Why do metal roof panels rust prematurely even with anti-corrosion coating?

Many industrial owners and engineers face a common dilemma: Why do metal roof panels rust prematurely even with anti-corrosion coating?
The core difference lies in the base substrate: ordinary galvanized steel vs galvalume steel. 
Today we make a detailed professional comparison for steel structure & roofing project selection.

 Galvanized Steel (Z Coating)
 Coating composition: pure zinc layer. It only relies on sacrificial anode protection. The zinc layer consumes continuously to protect the steel substrate, with no dense isolation barrier. 
Galvalume Steel (AZ Coating)
 Coating composition: 55% Al + 43.4% Zn + 1.6% Si. It adopts dual protection: aluminum forms a dense physical barrier to isolate water & oxygen, while zinc provides sacrificial anti-rust protection. Silicon enhances coating adhesion and processing resistance.

Core Anti-corrosion Performance Data Comparison
 1. Salt Spray Resistance: Galvalume is 3-4 times better than ordinary galvanized steel
 2. Heat Resistance: Galvalume withstands high temperature and ultraviolet aging, suitable for long-term outdoor exposure
 3. Edge Self-healing: Galvalume alloy ions can migrate to cover cutting notches, effectively preventing edge rust; galvanized sheets fail rapidly at cut edges
 4. Weather Resistance: In coastal & industrial corrosive environments, galvalume avoids peeling and blistering in short-term use

In short, galvanized sheets are cost-effective for ordinary inland projects; while galvalume sheets are the optimal choice for high-corrosion and long-life demand projects. 

What kind of substrate do you usually choose for industrial roofing projects? Welcome to share your experience in the comment section!