Three Japanese-standard designations, SGCC, SECC and SGLD, dominate specifications for coated steel sheet in appliances, electronics and reflectors. They look similar but are produced by completely different processes, with different coating thicknesses, corrosion mechanisms and formability. Mis-specifying them is one of the most common causes of field failures in stamped and painted parts. This article explains each grade per JIS G3302, JIS G3313 and JIS G3321 and shows how to select correctly.
1. SGCC: Hot-Dip Galvanized Sheet per JIS G3302
SGCC is a hot-dip galvanized sheet: the strip is immersed in molten zinc, producing a bright white surface with visible hexagonal spangles. The coating class is denoted by Z plus a number equal to ten times the coating mass in g/m² total for both sides. For example, Z12 means 120 g/m² total (about 8-9 µm per side). Common classes range from Z06 to Z90, and structural grades SGC340 to SGC570 are specified for load-bearing use. SGCC offers robust corrosion protection with cathodic action at cut edges, good weldability and low cost, and is widely used for roofing accessories, cable trays, air ducts and general sheet-metal parts. Its surface is less uniform than electrogalvanized sheet, so very smooth painted finishes are better achieved on SECC.
2. SECC: Electrogalvanized Sheet per JIS G3313
SECC is an electrolytic zinc-coated sheet. Zinc is deposited from an electrolyte, giving a thin, uniform gray coating (classes E16, E24, E32 per JIS G3313, corresponding to roughly 16, 24 and 32 g/m²), typically a few microns per side. Because the substrate is a cold-rolled quality sheet and the coating is thin, SECC retains the excellent surface finish, flatness and formability of cold-rolled steel, is fingerprint-resistant when additionally passivated, and paints extremely well. It is the standard material for computer cases, home appliance casings, switchgear panels, door skins and other appearance-critical parts. The trade-off is much lower corrosion resistance than SGCC in humid or outdoor service, so SECC is an indoor material.
3. SGLD: Aluminum-Zinc Alloy Coated Sheet per JIS G3321
SGLD-type products belong to the 55% aluminum-zinc alloy coated family standardized in JIS G3321 (grades SGLCC, SGLCD, SGLCE) and in ASTM A792 as AZ coatings. The coating contains approximately 55% Al, 43.4% Zn and 1.6% Si. Compared with SGCC it offers better atmospheric corrosion resistance, better heat resistance, higher thermal conductivity and reflectivity, and improved formability and weldability in thin-gauge applications. These properties make SGLD the preferred choice for heat-reflective parts such as oven reflector plates, cooker liners and lamp housings, as well as for long-life roofing where edge creep must be minimized. Common coating classes are AZ120 to AZ185 (120-185 g/m² total both sides).
4. Selection Guide
| Property | SGCC | SECC | SGLD (55% Al-Zn) |
|---|---|---|---|
| Process | Hot-dip galvanizing | Electrogalvanizing | Hot-dip Al-Zn alloy |
| Standard | JIS G3302 | JIS G3313 | JIS G3321 / ASTM A792 |
| Typical coating | Z12-Z27 (120-270 g/m²) | E16-E32 (16-32 g/m²) | AZ120-AZ185 |
| Corrosion resistance | Good (cathodic at edges) | Moderate (indoor) | Best in atmosphere |
| Surface finish | Spangled, bright | Uniform, smooth | Spangled, silvery |
| Typical use | Ducts, roofing, general parts | Electronics, appliance casings | Oven reflectors, roofing, heat shields |
Frequently Asked Questions
What does Z12 mean on an SGCC sheet? Per JIS G3302, Z12 means a total zinc coating mass of 120 g/m² for both sides combined, approximately 8-9 µm per side; the number is ten times the mass in g/m².
Is SECC suitable for outdoor use? No. With only 16-32 g/m² of zinc it is designed for indoor appliances and electronics; outdoor exposure needs SGCC, SGLD or a hot-dip product with a higher coating class.
Why does SGLD reflect heat better than SGCC? The 55% aluminum-zinc coating has a higher infrared reflectivity and thermal conductivity than pure zinc, making it the standard choice for oven and cooker reflector parts.
Can SECC be painted directly? Yes, after passivation it gives one of the best paint bases among coated steels thanks to its smooth, uniform surface; this is why computer and appliance housings use SECC.
Which grade should I choose for a coastal building roof? n aluminum-zinc coated sheet (SGLD per JIS G3321 or AZ150 per ASTM A792) is the better long-term choice in coastal atmospheres because its oxide film suppresses cut-edge corrosion.
Do SGCC and SGLD have the same weldability? Both weld well by resistance welding. For painted or thin-gauge parts, SGLD's aluminum-rich coating needs slightly adapted welding parameters; for general framing, SGCC is the simpler choice.