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.
- Preparation
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.
- Cooling
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.
- Sampling
Section 7 of ASTM A123 establishes the number of items to be tested based on the number of pieces in a lot.
- Thickness
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
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.
- Cracking
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.
