Introduction – Galvanizing is the application of a zinc coating to iron and steel by the hot dipping process. The process involves the dipping of both fabricated and unfabricated pieces into hot molten zinc, usually about 840° F (449°C). Quality galvanizing starts before the item is dipped into the molten zinc and in some cases even before fabrication is begun. The emphasis in this presentation is on A123/A123M-17, Standard Specification for Zinc (Hot-Dip Galvanized Coatings on Iron and Steel Products and ASTM A780 Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings.
Inspection Steps – In order for the galvanizing to perform as intended, it is important that each step of the process be monitored, and the final product be inspected. Inspection defined in ASTM A123, which would be reviewed in its entirety before inspecting the quality of galvanizing. The following is a summary of some of the inspections that should be performed by the galvanizer’s QC and/or the Owner’s QA, depending on the contract requirements.
- Receipt inspection
The incoming material needs to be inspected for handling damage. The contract documents may also require evidence of fabrication release (e.g., a stamped delivery ticket or inspection stamp on the article), abrasive blast cleaning completed prior to processing, and marking identifications to be maintained throughout the galvanizing process.
It should be known if the steel is reactive or if reactive steel is present in fabricated assemblies before galvanizing begins. See topic 2 of this Galvanizing Cleaning and Painting series for a discussion of reactive steel.
Lifting chains locations need to be considered if no lifting lugs are present. The galvanizer should never create a lifting hole without the permission of the owner/engineer since doing so could affect the serviceability of the pieces.
Hanging of the pieces should be such that, if more than one piece on the rack, they will not interfere with each other preventing the molten zinc from coming into contact with the metal surface.
For galvanizing to be successful, there should be no dirt, oil, grease, rust, paint or mill scale on the pieces. Cleaning is usually accomplished by dipping the pieces in a tank with alkaline cleaner followed by a dipped rinse. Then dipping in acid bath (generally hydrochloric or sulfuric acid) followed by a dipped rinse. The last step is the application of a flux to prevent formation of any rust and to aid in the alloying of the zinc. The flux may be applied in a dry or wet method. The dry method consists of immersing the piece in a tank of liquid flux whereas the wet method has a layer of flux floating on the top of the molten zinc. Either method works, except for the wet method the flux has to be skimmed to the side of the tank so that, as the piece exits from the zinc bath, the flux will not be on the exterior of the coating. Even with dry method, skimmings (oxides which form on the surface of the molten zinc) need to be skimmed to the side to prevent being frozen onto the surface.
If the piece is too large for the zinc tank, progressive dipping (sometimes called double dipping) is required. The piece is inserted into the bath approximately halfway, removed from the bath, turned over or around and the uncoated portion reinserted into the zinc bath. The interface between the two dips needs be inspected for compliance with ASTM A123.
It is important that the galvanizer know the final use of the item being galvanized. ASTM A123 allows extra deposits of zinc (lumps and bumps) if they do not interfere with intended use. However, if the item is be coated, both D6386 and D7803 require smoothing to remove all the lumps and bumps. The galvanizer can minimize the amount of future grinding through controls of the galvanizing process but needs to know this in advance. The contract documents should also establish who is responsible for the smoothing, the galvanizer or the coater. Good acceptable galvanizing per ASTM A123 is not always acceptable for coating. The coating standards require that the item to be coated not be water quenched or chromate quenched since quenching leaves a coating on the surface that has poor adhesion and would act as a bond breaker for subsequent required coatings.
After the piece has been removed from the zinc bath, it needs to be cooled prior to inspection of the final product. For large, thick pieces this may be the next day.
Section 7 of ASTM A123 establishes the number of items to be tested based on the number of pieces in a lot.
Measurement of the thickness of galvanizing can be quite complex. Following is a brief summary of the thickness requirements, but specific details on the requirements for measurement are presented in Topic 3 of this Hot-Dip Galvanizing Cleaning and Painting Series. Minimum average zinc thickness is established in Table 1 of ASTM A123. Zinc thickness requirements are provided for steel thickness from <1/16” to >5/8” in the following categories: Structural Shapes, Strip and Bar, Plate, Pipe and Tubing, Wire, and Reinforcing Bar. Minimum average zinc thickness across the combinations of steel thickness >1/8 inch and all categories except wire range from 75µm to 100 µm (3.0mils to 4.0 mils). Unless otherwise specified, A123 does not allow for rejection of coating for heavy zinc that does not interfere with the intended use.
