In this article, I will introduce how stainless steel, carbon steel and galvanized steel differ, how each one deals with corrosion, when a buyer orders each series, and what to send so the offer matches the drawing.
What are the three materials?
They are three different orders, not three finishes of one steel. Stainless steel is an iron alloy with enough chromium, about 10.5 percent or more, for a passive oxide film to reform on a clean surface. Carbon steel is iron with carbon and without that stainless chromium level. Left bare and wet, it rusts. Galvanized steel is carbon steel with a zinc coating. The zinc is the protection. The steel underneath is still carbon steel.
A note that says only steel cannot be quoted. The series decides the mill route, the certificate and the corrosion behavior. The form, sheet, coil, pipe or profile, is chosen after the series, because a stainless pipe and a galvanized pipe can share an outline and still be different products.
How each one resists corrosion
Stainless resists in the metal. If the film is scratched, chromium in the alloy helps it reform, provided the service is one the grade was selected for. 304 is the usual general grade. 316 is the usual step up when the part sees chlorides. The grade has to be named. The word stainless is not a grade.
Bare carbon steel has no such film. Paint, oil or a dry indoor location is how fabricators keep it from rusting, and that protection is applied by the process the drawing describes. Structural names buyers write include ASTM A36, EN S235 and the Chinese Q235 or Japanese SS400. Those names are not interchangeable. Copy the one the drawing prints.
Galvanized steel resists by the zinc layer. Zinc is a barrier, and at a scratch it is sacrificed ahead of the steel. On continuously hot-dip coated coil, EN 10346 writes the coating as a mass on both sides together. Z275 means a nominal 275 grams per square metre total, with about 20 micrometres a side used only as a thickness guide. ASTM A653 G90, 0.90 ounce per square foot, is often discussed next to Z275. The units differ, and one is not an automatic substitute for the other. The drawing should name the coating code and the standard.
What the series choice protects
The series is the corrosion method. Getting it wrong is not corrected by a thicker gauge.
A surface that does not rely on a coating
Stainless is specified when the part is washed, coastal, food-plant or expected to stay bright without paint. The certificate is a stainless grade and a stainless standard, such as ASTM A240 for sheet or ASTM A312 for pipe. A galvanized certificate does not satisfy that line.
A lower-cost base metal
Carbon steel is specified when strength and cost come first and corrosion is handled by paint, oil or by the part staying dry. It is the usual metal for internal structure, machining bar and parts the fabricator will coat. It is the wrong order for a part the drawing already calls stainless.
A zinc layer for outdoor steel
Galvanized steel is specified when the drawing asks for zinc and stainless would be more than the service needs. Building profiles, duct, guards and general outdoor fabrication are the usual conversation. Cut edges and welds remove or burn the coating locally. How those areas are repaired belongs on the specification, not in a default.
When each series is the right order
Choose stainless when the drawing names a stainless grade, or when the agreed service needs the alloy itself. Choose plain carbon steel for structure and for parts that will be painted or oiled after fabrication. Choose galvanized when the drawing asks for a zinc coating mass, such as a Z or G code, and the part does not need stainless.
If the coil must arrive already painted, that is PPGI: paint on a zinc-coated substrate, in systems such as PE, SMP or PVDF. It is not bare galvanized coil, and it is not stainless. Paint on an aluminium-zinc substrate is a different product and is not this site's PPGI series. Name the paint system and the substrate the drawing uses.
How the three compare
Compare them on the corrosion method, then on the form. Stainless costs more as an alloy and needs no zinc. Carbon steel is the base metal and rusts if it is left bare and wet. Galvanized steel adds a coating step to carbon steel and is checked by coating mass, not by a stainless grade. A thicker carbon plate does not become stainless. A heavier zinc coating does not become 316.
After the series, the form still has to match. Sheet is thickness, width and length. Coil adds coil weight and inner diameter. Pipe replaces those with outside diameter and wall, and with welded or seamless. Angle, channel and beam are profiles, sized by the section. A stainless pipe and a galvanized pipe are two enquiries even when the sketch looks alike.
FAQ
These are the lines that keep the three series from being mixed on one offer.
Will bare carbon steel stay bright indoors?
Only while it stays dry. Moisture and oxygen rust bare carbon steel. If the part must stay bright without paint, the drawing is usually a stainless grade, not a carbon grade.
Is galvanized steel a kind of stainless steel?
No. Galvanized steel is carbon steel plus zinc. The protection is the coating. Stainless resists by the alloy. The certificates, the standards and the prices are different.
What does Z275 mean?
Under EN 10346 it is a nominal zinc coating of 275 grams per square metre on both sides together. It is a coating mass, not a steel thickness. The steel gauge is a separate line on the order.
Can a galvanized coil arrive already painted?
Bare galvanized coil has zinc and no paint. A coil that arrives painted is PPGI, and the enquiry should name the paint system. Ordering galvanized when the line expects a painted coil, or the reverse, is a different product.
Which form comes after the series?
Sheet, coil, pipe or profile. The series does not imply the form. Write both, plus the grade or coating code, the standard, the size and the quantity.
What to send with the enquiry
Name the series first: stainless, carbon or galvanized. Then the form, the grade or the coating code, and the standard exactly as the drawing prints them. Add size, quantity, surface or paint system, and whether a certificate is required. A stainless pipe, a carbon pipe and a galvanized pipe are confirmed as three separate lines. They are not assumed from one sketch.