the skill and knowledge of the operator.

Additional Tests
Adherence Testing

Testing the adhesion of the zinc coating to the steel is done using a stout knife. If the coating flakes off and exposes the base metal before the knife point touches it, it is considered ‘not adherent’. It's important to note that the test's purpose is not to cut or shape the zinc coating. If the coating is adherent, the knife should leave a minor mark on the zinc's surface, but it shouldn't lead to any separation of the coating layers.

Adhesion Test with a Stout Knife

• Push down the point of the stout knife

• The coating must not flake off, exposing the base metal

• Do not perform at the edges or corners of the product

• No paring or whittling with the knife is acceptable

Bending Testing
Peeling and flaking of the coating during the bending of rebar after the galvanising process are not reasons to consider it unacceptable; these issues can be fixed. The purpose of bending tests on steel structures is primarily to assess their brittleness. It's recommended to bend the material with a radius three times larger than its thickness. Various tests are employed to evaluate the malleability of steel when subjected to bending. One method might involve determining the smallest acceptable radius or diameter for a satisfactory bend. Another approach could measure the number of bends a material can endure without breaking, considering the angle and radius of the bend. Before being hot-dip galvanised, rebar is frequently bent. Before galvanising, cold-bending steel reinforcement bars should be shaped with a bend diameter equal to or exceeding the specified value in the standard. Nevertheless, bending these bars to tighter diameters is possible if they undergo stress relief at temperatures of 900 to 1050 F (480 to 560 C) for one hour per inch (25mm) of diameter.
Embrittlement Testing
If there is a concern about potential brittleness in a product, it might be necessary to examine a small sample of the products to assess their flexibility. These tests typically involve damaging the zinc coating and possibly the product itself. Products suspected of becoming brittle should undergo testing as outlined in the standard. Depending on the conditions the product will face in its intended use, one of three tests for brittleness — the similar bend radius test, the sharp blow test, or the steel angle test — may be required. The brittleness test applies a known force to create stress that should be lower than the part's yield stress. The parts must be considered unacceptable if the testing process leads to a fracture or permanent damage.
Passivation Testing
This test includes putting drops of a lead acetate solution on the product's surface, waiting for five seconds and then carefully dabbing it. If this solution leaves a dark mark or black stain, it means there is untreated zinc. A clean result suggests the presence of a chromate protective coating.
Field Inspection
Unknown Deposits on the Surface
The examination of hot-dip galvanised steel products doesn't conclude once approved at the galvanising facility or construction site. Throughout the installation process and once in position, a sound corrosion protection plan involves periodic scrutiny and upkeep to ensure the protective coating functions as intended. When assessing hot-dip galvanised steel on-site, the inspector should pay attention to areas prone to quicker corrosion and any visual surface conditions, considering whether they pose a concern. During the field inspection of a galvanised coating, determining how many years remain before the coating requires repair or replacement is the primary consideration. The good news is that estimating the remaining time until the initial maintenance is needed for hot-dip galvanised coatings in outdoor conditions is a relatively straightforward process.
Corrosion Prone Areas

Aside from measuring the thickness of the coating, it's also important to visually examine the galvanised coating for indications of increased corrosion in certain regions. Thickness measurements must be conducted in these areas to confirm whether sufficient zinc coating is remaining or if a touch-up is necessary. Areas prone to corrosion that require closer inspection encompass:


When corrosive substances like water seep into gaps, the restricted air movement can cause electrical potential variations, resulting in corrosion-prone and corrosion-resistant sections. This situation can lead to corrosion. Some usual spots susceptible to this issue include places where materials overlap, sections where fasteners are joined, and points where the galvanised coating meets another surface like wood, concrete, or asphalt. Whenever feasible, it's advisable to prevent the formation of gaps during the design phase.

Dissimilar Metals in Contact

When different types of metals come into contact, a process called galvanic corrosion can occur. The zinc in the galvanised coating is positioned high in the Galvanic Series of Metals. As a result, it tends to corrode before nearly any other metal it comes into contact with. To minimise the risk of galvanic corrosion, it's advisable to avoid direct contact between dissimilar metals during the design stage. One effective method to prevent galvanic corrosion is to separate dissimilar metals from each other using plastic or rubber grommets or by applying paint to the metal that is more prone to corrosion (the cathode). If the cathode has a much larger surface area than the anode, the process of galvanic corrosion can swiftly deteriorate the anodic material.