Adhesion of the galvanizing is tested by cutting or prying at the zinc with the point of a stout knife, attempting to lift it from the surface. The adhesion is considered to fail if the coating flakes off in the form of a layer, exposing the steel.
ASTM A385, Standard Practice for Providing High-Quality Zinc Coatings (Hot-Dip) addresses cracking. Upon removal from the zinc tank, the surfaces should be examined for cracking in areas of high residual stress. Potential locations of high residual stress include the heat affected zone at welds, which is typically harder and stronger than the surrounding steel. Unless stress-relieved, other causes of hardening include heavy cold working like bending, punching of holes and shearing of the steel during fabrication, and thermally cut edges that have not been smoothed. Cracking can also occur at framed areas and when welding steel of different thickness. The differential expansion can cause cracks if restrained. Cracking can also occur at burnt holes that have not been reamed or smoothed.
- Hydrogen Embrittlement
Hydrogen Embrittlement occurs when the steel being galvanized exceeds a tensile value of 150 ksi or has been severely cold worked, leading to a tensile value of 150 ksi or greater. Upon entering the acid bath, hydrogen is generated which is absorbed by the steel. It is generally expelled by immersion in the zinc tank; however, the small grain structure of the high tensile value steel prevents this from occurring and it remains trapped. When the steel is put under stress, hydrogen embrittlement may occur and result in cracks under use. Unfortunately, this problem is not evident upon removal from the tank, but ASTM A143, Standard Practice for Safeguarding Against Embrittlement of Hot-Dip Galvanized Structural Steel Products and Procedure for Detecting Embrittlement provides procedures to guard against embrittlement and quality checks to ensure that the embrittlement does not occur.
- Appearance and Renovation
The attached recommendations from the American Galvanizers Association (AGA) “Inspection of Hot Dip Galvanizing Steel Products” address inspections of the visual appearance of the galvanizing. Renovation (Repair) of deficiencies is also addressed.
- Renovation (Repair)
After the inspections and examinations are completed, including those in the attached list from the AGA, decisions have to be made regarding repairs. ASTM A123 refers to uncoated areas requiring renovation and has a limit on the amount of repair based on area. Only very small bare areas, less than 1 inch in the narrowest dimension with a total of no more than 0.5% of the accessible surface area or 36 square inches per short ton, may be renovated using ASTM A780, Standard Practice for Repair of Damaged and Uncoated Areas of Hot-Dip Galvanized Coatings. Quantities outside these amounts are rejected. The extent of the bare areas needs to be measured prior to the repair since the repair coating may overlap onto otherwise good galvanizing and give a false impression of the amount of repair performed.
There are basically three types of zinc repair materials.
- Paint containing zinc dust-these are typically organic binders, pre-mixed and formulated for use on steel. The paint typically has 65-69% or above 92% zinc dust in the dried film. Contract documents have to be consulted to determine if a specific type is required or if a different repair paint is required. If the item is used in immersion service, the surfaces are prepared by blast cleaning to SSPC-SP10, Near White. For less critical service, Power Tool Cleaning to Bare Metal per SSPC-SP11 is acceptable. If blasting or power tool cleaning are not allowed, SSPC-SP2, Hand Tool Cleaning can be used, but the performance may be reduced.
- Zinc Based Solder There are zinc-cadmium, zinc-tin-lead and zinc-tin-copper solders. It is important to note whether contract documents limit the use of any of the solders. The surfaces may be cleaned by wire brushing, light grinding, or light blasting.
- Sprayed Zinc-This is metallizing using zinc wire or powder. Surface preparation is by blast cleaning to SSPC-SP5, White Metal.
