Corrosion processes begin immediately following a disaster, but corrosion does not proceed at a uniform rate over time. The rate is greatest at or about the time when the fire is extinguished or the flood water has drained. Thereafter, the corrosion rate decreases gradually over time but never reaches zero.
At the conclusion of the fire-fighting effort, conditions are optimum for rapid corrosion: hot, wet, acidic and highly halogenated. It is not entirely surprising to see moist brown rust appearing on steel surfaces even before the firemen have left the building. Fortunately, this rapid corrosion rate is not maintained for long. The temperature will drop, and ventilation will produce airflow that removes humidity and most of the acidic halogen gases. Within a few hours of the fire, it would be difficult to detect any HCI or HBr in the air. On surfaces inside the building, however, they remain easy to detect in puddles of water on the floor, inside equipment, in brick, concrete, wall cavities, fabric, dust, soot and other hydroscopic materials.
Bacteria and fungi will produce corrosion even on stainless steels and copper/nickel alloys, as well as cast iron, aluminum, and concrete. The sulfate reducing bacteria, such as Desulfovibrio, are supported by sulfate ion, which is produced in fires involving coal, paper, wood or heating oil. Furthermore, electrolytic corrosion occurs whenever two dissimilar metals are in contact with an external electrically conductive liquid. This can cause rapid failure of soldered, brazed or welded joints.