GI GL and PPGI Coated Steel Coil Compared

What the Letters Stand For

GI is galvanized coil, a steel substrate carrying a zinc coating applied either by hot dipping or by an electro process. GL is galvalume, where the coating is an aluminum zinc alloy instead of zinc alone. PPGI and PPGL are the pre painted versions of those two, supplied with the paint already on the substrate so a roll former can run finished panels without a paint line.

The prefix tells you nothing about the steel underneath. The same substrate grades appear across all four products, which is why the grade and the coating weight both have to be named on the order.

• GI carries a zinc coating applied hot dipped or electro
• GL carries an aluminum zinc coating and is chosen for outdoor exposure
• PPGI is galvanized substrate with the mill applied paint system
• PPGL is galvalume substrate with the mill applied paint system
• Galvanized steel strip is the slit version of GI for narrow profiles

How the Coating Chemistry Changes Behavior

Zinc protects steel sacrificially, which is why a scratch in a galvanized panel does not immediately undercut. An aluminum zinc coating trades some of that sacrificial action for a more stable barrier, and galvalume steel coil is listed as having higher corrosion resistance than standard galvanized steel coil in outdoor environments, resisting corrosion better when exposed to snow, ice and rain.

That difference shows up on roofing and exterior wall panels long before it shows up in a salt spray report. A cladding job that sits under repeated wet and dry cycles is the case where galvalume earns its price over plain galvanized.

• Zinc gives sacrificial protection at scratches and cut edges
• Aluminum zinc gives a more stable barrier under repeated wetting
• Galvalume is named for exterior metal paneling and roofing
• Paint on PPGI and PPGL adds a further barrier over the metallic coating

Reading the Coating Weight Designation

Coating weight is ordered in one of two systems and both are quoted here. The G system runs G20 G30 G40 G60 and G90. The Z system runs Z40 Z60 Z80 Z100 Z120 Z150 and then Z275 and Z350 at the heavy end.

A coil is only as durable as the coating layer on it, so a buyer who names the substrate grade but leaves the coating weight blank is leaving the most important variable to the mill. Fix the designation on the purchase order and the comparison between two quotations becomes meaningful.

• G system available as G20 G30 G40 G60 and G90
• Z system available as Z40 Z60 Z80 Z100 Z120 Z150 Z275 and Z350
• Heavier Z275 and Z350 coatings suit exposed structures
• Lighter coatings suit interior partitions and painted indoor parts
• Name the designation on the order rather than asking for galvanized coil

Choosing the Substrate Grade

The structural grades quoted across these coils are G250 G350 G450 and G550, alongside A36 SS400 S275jr S235 A653 Q195 Q215 Q235B Q345B SGCC and DX51D. The soft end forms deep profiles without splitting. The hard end holds a thin high rib profile that would otherwise buckle.

Galvalume and pre painted galvalume also list Q195 Q215 Q235B Q345B S235JR S355JR SS400 SS490 SM400 SM490 G350 and G550, which covers most European Japanese and Chinese drawing designations without a substitution letter.

• G250 and DX51D type grades for deep drawn and folded profiles
• G450 and G550 for thin gauge high rib roofing profiles
• SGCC and A653 for buyers working to Japanese or American drawings
• Q195 through Q345B and S235JR through S355JR for structural sections

Where Zn Al Mg Fits Above All Three

Zn-Al-Mg coil, also called AZM or zinc aluminum magnesium coated coil, adds magnesium and a small amount of silicon to the zinc aluminum coating. The stated corrosion resistance is 10 to 20 times that of hot dip galvanized steel coil at less coating adhesion, which is the point of the product.

It also self heals at cut edges. Magnesium aluminum and zinc dissolving at a scratch form a dense film of zinc hydroxide, alkaline zinc chloride and magnesium hydroxide that covers the exposed steel and separates it from the corrosive medium. Paintability is similar to galvalume and welding is slightly worse than hot rolled and cold rolled sheet but workable with a reasonable procedure, with the weld repairable using zinc aluminum paint.

• Corrosion resistance stated at 10 to 20 times hot dip galvanized coil
• Self healing film forms over scratches and cut edges
• Alkali resistance strong enough to replace stainless steel in individual fields
• Good scratch resistance on handling and roll forming
• Paintability similar to galvalume for a further coated finish

Matching the Coil to the Exposure

Work backwards from where the panel ends up. Interior framing and ceiling sections take a light zinc coating on a soft grade. Roof sheets on an inland warehouse take a mid Z coating on GI or PPGI. Coastal cladding, agricultural buildings with ammonia exposure and solar mounting rails are where galvalume or Zn-Al-Mg justify the step up.

Color is a separate decision from corrosion. Pre painted coil removes a paint booth and a rework rate from your line, but the metallic coating underneath is still what decides service life.

• Interior sections take lighter coating on a forming grade
• Inland roofing takes GI or PPGI at a mid coating weight
• Coastal and wet cycle exposure moves the choice to GL or PPGL
• Structures expected to stand for decades move to Zn-Al-Mg
• Pre painted coil removes the paint shop step from panel production

What to Put on the Purchase Order

A complete coated coil line reads as coating type, substrate grade, coating weight designation, thickness, width, coil dimensions and finish. Anything left off becomes a mill decision. Thickness on the coated coil range is listed from 0.1mm and is customized as required, so the gauge belongs on the order line rather than in a follow up email.

Ask for the mill test certificate with the shipment so the grade and coating weight are documented for the building file. Certification available on request covers ISO9001, BV, SGS and RoHS.

• Coating type GI GL PPGI PPGL or Zn-Al-Mg
• Substrate grade in the designation used on your drawing
• Coating weight in the G or Z system
• Thickness width and strip width where strip is needed
• Paint system and color reference for pre painted coil
• Mill test certificate requested with the shipment