The materials used have to be capable
of being applicable to a 2.0 mil dry film thickness in one coat, provide a
barrier and preferably anodic to steel and be applicable in shop or field
conditions. Per ASTM A123 the thickness of the applied repair coating shall be
50% greater than that required in Table 1 o the standard, not to exceed 4 mils. It is important that the inspector knows the
final use of the galvanizing.
NOTE 1: Galvanizing which is to be powder coated may have restrictions on the use of repair paint since the items will be heated in the powder coating process.
NOTE 2: Zinc is required to meet the requirements of ASTM B6 “Specification for Zinc” which may contain traces of lead. While worker exposure issues during the galvanizing dipping process may not occur since the temperatures do not reach the volatile stage, surface preparation during repairs may create a dust that could be an issue if vacuum collection or other controls are not used. Worker exposure monitoring will indicate the extent of controls that may be needed.
Conclusion – Hot-dip galvanizing can provide long-lasting corrosion protection. Zinc is an excellent galvanic protector that sacrifices itself to protect the steel. If the quality of both the galvanizing process and finished product is not controlled and verified, then its ability to protect is compromised and the expected service life may not be achieved. Zinc has to weather in order to form a carbonate film on the surface which becomes a natural barrier protector for the zinc. This carbonate film forms by reaction with carbon dioxide in the atmosphere during normal wetting and drying cycles. But storage of items or installation in situations where the item does not dry out will prevent this carbonate film from forming and will lead to a shortening of the expected life of the even the best zinc application. In these instances, the longevity of the galvanizing can be increased by painting it with a liquid coating. Cleaning and painting galvanizing is addressed in other topics of this series.
Other topics in this galvanizing series:
Topic 1 – Inspection of the galvanizing process
Topic 2 – Problems with galvanizing reactive steel
Topic 3 – Measurement of galvanizing thickness
Topic 4 – Preparation of galvanizing for painting
Topic 5 – Coatings for galvanizing
Topic 6 – Inspection of surface preparation and coating application
Attachment – Inspection of Appearance
The following is from the American Galvanizers Association (AGA), “Inspection of Hot Dip Galvanizing Steel Products.” Galvanized items should be visually inspected for the following:
|Bare spots, defined as uncoated areas on the steel surface, are the most common surface defect and occur because of inadequate surface preparation, welding slag, sand embedded in castings, excess aluminum in the galvanizing kettle, or lifting aids that prevent the coating from forming in a small area. Only very small areas, less than 1 inch in the narrowest dimension with a total of no more than 0.5% of the accessible surface area, shall be renovated using ASTM A 780. If the size of the bare spot or total surface area is exceeded, the item is rejected.|
|Chain and wire marks||Another type of surface defect occurs when steel is lifted and transported around the galvanizing plant using a chain or wire. These lifting aids can leave uncoated areas on the finished product that will need to be repaired. If the marks are superficial, that are not grounds for rejection as long as marks can be repaired. If area is bare, ASTM specifications do not allow any bare spots on the finished galvanized part and the area is included in the dimension limits for renovation.|
|Clogged Holes||Clogged holes are holes partially or completely clogged with zinc metal. Clogs can occur because liquid zinc will not drain easily from holes less than 3/10” (8mm) in diameter due to its high surface tension. Clogged holes can be minimized by making all holes as large as possible. The trapped zinc can be removed by using active fettling when the part is in the galvanizing kettle, vibrating the cranes to jostle the parts, or blowing compressed air onto the galvanized products. This condition is not a cause for rejection, unless it prevents the part from being used for its intended purpose.|
|Clogged Threads||Clogged threads are caused by poor drainage of a threaded section after the product is withdrawn from the galvanizing kettle. Clogged threads, can be cleaned by using post-galvanizing cleaning operations such as a centrifuge or by heating them with a torch to about 500 F (260 C) and then brushing them off with a wire brush to remove the excess zinc. Clogged threads must be cleaned before the part can be accepted.|
