ISO 9224:2012 specifies guiding values of corrosion attack for metals and alloys exposed to natural outdoor atmospheres for exposures greater than one year. It is intended to be used in conjunction with ISO 9223. Guiding corrosion values for standard structural materials can be used for engineering calculations. The guiding corrosion values specify the technical content of each of the individual corrosivity categories for these standard metals. ISO 9224:2012 provides examples of calculated maximum corrosion attack after extended exposure (up to 20 years) for six standardized corrosivity categories. It provides presumed average initial and steady-state corrosion rates of standard metals in intervals relative to six standardized corrosivity categories. It provides the calculation procedure for corrosion attack of steels in regard to their composition.