When aluminium metal reacts with steam it produces: 

When aluminium metal reacts with water it produces an oxide known as aluminium oxide.
Water has no action on aluminium because of the layer of oxide on it and the steam is passed over pure heated aluminium then hydrogen is produced and because of the highly negative redox potential, the aluminium reacts with water to produce hydrogen gas according to the equation.
The reaction involved is:
$2\mathit{Al} + 6H_{2}O \to 2{\mathit{Al}}_{\mathit{2}}{O}_3+ 3H_2$