|Delamination||Delamination or peeling creates a rough coating on the steel where the zinc has peeled off. There are a number of causes for zinc peeling. Many large galvanized parts take a long time to cool in the air and continue to form zinc-iron layers after they have been removed from the galvanizing kettle. This continued coating formation leaves behind a void between the top two layers of the galvanized coating. If there are many voids formed, the top layer of zinc can separate from the rest of the coating and peel off the part. If the remaining coating still meets the minimum specification requirements, then the part is still acceptable. If the coating does not meet the minimum specified requirements, the part must be rejected, subject to the renovation limit (see bare spots above). If delamination, occurs as a result of fabrication after galvanizing, such as blasting before painting, then the galvanizer is not responsible for the defect.|
|Distortion||Distortion is defined as the buckling of a thin, flat steel plate or other flat material such as wire mesh. The cause of this is differential thermal expansion and contraction rates for the thin, flat plate and mesh than the thicker steel of the surrounding frame. In order to avoid distortion, use a thicker plate, ribs, or corrugations to stiffen flat sections or make the entire assembly out of the same thickness steel. Distortion is acceptable, unless distortion changes the part so that it is no longer suitable for its intended use.|
|Drainage spikes||Drainage spikes or drips are spikes or tear drops of zinc along the bottom edges of the product. These result when the surfaces of the product are processed horizontal to the galvanizing kettle, preventing proper drainage of the zinc from the surface as the product is withdrawn from the kettle. Drainage spikes are typically removed during the inspection stage by a buffing or grinding process. Drainage spikes or drips are excess zinc and will not affect corrosion protection but are potentially dangerous for anyone who handles the parts. These defects must be removed before the part can be accepted.|
|Dross Inclusions||Dross inclusions are a distinct zinc-iron intermetallic alloy that becomes entrapped or entrained in the zinc coating. This is caused by picking up zinc-iron particles from the bottom of the kettle. Dross may be avoided by changing the lifting orientation or redesigning the product to allow for proper drainage. If the dross particles are small and completely covered by zinc metal, they will not affect the corrosion protection and are acceptable. If the dross particles are large, then the dross must be removed, and the area repaired.|
|Excess Aluminum in the zinc bath||Another type of surface defect is caused by an excess amount of aluminum in the galvanizing bath. This creates bare spots and black marks on the surface of the steel. The excess aluminum can be avoided by ensuring proper control of the aluminum level in the galvanizing bath by means of regular sampling and analysis, and by adjusting the levels in a regular and controlled manner. For small areas of bare spots, the part may be repaired as detailed in the specification. If this condition occurs over the entire part, then it must be rejected.|
|Fish boning||Fish boning is similar to striations and is an irregular pattern over the entire surface of the steel part. This is caused by differences in the surface chemistry of a large diameter steel piece and variations in the reaction rate between the steel and zinc. These reaction differences cause the thickness of the galvanized coating to vary in sharply defined zones across the surface. Fish boning has no effect on the corrosion protection provided by the zinc coating and is not cause for rejection of the hot-dip galvanized part unless it interferes with its function, according to the AGA.|
|Flaking||Flaking results when heavy coatings develop in the galvanizing process, usually 12 mils or greater. This generates high stresses at the interface of the steel and the galvanized coating and causes the zinc to become flaky and separate from the surface of the steel. Flaking can be avoided by minimizing the immersion time in the galvanizing kettle and cooling of the galvanized steel parts as quickly as possible. In addition, using a different steel grade, if possible, may also help avoid flaking. If the area of flaking is small, it can be repaired and the part can be accepted; however, if the area of flaking is larger than allowed by the specifications, the part must be rejected and re-galvanized.|
|Flux Inclusions||Flux inclusion can be created by the failure of the flux to release during the hot-dip galvanizing process. If this occurs, the galvanized coating will not form under this flux spot. If the area is small enough, it must be cleaned and repaired; otherwise, the part must be rejected. Flux spots can increase if the flux is applied using the wet galvanizing method, which is when the flux floats on the zinc bath surface. Flux deposits on the interior of a hollow part, such as a pipe or tube, cannot be repaired, thus the part must be rejected. Any flux spots or deposits picked up during withdrawal from the galvanizing kettle do not warrant rejection if the underlying coating is not harmed, and the flux is properly removed.|
