An Introduction to Z120 Galvanized Steel Coil

Z120 is a coating designation for continuously hot-dip galvanized steel coil, where the number 120 indicates a double-sided zinc coating weight of 120 grams per square metre (g/m²). The letter Z stands for zinc. This coating weight corresponds to an approximate single-side thickness of 12 micrometres (μm) of pure zinc. In the context of steel standards such as EN 10346, JIS G 3302, and ASTM A653, Z120 is considered a medium-level coating weight that provides a balanced compromise between corrosion resistance and cost efficiency. The product is produced by immersing cold-rolled steel strip into a molten zinc bath at approximately 450 degrees Celsius (around 840 degrees Fahrenheit), which forms a metallurgically bonded zinc layer. This zinc coating acts as a barrier against moisture and oxygen and also serves as a sacrificial anode, meaning the zinc corrodes preferentially to protect the underlying steel even if the coating is scratched.

Zinc Coating Specification

The Z120 coating has a nominal double-sided weight of 120 g/m², which under the three-point test method averages approximately 120 g/m² across three measurement points on both sides of the coil. In single-point test measurements, the coating weight is typically around 90 g/m². For comparison, lighter coatings such as Z60 and Z80 are rated for ordinary indoor environments with 6 to 8 μm thickness, while Z100 with 10 μm thickness is used for outdoor structural components. Z120 offers 12 μm of zinc per side and is specifically recommended for high salt spray environments, including coastal areas. Heavier coatings such as Z150, Z180, and Z275 with thicknesses of 15 μm and above are specified for extreme corrosive conditions such as power towers and chemical plants. The coating is applied through the continuous hot-dip galvanising process, where the steel strip passes through a bath of molten zinc, and the coating weight is precisely controlled by air knives that wipe off excess zinc as the strip exits the bath.

Key Properties and Performance

Z120 galvanised steel provides moderate to good corrosion resistance suitable for many applications. Under salt spray testing, DX51D+Z120 typically achieves approximately 500 to 800 hours before red rust appears. This compares to approximately 1,000 to 1,500 hours for the heavier Z275 coating. In terms of formability, the zinc coating on Z120 is sufficiently ductile to withstand moderate bending, stamping, and profiling without cracking or delamination. The base steel, typically a low-carbon grade such as DX51D or SGCC, provides a minimum tensile strength of 270 to 350 MPa, yield strength of at least 140 MPa, and minimum elongation of 22 percent when measured on an 80 mm gauge length. The steel can be cold bent at 180 degrees without cracking, and the coating adhesion is good, with no flaking observed after bending or impact testing. Hardness typically ranges from 70 to 130 HV. The material offers good weldability using standard resistance welding methods and provides excellent paint adhesion when surface preparation is performed. However, for outdoor applications in persistently wet or chemically aggressive environments, Z120 may require additional protective coatings to achieve extended service life.

Surface Finishes and Treatments

Z120 galvanised steel is available with several surface finish options that affect both appearance and performance. Regular spangle finishes display visible crystalline zinc patterns formed during cooling, which can be aesthetically desirable for architectural applications. Zero or minimised spangle finishes are produced through controlled cooling or the addition of alloying elements, resulting in a smooth, uniform surface that is preferred for painting and powder coating. Surface treatments can be applied to the zinc coating, including chromate passivation for improved corrosion resistance, non-chromate passivation for environmentally compliant applications, and oiling for temporary protection during storage and handling. Skin-passed finishes, which involve a light cold rolling after galvanising, improve surface flatness and uniform appearance. The product can also be supplied as unoiled or dry for applications requiring immediate painting or adhesive bonding.

Typical Applications

Z120 galvanised steel is widely specified for both general industrial use and more demanding environments due to its balanced corrosion protection and cost efficiency. In the construction sector, it is used for roofing coils, wall cladding, prefabricated building components, corrugated panels, structural keels, garage doors, and light gauge steel framing. In the household appliance industry, applications include refrigerator outer panels, washing machine housings, microwave oven shells, and air conditioner casings. For general industry, Z120 is found in HVAC ductwork, cable trays, storage shelving, industrial enclosures, and electrical cabinets. In agriculture, the material is used for greenhouse frames and simple grain storage silos. The automotive sector utilises Z120 for certain internal components and spare parts where exposure is not extreme. The product is particularly recommended for dry or inland environments and for sheltered outdoor conditions. For coastal regions or high-humidity environments where salt spray exposure is significant, Z120 provides a recommended level of protection, though heavier coatings such as Z180 or Z275 would offer a longer service life.

Available Steel Grades and Standards

Z120 coating can be applied to various base steel grades, each suited to different forming and strength requirements. Under the European EN 10346 standard, common grades include DX51D+Z for general forming and bending, DX52D+Z for drawing quality, DX53D+Z for deep drawing, and DX54D+Z for very deep drawing. Structural grades such as S220GD+Z, S250GD+Z, S280GD+Z, S320GD+Z, and S350GD+Z are available where higher strength is required. Under the Japanese JIS G 3302 standard, SGCC is the commercial quality grade equivalent to DX51D+Z, while SGCD1, SGCD2, and SGCD3 are drawing grades with increasing formability. Structural Japanese grades include SGC340, SGC400, SGC440, and SGC490. Under the American ASTM A653/A653M standard, Z120 coating corresponds to a minimum triple-spot requirement of 120 g/m². The standard also specifies various steel grades, including CS Type B for commercial quality, FS for forming steel, DDS for deep drawing steel, and EDDS for extra deep drawing steel, as well as structural grades from 33 to 80 ksi.