These corner braces are perfect for reinforcing right angle corner joints. They can be used both indoors and outdoors and are rust-resistant.
Prosource CB-Z10-013L Corner Brace, 10 in L, 10 in W, 1-1/4 in H, Steel, Zinc-Plated, 5.3 mm Thick Material
National Hardware 115BC Series N220-145 Corner Brace, 4 in L, 7/8 in W, Steel, Zinc, 0.12 Thick Material
National Hardware N272-740 Mending Brace, 5 in L, 5/8 in W, Steel, Zinc, Screw Mounting
ProSource MP-Z02-013L Mending Plate, 2 in L, 5/8 in W, Steel, Screw Mounting