|Oxide lines||Oxide lines are light colored oxide film lines on the galvanized steel surface. Oxide lines are caused when the product is not removed from the galvanizing kettle at a constant rate. This may be due to the shape of the product or the drainage conditions. Oxide lines fade over time as the entire zinc surface oxidizes. They will have no effect on the corrosion performance; only the initial appearance will be affected. This condition is not a cause for rejection of the hot-dip galvanized parts|
|Product in contact||Another type of surface defect is caused by products that come in contact with each other or are stuck together. This usually occurs when many small products are hung on the same fixture, which creates the chance products may become connected or overlapped during the galvanizing process. The galvanizer is responsible for proper racking/handling of all products in order to avoid this defect. In addition, if the surface of a product has a larger bare area than the specified repair requirement allows, then that product must be rejected.|
|Rough Surfaces||Rough surface condition or appearance is a uniformly rough coating with a textured appearance over the entire product. The cause for this rough surface condition is hot-rolled steel with a high level of silicon content. This can be avoided by purchasing steel with a silicon content less than 0.03% of the steel by weight. Rough surface condition can actually have a positive effect on corrosion performance because of the thicker zinc coating produced. One of the few situations where rough coating is cause for rejection is if it occurs on handrails. The corrosion performance of galvanized steel with rough coatings is not affected by the surface roughness.|
|Runs||Runs are localized thick areas of zinc on the surface. Runs occur when zinc freezes on the surface of the product during removal from the zinc bath. This is more likely to occur on thinner sections with large surface areas that cool quickly. In order to avoid runs, adjustments of the dipping angles can be made, if possible, to alter the drainage pattern to a more acceptable mode. If runs are unavoidable and will interfere with the intended application, they can be buffed. Runs are not cause for rejection.|
|Rust Bleed||Rust bleeding appears as a brown or red stain that leaks from unsealed joints after the product has been hot-dip galvanized. It is caused by pre-treatment chemicals that penetrate an unsealed joint. During galvanizing of the product, moisture boils off the trapped treatment chemicals leaving anhydrous crystal residues in the joint. Over time, these crystal residues absorb water from the atmosphere and attack the steel on both surfaces of the joint, creating rust that seeps out of the joint. Rust bleed can be avoided by seal welding the joint where possible or by leaving a gap greater than 3/32” (2.4mm) wide in order to allow solutions to escape and zinc to penetrate during hot-dip galvanizing. If bleeding occurs, it can be cleaned up by washing the joint after the crystals are hydrolyzed. Bleeding from unsealed joints is not the responsibility of the galvanizers and is not cause for rejection.|
|Sand in Castings||Another type of surface defect occurs when sand becomes embedded in the castings and creates rough or bare spots on the surface of the galvanized steel. Sand inclusions are not removed by conventional acid pickling, so abrasive cleaning should be done at the foundry before the products are sent to the galvanizer. This type of defect also leaves bare spots and must be cleaned and repaired or the part must be rejected, stripped, and re-galvanized.|
|Striations||Striations are characterized by raised parallel ridges in the galvanized coating, mostly in the longitudinal direction. This can be caused when sections of the steel surface are more highly reactive then the areas around them. These sections are usually associated with segregation of steel impurities, especially phosphorous, created during the rolling process in steel making. Striations are related to the type of steel galvanized and while the appearance is affected, the performance of the corrosion protection is not. Striations are acceptable on most parts; however, if the striations happen to occur on handrails, then the parts must be rejected and re-galvanized. Sometimes re-galvanizing does not improve the striations and the handrail must be refabricated using a higher quality steel.|
|Surface Contaminant||When surface contaminants create an ungalvanized area where the contaminant was originally applied, a surface defect may occur. This is caused by paint, oil, wax, or lacquer not removed during the pretreatment cleaning steps. Surface contaminants should be mechanically removed prior to the galvanizing process. If they result in bare areas, then the repair requirements apply and small areas may be repaired, but a large area is grounds for rejection|
