There is no concern there. In order to burn, you would have grind those frames up into fine particles to increase the oxygen to magnesium contact ratio. It burns only as a powder or very thin ribbon. A larger, solid piece will not ignite. Think of it. It is used in all kinds of lightweight, high temperature applications like engine blocks without concern. When a race car bursts into flames, the magnesium rims, transmission casings, engine blocks, etc, do not go up in flames with it. If they did, few very drivers would survive and most race car fans would be suffering from varying degrees of blindness.