Galvanization is a widely used industrial process that applies a protective zinc coating to iron or steel materials. This metallic layer shields the underlying ferrous metal from corrosion, significantly extending its service life. The thickness of this zinc coating is the most important factor determining the duration of its protection. A thicker coating provides a greater reservoir of sacrificial zinc, which translates directly to a longer lifespan.
Factors Influencing Coating Thickness
The final thickness of a galvanized coating is not uniform across all products. It is a complex outcome shaped by the specific galvanizing method and the chemistry of the base steel. Hot-dip galvanization, where steel is immersed in molten zinc, produces the thickest and most durable coatings available. This process creates a metallurgical bond, forming zinc-iron alloy layers that are harder than the base steel.
The composition of the steel substrate plays a substantial role, particularly its silicon and phosphorus content. Steels containing silicon in the “Sandelin curve” range (approximately 0.04% to 0.13%) exhibit a highly accelerated reaction with molten zinc. This often results in much thicker, but sometimes less uniform, coatings. Conversely, low-silicon steels typically form a more controlled and thinner layer of zinc-iron alloys.
Process parameters such as the temperature of the molten zinc bath and the duration of immersion also affect the final thickness. Higher bath temperatures accelerate the reaction rate between the iron and zinc, leading to a faster formation of the zinc-iron alloy layers. However, for low-silicon steels, extending the immersion time often does not significantly increase the coating thickness.
Other galvanizing methods produce coatings considerably thinner than the hot-dip process. Electro-galvanization deposits zinc using an electric current, yielding a thin coating typically in the range of 5 to 25 micrometers (µm). Mechanical plating, which uses a tumbling process with zinc powder, also results in a thinner coating, usually between 12 and 25 µm. These thinner coatings are better suited for applications requiring dimensional precision but offer less long-term corrosion protection.
Industry Standards for Minimum Thickness
The required minimum thickness for a galvanized coating is defined by industry standards, ensuring the material is fit for its intended use. The two primary specifications in the United States are ASTM A123, which covers coatings on fabricated structural steel products, and ASTM A153, which specifies coatings for hardware items like fasteners and castings. These standards establish minimum thickness requirements proportional to the thickness or size of the underlying steel material.
For structural steel products covered by ASTM A123, the minimum average coating thickness is measured in micrometers (µm) or mils. A common requirement for steel sections greater than 6.0 millimeters thick is a minimum average thickness of 100 µm (3.9 mils). This requirement is based on the principle that thicker steel retains heat longer, promoting the growth of the zinc-iron alloy layers.
Thinner structural materials, such as plate steel between 1.5 and 3.0 millimeters thick, require a lower minimum average coating of 65 µm (2.6 mils). The coating weight is sometimes expressed in ounces per square foot (\(\text{oz}/\text{ft}^2\)). For example, a 100 µm coating is roughly equivalent to 2.3 ounces per square foot. All thickness requirements in these specifications are minimums, reflecting the direct link between coating thickness and expected service life.
For hardware and fasteners covered by ASTM A153, products are categorized into classes with differing minimum thickness requirements. A large fastener, classified as Class C (over 3/8 inch in diameter), must have a minimum average thickness of 2.1 mils. Smaller hardware, such as nails and rivets, are in Class D and require a minimum average thickness of 1.7 mils. These minimums reflect that hardware is often centrifuged after galvanizing to remove excess zinc, which preserves the threading.
Methods for Measuring Coating Thickness
Quality control procedures verify that the final galvanized coating meets the minimum thickness requirements set by governing standards. The most common and practical method for field inspection is a non-destructive test utilizing a magnetic induction gauge. This electronic instrument measures the magnetic flux created between the probe and the underlying steel. This measurement is inversely proportional to the thickness of the non-magnetic zinc coating.
Magnetic induction gauges allow for quick, simple, and non-destructive measurement of thickness on a finished product. Standards like ASTM A123 require multiple local thickness readings across a component to determine a comprehensive average thickness. This average is then compared against the specified minimum to ensure compliance.
While magnetic gauges are used for routine inspection, destructive methods are sometimes employed for laboratory verification or in cases of dispute. One destructive technique is the stripping and weighing method. Here, the zinc coating is chemically removed from a known surface area, and the mass of the removed zinc is measured to calculate the coating weight and thickness. Optical microscopy is another destructive method that involves cross-sectioning a sample and visually measuring the coating thickness under magnification.