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 | 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| Clogged Threads | Clogged threads are caused by poor drainage of a threaded section after the product is withdrawn from the galvanizing kettle.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. |
