Many boundary wall grills are made of mild steel or iron; sometimes cast iron elements. For example, a Mumbai manufacturer lists “iron, material: iron, colour: black” for boundary wall grills
Finish is extremely important—especially outdoors in Maharashtra (humidity, sea‐air). A powder-coated finish helps protect against rust and corrosion. One listing notes “Powder Coating” finish for mild steel boundary wall grill.
Thickness & strength: In design advice, it's suggested to use solid bars (12-16 mm) so it is safe, strong and not overly climbable.
The design can vary: from decorative scroll / floral motifs to more modern minimalist straight bars. Your choice depends on your home style.
According to one article, design considerations include balancing privacy, ventilation, load & wind‐resistance.