Which of the following statement is incorrect about duralumin?

In addition to aluminum, the main materials in duralumin are copper, manganese and magnesium.
Although the addition of copper improves strength,
it also makes these alloys susceptible to corrosion.
For sheet products, corrosion resistance can be greatly enhanced by metallurgical bonding of a high-purity aluminium surface layer. These sheets are referred to as alclad, and are commonly used by the aircraft industry