There are a number of types of aluminum anodize - Sulfuric, Sulfuric Hard Coat, Chromic Acid, Phosphoric Acid, and Boric Sulfuric Acid, and Thin Film Sulfuric are the most common in the finishing industry. They all have their uses and some are being phased out of the finishing industry like Chromic due to all the environmental and health problems with Cr+6.
What any anodize does is to produce an aluminum oxide coating onto the parts surface. This coating changes with the type of anodize and can have any number of thickness. Some are better for bonding to, others are better for corrosion protection.
What you are really looking at here is the difference between Sulfuric Acid Anodize and Sulfuric Acid Hard Coat. Sulfuric Acid anodize is the most common anodize you will run into outside of aerospace as it can be dyed many different colors such as blue, red, black, etc. It has good corrosion protection, is fairly hard, and its thickness will not generally effect the fatigue hardness of a fatigue loaded part.
Sulfuric Hard coat has GREAT corrosion protection, is hard as hell, not all that easy or cheep to apply and can not be dyed. It is very thick, anodize coating wise, and will lower fatigue life of loaded parts. If you have a plate that is Hard Coated it should be a light to medium grey unless it has been painted or powder coated. As the coating is nonconductive it will much better withstand galvanic effects from dissimilar metals.
But as a poster said, if you rinse and dry your plate well every time you will not notice the difference between Sulfuric Acid Anodize and Sulfuric Hard Coat in any reasonable time frame.