|Touch marks||Another type of surface defect is known as touch marks, which are damaged or uncoated areas on the surface of the product. Touch marks are caused by galvanized products resting on each other or by the material handling equipment used during the galvanizing operation. Touch marks are not cause for rejection if they meet the size criteria for repairable areas. They must be repaired before the part is accepted.|
|Weeping weld||Weeping welds stain the zinc surface at the welded connections on the steel. They are caused by entrapped cleaning solutions that penetrate the incomplete weld. In order to avoid weeping welds for small overlapping surfaces, completely seal weld the edges of the overlapping area. For larger overlapping areas, the area cannot be seal welded since the volume expansion of air in the trapped area can cause explosions in the galvanizing kettle. To avoid weeping welds in large overlapping areas, the best plan is to provide a 3/32” (2.4mm) or larger gap between the two pieces when welding them and let the zinc fill the gap between the pieces or to seal weld the gap. Weeping welds are not the responsibility of the galvanizer and are not cause for rejection.|
|Welding blow outs||Welding blowout is a bare spot around a weld or overlapping surface hole. These are caused by pre-treatment liquids penetrating the sealed and overlapped areas that boil out during immersion in the liquid zinc. This causes localized surface contamination and prevents the galvanized coating from forming. In order to avoid welding blowouts, check weld areas for complete welds to insure there is no fluid penetration. In addition, products can be preheated prior to immersion into the galvanizing kettle in order to dry out overlap areas as much as possible. Welding blowouts cause bare areas that must be repaired before the part is acceptable|
|Welding spatter||Welding spatter appears as lumps in the galvanized coating adjacent to weld areas. It is created when welding spatter is left on the surface of the part before it is hot dip galvanized. In order to avoid welding spatter, welding residues should be removed prior to hot dip galvanizing. Welding spatter, as seen in Figure 48, appears to be covered by the zinc coating, but the coating does not adhere well and can be easily removed. This type of defect can leave an uncoated area or bare spot if the zinc coating is damaged and must be cleaned and properly repaired.|
|Wet Storage Stain||Wet storage stain is a white, powdery surface deposit on freshly galvanized surfaces. It is caused by newly galvanized surfaces being exposed to fresh water, such as rain, dew, or condensation that react with the zinc metal on the surface to form zinc oxide and zinc hydroxide. It is found most often on tightly stacked and bundled items, such as galvanized sheets, plates, angles, bars, and pipes. Wet storage stain can have the appearance of light, medium, or heavy white powder on the galvanized steel product. One method to avoid wet storage stains is to passivate the product after galvanizing by using a chromate quench solution. Another precaution is to avoid stacking products in poorly ventilated, damp conditions. Light or medium wet storage stain will weather over time in service and is acceptable. In most cases, wet storage stain does not indicate serious degradation of the zinc coating, nor does it necessarily imply any likely reduction in the expected life of the product. However, heavy wet storage stain should be removed mechanically or with appropriate chemical treatments before the galvanized part is put into service. Heavy storage stain must be removed or the part must be rejected and re-galvanized.|
|Zinc Skimmings||Skimming deposits are usually caused when there is no access to remove the skimmings during the withdrawal of the steel from the galvanizing kettle. The skimmings on the liquid zinc surface are trapped on the zinc coating. In order to remove zinc skimmings without harming the soft zinc coating underneath, lightly brush them off the surface of the galvanized steel during the in-house inspection stage with a nylon-bristle brush. Zinc skimmings are not grounds for rejection. The zinc coating underneath is not harmed during their removal and it meets the necessary specifications.|
|Zinc Spatter||Zinc splatter is defined as splashes and flakes of zinc that loosely adhere to the galvanized coating surface. Zinc splatter is created when moisture on the surface of the galvanizing kettle causes liquid zinc to “pop” and splash droplets onto the product. These splashes create flakes of zinc loosely adherent to the galvanized surface. Zinc splatter will not affect the corrosion performance of the zinc coating and is not cause for rejection. The splatter does not need to be cleaned off the zinc coating surface, but can be if a consistent, smooth coating is required.|
|Cracking||Visually Inspect weld locations for evidence of cracking due to residual stress. Any observed cracking has to be investigated to determine if it is in the base metal. If in the zinc, it can be repaired. If in the base metal, the item requires base metal repairs. *See below for more information on cracking.|