Areas Where Water Pools

Level surfaces can gather water and other substances that promote corrosion, and they may experience more significant corrosion rates compared to upright surfaces. Thoroughly examining the flat sections of galvanised steel and measuring the coating thickness will confirm that sufficient protection against corrosion is present. When feasible, it's a good idea to tackle areas where water accumulates by incorporating drainage holes to prevent moisture from collecting on the surface for extended periods. In cases where drainage holes are present, regularly inspect these openings on the galvanised steel for signs of corrosion and perform any needed touch-up work.

Previously Touched-Up Areas

Sections of hot-dip galvanised steel that have been repaired either after the initial coating or during installation can sometimes experience accelerated corrosion compared to the surrounding zinc coating. It's important to visually inspect and assess these areas using a magnetic thickness gauge.

Visual Observations

You might notice several typical cosmetic concerns while visually examining galvanised steel in outdoor settings. Most of these are surface-related and don't warrant worry. However, some could necessitate care or upkeep. The prevalent visual problems often observed on galvanised steel that has been in use for a period of time consist of the following:

Brown Staining

Frequently confused with corrosion, brown staining is a surface flaw that emerges when the iron within the zinc-iron alloy layers oxidises. As highlighted in this document earlier, there are instances when hot-dip galvanised coatings develop without a free zinc layer, resulting in intermetallic layers on the surface. Additionally, as galvanised steel ages, the eta layer can erode, leading to this occurrence. Brown staining arises when unbound iron within these intermetallic layers reacts with moisture in the environment, causing oxidation that alters the colour of the nearby zinc coating. A simple test using a magnetic thickness gauge can be performed to differentiate between red rust and brown staining. If the gauge reading displays a coating thickness, it indicates brown staining and the protective properties of the galvanised coating are unaffected. Since brown staining is purely a visual matter, there's no need for touch-ups in the stained area. The staining can be eliminated by gently brushing with a nylon bristle brush. However, it's worth noting that wire brushing only provides a temporary removal of brown stains and might not be the best recommendation.

Wet Storage Stain

Improperly storing and tightly stacking galvanised items can result in the formation of wet storage stains or the buildup of zinc oxide and hydroxide on the surface, as previously discussed. If you plan to store galvanised products before installing them, it's crucial to ensure proper ventilation of the bundle to prevent the emergence of wet storage stains. Just as wet storage stains can develop during storage, galvanised products installed in areas with surface moisture and limited airflow can accumulate oxides and hydroxides that resemble wet storage stains. A typical scenario is on surfaces where accumulated snow melts and pools or in places where water remains stagnant for extended periods without drying out. A wet storage stain will likely occur within the initial month after galvanisation.

Weeping Welds

Weeping Welds frequently appear after the steel has been put to use. As mentioned earlier, weeping welds are primarily a matter of appearance, yet the presence of liquid and rust bleeding can accelerate corrosion in the affected region. To address weeping welds, the outer layer's oxides can be cleansed, followed by the application of epoxy or caulk to prevent water from infiltrating the crevices in the future.

Bare Spots

The galvanised layer's integrity can be compromised by bare spots during transportation, handling, installation, and while the steel is in use. Although some safeguarding is provided to exposed steel parts by the nearby galvanised coating, these sections can still corrode if they are overly broad or corrosive substances frequently come into contact with the steel. Research indicates that the galvanised coating supplies protective cathodic action to exposed sections ranging from 1 mm to 5 mm wide, depending on the electrolyte that electrically connects the galvanised coating to the exposed region.

