Iron alloys can corrode outdoors, and scientists use simulations to predict how they will hold up. These simulations can give a good idea of how well the alloys resist corrosion, but the rate of corrosion in simulations is faster than in real life. The composition of the corrosion products and their structure can vary depending on the corrosion process. The outer layers of the products are likely to be hydrated oxides and oxy-hydroxides, while the inner layers contain less voluminous oxides. However, the macroscopic structure may not fully represent the layered structure, thickness, or mechanical properties of the corrosion products. Near cracks in the products, the structure is more similar to the surface.