**2.7. Aluminum and their coating**

In terms of cost and density, aluminum offers a better substitute compared to other bipolar plate materials. It also has an inherent problem of developing an oxide film like stainless steel and titanium. This reduces its surface conductivity and rendered it incompetent as bipolar plate material except used in combination with other metals or with suitable coating blend.

#### **2.8. Nickel and their coatings**

Nickel is comparatively inexpensive, and it exhibits good ductility and ease of manufacturing. Pure nickel does not form protective oxide layer like other known bipolar plate materials but is very susceptible to severe corrosion. Therefore, there is need for alloying it with chromium to be very stable at minimum corrosion rate and low electrical resistivity compared to stainless steel alloys.