Repair of Surface
Maintaining consistent protection and durability is crucial for the touch-up and mending of hot-dip galvanised steel coatings. Despite the robustness of the hot-dip galvanised coating, minor gaps or imperfections can arise during the galvanisation process or due to mishandling of the steel after galvanising. Both newly galvanised steel and those in service for extended periods require straightforward touch-ups and repairs. The procedure remains unchanged, although the permissible size limits relate to fixes conducted at the manufacturing plant. Once the product has been approved, there are no size restrictions for repairs performed in the field — whether during construction or after years of service. The touch-up and repair process for galvanised steel adheres to the standard practice for repair of damaged and uncoated areas of hot-dip galvanised coatings. This specification outlines three approved techniques for mending and restoring hot-dip galvanised steel surfaces: zinc-based solder, zinc-rich paints, and zinc-spray metallising. Additional details can be found in the Touch-Up & Repair Methods section on this site.
In-Plant Repairs
To uphold consistent protection and ensure lasting defence against corrosion, it is essential to address small gaps or imperfections that may arise during the galvanisation process. This involves applying touch-ups or repairs to the galvanised coating, maintaining a uniform barrier and providing cathodic protection. Several common situations necessitate touch-up work at the galvanising plant, such as addressing exposed areas discovered post-galvanisation, rectifying marks caused by steel pieces coming into contact during the galvanising process, or attending to marks left by chains and wires. When repairing freshly galvanised materials at the galvanising plant, the primary limitation concerns the size of the affected area. The specific size restrictions are outlined in the galvanising standard EN ISO 1461. Any method for touch-up and repair detailed in the standard (including zinc-based solder, zinc-rich paint, or metallising) is permissible unless agreed otherwise. The thickness requirements for the repair materials are also provided in the standard and are determined by the material category.
Allowable Size
The main restriction in the specification for repairing newly galvanised material is the area's size outlined in the product galvanising specifications. Another tenet of the specification for touch-up and repair is the coating thickness of the repair area. Touch-up materials are required to meet a coating thickness of at least 2.0 mils (50.8 µm) for one application, and the final coating thickness of the repair area is dictated by the material used to repair, outlined in the Touch-Up & Repair Methods section.
Touch-Up and Repair Methods
Zinc-Based Solders
The standard outlines three approved approaches for fixing and restoring hot-dip galvanised steel: utilising zinc-based solder, zinc-rich paints, or zinc-spray metallising. Zinc-based soldering involves the application of zinc alloy in either stick or powder form. To carry out this repair, the area requiring attention should be heated to around 600°F (315°C). The specification provides the acceptable compositions of solders suitable for repair work. The final coating thickness for this repair must align with the specification requirement for the steel part's material category being repaired, with a maximum thickness of 4 mils (100 µm). The thickness measurement should be performed using any non-destructive methods outlined in the standard. Zinc-based solder products closely match the nearby zinc and seamlessly integrate with the existing coating appearance.
Zinc-Rich Paints
The standard provides three approved techniques for mending and restoring hot-dip galvanised steel: zinc-based solder, zinc-rich paints, and zinc-spray metallising. Zinc-rich paint is applied to a clean and dry steel surface using a brush or spray. These paints must contain either 65% to 69% metallic zinc by weight or over 92% metallic zinc by weight in the dry film. Zinc dust-based paints fall into either organic or inorganic categories, based on their binding agents. Inorganic binders are especially suitable for paints used in touch-up applications for undamaged sections of hot-dip galvanised areas. The paint's coating thickness should be 50% greater than the surrounding coating thickness, not exceeding 4.0 mils (100 µm). To ensure adherence to the specification, measurements should be taken using a magnetic, electromagnetic, or eddy current gauge.
Zinc Spray (Metalising)
The standard outlines three approved methods for addressing touch-up and repair hot-dip galvanised steel: zinc-based solder, zinc-rich paints, or zinc-spray metallising. Zinc spray, known as metallising, involves melting zinc powder or zinc wire in a flame or electric arc and projecting the resulting molten zinc droplets onto the intended surface using air or gas. The zinc used should be of nominal purity of 99.5% or better. The renewed portion must have a zinc coating thickness at least as thick as the requirement specified in the standard for the corresponding material category. For optimal outcomes, thickness measurements of the metallised coating should be taken using either a magnetic or electromagnetic gauge.
In-Field or Jobsite Repairs
The hot-dip galvanised coating demonstrates remarkable resistance against abrasion. Nevertheless, instances of damage can arise during transportation, rough handling, installation techniques, or while in use at a job site. It's crucial to undertake touch-ups and repairs for compromised hot-dip galvanised steel coatings to ensure their endurance and sustained protection against corrosion. Upon delivery of galvanised steel to a job site, some typical field repairs encompass touch-ups following welding, rectifying bends in rebar or pipes, and addressing scratches or flaked coatings due to rough lifting, impacts, or other installation methods. The cathodic nature of hot-dip galvanising offers partial protection to uncoated areas, but it's advisable to perform touch-ups on these sections to extend the coating's lifespan. While there are no specific limitations on permissible repair sizes for galvanised coatings that have already been accepted and transported to a job site, any harm to or removal of the galvanised coating in the field should be minimised to the greatest extent feasible. Defects of any size can be rectified on-site using any of the three approved materials outlined in the standard: zinc-based solder, zinc-rich paint, or metallising. The required thickness for the repair materials is detailed in the standard.