In this article, I will introduce what galvanized steel is, how the zinc coating is designated, where sheet, coil and pipe are used, and how plain galvanized steel differs from painted coil.
What is galvanized steel?
Galvanized steel is carbon steel protected by a zinc coating. The zinc corrodes preferentially and slows rust on the steel underneath. On flat product the usual industrial route is continuous hot-dip coating: the strip passes through a zinc bath, and the coating mass is controlled on the line. That product is often called GI, or Z in the EN 10346 designation. Electro-galvanized sheet, a thinner zinc applied by current, is a different product and should be named when the drawing asks for it.
The coating designation is the order, not the word galvanized. Under EN 10346 the number is the nominal coating mass in grams per square metre for both surfaces together. Z275 means 275 grams per square metre including both sides, with a typical guidance thickness of about 20 micrometres on each side. ASTM A653 writes a related idea as a G number. G90, 0.90 ounces per square foot total, is the designation often set beside Z275. They are not automatically the same order unless the drawing accepts that equivalence. Heavier codes, such as Z450 or Z600, exist. Thicker zinc can limit forming and welding, which EN 10346 itself notes, so the coating mass has to suit the fabrication, not only the corrosion wish.
How the coating and the form are specified
EN 10346 also separates coating types. Z is zinc. ZF is a zinc-iron alloy coating, the galvannealed surface used when the next step is painting and a smoother, more paintable face is required. ZA, ZM and AZ are other hot-dip alloys, including zinc-aluminium and aluminium-zinc. An aluminium-zinc coating is the product many buyers call galvalume. It is not Z275. If the drawing says AZ or galvalume, a plain zinc coil is the wrong quote.
Flat product is sold as coil or as sheet. Coil feeds roll forming, slitting and cut-to-length. Sheet is already cut. The steel grade under the zinc still matters: a structural grade and a forming grade do not bend the same way. Spangle is the visible zinc crystal. Regular, minimized and zero spangle are appearance choices, and zero spangle is the usual request when the coil will be painted. Surface treatment, such as passivation or oil, is a further line because it affects storage stain and later painting.
Galvanized pipe is a different line. The buyer needs outside diameter, wall and length, not a coil weight. A pipe coated after forming is not a coil that was slit and welded without checking the coating on the weld. Do not convert a coil enquiry into a pipe enquiry by changing only the size words.
What a stated coating mass is for
Zinc weight is the corrosion allowance. Leaving it off the enquiry makes two offers impossible to compare.
Corrosion life tied to a number
A light coating and a heavy coating can both be called galvanized. Z275 and a heavier Z code are different amounts of zinc. The service and the drawing pick the code. Outdoor life is not a single year-count that travels from one climate to another.
A surface the next process can use
Painting, roll forming and welding care about spangle, treatment and coating thickness. A minimized or zero spangle, and a ZF coating when the drawing asks for galvanneal, keep the next process on the surface it was planned around.
The right product shape
Coil, sheet and pipe do not share a size line. Naming the form first stops a coil price being read as a pipe price.
When galvanized steel is specified
Galvanized sheet and coil are specified for ducting, purlins, light structure, appliance inners and general outdoor parts where the drawing asks for zinc and stainless would be more than the service needs. Pipe is specified when the drawing is a galvanized hollow section. Order plain galvanized product when the factory will paint locally, or when no paint is required.
Order PPGI instead when the coil must arrive already painted. Order stainless when the drawing names a stainless grade, or when a coating that can be scratched through is not the protection the duty allows. Order bare carbon steel when the fabricator will oil or paint the part and the drawing does not ask for zinc.
Galvanized steel versus PPGI
Galvanized steel stops at the zinc. PPGI is that galvanized strip with a paint system applied at the mill. The paint has its own type, color, thickness and back coat. A galvanized coil cannot be invoiced as PPGI because the buyer plans to paint it later, and a painted coil is not a substitute for a bare Z275 coil that will be formed and then painted on a different system.
Inside galvanized steel, Z, ZF and AZ are also different. Zinc, galvanneal and aluminium-zinc change corrosion behavior, weld appearance and paint adhesion. The letters on the drawing are the comparison. The price is not.
FAQ
These answers keep a galvanized enquiry from collapsing into one word.
What does Z275 mean?
In EN 10346 it is a zinc coating whose nominal mass is 275 grams per square metre counting both sides. It is not a steel grade. The grade is named separately.
Is G90 the same as Z275?
G90 is the ASTM A653 way of writing a total zinc mass of 0.90 ounces per square foot, often set beside Z275. Use the designation the drawing prints. Do not swap the two unless the specification allows it.
What is galvanneal?
It is the ZF, zinc-iron coating. The surface is duller and is often chosen for painting. It is not a heavier Z coating.
Does a thicker coating always last longer?
More zinc is more corrosion allowance, but EN 10346 notes that thicker coatings can limit forming and welding. The drawing balances the two. Climate and design details change the life as well.
Sheet, coil or pipe?
Coil for a forming line, sheet for cut lengths, pipe for a hollow section. Each one has its own size line. Painted coil is PPGI, not a fourth finish of the same coil.
What to send with the enquiry
Name the form, the steel grade, the standard, the coating letters and number, the spangle, the surface treatment, the size and the quantity. Say whether the coil will be painted later. Those items are checked on the enquiry before the coil is produced. The size range is confirmed from the drawing, not from the coating code.