There are about 2 type Galvalume coil in China, maybe they look like no big difference ,but the price is 

1. Normal Galvalume coil .  

For normal Galvalume coil , the coating chemical composition is 55% aluminum, 43% zinc, and 1.6% silicon ， with  high corrosion resistance. But the price is also higher.

2. PPGL base plate galvalume coil 

For this kind , the aluminum content is just about 20 to 30%. So corrosion resistance performance is not so good. With lower price. 

For some customers who only want low price , even some suppliers told you the aluminum content is 55% , but at last , what they get is just 20% to 30%. 

In a word , you will get what you paid.